1 CS716 Advanced Computer Networks By Dr. Amir Qayyum

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 8 th to 13 th 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 )

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 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 Transceiver Bus Topology

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 Ethernet Packet Frame Preamble allows the receiver to synchronize with signal Frame must contain at least 46 bytes to detect collision 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 Dest addr CRCPreamble Src addr TypeBody 1648

26 Ethernet Address Addresses –Unique, 48-bit unicast address assigned to each adapter –Example: 8:0:e4:b1:2 –Broadcast: all 1 s –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 How to ensure that my-machine knows about the collision? Start transmission at time 0 my-machine your-machine 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 –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