17 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 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

17 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 2 LAN Topologies Basic topologies revisited: Hub

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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… …

17 - Winter 2005 ECE 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 A1 A2 B1 B2 C1 A B C

17 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 6 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 t0t0 t 0 +tt 0 +2tt 0 +3t Vulnerable period

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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 t 0, station 1 starts sending –At t 0 + τ, station 2 sends its first bit, causes collision, stops sending –Station 1 detects collision at t 0 +2 τ –Hence, the contention period is 2 τ

17 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 12 Project Logical Link Control (LLC) Internetworking CSMA/CD Token Bus Token Ring... Project 802 Network Data Link Layer Physical OSI Model

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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 = 2 τ  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

17 - Winter 2005 ECE 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 k th collision, randomly wait 0-(2 k -1) slot times –Maximum slot time to be waited is 1023 –Give up after 16 consecutive collisions

17 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 17 Ethernet (802.3) Frame format –Preamble used for sender/receiver clock synchronization –MSB of destination address marks single (0) or group communication (1) Preamble Destination Address Source Address DataPadChecksum Bytes Start of frame delimiter Length of data

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE 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

17 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 20 Token Ring (802.5) Token format Frame format SDACED SD/ED: Starting/Ending Delimiter AC: Access control 1 byte each SD Destination Address Source Address DataFCS 16 6No limit1 4 Bytes ACFCEDFS 1 11 Start of Frame Sequence FCS CoverageEnd of Frame Sequence FC: Frame Control FCS: Frame Check Sequence FS: Frame Status

17 - Winter 2005 ECE ECE 766 Computer Interfacing and Protocols 21 Token Ring (802.5) Starting Delimiter: J K 0 J K –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?

17 - Winter 2005 ECE 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