Computer Networks Chapter 2 Getting Connected Problems n

Computer Networks Chapter 2: Getting Connected

Problems n In Chapter 1 we saw networks consists of links interconnecting nodes. How to connect two nodes together ? n We also introduced the concept of “cloud” abstractions to represent a network without revealing its internal complexities. How to connect a host to a cloud ? CN_2. 2

Chapter Outline n Perspectives on Connecting nodes n Encoding n Framing n Error Detection n Reliable Transmission n Ethernet and Multiple Access Networks n Wireless Networks CN_2. 3

Chapter Goal n Exploring different communication medium over which we can send data n Understanding the issue of encoding bits onto transmission medium so that they can be understood by the receiving end n Discussing the matter of delineating the sequence of bits transmitted over the link into complete messages that can be delivered to the end node n Discussing different technique to detect transmission errors and take the appropriate action CN_2. 4

Chapter Goal (contd. ) n Discussing the issue of making the links reliable in spite of transmission problems n Introducing Media Access Control (MAC) Problem n Introducing Carrier Sense Multiple Access (CSMA) networks n Introducing Wireless Networks with different available technologies and protocols CN_2. 5

2. 1 Perspectives on Connecting An end-user’s view of the Internet CN_2. 6

Link Capacity and Shannon-Hartley Theorem n Gives the upper bound to the capacity of a link in terms of bits per second (bps) as a function of signal-to-noise ratio of the link measured in decibels (d. B). n C = B log 2(1+S/N) Where B = 3300 – 300 = 3000 Hz, S is the signal power, N the average noise. l The signal to noise ratio (S/N) is measured in decibels is related to d. B , SNR = 10 x log 10(S/N). If there is 30 d. B of noise then S/N = 1000. l Now C = 3000 x log 2(1001) = 30 kbps. l How can we get 56 kbps? l CN_2. 7

Links n All practical links rely on some sort of electromagnetic radiation propagating through a medium or, in some cases, through free space n One way to characterize links, then, is by the medium they use l Typically copper wire in some form (as in Digital Subscriber Line (DSL) and coaxial cable), l Optical fiber (as in both commercial fiber-to-the home services and many long-distance links in the Internet’s backbone), or l Air/free space (for wireless links) CN_2. 8

Links n Another important link characteristic is the frequency l Measured in hertz, with which the electromagnetic waves oscillate n Distance between the adjacent pair of maxima or minima of a wave measured in meters is called wavelength Speed of light divided by frequency gives the wavelength. l Frequency on a copper cable range from 300 Hz to 3300 Hz; Wavelength for 300 Hz wave through copper is speed of light on a copper / frequency l 2/3 x 108 /300 = 667 x 103 meters. l CN_2. 9

Links n Placing binary data on a signal is called encoding. n Modulation involves modifying the signals in terms of their frequency (FM), amplitude (AM), and phase (PM). CN_2. 10

Links Electromagnetic spectrum CN_2. 11

Links Common services available to connect your home CN_2. 12

2. 2 Encoding Signals travel between signaling components; bits flow between adaptors NRZ encoding of a bit stream CN_2. 13

Encoding (NRZ) n Problem with NRZ l Baseline wander 4 The receiver keeps an average of the signals it has seen so far 4 Uses the average to distinguish between low and high signal 4 When a signal is significantly low than the average, it is 0, else it is 1 4 Too many consecutive 0’s and 1’s cause this average to change, making it difficult to detect CN_2. 14

Encoding (NRZ) n Problem with NRZ l Clock recovery 4 Frequent transition from high to low or vice versa are necessary to enable clock recovery 4 Both the sending and decoding process is driven by a clock 4 Every clock cycle, the sender transmits a bit and the receiver recovers a bit 4 The sender and receiver have to be precisely 4 Too many consecutive 0’s and 1’s cause problem (non-synchronized) synchronized clock recovery CN_2. 15

Encoding (NRZI) n NRZI l Non Return to Zero Inverted l Sender makes a transition from the current signal to encode 1 and stay at the current signal to encode 0 l Solves for consecutive 1’s problem CN_2. 16

Encoding (Manchester) n Manchester encoding l Merging the clock with signal by transmitting XOR of the NRZ encoded data and the clock l Clock is an internal signal that alternates from low to high, a low/high pair is considered as one clock cycle l In Manchester encoding 4 0: low high transition 4 1: high low transition CN_2. 17

Encoding (Manchester) n Problem with Manchester encoding l Doubles the rate at which the signal transitions are made on the link 4 Which means the receiver has half of the time to detect each pulse of the signal l The rate at which the signal changes is called the link’s baud rate l In Manchester the bit rate is half the baud rate CN_2. 18

Different Encoding Strategies Different encoding strategies CN_2. 19

Encoding (4 B/5 B) n 4 B/5 B encoding l Insert extra bits into bit stream so as to break up the long sequence of 0’s and 1’s l Every 4 -bits of actual data are encoded in a 5 - bit code that is transmitted to the receiver l 5 -bit codes are selected in such a way that each one has no more than one leading 0(zero) and no more than two trailing 0’s. l No pair of 5 -bit codes results in more than three consecutive 0’s CN_2. 20

Encoding (4 B/5 B) n 4 B/5 B encoding 0000 11110 16 left 0001 01001 11111 – when the line is idle 0010 10100 00000 – when the line is dead . . 00100 – to mean halt . . 1111 11101 13 left : 7 invalid, 6 for various control signals CN_2. 21

2. 3 Framing n We are focusing on packet-switched networks, which means that blocks of data (called frames at this level), not bit streams, are exchanged between nodes. n It is the network adaptor that enables the nodes to exchange frames. Bits flow between adaptors, frames between hosts CN_2. 22

Framing n When node A wishes to transmit a frame to node B, it tells its adaptor to transmit a frame from the node’s memory. This results in a sequence of bits being sent over the link. n The adaptor on node B then collects together the sequence of bits arriving on the link and deposits the corresponding frame in B’s memory. n Recognizing exactly what set of bits constitute a frame—that is, determining where the frame begins and ends—is the central challenge faced by the adaptor CN_2. 23

Framing n Byte-oriented Protocols To view each frame as a collection of bytes (characters) rather than bits l BISYNC (Binary Synchronous Communication) Protocol l 4 Developed by IBM (late 1960) PPP (Point-to-Point) Protocol l DDCMP (Digital Data Communication Protocol) l 4 Used in DECNet CN_2. 24

Framing (BISYNC) n BISYNC – sentinel approach l Frames transmitted beginning with leftmost field l Beginning of a frame is denoted by sending a special SYN (synchronize) character l Data portion of the frame is contained between special sentinel character STX (start of text) and ETX (end of text) l SOH : Start of Header l DLE : Data Link Escape l CRC: Cyclic Redundancy Check BISYNC Frame Format CN_2. 25

Framing (PPP) n Recent PPP (Point-to-Point Protocol) also uses sentinel approach l Special start of text character: Flag = 0 1 1 1 0 l Address, control : default numbers l Protocol for demux : IP / IPX l Payload : negotiated (1500 bytes) l Checksum : for error detection PPP Frame Format CN_2. 26

Framing (DDCMP) n Byte-counting approach l DDCMP l Count : how many bytes are contained in the frame body l If count is corrupted 4 Framing error DDCMP Frame Format CN_2. 27

Framing (HDLC) n Bit-oriented Protocol l HDLC : High Level Data Link Control 4 Beginning and Ending Sequences : 01111110 HDLC Frame Format 01111110 What to do if some data in the body identical to 01111110 ? Bit stuffing !! CN_2. 28

Framing (HDLC) n HDLC Protocol (Bit Stuffing) l On the sending side, any time five consecutive 1’s have been transmitted from the body of the message (i. e. excluding when the sender is trying to send the distinguished 01111110 sequence) 4 The sender inserts 0 before transmitting the next bit 011111100111111011101000 01111101001111101011101000 CN_2. 29

