Chapter 3 Underlying Technology Objectives Upon completion you

Chapter 3 Underlying Technology Objectives Upon completion you will be able to: • Understand the different versions of wired Ethernet • Understand wireless Ethernet • Understand the types of point-to-point WANs • Understand the types of switched WANs, especially ATM • Differentiate between repeaters, bridges, routers, and hubs TCP/IP Protocol Suite 1

Figure 3. 1 TCP/IP Protocol Suite Internet model 2

3. 1 Local Area Networks A local area network (LAN) is a data communication system that allows a number of independent devices to communicate directly with each other in a limited geographic area such as a single department, a single building, or a campus. A large organization may need several connected LANs. The most popular LANs are Ethernet and wireless LANs. We briefly review these technologies in this section. The topics discussed in this section include: Wired LANs: Ethernet Wireless LANs: IEEE 802. 11 TCP/IP Protocol Suite 3

Ethernet n Xerox (Bob Metcalfe) performed initial development of Ethernet in 1976 (2. 94 Mbps over 100 personal workstations, 1 -km long cable) and was later joined by the Digital Equipment Corporation (DEC) and Intel to define the Ethernet 1 specification in 1980. The same group subsequently released the Ethernet 2 specification in 1984. The Ethernet specification describes a packet switching CSMA/CD LAN. TCP/IP Protocol Suite 4

Original Ethernet architecture station interface Data link layer data encapsulation link management Network Interface Card (NIC) encoding and decoding AUI cable Physical layer transmission and receipt tap transceiver BNC connector 0. 5 “ Coax TCP/IP Protocol Suite 5

Carrier Sense Multiple Access Carrier sense: station listens to media before transmitting n. Multiple access: multiple stations may access at same time n Media idle? YES NO -- wait Transmit immediately Listen until the media is idle, then transmit

Collision More than one station may send a frame during overlapping times. How does a station know that a collision occurred? What does the station do after a collision?

TCP/IP Protocol Suite 8

Collision Detection Rules. 1 Stations must listen to the cable while transmitting in order to detect a collision. . 2 A frame must be at least 64 bytes (512 bits, 51. 2 msec) long to ensure sender “hears” a collision before he finishes. (The transmission time must be more than the RTT(. . 3 If a collision is detected, send a brief jamming signal and then wait before retransmitting. Jamming signal

Worst Case Collision Timing 500 meters n n n n 2, 500 meters Assume delay for repeater is 1 msec; for transceiver 0. 5 msec. Component Propagation Time. Microsecs Five 500 meter segments 2500 m/0. 77 c Four repeaters 4 x 1 msec 4. 0 Nine 50 meter AUI cables 450 m/0. 65 c Nine transceivers 9 x 0. 5 msec 4. 5 Total one-way time 21. 6 TCP/IP Protocol Suite 10. 8 2. 3 10

Worst Case Collision Timing (cont(. 500 meters 43. 2 microsecond RTT Under these assumptions, the round trip time would be 43. 2 microseconds. n Allowing some tolerance for equipment, IEEE chose 51. 2 microseconds, equal to 512 bit-times, as the collision detection interval. n This is why all frames must be at least 512 bits (64 bytes) long. n TCP/IP Protocol Suite 11

How does a node detect a collision? Transceiver: A node monitors the media while transmitting. If the observed power is more than transmitted power + attenuated reflection of its own signal, it indicates a collision. n Transmitted signal Collision! Observed signal Hub: if input occurs simultaneously on two ports, it indicates a collision. Hub sends a collision presence signal on all ports. n Simultaneous input on two ports Output “collision presence” on all ports

Late Collisions A late collision occurs when sender finishes transmission before detecting collision presence. Usual causes: • cable too long • too many repeaters between stations Solution: higher layer protocol must detect packet loss and retransmit.

