Efficient Local Area Network Protocols for High-Speed Communications

1. Local Area Networks • Need for high performance communications for physically close devices (e.g. office environment) • Why “local”? • Volume of locally created data • Increasing local computation power • Advantage of locality: High Speed! • Design goals: • High speed and high bandwidth • Simple, maintainable, flexible, extendable • Low cost ECE 766 Computer Interfacing and Protocols

2. Hub LAN Topologies • Basic topologies revisited: ECE 766 Computer Interfacing and Protocols

3. Channel Access • Line discipline is important factor of overall performance • Static allocation of resources results in poor performance • From queuing theory: dividing resources in N equal parts multiplies the mean waiting time by N • Polling vs. contention techniques • Polling: Asking everyone if they have something to send • Can be centralized or distributed (how?) • Contention:Try to access the channel without prior arrangement ECE 766 Computer Interfacing and Protocols

4. Multiple Access Protocols • Several different kinds of multiple access protocols exist: • Aloha • Pure, slotted • Carrier Sense Multiple Access (CSMA) • Persistent, non-persistent, p-persistent, CD • Collision-Free Protocols • Bit map, binary count… • Limited Contention Protocols • Adaptive tree walk… … ECE 766 Computer Interfacing and Protocols

5. Pure Aloha • Users send their frames as soon as they are available • Collisions will occur, but wait for a random amount of time and send the frame once again A A1 A1 A2 A2 B B1 B1 B2 C C1 C1 ECE 766 Computer Interfacing and Protocols

6. t0 t0+t t0+2t t0+3t Vulnerable period Pure Aloha • Performance of Pure Aloha • When sending a frame, we hope that no one else is transmitting from 1 frame time before we start transmission until our transmission is over ECE 766 Computer Interfacing and Protocols

7. Slotted Aloha • Different from Pure Aloha in the timing of channel access • Time is partitioned into slots • When a host receives a frame, it waits until the beginning of the next slot to transmit • The vulnerable period is reduced to half of Pure Aloha • A maximum of one slot waiting time is possible ECE 766 Computer Interfacing and Protocols

8. CSMA Protocols • Based on sensing the channel before sending the frame • Send the frame if channel is free • Behavior after detecting a busy channel determines the kind of CSMA protocol • 1-persistent: Send the frame if channel is available. If busy, transmit the frame with probability 1 as soon as the channel is free. If collision occurs, wait a random amount of time and start over • Nonpersistent: Send the frame if channel is available. If busy, wait a random amount of time and try sending once again ECE 766 Computer Interfacing and Protocols

9. CSMA Protocols • P-persistent: Used in slotted channels. Send the frame with probability p if channel is available, defer to the next slot with probability 1-p. If busy, wait until the next slot and repeat the algorithm. • CSMA protocols have higher throughput than Aloha protocols • Nonpersistent protocol has higher throughput and delay than 1-persistent • Performance of p-persistent depends on the value of p ECE 766 Computer Interfacing and Protocols

10. CSMA/CD • Carrier Sense Multiple Access / Collision Detection • Sense the channel before sending • If collision is detected, stop the transmission (frame is damaged anyway) • Wait for a random amount of time before the next attempt • Collision detection is done by comparing the transmitted power to the received one ECE 766 Computer Interfacing and Protocols

11. CSMA/CD • How long does it take for a station to conclude that it seized the channel, i.e., what is the contention period? • Consider the worst case scenario • Largest propagation delay = τ • At t0, station 1 starts sending • At t0+ τ , station 2 sends its first bit, causes collision, stops sending • Station 1 detects collision at t0+2τ • Hence, the contention period is 2τ ECE 766 Computer Interfacing and Protocols

12. Network 802.1 Internetworking Data Link Layer 802.2 Logical Link Control (LLC) 802.3CSMA/CD 802.4Token Bus 802.5Token Ring ... Physical OSI Model Project 802 Project 802 ECE 766 Computer Interfacing and Protocols