Framing n HDLC Protocol (Bit Stuffing) l On the receiving side 4 5 consecutive 1’s – Next bit 0 : Stuffed, so discard it 1 : Either End of the frame marker Or Error has been introduced in the bitstream Look at the next bit If 0 ( 01111110 ) End of the frame marker If 1 ( 01111111 ) Error, discard the whole frame The receiver needs to wait for next 01111110 before it can start receiving again CN_2. 30

2. 4 Error Detection n Bit errors are introduced into frames l Because of electrical interference and thermal noises n Detecting Error n Correction Error n Two approaches when the recipient detects an error l Notify the sender that the message was corrupted, so the sender can send again. 4 If the error is rare, then the retransmitted message will error-free l be Using some error correct detection and correction algorithm, the receiver reconstructs the message CN_2. 31

Error Detection n Common technique for detecting transmission error l CRC (Cyclic Redundancy Check) 4 Used in HDLC, DDCMP, CSMA/CD, CSMA/CA, Token Ring l Other approaches 4 Two Dimensional Parity (BISYNC) 4 Checksum (IP) CN_2. 32

Error Detection n Basic Idea of Error Detection l To add redundant information to a frame that can be used to determine if errors have been introduced l Imagine (Extreme Case) 4 Transmitting two complete copies of data – Identical No error – Differ Error – Poor Scheme ? ? ? » n bit message, n bit redundant information » Error can go undetected 4 In general, we can provide strong error detection technique – k redundant bits, n bits message, k << n – In Ethernet, a frame carrying up to 12, 000 bits of data requires only 32 -bit CRC CN_2. 33

Error Detection n Extra bits are redundant l They add no new information to the message l Derived from the original message using some algorithm l Both the sender and receiver know the algorithm Sender m r Receiver computes r using m If they match, no error CN_2. 34

Two-dimensional parity n Two-dimensional parity is exactly what the name suggests n It is based on “simple” (one-dimensional) parity, which usually involves adding one extra bit to a 7 -bit code to balance the number of 1 s in the byte. For example, l Odd parity sets the eighth bit to 1 if needed to give an odd number of 1 s in the byte, and l Even parity sets the eighth bit to 1 if needed to give an even number of 1 s in the byte CN_2. 35

Two-dimensional parity n Two-dimensional parity does a similar calculation for each bit position across each of the bytes contained in the frame Even Parity CN_2. 36

Two-dimensional parity n This results in an extra parity byte for the entire frame, in addition to a parity bit for each byte n Two-dimensional parity catches all 1 -, 2 -, and 3 -bit errors and most 4 -bit errors CN_2. 37

Internet Checksum Algorithm n Not used at the link level n Add up all the words that are transmitted and then transmit the result of that sum l The result is called the checksum n The receiver performs the same calculation on the received data and compares the result with the received checksum n If any transmitted data, including the checksum itself, is corrupted, then the results will not match, so the receiver knows that an error occurred CN_2. 38

Internet Checksum Algorithm n Consider the data being checksummed as a sequence of 16 -bit integers. n Add them together using 16 -bit ones complement arithmetic (explained next slide) and then take the ones complement of the result. n That 16 -bit number is the checksum CN_2. 39

Internet Checksum Algorithm n In ones complement arithmetic, a negative integer −x is represented as the complement of x; l Each bit of x is inverted. n When adding numbers in ones complement arithmetic, a carryout from the most significant bit needs to be added to the result. CN_2. 40

Internet Checksum Algorithm n Consider, for example, the addition of − 5 and − 3 in ones complement arithmetic on 4 -bit integers l +5 is 0101, so − 5 is 1010; +3 is 0011, so − 3 is 1100 n If we add 1010 and 1100 ignoring the carry, we get 0110 n In ones complement arithmetic, the fact that this operation caused a carry from the most significant bit causes us to increment the result, giving 0111, which is the ones complement representation of − 8 (obtained by inverting the bits in 1000), as we would expect CN_2. 41

Cyclic Redundancy Check (CRC) n Reduce the number of extra bits and maximize protection n Given a bit string 110001 we can associate a polynomial on a single variable x for it. 1. x 5+1. x 4+0. x 3+0. x 2+0. x 1+1. x 0 = x 5+x 4+1 and the degree is 5. A k-bit frame has a maximum degree of k-1 n Let M(x) be a message polynomial and C(x) be a generator polynomial. CN_2. 42

Cyclic Redundancy Check (CRC) n Let M(x)/C(x) leave a remainder of 0. n When M(x) is sent and M’(x) is received we have M’(x) = M(x)+E(x) n The receiver computes M’(x)/C(x) and if the remainder is nonzero, then an error has occurred. n The only thing the sender and the receiver should know is C(x). CN_2. 43

Cyclic Redundancy Check (CRC) Polynomial Arithmetic Modulo 2 Any polynomial B(x) can be divided by a divisor polynomial C(x) if B(x) is of higher degree than C(x). l Any polynomial B(x) can be divided once by a divisor polynomial C(x) if B(x) is of the same degree as C(x). l The remainder obtained when B(x) is divided by C(x) is obtained by subtracting C(x) from B(x). l To subtract C(x) from B(x), we simply perform the exclusive-OR (XOR) operation on each pair of matching coefficients. l 10111001 11010010 01101011 CN_2. 44

Cyclic Redundancy Check (CRC) n Let M(x) be a frame with m bits and let the generator polynomial have less than m bits say equal to r. n Let r be the degree of C(x). Append r zero bits to the low-order end of the frame, so it now contains m+r bits and corresponds to the polynomial xr. M(x). n M(x) = 1001011 = x 6 + x 3 + x 1 + 1 nr=3 n xr M(x) = x 3 M(x) = 1001011000 = x 9 + x 6 + x 5 + x 4 CN_2. 45

Cyclic Redundancy Check (CRC) n Divide the bit string corresponding to xr. M(x) by the bit string corresponding to C(x) using modulo 2 division. n Subtract the remainder (which is always r or fewer bits) from the string corresponding to xr. M(x) using modulo 2 subtraction (addition and subtraction are the same in modulo 2). n The result is the checksummed frame to be transmitted. Call it polynomial M’(x). CN_2. 46

Cyclic Redundancy Check (CRC) CRC Calculation using Polynomial Long Division CN_2. 47

Cyclic Redundancy Check (CRC) n Properties of Generator Polynomial l Let P(x) represent what the sender sent and P(x) + E(x) is the received string. A 1 in E(x) represents that in the corresponding position in P(x) the message the bit is flipped. l We know that P(x)/C(x) leaves a remainder of 0, but if E(x)/C(x) leaves a remainder of 0, then either E(x) = 0 or C(x) is factor of E(x). When C(x) is a factor of E(x) we have problem; errors go unnoticed. l If there is a single bit error then E(x) = xi, where i determines the bit in error. If C(x) contains two or more terms it will never divide E(x), so all single bit errors will be detected. l CN_2. 48

Cyclic Redundancy Check (CRC) n Properties of Generator Polynomial l In general, it is possible to prove that the following types of errors can be detected by a C(x) with the stated properties 4 All single-bit errors, as long as the xk and x 0 terms have nonzero coefficients. 4 All double-bit errors, as long as C(x) has a factor with at least three terms. 4 Any odd number of errors, as long as C(x) contains the factor (x+1). 4 Any “burst” error (i. e. , sequence of consecutive error bits) for which the length of the burst is less than k bits. (Most burst errors of larger than k bits can also be detected. ) CN_2. 49

