Internet Protocols Fall Outline

Transcription

1 Internet Protocols Fall 2004 Lecture 4 Andreas Terzis Outline FEC MAC Protocols Channel Partitioning vs. Random Access Protocols Aloha and Slotted Aloha CSMA CSMA/CD (Ethernet) CS 449/Fall

2 Forward Error Correction FEC is an error detection and error correction mechanism Adds redundant information in the frame so the receiver can reconstruct the frame Which is better? Go-Back-N FEC CS 449/Fall 04 3 Space Communications Example CS 449/Fall

3 MAC Protocols Problem: How to share medium among multiple senders Different Solutions Channel Partitioning (FDMA, TDMA,CDMA) Taking Turns (Token Ring) Random Access (Aloha, CSMA,CSMA/CD) CS 449/Fall 04 5 Goals of MAC Protocols MAC Protocols arbitrate access to a common shared channel among a population of users 1. Fair among users 2. High efficiency 3. Low delay 4. Fault tolerant CS 449/Fall

4 ALOHA Packet radio network created by the University of Hawaii in the 70 s Basic Idea: Let a node transmit when it has data to send If frame was destroyed then retransmit after a random period of time CS 449/Fall 04 7 ALOHA Efficiency Assumptions Infinite number of users Frame arrivals are modeled by a Poisson process with rate λ Retransmissions can also be modeled by a Poisson process with rate G > λ Throughput S=GP 0 P 0 = probability frame does not suffer a collision CS 449/Fall

5 ALOHA: Collision Probability Frame tx time:t Vulnerable interval:2t Prob k frames generated: Pr[k] = Gk e G P 0 =e -2G S=Ge -2G Max throughput is when G=0.5 k! t 0 t 0 +t t 0 +2t t 0 +3t Vulnerable Interval CS 449/Fall 04 9 Slotted ALOHA Time is divided into slots equal to the frame transmission time Vulnerable time is equal to t Throughput S=Ge -G (max =0.37, G=1) Expected number of retransmissions: E = kp k = ke G (1 e G ) k 1 = e G k=1 k=1 CS 449/Fall

6 Comparison CS 449/Fall CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don t interrupt others! CS 449/Fall

7 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability spatial layout of nodes CS 449/Fall CSMA Variants Persistent CSMA: Backlogged nodes will immediately transmit after channel becomes idle Higher probability of collisions at high loads P-Persistent CSMA: If channel is idle transmit with probability p Higher delay at low loads CS 449/Fall

8 Performance CS 449/Fall CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting human analogy: the polite conversationalist CS 449/Fall

9 CSMA/CD collision detection CS 449/Fall Ethernet uses CSMA/CD Adapter doesn t transmit if it senses that some other adapter is transmitting, that is, carrier sense transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection Before attempting a retransmission, adapter waits a random time, that is, random access CS 449/Fall

10 Ethernet CSMA/CD algorithm 1. Adapter gets datagram from and creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,,2 m -1}. Adapter waits K*512 bit times and returns to Step 2 CS 449/Fall Ethernet s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Bit time:.1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K x 512 bit transmission times after second collision: choose K from {0,1,2,3} after ten collisions, choose K from {0,1,2,3,4,,1023} CS 449/Fall

11 CSMA/CD efficiency Assume: k stations, prob transmit=p Pr[node acquires channel] = A = kp(1 p) k 1 p = 1 k,a 1 e,k Pr[contention interval has j slots] = A(1 A) j 1 w = 1 A mean contention interval = 2τ A P P S = P + 2τ P + 5.4τ A 1 S = 1+ 2BLe cf CS 449/Fall Efficiency as a function of τ CS 449/Fall

12 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Bytes: Preamble PreambleSFD SFD DA DA SA SA Type Type Data Data Pad Pad CRC CRC Ethernet Bytes: 2 Preamble: Preamble Preamble SFD SFD DA DA SA Length Length AAAA AAAAcntl cntl Org Org cd cd Type Type SA 802.2LLC SNAP 7 bytes with pattern followed by one byte with pattern used to synchronize receiver, sender clock rates Data Data CRC CRC CS 449/Fall