CS571: Computer Networks. Medium Access Control Protocols / Ethernet

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1 CS571: Computer Networks Medium ccess Control Protocols / Ethernet

2 Medium ccess Control Protocols The problem llocate a single broadcast channel among competing users. Fairness Performance Fault-tolerance Scheduling methods Take turns Random access MC protocols node transmits whenever it has data to be sent. No a priori coordination among nodes. Two or more nodes transmitting -- collision

3 Time-Space Diagram Signals collide iff they reach some receiver at the same time (Station C in this example) Waste bandwidth

4 Random ccess MC Protocols Non-Carrier-Sense protocols: doesn t listen to the channel before transmitting LOH Carrier-Sense protocols: senses the channel before transmitting CSM (Carrier Sense Multiple ccess): does not detect collision. CSM/CD (Ethernet): node listens while transmitting to determine whether a collision happens.

5 LOH -- asics Radio-based communication network Developed in 1970s at the Univ of Hawaii asic idea: transmit when a node has data to be sent. Receiver sends CK for data Detect collisions by timing out for CK Recover from collision by trying after random delay Too short large number of collisions Too long underutilization

6 Random ackoff Motivation collides Rexmits, collides again Rexmits, collides again detects & rexmits detects & rexmits

7 Random ackoff collides acks off 2 frame times, rexmit detects acks off 1 frame time, rexmit common backoff algorithm: randomly choose an interger in the interval [0,k], and wait frame times. =0may be chosen.

8 loha -- Performance Collides with start of i s frame Collides with end of i s frame Node i t 0 -T t 0 t 0 +T Vulnerable period: 2T T: frame time Collision probability increases when network load increases Channel capacity is wasted due to collisions verage delay increases rapidly.

9 CSM/CD Ethernet Listen to wire before transmission Vulnerable period: T (1 frame time) bort transmission as soon as collision is detected Does not waste bandwidth on corrupted frames Why didn t loha have this? Very difficult for radios to listen and transmit

10 Ethernet MC If line is idle (no carrier sensed) send packet immediately If line is busy (carrier sensed) wait until idle and transmit packet immediately If collision detected Stop sending and jam signal Jam signal: make sure all other transmitters are aware of collision Wait a random time (Exponential backoff), and try again

11 Collision Detection daptors sense collision based on voltage differences starts at time 0 Propogation delay: T d How does know about collision? Message almost there at time T d when starts collision!

12 Collision Detection time = 0 time = T d time = 2T d detects collision at 2T d in worst case

13 Collision Detection To give a node enough time to detect collision Minimum frame size: 2T d x transmit rate In 10M eternet: 512 bits (64ytes) If a node has less bytes to send, the adaptor pads (adds) bytes.

14 Exponential ackoff If a collision is detected, delay and try again Delay time is selected using binary exponential backoff More senders increase wait time First collision: choose K from {0,1}; delay is K x frame time Second collision: choose K from {0,1,2,3} nth time: delay = K x frame time, for K=0..2 n 1 Note max value for k = 1023 Give up after several tries (usually 16) Report transmit error to host

15 t Receiver Side Senders handle all access control Receivers simply read frames with acceptable address ddress to host ddress to broadcast ddress to multicast to which host belongs ll frames if host is in promiscuous mode

16 Ethernet Performance Ethernets work best under light loads Utilization over 30% is considered heavy Peak throughput worse with More hosts More collisions needed to identify single sender Smaller packet sizes More frequent arbitration Longer links Collisions take longer to observe, more wasted bandwidth Efficiency is improved by avoiding these conditions

17 Ethernet Capture Effect If one node starts sending an excessive number of frames, it may dominate ( capture ) the channel. Rexmit[0,1] Rexmit[0,1] Collide Rexmit[0,1] Rexmit[0,1,2,3] Rexmit[0,1] Rexmit[0,1,2,3, 4,5,6,7]

18 Ethernet frame structure IEEE802.3 ytes Preamble Dest. ddr Source ddr Len./ Type Data Pad CRC Start of frame delimiter Preamble: a sequence of 7 bytes To synchronize receiver Start of frame delimiter: ddress Unique, 48-bit unicast address assigned to each adapter Each manufacturer gets their own address range CRC: generated based on dest addr, source addr, length and data fields.

19 Ethernet Technologies 10ase5: also called thick-net. 10 Mbps cable is used in a baseband system up to 500m Coax cable 10ase2: up to 200m, thin-net

20 Ethernet Technologies 10aseT: T -> twisted pair Hub is used to connect multiple nodes. Hub Hub

Direct Link Networks. Introduction. Physical Properties. Lecture - Ethernet 1. Areas for Discussion. Ethernet (Section 2.6)

Direct Link Networks. Introduction. Physical Properties. Lecture - Ethernet 1. Areas for Discussion. Ethernet (Section 2.6) reas for Discussion Direct Link Networks Joseph Spring School of Computer Science Sc - Computer Network Protocols & rch s ased on Chapter 2, Peterson & Davie, Computer Networks: Systems pproach, 5 th Ed