Cyclic Redundancy Check (CRC) n Six generator polynomials that have become international standards are: l CRC-8 = x 8+x 2+x+1 l CRC-10 = x 10+x 9+x 5+x 4+x+1 l CRC-12 = x 12+x 11+x 3+x 2+x+1 l CRC-16 = x 16+x 15+x 2+1 l CRC-CCITT = x 16+x 12+x 5+1 l CRC-32 = x 32+x 26+x 23+x 22+x 16+x 12+x 11+x 10+x 8+x 7+x 5+x 4+x 2+x+1 CN_2. 50

2. 5 Reliable Transmission n CRC is used to detect errors. n Some error codes are strong enough to correct errors. n The overhead is typically too high. n Corrupt frames must be discarded. n A link-level protocol that wants to deliver frames reliably must recover from these discarded frames. n This is accomplished using a combination of two fundamental mechanisms l Acknowledgements and Timeouts CN_2. 51

Reliable Transmission n An acknowledgement (ACK for short) is a small control frame that a protocol sends back to its peer saying that it has received the earlier frame. l A control frame is a frame with header only (no data). n The receipt of an acknowledgement indicates to the sender of the original frame that its frame was successfully delivered. CN_2. 52

Reliable Transmission n If the sender does not receive an acknowledgment after a reasonable amount of time, then it retransmits the original frame. n The action of waiting a reasonable amount of time is called a timeout. n The general strategy of using acknowledgements and timeouts to implement reliable delivery is sometimes called Automatic Repeat re. Quest (ARQ). CN_2. 53

Stop and Wait Protocol n Idea of stop-and-wait protocol is straightforward l After transmitting one frame, the sender waits for an acknowledgement before transmitting the next frame. l If the acknowledgement does not arrive after a certain period of time, the sender times out and retransmits the original frame CN_2. 54

Stop and Wait Protocol Timeline showing four different scenarios for the stop-and-wait algorithm. (a) The ACK is received before the timer expires; (b) the original frame is lost; (c) the ACK is lost; (d) the timeout fires too soon CN_2. 55

Stop and Wait Protocol n If the acknowledgment is lost or delayed in arriving l The sender times out and retransmits the original frame, but the receiver will think that it is the next frame since it has correctly received and acknowledged the first frame l As a result, duplicate copies of frames will be delivered n How to solve this ? l Use 1 bit sequence number (0 or 1) l When the sender retransmits frame 0, the receiver can determine that it is seeing a second copy of frame 0 rather than the first copy of frame 1 and therefore can ignore it (the receiver still acknowledges it, in case the first acknowledgement was lost) CN_2. 56

Stop and Wait Protocol Timeline for stop-and-wait with 1 -bit sequence number CN_2. 57

Stop and Wait Protocol n The sender has only one outstanding frame on the link at a time l This may be far below the link’s capacity n Consider a 1. 5 Mbps link with a 45 ms RTT l The link has a delay bandwidth product of 67. 5 Kb or approximately 8 KB l Since the sender can send only one frame per RTT and assuming a frame size of 1 KB l Maximum Sending rate 4 Bits per frame Time per frame = 1024 8 0. 045 = 182 Kbps Or about one-eighth of the link’s capacity l To use the link fully, then sender should transmit up to eight frames before having to wait for an acknowledgement CN_2. 58

Sliding Window Protocol Timeline for Sliding Window Protocol CN_2. 59

Sliding Window Protocol n Sender assigns a sequence number denoted as Seq. Num to each frame. l Assume it can grow infinitely large n Sender maintains three variables l Sending Window Size (SWS) 4 Upper bound on the number of outstanding (unacknowledged) frames that the sender can transmit l Last Acknowledgement Received (LAR) 4 Sequence number of the last acknowledgement received l Last Frame Sent (LFS) 4 Sequence number of the last frame sent CN_2. 60

Sliding Window Protocol n Sender also maintains the following invariant LFS – LAR ≤ SWS Sliding Window on Sender CN_2. 61

Sliding Window Protocol n When an acknowledgement arrives l the sender moves LAR to right, thereby allowing the sender to transmit another frame n Also the sender associates a timer with each frame it transmits l It retransmits the frame if the timer expires before the ACK is received n Note that the sender has to be willing to buffer up to SWS frames l WHY? CN_2. 62

Sliding Window Protocol n Receiver maintains three variables l Receiving Window Size (RWS) 4 Upper bound on the number of out-of-order frames that the receiver is willing to accept l Largest Acceptable Frame (LAF) 4 Sequence number of the largest acceptable frame l Last Frame Received (LFR) 4 Sequence number of the last frame received CN_2. 63

Sliding Window Protocol n Receiver also maintains the following invariant LAF – LFR ≤ RWS Sliding Window on Receiver CN_2. 64

Sliding Window Protocol n When a frame with sequence number Seq. Num arrives, what does the receiver do? l If Seq. Num ≤ LFR or Seq. Num > LAF 4 Discard it (the frame is outside the receiver window) l If LFR < Seq. Num ≤ LAF 4 Accept it 4 Now the receiver needs to decide whether or not to send an ACK CN_2. 65

Sliding Window Protocol l Let Seq. Num. To. Ack 4 Denote the largest sequence number not yet acknowledged, such that all frames with sequence number less than or equal to Seq. Num. To. Ack have been received l The receiver acknowledges the receipt of Seq. Num. To. Ack even if high-numbered packets have been received 4 This acknowledgement is said to be l cumulative. The receiver then sets 4 LFR = Seq. Num. To. Ack and adjusts 4 LAF = LFR + RWS (Window sliding) CN_2. 66

Sliding Window Protocol n For example, suppose LFR = 5 and RWS = 4 (i. e. the last ACK that the receiver sent was for seq. no. 5) LAF = 9 n If frames 7 and 8 arrive, they will be buffered because they are within the receiver window n But no ACK will be sent since frame 6 is yet to arrive n Frames 7 and 8 are out of order n Frame 6 arrives (it is late because it was lost first time and had to be retransmitted) n Now Receiver Acknowledges Frame 8 and bumps LFR to 8 and LAF to 12 (Window sliding) CN_2. 67

Issues with Sliding Window Protocol n When timeout occurs, the amount of data in transit decreases l Since the sender is unable to advance its window n When the packet loss occurs, this scheme is no longer keeping the pipe full l The longer it takes to notice that a packet loss has occurred, the more severe the problem becomes n How to improve this Negative Acknowledgement (NAK) l Additional Acknowledgement l Selective Acknowledgement l CN_2. 68

Issues with Sliding Window Protocol n Negative Acknowledgement (NAK) l Receiver sends NAK for frame 6 when frame 7 arrive (in the previous example) 4 However this is unnecessary since sender’s timeout mechanism will be sufficient to catch the situation n Additional Acknowledgement l Receiver sends additional ACK for frame 5 when frame 7 arrives 4 Sender uses duplicate ACK as a clue for frame loss n Selective Acknowledgement l Receiver will acknowledge exactly those frames it has received, rather than the highest number frames 4 Receiver will acknowledge frames 7 and 8 4 Sender knows frame 6 is lost 4 Sender can keep the pipe full (additional complexity) CN_2. 69

Issues with Sliding Window Protocol How to select the window size ? l SWS is easy to compute 4 Delay Bandwidth l RWS can be anything 4 Two common setting » RWS = 1 No buffer at the receiver for frames that arrive out of order » RWS = SWS The receiver can buffer frames that the sender transmits It does not make any sense to keep RWS > SWS WHY? CN_2. 70

Issues with Sliding Window Protocol n Finite Sequence Number l Frame sequence number is specified in the header field 4 Finite size » 3 bits: eight possible sequence number: 0, 1, 2, 3, 4, 5, 6, 7 4 It is necessary to wrap around CN_2. 71