Figure 3. 3 TCP/IP Protocol Suite Ethernet layers 14

Figure 3. 4 TCP/IP Protocol Suite Ethernet frame 15

Figure 3. 5 TCP/IP Protocol Suite Ethernet implementations 16

10 Base 5 • tap : cable does not need to be cut • transceiver : send/receive, collision detection, electronics isolation • AUI : Attachment Unit Interface • Use for backbone networks vampire tap Æ 0. 5“ Coax BNC connector maximum segment length=500 m maximum number of stations per segment=100 transceiver AUI cable minimum distance between two stations = 2. 5 m NIC maximum network distance between two stations = 2. 8 km TCP/IP Protocol Suite 17

10 Base 2 • BNC connector • No drop cable • use for office LAN • What is its benefit since length < 500 m? 0. 25 “ Coax BNC T-connector NIC maximum segment length=185 m maximum number of stations per segment=30 minimum distance between two stations = 0. 5 m maximum network distance between two stations = 925 m TCP/IP Protocol Suite 18

10 Base. T n n A hub functions as a repeater UTP category 5 uses 2 pairs of wires terminated by an eight-bin (RJ-45 style) connector. This means that 4 pins of the 8 -pin are used. The transmit and receive data signal on each pair of the segment are polarised, with one wire of the signal pair carrying the positive (+) signal and the other carrying the negative (-). Medium Dependent Interface (MDI), RJ 45 hub maximum segment length = 100 m TCP/IP Protocol Suite NIC 19

Ethernet Charecteristics n Half Duplex / Full Duplex n n Ethernet they were initially implemented in a half-duplex manner with the transceiver detecting a collision if an attempt was made to transmit and receive simultaneously and looping back data to the host so it could hear itself transmit (as it would on a shared medium. ( However if both ends of the link are not hubs, and the hardware supports it, the two channels can be split and used to make a fullduplex link. Unfortunately if autonegotiation is enabled on one end and forced full-duplex on the other, the end with autonegotiation will detect the link as half-duplex causing large numbers of errors due to duplex mismatch. TCP/IP Protocol Suite 20

Ethernet Charecteristics n n Collision Domain - Segment Broadcast Domain – Subnet Hub Switch TCP/IP Protocol Suite 21

Figure 3. 6 TCP/IP Protocol Suite Fast Ethernet implementations 22

Figure 3. 7 TCP/IP Protocol Suite Gigabit Ethernet implementations 23

Wireless LAN (WLAN) n n n A WLAN is a shared network. An access point is a shared device and functions like a shared Ethernet hub. Data is transmitted over radio waves. Two-way radio communications (half-duplex) are used. The same radio frequency is used for sending and receiving (transceiver). TCP/IP Protocol Suite 24

What Are WLANs? n They are: n n n Local In building or campus for mobile users Radio or infrared Not required to have RF licenses in most countries Using equipment owned by customers n They are not: n n n WAN or MAN networks Cellular phones networks Packet data transmission via celluar phone networks n n n Cellular digital packet data (CDPD) General packet radio service (GPRS) 2. 5 G to 3 G services

Similarities Between WLAN and LAN n A WLAN is an 802 LAN. n n n Transmits data over the air vs. data over the wire Looks like a wired network to the user Defines physical and data link layer Uses MAC addresses The same protocols/applications run over both WLANs and LANs. n n n IP (network layer) IPSec VPNs (IP-based) Web, FTP, SNMP (applications) TCP/IP Protocol Suite 26

Differences Between WLAN and LAN n WLANs use radio waves as the physical layer. n n WLANs use CSMA/CA instead of CSMA/CD to access the network. Radio waves have problems that are not found on wires. n Connectivity issues. n n n Privacy issues. WLANs use mobile clients. n n n Coverage problems Multipath issues Interference, noise No physical connection. Battery-powered. WLANs must meet country-specific RF regulations. TCP/IP Protocol Suite 27

WLAN and LAN TCP/IP Protocol Suite 28

Service Set Identifier (SSID) n n n SSID is used to logically separate WLANs. The SSID must match on client and access point. Access point broadcasts one SSID in beacon. Client can be configured without SSID. Client association steps: 1. 2. 3. 4. 5. Client sends probe request. A point sends probe response. Client initiates association. A point accepts association. A point adds client MAC address to association table. TCP/IP Protocol Suite 29

WLAN Access Topology

Wireless Repeater Topology

Workgroup Bridge Topology TCP/IP Protocol Suite 32

Alternative Peer-to-Peer Topology

Service Sets and Modes n Ad hoc mode n Independent Basic Service Set (IBSS) n n Mobile clients connect directly without an intermediate access point. Infrastructure mode n Basic Service Set n n Mobile clients use a single access point for connecting to each other or to wired network resources. Extended Services Set n Two or more Basic Service Sets are connected by a common distribution system. 34

