EEC 189Q Medium Access Sublayer Computer Networks Reading: Chapter Communication Links Remarks on MAC Sublayer

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1 EEC 189Q Computer Networks Medium Access Sublayer Medium Access Control (MAC) Random vs. controlled access Random Access Aloha, CSMA, CSMA/CD Network Data Link Physical Medium Access Sublayer Medium Access Control (MAC) Protocols --- distributed algorithm that arbitrate access to a common shared channel among a population of users, i.e., determine when a node can transmit Reading: Chapter Chuah, Fall Communication Links Remarks on MAC Sublayer Point to point link - One sender, one receiver - Dedicated channel Multi-access broadcast link - Multiple senders, multiple potential receivers - Shared media MAC is not important on point-to-point links MAC is only used in broadcast or shared channel networks - communication about sharing must use the channel itself! Examples: - Packet-switched Radio Network (Aloha) - Ethernet IEEE (CSMA/CD) - Token Ring IEEE 802.5, FDDI (Token Passing) - Cellular, Satellite, Wireless LAN (MACAW) Chuah, Fall Chuah, Fall Page 1

2 Broadcast Links: Multiple Access Contention Access Protocols Single shared communication channel Only one can send successfully at a time Two or more simultaneous transmissions - Interference How to share a broadcast channel - Humans use multi-access protocols all the time General Characteristics - Single channel/medium shared by a large number of hosts - No coordination between hosts - Control is completely distributed How does it compared to complete control with switching? Chuah, Fall Chuah, Fall Design Goals Taxonomy of MAC Protocols Fully decentralized Fairness among users High efficiency Low delay Fault tolerance Chuah, Fall Chuah, Fall Page 2

3 Random Access Controlled Access Stations contend for channels Overlapping transmissions (collisions) can occur - Carrier sensing? - Collision detection? Protocols - Aloha - Slotted Aloha - Carrier Sense Multiple Access: Ethernet Stations reserve or are allocated channel - No collisions - Allocation: static or dynamic Protocols - Static channel allocation Time division multiple access - Demand adaptive channel allocation Reservation protocols Token passing (token bus, token ring) Chuah, Fall Chuah, Fall Pure Aloha Originally developed for packet radio communications at the campuses of U. of Hawaii in between a central computer & various data terminals All nodes transmit on one frequency, f o Central node relays packets on the other frequency, f 1 Central Node f o f 1 Pure Aloha Algorithm 1. Users transmit whenever they have something to send If more than one node transmits at the same time => Collision! If there is a collision, both nodes need to retransmit Central Node X node 1 node 2 node 3 node n Chuah, Fall Chuah, Fall Page 3

4 Pure Aloha Algorithm (cont d) 2. Listen to the broadcast Assume the receiver rebroadcasts the received signal, so the sender can find out if its packet was destroyed just by listening to downward broadcast one round-trip time after sending it 3. If packet was destroyed, wait a random amount of time, and send it again prevent the same packet from colliding over and over again Contention Period in Pure Aloha Just send: no waiting for beginning of slot If first bit of a new packet overlaps with last bit of a packet almost finished, both packets are destroyed. t: one packet transmission time (L/R) t 0 -t t 0 t 0 +t Vulnerable period: 2t Chuah, Fall Chuah, Fall Slotted Aloha How to reduce vulnerability/contention period? Time is divided into equal size slots Nodes transmit at the beginning of a slot - Packets must be transmitted within a slot. If collision, retransmit later Contention Period in Slotted Aloha t: one packet transmission time (L/R) = slot duration Packet could arrive in the middle of a slot Potential collision with packets sent within the same slot t 0 -t t 0 t 0 +t Success (S), Collision (C), Empty (E) slots Chuah, Fall Vulnerable period: t Chuah, Fall Page 4

5 Performance of Aloha Protocols Slotted vs Pure Aloha S = throughput = goodput (success rate) Pure Aloha Slotted Aloha Synchronous system Have to wait till start of next slot Packets overlap completely or not at all Maximum throughput 0.37 (1/e) Simpler, no synchronization No waiting for beginning of slot Packets may overlap partially Maximum throughput 0.18 (1/2e) G = offered load = Np Chuah, Fall Chuah, Fall Carrier Sense Multiple Access Assumptions with CSMA Networks Aloha is inefficient (and rude) - Doesn t listen before talking CSMA: Listen before transmit - If channel idle, transmit entire packet - If busy, defer transmission How long should we wait? - Human analogy: don t interrupt others Can carrier sense avoid collisions completely? Constant length packets No errors, except those caused by collisions Each host can sense the transmission of all other hosts The propagation delay is small compared to the transmission time. Chuah, Fall Chuah, Fall Page 5