Issues with Sliding Window Protocol n How to distinguish between different incarnations of the same sequence number? l Number of possible sequence number must be larger than the number of outstanding frames allowed 4 Stop and Wait: One outstanding frame » 2 distinct sequence number (0 and 1) 4 Let Max. Seq. Num be the number of available sequence numbers 4 SWS + 1 ≤ Max. Seq. Num – – Is this sufficient? Depends on RWS If RWS = 1, then sufficient If RWS = SWS, then not good enough CN_2. 72

Issues with Sliding Window Protocol For example, we have eight sequence numbers 0, 1, 2, 3, 4, 5, 6, 7 RWS = SWS = 7 Sender sends 0, 1, …, 6 Receiver receives 0, 1, … , 6 Receiver acknowledges 0, 1, …, 6 ACK (0, 1, …, 6) are lost Sender retransmits 0, 1, …, 5, 6 Receiver is expecting 7, 0, …. , 5 CN_2. 73

Issues with Sliding Window Protocol To avoid this, If RWS = SWS < (Max. Seq. Num + 1)/2 CN_2. 74

Issues with Sliding Window Protocol n Serves three different roles l Reliable l Preserve the order 4 Each frame has a sequence number 4 The receiver makes sure that it does not pass a frame up to the next higher-level protocol until it has already passed up all frames with a smaller sequence number l Frame control 4 Receiver is able to throttle the sender – Keeps the sender from overrunning the receiver » From transmitting more data than the receiver is able to process CN_2. 75

2. 6 Ethernet n Most successful local area networking technology of last 20 years. n Developed in the mid-1970 s by researchers at the Xerox Palo Alto Research Centers (PARC). n Uses CSMA/CD technology Carrier Sense Multiple Access with Collision Detection. l A set of nodes send and receive frames over a shared link. l Carrier sense means that all nodes can distinguish between an idle and a busy link. l Collision detection means that a node listens as it transmits and can therefore detect when a frame it is transmitting has collided with a frame transmitted by another node. l CN_2. 76

Ethernet n Uses ALOHA (packet radio network) as the root protocol Developed at the University of Hawaii to support communication across the Hawaiian Islands. l For ALOHA the medium was atmosphere, for Ethernet the medium is a coax cable. l n DEC and Intel joined Xerox to define a 10 -Mbps Ethernet standard in 1978. n This standard formed the basis for IEEE standard 802. 3 n More recently 802. 3 has been extended to include a 100 - Mbps version called Fast Ethernet, a 1000 -Mbps version called Gigabit Ethernet, and also 10 Gigabit Ethernet. CN_2. 77

Ethernet (10 Base 5) n An Ethernet segment is implemented on a coaxial cable of up to 500 m. l This cable is similar to the type used for cable TV except that it typically has an impedance of 50 ohms instead of cable TV’s 75 ohms. n Hosts connect to an Ethernet segment by tapping into it. n A transceiver (a small device directly attached to the tap) detects when the line is idle and drives signal when the host is transmitting. n The transceiver also receives incoming signal. n The transceiver is connected to an Ethernet adaptor which is plugged into the host. But now most are built in into the computers. n The protocol is implemented on the adaptor. CN_2. 78

Ethernet (10 Base 5) Terminator Ethernet transceiver, adaptor, and terminator CN_2. 79

Ethernet n Multiple Ethernet segments can be joined together by repeaters. n A repeater is a device that forwards digital signals. n No more than four repeaters may be positioned between any pair of hosts. l An Ethernet has a total reach of only 2500 m. CN_2. 81

Network Configuration Example 3 (Five segments, maximum) A 1 D 區段 1 區段 2 B E Repeater C 2 F H G 3 區段 3 4 L 半訊號增益器 5 I 半訊號增益器間電纜 J 區段 4 K （最長 1000 公尺） 區段 5 M N 6 CN_2. 83

Ethernet n Any signal placed on the Ethernet by a host is broadcast over the entire network l Signal is propagated in both directions. l Repeaters forward the signal on all outgoing segments. l Terminators attached to the end of each segment absorb the signal. n Ethernet uses Manchester encoding scheme. CN_2. 84

Ethernet (10 Base 2) n New Technologies in Ethernet l Instead of using coax cable, an Ethernet can be constructed from a thinner cable known as 10 Base 2 (the original was 10 Base 5) 4 10 means the network operates at 10 Mbps 4 Base means the cable is used in a baseband system 4 2 means that a given segment can be no longer than 200 m CN_2. 85

Ethernet (10 Base. T) n New Technologies in Ethernet l Another cable technology is 10 Base. T 4 T stands for twisted pair 4 Limited to 100 m in length l With 10 Base. T, the common configuration is to have several point to point segments coming out of a multiway repeater, called Hub CN_2. 86

Ethernet HUB 1 HUB 2 Ethernet Hub CN_2. 87

Access Protocol for Ethernet n The algorithm is commonly called Ethernet’s Media Access Control (MAC). l It is implemented in Hardware on the network adaptor. n Frame format l Preamble (64 bit): allows the receiver to synchronize with the signal (sequence of alternating 0 s and 1 s). l Source and Destination Address (48 bit each). l Packet type (16 bit): acts as demux key to identify the higher level protocol. l Data (up to 1500 bytes) 4 Minimally a frame must contain at least 46 bytes of data. 4 Frame must be long enough to detect collision. l CRC (32 bit) CN_2. 88

Ethernet Frame Format 7 1 Preamble SFD l l l l l 6 DA 6 2 SA LEN 4 LLC PAD 位元組 FCS Preamble: (101010. . . 1010) for Synchronization SFD: Start Frame Delimiter (10101011) DA: Destination Address SA: Source Address Length: Length of LLC-Frame: Up to 1500 bytes PAD: Padding when LLC-Frame < 46 bytes FCS: Frame Check Sequence (CRC-32) MAC-frame size -- from DA to FCS l Min 64 bytes to distinguish from collision l Max 1518 bytes to prevent dominating bandwidth CN_2. 89

Ethernet Addresses n Each host on an Ethernet (in fact, every Ethernet host in the world) has a unique Ethernet Address. n The address belongs to the adaptor, not the host. l It is usually burnt into ROM. n Ethernet addresses are typically printed in a human readable format l As a sequence of six numbers separated by colons. l Each number corresponds to 1 byte of the 6 byte address and is given by a pair of hexadecimal digits, one for each of the 4 -bit nibbles in the byte l Leading 0 s are dropped. l For example, 8: 0: 2 b: e 4: b 1: 2 is 4 00001000 0000 00101011 11100100 10110001 00000010 CN_2. 90

Ethernet Addresses n To ensure that every adaptor gets a unique address, each manufacturer of Ethernet devices is allocated a different prefix that must be prepended to the address on every adaptor they build 4 AMD has been assigned the 24 bit prefix 8: 0: 20 CN_2. 91

Ethernet Addresses n Each frame transmitted on an Ethernet is received by every adaptor connected to that Ethernet. n Each adaptor recognizes those frames addressed to its address and passes only those frames on to the host. n In addition to unicast address, an Ethernet address consisting of all 1 s is treated as a broadcast address. l All adaptors pass frames addressed to the broadcast address up to the host. n Similarly, an address that has the first bit set to 1 but is not the broadcast address is called a multicast address. l A given host can program its adaptor to accept some set of multicast addresses. CN_2. 92

Ethernet Addresses n To summarize, an Ethernet adaptor receives all frames and accepts Frames addressed to its own address l Frames addressed to the broadcast address l l Frames addressed to a multicast addressed if it has been instructed CN_2. 93

Ethernet Transmitter Algorithm n When the adaptor has a frame to send and the line is idle, it transmits the frame immediately. l The upper bound of 1500 bytes in the message means that the adaptor can occupy the line for a fixed length of time. n When the adaptor has a frame to send and the line is busy, it waits for the line to go idle and then transmits immediately. n The Ethernet is said to be 1 -persistent protocol because an adaptor with a frame to send transmits with probability 1 whenever a busy line goes idle. CN_2. 94