Figure 3. 9 TCP/IP Protocol Suite ESS 35

Client Roaming • Maximum data retry count exceeded • Too many beacons missed • Data rate shifted • Periodic intervals • Roaming without interruption requires the same SSID on all access points. TCP/IP Protocol Suite 37

802. 11 b Standard n n Standard was ratified in September 1999 Operates in the 2. 4 -GHz band Specifies direct sequence spread spectrum (DSSS) Specifies four data rates up to 11 Mbps n n 1, 2, 5. 5, 11 Mbps Provides specifications for vendor interoperability (over the air) Defines basic security, encryption, and authentication for the wireless link Is the most commonly deployed WLAN standard TCP/IP Protocol Suite 38

2. 4 -GHz Channels Regulatory Domain Channel Identifier Channel Center Frequency Channel Frequency Range [MHz] Americas Europe, Middle East, and Asia Japan 1 2412 MHz 2401 – 2423 X X X 2 2417 MHz 2406 – 2428 X X X 3 2422 MHz 2411 – 2433 X X X 4 2427 MHz 2416 – 2438 X X X 5 2432 MHz 2421 – 2443 X X X 6 2437 MHz 2426 – 2448 X X X 7 2442 MHz 2431 – 2453 X X X 8 2447 MHz 2436 – 2458 X X X 9 2452 MHz 2441 – 2463 X X X 10 2457 MHz 2446 – 2468 X X X 11 2462 MHz 2451 – 2473 X X X 12 2467 MHz 2466 – 2478 X X 13 2472 MHz 2471 – 2483 X X 14 2484 MHz 2473 – 2495 X

2. 4 -GHz Channel Use • Each channel is 22 MHz wide. • North America: 11 channels. • Europe: 13 channels. • There are three nonoverlapping channels: 1, 6, 11. • Using any other channels will cause interference. • Three access points can occupy the same area. TCP/IP Protocol Suite 40

802. 11 b/g (2. 4 GHz) Channel Reuse TCP/IP Protocol Suite 41

802. 11 b Access Point Coverage TCP/IP Protocol Suite 42

802. 11 g Standard n n Standard was ratified June 2003 Operates in the 2. 4 -GHz band as 802. 11 b n n n DSSS (CCK) and OFDM transmission 12 data rates of up to 54 Mbps n n n Same three nonoverlapping channels: 1, 6, 11 1, 2, 5. 5, 11 Mbps (DSSS / 802. 11 b) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (OFDM) Full backward compatiblity to 802. 11 b standard TCP/IP Protocol Suite 43

Range Comparisons TCP/IP Protocol Suite 44

Figure 3. 10 Physical layer TCP/IP Protocol Suite 45

Figure 3. 11 n FHSS Frequency Hopping n n Sequential use of multiple frequencies Hop sequence and rate will vary TCP/IP Protocol Suite 46

Figure 3. 12 n DSSS Direct Sequence n n Each symbol is transmitted over multiple frequencies at the same time Very efficient (no overhead) Higher speed than FH at comparable distances System capacity (multiple channels) higher than FH TCP/IP Protocol Suite 47

Figure 3. 13 TCP/IP Protocol Suite MAC layers in IEEE 802. 11 standard 48

802. 11 Collisions Avoidance n Similar to CSMA/CD (Ethernet) n n Transmit when medium is idle, back off on collision Problem: medium is not fully shared A TCP/IP Protocol Suite B C D 49

802. 11 Collisions Avoidance contd. n Hidden node problem: A and C cannot hear each other n If they both transmit to B at the same time, there will be a collision which won’t be detected by A or C TCP/IP Protocol Suite 50

802. 11 Collisions Avoidance contd. n Exposed node problem: C could send to D while B is sending to A n C is blocked when B is sending TCP/IP Protocol Suite 51

“Hidden stations” the solution A B C RTS: I want to send to B 500 bytes CTS: OK A, go ahead, so everybody quiet Data: the 500 bytes of data from A to B ACK: B received the data OK, so an ACK n IEEE 802. 11 defines: n n MAC level RTS/CTS protocol (Request to Send / Clear to Send) Can be switched off to reduce overhead (when no hidden nodes exist) More robustness, and increased reliability No interruptions when large files are transmitted