6 CSMA Protocols 1-Persistent CSMA There several types of CSMA protocols - 1-Persistent CSMA - Non-Persistent CSMA - P-Persistent CSMA Sense the channel - If busy, keep listening to the channel and transmit immediately when the channel becomes idle. - If idle, transmit a packet immediately. If collision occurs - Wait a random amount of time and start over again Greedy algorithm - It s called 1-persistent because the host transmits with probability of 1 whenever it finds the channel idle. Chuah, Fall Chuah, Fall The Effect of Propagation Delay Contention Period A B Sense channel: idle => transmit Recall contention period in - Pure Aloha: 2 xpacket transmission time - Slotted Aloha: 1 packet transmission time Contention period in CSMA - up to 2 xend-to-end propagation delay Collision! Chuah, Fall Chuah, Fall Page 6

7 Brain Teaser 1-Persistent CSMA Can we use 1-Persistent in Satellite Network Satellite system has LONG propagation delay - time taken to sense the channel can be as long as 270 ms - vulnerability period = 540ms (1/2 a second) Even if propagation delay is zero, there will still be collisions. Why? - Nodes are greedy! They send packets as soon as they hear that the channel is idle! => CSMA doesn t make sense here Chuah, Fall Chuah, Fall Persistent and Non-persistent CSMA CSMA/CD Algorithm p-persistent - If idle, transmit with probability p - If busy, wait till it becomes idle - If collision, wait random amount of time Non-persistent - If idle, transmit - If busy, wait random amount of time Sense the channel - If idle, transmit immediately - If busy, wait until the channel becomes idle Collision detection - Abort a transmission immediately if a collision is detected - Try again later after waiting a random amount of time. Chuah, Fall Chuah, Fall Page 7

8 CSMA/CD Worst Case Collision Detection Time In CSMA protocols: If two colliding stations will transmit their complete packet, wasting the channel for the entire packet time. CSMA with collision detection (CD) Listen while talking Stop transmitting when collision detected - Compare transmitted and received signals Human analogy - Polite conversationalist Worst case time to detect a collision? Chuah, Fall Chuah, Fall Performance of CSMA/CD Performance of CSMA/CD (cont d) Assume time -slotted channel Probability that exactly one node transmits in a given time slot s( p) = s max N ( ) p(1 p) N 1 when p = 1/N Average number of time slots A wasted before a packet is transmitted successfully A = s.0 + (1-s)(1+A) Relevant parameters - cable length, signal speed, frame size, bandwidth More accurately (from simulation), channel efficiency under heavy load Efficiency = 1/ ( a) PROP cablelengt h bandwidth a = = TRANS signalspeed framesize When s = 0.4, A = 1.5 Chuah, Fall Chuah, Fall Page 8

9 Ethernet and IEEE Backoff After Collision 1-persistent CSMA/CD Carrier sense: station listens to channel first - Listen before talking If idle, station may initiate transmission - Talk if quiet Collision detection: continuously monitor channel - Listen while talking If collision, stop transmission - One talker at a time When collision occurs - Wait random time [0,1] slots If second collision occurs - Wait random time [0,3] slots Double range for each successive collision Called Exponential backoff Chuah, Fall Chuah, Fall Binary Exponential Backoff Algorithm Binary Exponential Backoff (cont d) If collision - choose one slot randomly from 2 k slots, {0,1,2,,2 k -1}, where k is the number of collisions the frame has suffered. - One contention slot length usually ~ 512 bit times (minimum frame size = 64 bytes) In Ethernet - Binary exponential backoff will allow maximum of 15 retransmission - If 16 backoffs occur, the transmission of the frame is considered a failure. Can adapt to varying network load Example - if k = 1, choose from {0, 1} - if k = 2, choose from {0, 1, 2, 3} - if k = 10, choose from {0, 1, 2, } Chuah, Fall Chuah, Fall Page 9

10 Ethernet Frame Format Ethernet Addressing Preamble Dest addr Src addr Type Body CRC 48-bit addresses All adaptors receive all packets - Dropped if address do not match Broadcast address - All 1 s: received and processed by all stations Multicast addresses - First bit is 1 Chuah, Fall Chuah, Fall IEEE Parameters Ethernet Summary 1 bit time = time to transmit one bit - 10 Mbps 1 bit time = 0.1 µs Maximum network diameter 2.5km - Maximum 4 repeaters Worst case collision detection time: 51.2 µs Slot time µs = 512 bits = 64 bytes 1-persistent CSMA/CD 51.2 µs to seize the channel Collision not possible after 51.2 µs Minimum frame size of 64 bytes Binary exponential backoff Works better under light load Delivery time non-deterministic Chuah, Fall Chuah, Fall Page 10

11 Pros and Cons of Contention Access Advantages: Short delay for bursty traffic Simple Flexible to fluctuations in the number of hosts Fairness Disadvantages: Low channel efficiency Not good for continuous traffic Cannot support priority traffic High variance in transmission delays Chuah, Fall Page 11

LANs. Local Area Networks. via the Media Access Control (MAC) SubLayer. Networks: Local Area Networks

LANs. Local Area Networks. via the Media Access Control (MAC) SubLayer. Networks: Local Area Networks LANs Local Area Networks via the Media Access Control (MAC) SubLayer 1 Local Area Networks Aloha Slotted Aloha CSMA (non-persistent, 1-persistent, p-persistent) CSMA/CD Ethernet Token Ring 2 Network Layer