Ethernet Transmitter Algorithm n Since there is no centralized control it is possible for two (or more) adaptors to begin transmitting at the same time, l Either because both found the line to be idle, l Or, both had been waiting for a busy line to become idle. n When this happens, the two (or more) frames are said to be collide on the network. CN_2. 95

Ethernet Transmitter Algorithm n Since Ethernet supports collision detection, each sender is able to determine that a collision is in progress. n At the moment an adaptor detects that its frame is colliding with another, it first makes sure to transmit a 32 -bit jamming sequence and then stops transmission. l Thus, a transmitter will minimally send 96 bits in the case of collision 464 -bit preamble + 32 -bit jamming sequence CN_2. 96

Ethernet Transmitter Algorithm n One way that an adaptor will send only 96 bits (called a runt frame) is if the two hosts are close to each other. n Had they been farther apart, l They would have had to transmit longer, and thus send more bits, before detecting the collision. CN_2. 97

Ethernet Transmitter Algorithm n The worst case scenario happens when the two hosts are at opposite ends of the Ethernet. n To know for sure that the frame its just sent did not collide with another frame, the transmitter may need to send as many as 512 bits. l Every Ethernet frame must be at least 512 bits (64 bytes) long. 4 14 bytes of header + 46 bytes of data + 4 bytes of CRC CN_2. 98

Ethernet Transmitter Algorithm n Why 512 bits? l Why is its length limited to 2500 m? n The farther apart two nodes are, the longer it takes for a frame sent by one to reach the other, and the network is vulnerable to collision during this time CN_2. 99

Ethernet Transmitter Algorithm n n n n A begins transmitting a frame at time t d denotes the one link latency The first bit of A’s frame arrives at B at time t + d Suppose an instant before host A’s frame arrives, host B begins to transmit its own frame B’s frame will immediately collide with A’s frame and this collision will be detected by host B Host B will send the 32 -bit jamming sequence Host A will not know that the collision occurred until B’s frame reaches it, which will happen at t + 2 * d Host A must continue to transmit until this time in order to detect the collision l Host A must transmit for 2 * d to be sure that it detects all possible collisions CN_2. 100

Ethernet Transmitter Algorithm Worst-case scenario: (a) A sends a frame at time t; (b) A’s frame arrives at B at time t + d; (c) B begins transmitting at time t + d and collides with A’s frame; (d) B’s runt (32 -bit) frame arrives at A at time t + 2 d. CN_2. 101

Ethernet Transmitter Algorithm n Consider that a maximally configured Ethernet is 2500 m long, and there may be up to four repeaters between any two hosts, the round trip delay has been determined to be 51. 2 s l l Which on 10 Mbps Ethernet corresponds to 512 bits 10 Mbps x 51. 2 s = 512 bits n The other way to look at this situation, l We need to limit the Ethernet’s maximum latency to a fairly small value (51. 2 s) for the access algorithm to work 4 Hence the maximum length for the Ethernet is on the order of 2500 m. CN_2. 102

Ethernet Transmitter Algorithm n Once an adaptor has detected a collision, and stopped its transmission, it waits a certain amount of time and tries again. n Each time the adaptor tries to transmit but fails, it doubles the amount of time it waits before trying again. n This strategy of doubling the delay interval between each retransmission attempt is known as Exponential Backoff. CN_2. 103

Ethernet Transmitter Algorithm n The adaptor first delays either 0 or 51. 2 s, selected at random. n If this effort fails, it then waits 0, 51. 2, 102. 4, 153. 6 s (selected randomly) before trying again; l This is k * 51. 2 for k = 0, 1, 2, 3 n After the third collision, it waits k * 51. 2 for k = 0… 23 – 1 (again selected at random). n In general, the algorithm randomly selects a k between 0 and 2 n – 1 and waits for k * 51. 2 s, where n is the number of collisions experienced so far. CN_2. 104

Experience with Ethernet n Ethernets work best under lightly loaded conditions. l Under heavy loads, too much of the network’s capacity is wasted by collisions. n Most Ethernets are used in a conservative way. l Have fewer than 200 hosts connected to them which is far fewer than the maximum of 1024. n Most Ethernets are far shorter than 2500 m with a round- trip delay of closer to 5 s than 51. 2 s. n Ethernets are easy to administer and maintain. l There are no switches that can fail and no routing and configuration tables that have to be kept up-to-date. l It is easy to add a new host to the network. l It is inexpensive. 4 Cable is cheap, and only other cost is the network adaptor on each host. CN_2. 105

2. 7 Wireless Links n Wireless links transmit electromagnetic signals l Radio, microwave, infrared n Wireless links all share the same “wire” (so to speak) l The challenge is to share it efficiently without unduly interfering with each other l Most of this sharing is accomplished by dividing the “wire” along the dimensions of frequency and space n Exclusive use of a particular frequency in a particular geographic area may be allocated to an individual entity such as a corporation CN_2. 106

Wireless Links n These allocations are determined by government agencies such as FCC (Federal Communications Commission) in USA n Specific bands (frequency) ranges are allocated to certain uses. l Some bands are reserved for government use l Other bands are reserved for uses such as AM radio, FM radio, televisions, satellite communications, and cell phones l Specific frequencies within these bands are then allocated to individual organizations for use within certain geographical areas. l Finally, there are several frequency bands set aside for “license exempt” usage 4 Bands in which a license is not needed CN_2. 107

Wireless Links n Devices that use license-exempt frequencies are still subject to certain restrictions l The first is a limit on transmission power l This limits the range of signal, making it less likely to interfere with another signal 4 For example, a cordless phone might have a range of about 100 feet. CN_2. 108

Wireless Links n The second restriction requires the use of Spread Spectrum technique l Idea is to spread the signal over a wider frequency band 4 So as to minimize the impact of interference from other devices 4 Originally designed for military use l Frequency hopping 4 Transmitting signal over a random sequence of frequencies ü First transmitting at one frequency, then a second, then a third… ü The sequence of frequencies is not truly random, instead computed algorithmically by a pseudorandom number generator ü The receiver uses the same algorithm as the sender, initializes it with the same seed, and is - Able to hop frequencies in sync with the transmitter to correctly receive the frame CN_2. 109

Wireless Links n A second spread spectrum technique called Direct sequence l Represents each bit in the frame by multiple bits in the transmitted signal. l For each bit the sender wants to transmit 4 It actually sends the exclusive OR of that bit and n random bits l The sequence of random bits is generated by a pseudorandom number generator known to both the sender and the receiver. l The transmitted values, known as an n-bit chipping code, spread the signal across a frequency band that is n times wider CN_2. 110

Wireless Links Example 4 -bit chipping sequence CN_2. 111

Wireless Links n Wireless technologies differ in a variety of dimensions l How much bandwidth they provide l How far apart the communication nodes can be n Four prominent wireless technologies l Bluetooth l Wi-Fi (more formally known as 802. 11) l Wi. MAX (802. 16) l 3 G cellular wireless CN_2. 112

Wireless Links Overview of leading wireless technologies CN_2. 113

Wireless Links n Mostly widely used wireless links today are usually asymmetric l Two end-points are usually different kinds of nodes 4 One end-point usually has no mobility, but has wired connection to the Internet (known as base station) 4 The node at the other end of the link is often mobile CN_2. 114

Wireless Links A wireless network using a base station CN_2. 115

Wireless Links n Wireless communication supports point-to-multipoint communication n Communication between non-base (client) nodes is routed via the base station n Three levels of mobility for clients l No mobility: the receiver must be in a fix location to receive a directional transmission from the base station (initial version of Wi. MAX) l Mobility is within the range of a base (Bluetooth) l Mobility between bases (Cell phones and Wi-Fi) CN_2. 116