Figure 3. 14 TCP/IP Protocol Suite CSMA/CA 53

Figure 3. 15 TCP/IP Protocol Suite Frame 54

Table 3. 1 Addresses in IEEE 802. 11 TCP/IP Protocol Suite 55

3. 2 Point-to-Point WANs A second type of network we encounter in the Internet is the point-topoint wide area network. A point-to-point WAN connects two remote devices using a line available from a public network such as a telephone network. We discuss the physical and data link layers of these technologies here. . The topics discussed in this section include: Physical Layer Data Link Layer TCP/IP Protocol Suite 56

Figure 3. 16 TCP/IP Protocol Suite 56 K modem 57

Note: ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. TCP/IP Protocol Suite 58

Figure 3. 17 TCP/IP Protocol Suite Bandwidth division 59

Figure 3. 18 TCP/IP Protocol Suite ADSL and DSLAM 60

Figure 3. 19 TCP/IP Protocol Suite Cable bandwidth 61

Figure 3. 20 TCP/IP Protocol Suite Cable modem configurations 62

Table 3. 2 T line rates TCP/IP Protocol Suite 63

Table 3. 3 SONET rates TCP/IP Protocol Suite 64

Figure 3. 21 TCP/IP Protocol Suite PPP frame 65

3. 3 Switched WANs The backbone networks in the Internet are usually switched WANs. A switched WAN is a wide area network that covers a large area (a state or a country) and provides access at several points to the users. Inside the network, there is a mesh of point-to-point networks that connects switches. The switches, multiple port connectors, allow the connection of several inputs and outputs. The topics discussed in this section include: X. 25 Frame Relay ATM TCP/IP Protocol Suite 66

Figure 3. 22 TCP/IP Protocol Suite Frame Relay network 67

Note: A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-size block of information. TCP/IP Protocol Suite 68

Figure 3. 23 TCP/IP Protocol Suite ATM multiplexing 69

Figure 3. 24 TCP/IP Protocol Suite Architecture of an ATM network 70

Figure 3. 25 TCP/IP Protocol Suite Virtual circuits 71

Note: Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI. TCP/IP Protocol Suite 72

Figure 3. 26 TCP/IP Protocol Suite An ATM cell 73

Figure 3. 27 TCP/IP Protocol Suite ATM layers 74

Note: The IP protocol uses the AAL 5 sublayer. TCP/IP Protocol Suite 75

Note: We will discuss IP over ATM in Chapter 23. TCP/IP Protocol Suite 76

3. 4 Connecting Devices LANs or WANs do not normally operate in isolation. They are connected to one another or to the Internet. To connect LANs or WANs, we use connecting devices. Connecting devices can operate in different layers of the Internet model. We discuss three kinds of connecting devices: repeaters (or hubs), bridges (or two-layer switches), and routers (or three -layer switches). Repeaters and hubs operate in the first layer of the Internet model. Bridges and two-layer switches operate in the first two layers. Routers and three-layer switches operate in the first three layers The topics discussed in this section include: Repeaters Hubs Bridges Router TCP/IP Protocol Suite 77

Figure 3. 28 TCP/IP Protocol Suite Connecting devices 78

Figure 3. 29 TCP/IP Protocol Suite Repeater 79

Note: A repeater connects segments of a LAN. TCP/IP Protocol Suite 80

Note: A repeater forwards every bit; it has no filtering capability. TCP/IP Protocol Suite 81

Note: A repeater is a regenerator, not an amplifier. TCP/IP Protocol Suite 82

Figure 3. 30 TCP/IP Protocol Suite Function of a repeater 83

Note: A bridge has a table used in filtering decisions. TCP/IP Protocol Suite 84

Figure 3. 31 TCP/IP Protocol Suite Bridge 85

Note: A bridge does not change the physical (MAC) addresses in a frame. TCP/IP Protocol Suite 86

Figure 3. 32 TCP/IP Protocol Suite Learning bridge 87

Note: A router is a three-layer (physical, data link, and network) device. TCP/IP Protocol Suite 88

Note: A repeater or a bridge connects segments of a LAN. A router connects independent LANs or WANs to create an internetwork (internet). TCP/IP Protocol Suite 89

Figure 3. 33 TCP/IP Protocol Suite Routing example 90

Note: A router changes the physical addresses in a packet. TCP/IP Protocol Suite 91