CS 716 Advanced Computer Networks By Dr Amir

CS 716 Advanced Computer Networks By Dr. Amir Qayyum 1

Lecture No. 9

Sequence Number Space • Seq. Num field is finite; sequence numbers wrap around • Sequence number space must be larger than number of outstanding frames (SWS) 3

Sequence Number Space • SWS <= Max. Seq. Num-1 is not sufficient – Suppose 3 -bit Seq. Num field (0. . 7); SWS=RWS=7 – Sender transmits frames 0. . 6; which arrive successfully (receiver window advances) – ACKs are lost; sender retransmits 0. . 6 – Receiver expecting 7, 0. . 5, but receives second incarnation of 0. . 5 assuming them as 4 8 th to 13 th frame

Required Sequence Number Space ? • Assume SWS=RWS (simplest, and typical) – Sender transmits full SWS – Two extreme cases at receiver • None received (waiting for 0…SWS-1) • All received (waiting for SWS… 2*SWS-1) 5

Required Sequence Number Space ? • All possible packets must have unique Seq. Num • SWS < (Max. Seq. Num+1)/2 or SWS+RWS < Max. Seq. Num+1 is the correct rule • Intuitively, Seq. Num “slides” between two halves of sequence number space 6

What Next ? • Arbitrating access to a shared medium • After that: network adapters and example protocols 7

Shared Access Networks Outline • • Bus (Ethernet) Token ring (FDDI) Wireless (802. 11) Network Adapter 8

Shared Access Media • Arbitrating access to a shared media – Multiple hosts on a single link 9

Multiple Access Media • Multiple senders on some media – Buses (Ethernet, including links in switched form) – Radio, satellite – Token rings … 10

Multiple Access Media • Need method to moderate access – Fair arbitration – Good performance 11

Shared Media • Communication needs vary – Over time – Between hosts • Network is not fully utilized 12

Shared Media • Recall methods for multiplexing – Frequency-division multiplexing (FDM, separate bands) – Time-division multiplexing (TDM, synchronous time slots) – Statistical TDM (STDM, time slots on demand) • STDM most appropriate with stated assumptions 13

Shared Media: Problems • Problem: demands can conflict, e. g. , two hosts send simultaneously – STDM does not address this problem centralized – Solution is a medium access control (MAC) algorithm 14

Shared Media: Solutions • Three solutions (out of many) – Carrier sense multiple access with collision detection (CSMA / CD) • Send only if medium is idle • Stop sending immediately if collision detected – Token ring/FDDI pass a token around a ring; only token holder sends – Radio / wireless (IEEE 802. 11) 15

Ethernet 16

History of Ethernet • Developed by Xerox PARC in mid-1970 s • Roots in Aloha packet-radio network • Standardized by Xerox / DEC / Intel in 1978 • Similar to IEEE 802. 3 standard • IEEE 802. 3 u standard defines Fast Ethernet (100 Mbps) • New switched Ethernet now popular 17

Ethernet Topologies • Bus— all nodes connected to a wire. . . • Star— all nodes connected to a central repeater • Combinations thereof 18

Ethernet Adaptor • • Segment of up to 500 m Nodes tap into segments Taps must be 2. 5 m apart Transceiver performs carrier sensing • Transceiver transmits and receive signals • Protocol is implemented in the adaptor 19

Ethernet – Alternative Technologies • Can be constructed from a thinner cable (10 Base 2) rather than 50 -ohm coax cable (10 Base 5) • Newer technology uses 10 Base. T (twisted pair) – Several point-to-point segments coming out of a multiway repeater called “hub” 20

Ethernet Components 10 Base 5 (Thick. Net) Controller (Ethernet Card) Vampire Tap Bus Topology Transceiver 21

Ethernet Components 10 Base 2 (Thin. Net) Controller (Ethernet Card) Transceiver BNC T-junction Bus Topology 22

Ethernet Components 10 Base. T (Twisted Pair) Controller (Ethernet Card) Hub Star Topology 23

Ethernet – Multiple Segments • Repeaters forward the broadcast signal on all out going segments (10 Base 5) • Maximum of 4 repeaters (2500 m), 1024 hosts … Repeater … Host … 24

Ethernet Packet Frame • Preamble allows the receiver to synchronize with signal • Frame must contain at least 46 bytes to detect collision • 802. 3 standard substitutes length with type field – Type field (demux key) is the first thing in data portion – A device can accept both frames: type > 1500 64 48 48 16 Preamble Dest addr Src addr Type 32 Body CRC 25

Ethernet Address • Addresses – Unique, 48 -bit unicast address assigned to each adapter – Example: 8: 0: e 4: b 1: 2 – Broadcast: all 1 s – Multicast: first bit is 1 – Promiscuous mode • Problem remains: A distributed algorithm that provides fair access 26

Ethernet MAC – CSMA/CD • Multiple access – Nodes send and receive frames over a shared link • Carrier sense – Nodes can distinguish between an idle and busy link • Collision detection – A node listens as it transmits to detect collision 27

CSMA/CD MAC Algorithm • If line is idle (no carrier sensed) – Send immediately – Upper bound message size of ~1500 bytes – Must wait 9. 6µs between back-toback frames 28

CSMA/CD MAC Algorithm • If line is busy (carrier sensed) … – Wait until the line becomes idle and then transmit immediately – Called 1 -persistent (special case of ppersistent) • If collision detected – Stop sending data and jam signal – Try again later 29

Collision Detection my-machine your-machine Start transmission at time 0 Start transmission at time T Almost there at time T Collision !!! How to ensure that my-machine knows about the collision? 30

Constraints on Collision Detection • In our example, consider – my-machine’s message reaches yourmachine at T – your-machine’s message reaches mymachine at 2 T • Thus, my-machine must still be transmitting at 2 T 31

Constraints on Collision Detection • Specifics of IEEE 802. 3 – Bounds 2 T to 51. 2 microseconds – Packet must be at least 64 B long • Jam after the collision, for 32 bits, then stop transmitting frame (runt frame of 96 bits) – Ensures that all hosts notice collision 32

Review Lecture 9 • • • Shared access networks Shared media: issues Ethernet Topologies, technologies Segments Frame format, Addresses MAC protocol: CSMA/CD 33