13. Ethernet (802.3) • Xerox, DEC, Intel • Properties: • Simple, low cost, low delay • High speed (10, 100, 1000 Mbps) • Aims data exchange at data link level • Fairness in channel access • Single node, group, broadcast addressing • No unused fields, no variants • Stability: increase in offered traffic should not choke the system ECE 766 Computer Interfacing and Protocols

14. Ethernet (802.3) • Properties (not so attractive ones): • Not full duplex • Limited error control • Detection of and recovery from collision • Error detection using CRC, retransmissions left to higher level • No security integrated • Best effort service • No measures against malicious users ECE 766 Computer Interfacing and Protocols

15. Ethernet (802.3) • Limit on cable length • Minimum frame size is 64 bytes • At 10Mbps, it takes 51.2μsec to transmit the shortest frame • 51.2μsec = 2τ 2500 meter cable length • To achieve 1Gbps: • Keep cable length at 2500m, minimum frame size becomes 6400 bytes • Keep minimum frame size at 64 bytes, maximum cable length becomes 25m ECE 766 Computer Interfacing and Protocols

16. Ethernet (802.3) • Binary Exponential Backoff Algorithm • Slot time = 51.2μsec • When collision occurs, wait 0 or 1 slot time • If another collision occurs, wait a random number of slot times between 0 and 3 • After kth collision, randomly wait 0-(2k-1) slot times • Maximum slot time to be waited is 1023 • Give up after 16 consecutive collisions ECE 766 Computer Interfacing and Protocols

17. 1 2-6 2-6 2 0-1500 0-46 4 Bytes 7 Preamble DestinationAddress SourceAddress Data Pad Checksum Start of framedelimiter Length of data Ethernet (802.3) • Frame format • Preamble used for sender/receiver clock synchronization • MSB of destination address marks single (0) or group communication (1) ECE 766 Computer Interfacing and Protocols

18. Other Ethernet Networks • Switched Ethernet: • Switch isolates communication between two stations • Medium is no longer truly broadcast medium • Fast Ethernet: • Reduce the cable size to 250m, increase the speed to 100Mbps • Gigabit Ethernet: • 1Gbps speed • 25m with cable, 550/5000m with multi/single mode optical fiber ECE 766 Computer Interfacing and Protocols

19. Token Ring (802.5) • Unidirectional ring • Stations are either active or let the frames pass • Medium access: • Station waits for token • Capture token and transmit your message instead • Wait until you get your own message, then place the token on the line ECE 766 Computer Interfacing and Protocols

20. 1 6 No limit 1 1 1 1 6 4 Bytes SD AC FC DestinationAddress SourceAddress Data FCS ED FS Token Ring (802.5) • Token format • Frame format SD/ED: Starting/Ending Delimiter AC: Access control 1 byte each SD AC ED FCS Coverage End of FrameSequence Start of FrameSequence FC: Frame Control FCS: Frame Check Sequence FS: Frame Status ECE 766 Computer Interfacing and Protocols

21. Token Ring (802.5) • Starting Delimiter: J K 0 J K 0 0 0 • Violations of Differential Manchester encoding • J: Cancel both transitions • K: Cancel middle transition only • Access Control: P P P T M R R R • P: Priority bits indicating which stations are allowed to use token • T: Token bit, 1 if token or abort, 0 if data or command • M: Monitor bit, used by active monitor station to detect orphan frames • R: Reservation bits to reserve the next token, cannot be set to less than priority of the frame How does it ever decrease? ECE 766 Computer Interfacing and Protocols

22. Token Ring (802.5) • Frame Control: • Used to distinguish data frames from control frames • Frame Status: • Includes A and C bits • A is set when destination passes the frame • C is set when destination copies the frame • AC=00: Destination not powered up or not present • AC=10: Destination present, but frame not accepted • AC=11: Destination present and frame accepted • Automatic acknowledgment of frames ECE 766 Computer Interfacing and Protocols