Wireless Links n Mesh or Ad-hoc network l Nodes are peers l Messages may be forwarded via a chain of peer nodes A wireless ad-hoc or mesh network CN_2. 117

IEEE 802. 11 n Also known as Wi-Fi n Like its Ethernet and token ring siblings, 802. 11 is designed for use in a limited geographical area (homes, office buildings, campuses) l Primary challenge is to mediate access to a shared communication medium – in this case, signals propagating through space n 802. 11 supports additional features l power management and l security mechanisms CN_2. 118

IEEE 802. 11 n Original 802. 11 standard defined two radio-based physical layer standard l One using the frequency hopping 4 l Second using direct sequence 4 l Over 79 1 -MHz-wide frequency bandwidths Using 11 -bit chipping sequence Both standards run in the 2. 4 -GHz and provide up to 2 Mbps n Then physical layer standard 802. 11 b was added l Using a variant of direct sequence 802. 11 b provides up to 11 Mbps l Uses license-exempt 2. 4 -GHz band n Then came 802. 11 a which delivers up to 54 Mbps using OFDM l 802. 11 a runs on license-exempt 5 -GHz band n Most recent standard is 802. 11 g which is backward compatible with 802. 11 b l Uses 2. 4 GHz band, OFDM and delivers up to 54 Mbps CN_2. 119

IEEE 802. 11 – Collision Avoidance n Consider the situation in the following figure where each of four nodes is able to send and receive signals that reach just the nodes to its immediate left and right l For example, B can exchange frames with A and C, but it cannot reach D l C can reach B and D but not A Example of a wireless network CN_2. 120

IEEE 802. 11 – Collision Avoidance n Suppose both A and C want to communicate with B and so they each send it a frame. l A and C are unaware of each other since their signals do not carry that far l These two frames collide with each other at B 4 But unlike an Ethernet, neither A nor C is aware of this collision l A and C are said to hidden nodes with respect to each other CN_2. 121

IEEE 802. 11 – Collision Avoidance The “Hidden Node” Problem. Although A and C are hidden from each other, their signals can collide at B. (B’s reach is not shown. ) CN_2. 122

IEEE 802. 11 – Collision Avoidance n Another problem called exposed node problem occurs l Suppose B is sending to A. Node C is aware of this communication because it hears B’s transmission. l It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission. l Suppose C wants to transmit to node D. l This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B. CN_2. 123

IEEE 802. 11 – Collision Avoidance Exposed Node Problem. Although B and C are exposed to each other’s signals, there is no interference if B transmits to A while C transmits to D. (A and D’s reaches are not shown. ) CN_2. 124

IEEE 802. 11 – Collision Avoidance n 802. 11 addresses these two problems with an algorithm called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). n Key Idea l Sender and receiver exchange control frames with each other before the sender actually transmits any data. l This exchange informs all nearby nodes that a transmission is about to begin l Sender transmits a Request to Send (RTS) frame to the receiver. 4 The RTS frame includes a field that indicates how long the sender wants to hold the medium - Length of the data frame to be transmitted l Receiver replies with a Clear to Send (CTS) frame 4 This frame echoes this length field back to the sender CN_2. 125

Frame Types n Class 1 frames l l Source Station Control Frames (1) RTS (2) CTS (3) ACK (4) Poll Management Frames (1) Probe Request/Response (2) Beacon (3) Authentication Destination Station Optional RTS CTS Data ACK CN_2. 126

IEEE 802. 11 – Collision Avoidance n Any node that sees the CTS frame knows that l it is close to the receiver, therefore l cannot transmit for the period of time it takes to send a frame of the specified length n Any node that sees the RTS frame but not the CTS frame l is not close enough to the receiver to interfere with it, and l so is free to transmit CN_2. 127

IEEE 802. 11 – Collision Avoidance n Using ACK in CSMA/CA n Receiver sends an ACK to the sender after successfully receiving a frame n All nodes must wait for this ACK before trying to transmit n If two or more nodes detect an idle link and try to transmit an RTS frame at the same time l Their RTS frame will collide with each other n 802. 11 does not support collision detection So the senders realize the collision has happened when they do not receive the CTS frame after a period of time l In this case, they each wait a random amount of time before trying again. l The amount of time a given node delays is defined by the same exponential backoff algorithm used on the Ethernet. l CN_2. 128

IEEE 802. 11 – Distribution System n 802. 11 is suitable for an ad-hoc configuration of nodes that may or may not be able to communicate with all other nodes. n Nodes are free to move around n The set of directly reachable nodes may change over time n To deal with this mobility and partial connectivity, l 802. 11 defines additional structures on a set of nodes l Instead of all nodes being created equal, 4 some nodes are allowed to roam 4 some are connected to a wired network infrastructure - they are called Access Points (AP) and they are connected to each other by a so-called distribution system CN_2. 129

IEEE 802. 11 – Distribution System n A distribution system that connects three access points, each of which services the nodes in the same region n Each of these regions is analogous to a cell in a cellular phone system with the APs playing the same role as a base station n The distribution network runs at layer 2 of the ISO architecture Access points connected to a distribution network CN_2. 130

IEEE 802. 11 – Distribution System n Although two nodes can communicate directly with each other if they are within reach of each other, the idea behind this configuration is l Each nodes associates itself with one access point l For node A to communicate with node E, A first sends a frame to its AP-1 which forwards the frame across the distribution system to AP-3, which finally transmits the frame to E Access points connected to a distribution network CN_2. 131

IEEE 802. 11 – Distribution System n How do the nodes select their access points ? n How does it work when nodes move from one cell to another ? n The technique for selecting an AP is called scanning l The node sends a Probe frame l All APs within reach reply with a Probe Response frame l The node selects one of the access points and sends that AP an Association Request frame l The AP replies with an Association Response frame n A node engages this protocol whenever l it joins the network, as well as l when it becomes unhappy with its current AP 4 4 This might happen, for example, because the signal from its current AP has weakened due to the node moving away from it Whenever a node acquires a new AP, the new AP notifies the old AP of the change via the distribution system CN_2. 132

IEEE 802. 11 – Distribution System n When node C moves from the cell serviced by AP-1 to the cell serviced by AP-2. n As it moves, it sends Probe frames, which eventually result in Probe Responses from AP-2. n At some point, C prefers AP-2 over AP-1 , and so it associates itself with that access point. l This is called active scanning since the node is actively searching for an access point Node Mobility CN_2. 133

IEEE 802. 11 – Distribution System n APs also periodically send a Beacon frame that advertises the capabilities of the access point; these include the transmission rate supported by the AP l This is called passive scanning l A node can change to this AP based on the Beacon frame simply by sending it an Association Request frame back to the access point. Node Mobility CN_2. 134

IEEE 802. 11 – Frame Format n Source and Destinations addresses: each 48 bits n Data: up to 2312 bytes n CRC: 32 bit n Control field: 16 bits l Contains three subfields (of interest) 4 6 bit Type field: indicates whether the frame is an RTS or CTS frame or being used by the scanning algorithm 4 A pair of 1 bit fields : called To. DS and From. DS Frame Format CN_2. 135

IEEE 802. 11 – Frame Format n Frame contains four addresses n How these addresses are interpreted depends on the settings of the To. DS and From. DS bits in the frame’s Control field n This is to account for the possibility that the frame had to be forwarded across the Distribution System n Simplest case l When one node is sending directly to another, both the DS bits are 0, Addr 1 identifies the target node, and Addr 2 identifies the source node CN_2. 136

