CS716 Advanced Computer Networks By Mr. Abdul Wahid Shaikh

1. CS716 Advanced Computer Networks By Mr. Abdul Wahid Shaikh 1

2. Lecture No. 9

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

4. Sequence Number Space • SWS <= MaxSeqNum-1 is not sufficient • Suppose 3-bit SeqNum 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 8th to 13th frame

5. 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)

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

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

8. Shared Access Networks • Outline • Bus (Ethernet) • Token ring (FDDI) • Wireless (802.11) • Network Adapter

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

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

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

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

13. 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

14. 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

15. 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)

16. Ethernet

17. History of Ethernet • Developed by Xerox PARC in mid-1970s • Roots in Aloha packet-radio network • Standardized by Xerox / DEC / Intel in 1978 • Similar to IEEE 802.3 standard • IEEE 802.3u standard defines Fast Ethernet (100 Mbps) • New switched Ethernet now popular

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

19. Ethernet Adaptor • Segment of up to 500 m • Nodes tap into segments • Taps must be 2.5m apart • Transceiver performs carrier sensing • Transceiver transmits and receive signals • Protocol is implemented in the adaptor

20. Ethernet – Alternative Technologies • Can be constructed from a thinner cable (10Base2) rather than 50-ohm coax cable (10Base5) • Newer technology uses 10BaseT (twisted pair) • Several point-to-point segments coming out of a multiway repeater called “hub”

21. Ethernet Components 10Base5 (ThickNet) Controller (Ethernet Card) Vampire Tap Bus Topology Transceiver

22. Ethernet Components 10Base2 (ThinNet) Controller (Ethernet Card) BNC T-junction Transceiver Bus Topology

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

24. Ethernet – Multiple Segments • Repeaters forward the broadcast signal on all out going segments (10Base5) • Maximum of 4 repeaters (2500m), 1024 hosts … Repeater … Host …

25. 64 48 48 16 32 Src Dest Preamble Type Body CRC addr addr 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

26. Ethernet Address • Addresses • Unique, 48-bit unicast address assigned to each adapter • Example: 8:0:e4:b1:2 • Broadcast: all 1s • Multicast: first bit is 1 • Promiscuous mode • Problem remains: A distributed algorithm that provides fair access

27. 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

28. 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-to-back frames

29. 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 p-persistent) • If collision detected • Stop sending data and jam signal • Try again later

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

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

32. Constraints on Collision Detection • Specifics of IEEE 802.3 • Bounds 2T to 51.2 microseconds • Packet must be at least 64B long • Jam after the collision, for 32 bits, then stop transmitting frame (runt frame of 96 bits) • Ensures that all hosts notice collision

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