IEEE 802. 11 – Frame Format n Most complex case l Both DS bits are set to 1 4 Indicates that the message went from 4 a wireless node l distribution system another wireless node With both bits set, 4 Addr 1 identifies the ultimate destination, 4 Addr 2 identifies the immediate sender (the one that forwarded the frame from the distribution system to the ultimate destination) 4 Addr 3 identifies the intermediate destination (the one that accepted the frame from a wireless node and forwarded across the distribution system) 4 Addr 4 identifies the original source CN_2. 137

IEEE 802. 11 – Frame Format A E Addr 1: E, Addr 2: AP-3, Addr 3: AP-1, Addr 4: A CN_2. 138

IEEE 802. 11 MAC Architecture 免競爭式服務 (具時限傳輸) 競爭式服務 (非同步傳輸) Point Coordination Function (PCF) MAC Extent Distributed Coordination Function (DCF) CN_2. 139

IEEE 802. 11 MAC Architecture n Distributed Coordination Function (DCF) l The fundamental access method for the 802. 11 MAC, known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). l Shall be implemented in all stations and APs. l Used within both configurations: 4 Ad hoc 4 Infrastructured CN_2. 140

IEEE 802. 11 MAC Architecture n Point Coordination Function (PCF) l An alternative access method l Shall be implemented on top of the DCF l A point coordinator (polling master) is used to determine which station currently has the right to transmit. l Shall be built up from the DCF through the use of an access priority mechanism. l Different accesses of traffic can be defined through the use of different values of IFS (Inter-Frame Space). CN_2. 141

IEEE 802. 11 MAC Architecture l Shall use a Point IFS (PIFS) < Distributed IFS (DIFS) l Point coordinated traffic shall have higher priority to access the medium, which may be used to provide a contention-free access method. l The priority access of the PIFS allows the point coordinator to seize control of the medium away from the other stations. CN_2. 142

IEEE 802. 11 MAC Architecture n Coexistence of DCF and PCF l Both the DCF and PCF shall coexist without interference. l They are integrated in a superframe in which a contention-free burst occurs at the beginning, followed by a contention period. CN_2. 143

IEEE 802. 11 MAC Architecture 超級訊框 Super Frame 免競爭訊框 需競爭訊框 CN_2. 144

Distributed Coordination Function n Allows for automatic medium sharing between PHYs through the use of CSMA/CA and a random backoff time following a busy medium condition. n All directed traffic uses immediate positive ack (ACK frame) where retransmission is scheduled by the sender if no ACK is received. n Carrier Sense shall be performed both through physical and virtual mechanisms. CN_2. 145

Distributed Coordination Function n The Virtual Carrier Sense mechanism is achieved by distributing medium busy reservation information through an exchange of special small RTS and CTS frames (contain a duration field) prior to the actual data frame. Unicast only, not used in multicast/broadcast. n The use of RTS/CTS is under control of RTS_Threshold (payload length, under which without any RTS/CTS prefix). CN_2. 146

Distributed Coordination Function n Physical Carrier Sense Mechanism l A physical carrier sense mechanism shall be provided by the PHY. n Virtual Carrier Sense Mechanism l Provided by the MAC, named Net Allocation Vector (NAV), which maintains a prediction of future traffic based on duration information announced in RTS/CTS frames. CN_2. 147

Distributed Coordination Function n MAC-Level Ack (Positive Acknowledgment) l To allow detection of a lost or errored frame an ACK frame shall be returned immediately following a successfully received frame. l The gap between the received frame and ACK frame shall be SIFS. l The frame types should be acknowledged with an ACK frame: 4 Data 4 Poll 4 Request 4 Response l The lack of an ACK frame means that an error has occurred. CN_2. 148

DCF -- Inter-Frame Space (IFS) n A station shall determine that the medium is free through the use of carrier sense function for the interval specified. n Three different IFS's are defined to provide priority levels. n Short-IFS (SIFS) l Shall be used for 4 an ACK frame, 4 a CTS frame, 4 by a station responding to any polling l Any STA intending to send only these frame types shall be allowed to transmit after the SIFS time has elapsed following a busy medium. CN_2. 149

DCF -- Inter-Frame Space (IFS) n PCF-IFS (PIFS) l Shall be used only by the PCF to send any of the Contention Free Period frames. l The PCF shall be allowed to transmit after it detects the medium free for the period PIFS, at the start of and during a CF-Burst. n DCF-IFS (DIFS) l Shall be used by the DCF to transmit asynchronous MPDUs. l A STA using the DCF is allowed to transmit after it detects the medium free for the period DIFS, as long as it is not in a backoff period. CN_2. 150

DCF -- Random Backoff Time n Before transmitting asynchronous MPDUs, a STA shall use the carrier sense function to determine the medium state. n If busy, the STA shall defer until after a DIFS gap is detected, and then generate a random backoff period for an additional deferral time (resolve contention). Backoff time = INT(CW * Random()) * Slot time CN_2. 151

DCF -- Random Backoff Time Where CW = An integer between CWmin and CWmax Random() = a random number Slot Time = Transmitter turn-on delay + medium propagation delay + medium busy detect response time CWmax CWmin 7 15 31 初始值 第一次重送 63 127 255 第三次重送 第二次重送 CN_2. 152

DCF Access Procedure n CSMA/CA Protocol n Used when there is no PCF detected and when in the Contention Period of a Superframe when using a PCF. n Basic Access l A STA with a pending MPDU may transmit when it detects a free medium for greater than or equal to a DIFS time. l If the medium is busy when a STA desires to initiate a Data, Poll, Request, or Response MPDU transfer, and only a DCF is being used (or a Contention Period portion of a Superframe is active), the Random Backoff Time algorithm shall be followed. CN_2. 153

DCF Access Procedure Immediate access when medium is free >= DIFS PIFS DIFS Contention Window SIFS Busy Medium Back off. Window Next Frame Slot time Defer Access CN_2. 154

DCF Access Procedure n Backoff Procedure l A backoff time is selected first. l The Backoff Timer shall be frozen while the medium is sensed busy and shall decrement only when the medium is free (resume whenever free period > DIFS). l Transmission shall commence whenever the Backoff Timer reaches zero. l A STA that has just transmitted a frame and has another frame ready to transmit (queued), shall perform the backoff procedure (fairness concern). l Tends toward fair access on a FCFS basis. CN_2. 155

DCF Access Procedure DIFS A B C D E DIFS Frame Backoff 10 us 19 us 4 us Frame 15 us Frame 5 us 7 us Frame 2 us CWindow = Contention Window = Backoff (後退) = Remaining Backoff (持續後退) CN_2. 156

DCF Access Procedure n Setting the NAV Through Use of RTS/CTS Frames l RTS and CTS frames contain a Duration field based on the medium occupancy time of the MPDU from the end of the RTS or CTS frame until the end of the ACK frame. CN_2. 157

DCF Access Procedure T 3 T 1 DIFS 起始 作站 Data RTS SIFS 目的地 作站 CTS SIFS ACK DIFS 其他 作站 NAV (RTS) NAV (CTS) Defer Access Contention Window Back off. Window Next Frame Back off after Defer CN_2. 158

DCF Access Procedure n Control of the Channel l The IFS is used to provide an efficient MSDU delivery mechanism. l Once a station has contended for the channel, it will continue to send fragments until either 4 all fragments of a MSDU have been sent, 4 an ack is not received, or 4 the station can not send any additional fragments due to a dwell time boundary. CN_2. 159

DCF Access Procedure DIFS Fragment Burst SIFS Src Dst Fragment 1 SIFS Fragment 2 Ack 1 PIFS SIFS Back off. Window Fragment 3 Ack 2 Ack 3 CN_2. 160

DCF Access Procedure n Control of the Channel l If the source station does not receive an ack frame, it will attempt to retransmit the fragment at a later time (according to the backoff algorithm). l When the time arrives to retransmit the fragment, the source station will contend for access in the contention window. CN_2. 161

DCF Access Procedure n RTS/CTS Usage with Fragmentation l The RTS/CTS frames defines the duration of the first frame and ack. l The duration field in the data and ack frames specifies the total duration of the next fragment and ack. l The last Fragment and ACK will have the duration set to zero. CN_2. 162

RTS/CTS Usage with Fragmentation DIFS PIFS SIFS Other NAV(RTS) NAV(Frag 1) NAV(CTS) SIFS NAV(ACK 1) SIFS Fragment 1 RTS Src Dst SIFS CTS NAV(ACK 2) SIFS Fragment 2 Ack 1 Back off. Window NAV(Frag 2) SIFS Fragment 3 Ack 2 Ack 3 CN_2. 163

DCF Access Procedure n RTS/CTS Usage with Fragmentation l Each Fragment and ACK acts as a virtual RTS and CTS. l In the case where an ack is not received by the source station, the NAV will be marked busy for next frame exchange. This is the worst case situation. CN_2. 164

DCF Access Procedure n RTS/CTS Usage with Fragmentation l If the ack is not sent by the destination, stations that can only hear the destination will not update their NAV and be free to access the channel. l All stations will be free to access the channel after the NAV from Fragment 1 has expired. l The source must wait until the NAV (Fragment 1) expires before attempting to contend for the channel after not receiving the ack. CN_2. 165

RTS/CTS Usage with Fragmentation DIFS PIFS SIFS Other NAV(RTS) NAV(Fragment 1) NAV(CTS) SIFS Src Dst SIFS Backoff. Window NAV(ACK 1) SIFS Fragment 1 RTS CTS Ack 1 CN_2. 166

DCF Access Procedure n Directed MPDU Transfer Procedure Using RTS/CTS l STA shall use an RTS/CTS exchange for directed frames only when the length of the MPDU is greater than the RTS_Threshold. n Directed MPDU Transfer Procedure Without RTS/CTS CN_2. 167

DCF Access Procedure DIFS 起始 作站 目的地 作站 Data SIFS ACK DIFS Contention Window Backoff. Window 其他 作站 Defer Access Next Frame Backoff after Defer CN_2. 168

Point Coordination Function(PCF) n The PCF provides contention free services. n It is an option for a station to become the Point Coordinator(PC), which generates the Superframe (SF). n Not all stations must be capable of becoming the PC and transmitting PCF data frames. n The SF consists of a Contention Free (CF) period and a Contention Period. n The length of a SF is a manageable parameter and that of the CF period may be variable on a per SF basis. CN_2. 169

Point Coordination Function(PCF) 超級訊框 Super Frame 免競爭訊框 需競爭訊框 CN_2. 170

PCF Access Procedure n The PCF protocol is based on a polling scheme controlled by one special STA per BSS called the Point Coordinator (PC). n The PC gains control of the medium at the beginning of the SF and maintains control for the entire CF period by waiting a shorter time between transmissions (PIFS). n CF-Down Frames and CF-UP Frames. CN_2. 171

PCF Access Procedure n At the beginning of the SF, the PC shall sense the medium. n If it is free the PC shall wait a PIFS time and transmit l a Data frame with the CF-Poll Subtype bit set, to the next station on the polling list, or l a CF-End frame, if a null CF period is desired. CN_2. 172

PCF Access Procedure n The PCF uses the PCF priority level of the CSMA/CA protocol. n The shorter PIFS gap causes a burst traffic with inter- frame gaps that are shorter than the DIFS gap needed by stations using the Contention period. n Each station, except the station with the PCF, shall preset it's NAV to the maximum CF-Period length at the beginning of every SF. n The PCF shall transmit a CF-End frame, at the end of the CF-Period, to reset the NAV of all stations in the BSS. CN_2. 173

PCF Transfer Procedure n PCF Transfers When the PCF Station is Transmitter or Recipient l Stations shall respond to the CF-Poll immediately when a frame is queued, by sending this frame after an SIFS gap. This results in a burst of Contention Free traffic (CFBurst). l For services that require MAC level ack, the ack is preferably done through the CF-Ack bit in the Subtype field of the responding CF-Up frame. CN_2. 174

PCF Transfer Procedure 超級訊框 免競爭週期 PIFS 媒介忙碌中 SIFS CF-D 1 SIFS CF-D 2 CF-U 1 SIFS CF-D 3 CF-U 2 SIFS 競爭週期 PIFS SIFS CF-End CF-D 4 CF-U 4 SIFS 重設 NAV CF-邊界 NAV Dx = Down Traffic Ux = Up Traffic CN_2. 175

PCF Transfer Procedure n PCF Transfers When the PCF Station is Neither Transmitter nor Recipient l A CF-aware station, when polled by the PCF, may send a Data frame to any station in the BSS an SIFS period after receiving the CF-Poll. l If the recipient of this transmission is not the PCF station, the Data frame is received and acknowledged in the same manner as a contention-based Data frame. l The PCF resumes (CF-Down) transmissions an SIFS period after the ACK frame. If not acknowledged, a PIFS period is employed. CN_2. 176

PCF Transfer Procedure 超級訊框 免競爭週期 PIFS 媒介忙碌中 SIFS CF-D 1 CF-End CF-D 2 S-To-S ACK SIFS 競爭週期 CF-U 2 SIFS 重設 NAV CF-邊界 NAV Dx = Down Traffic Ux = Up Traffic CN_2. 177

Bluetooth n Used for very short range communication between mobile phones, PDAs, notebook computers and other personal or peripheral devices n Operates in the license-free band at 2. 45 GHz n Has a range of only 10 m n Communication devices typically belong to one individual or group l Sometimes categorized as Personal Area Network (PAN) n Version 2. 0 provides speeds up to 2. 1 Mbps n Power consumption is low CN_2. 178

Bluetooth n Bluetooth is specified by an industry consortium called the Bluetooth Special Interest Group n It specifies an entire suite of protocols, going beyond the link layer to define application protocols, which it calls profiles, for a range of applications l There is a profile for synchronizing a PDA with personal computer l Another profile gives a mobile computer access to a wired LAN CN_2. 179

Bluetooth n The basic Bluetooth network configuration is called a piconet l Consists of a master device and up to seven slave devices l Any communication is between the master and a slave l The slaves do not communicate directly with each other l A slave can be parked: set to an inactive, low- power state CN_2. 180

Bluetooth A Bluetooth Piconet CN_2. 181

Zig. Bee n Zig. Bee is a new technology that competes with Bluetooth n Devised by the Zig. Bee alliance and standardized as IEEE 802. 15. 4 n It is designed for situations where the bandwidth requirements are low and power consumption must be very low to give very long battery life n It is also intended to be simpler and cheaper than Bluetooth, making it financially feasible to incorporate in cheaper devices such as a wall switch that wirelessly communicates with a ceiling -mounted fan CN_2. 182

Zig. Bee based IPv 6 enable Sensor node l Octopus II platform 4 CC 2420 -IEEE 802. 15. 4 wireless transceiver 4 MSP 430 -8 MHz Core microcontroller 4 Expandable flash memory – up to 1 MB 4 External oscillator – For the deep sleep mode 4 A connector with expansion I/O pins 4 Sensor devices – Light sensors – Temperature-humidity sensor CN_2. 183

Zig. Bee based IPv 6 enable Sensor n Octopus II CN_2. 184

Summary n We introduced the many and varied type of links that are used to connect users to existing networks, and to construct large networks from scratch. n We looked at the five key issues that must be addressed so that two or more nodes connected by some medium can exchange messages with each other l Encoding l Framing l Error Detecting l Reliability l Multiple Access Links 4 4 Ethernet Wireless 802. 11, Bluetooth CN_2. 185

End of Chapter 2