1. Advanced Computer Networks
CS716
Advanced Computer Networks
By Dr. Amir Qayyum 1

2. Lecture No. 10

3. Ethernet MAC – CSMA/CD Multiple access Carrier sense
Nodes send and receive frames over a shared link
Carrier sense
Nodes can distinguish between an idle and busy link
Collision detection
A node listens as it transmits to detect collision

4. If line is idle (no carrier sensed)
CSMA/CD MAC Algorithm
If line is idle (no carrier sensed)
Send immediately
Upper bound message size of ~1500 bytes
Must wait 9.6µs between back-to-back frames

5. CSMA/CD MAC Algorithm If line is busy (carrier sensed) …
Wait until the line becomes idle and then transmit immediately
Called 1-persistent (special case of p-persistent)
If collision detected
Stop sending data and jam signal
Try again later

6. start transmission at time 0 start transmission at time T
Collision Detection
my-machine
your-machine
start transmission at time 0
start transmission at time T
almost there at time T
collision !!!
How to ensure that my-machine knows about the collision?

7. Constraints on Collision Detection
In our example, consider
my-machine’s message reaches your-machine at T
your-machine’s message reaches my-machine at 2T
Thus, my-machine must still be transmitting at 2T

8. Constraints on Collision Detection
Specifics of IEEE 802.3
bounds 2T to 51.2 microseconds
packet must be at least 64B long
Jam after the collision, for 32 bits, then stop transmitting frame (runt frame of 96 bits)
ensures that all hosts notice collision

9. Ethernet Min. Frame Size
RTT on a maximally configured Ethernet of 2500m, with 4 repeaters is about 51.2 μs
2500m / 2 x 108 m/s = 12.5 us
2 x 12.5 = 25 us + repeater delays
51.2 μs on 10 Mbps corresponds to 512 bits (64 bytes)
Therefore, the minimum frame length for Ethernet is 64 bytes (header +46 bytes data)

10. start transmission at time 0 start transmission at time T
Collision Detection
my-machine
your-machine
start transmission at time 0
start transmission at time T
almost there at time T
collision
my-machine notices collision before transmission ends at 2T

11. Retry After the Collision
How long should a host wait to retry after a collision ?
Binary exponential backoff
maximum backoff doubles with each failure (exponential)
after N failures, pick an N-bit number
2N discrete possibilities from 0 to maximum

12. Retry After the Collision
Delay and try again algorithm
1st time: 0 or 51.2us
2nd time: 0, 51.2, or 102.4us
3rd time51.2, 102.4, or 153.6us
nth time: k x 51.2us, for randomly selected k=0..2n - 1
Give up after several tries (usually 16)

13. Binary Exponential Backoff
choices after 1 collision
choices after 2 collisions Ts
2Ts
3Ts
time
time of last collision
Why used fixed time slots ?
How long slots should be ?

14. Binary Exponential Backoff
Ts is 51.2 microseconds for IEEE 802.3
Consider that k hosts collide
each picks random number from 0 to 2( N-1)
if minimum value is unique
all other hosts see busy line
remember that Ethernet RTT < 51.2 microseconds

15. Binary Exponential Backoff
Consider that k hosts collide
if minimum value is not unique
hosts in minimum value slot collide again
following slot idle
consider next smallest backoff value

16. Ethernet Frame Reception
Sender handles all access control
Receiver simply pulls frames from network
Ethernet controller/card
sees all frames
selectively passes frames to host processor

17. Ethernet Frame Reception
Acceptable frames
addressed to host, or;
to multicast address to which host belongs or;
to a broadcast address
anything if in promiscuous mode (packet sniffing and tcpdump requirement)

18. Ethernet Collision Detection
Bus topology Ethernets
Transceiver handles
Carrier detection
Collision detection
Jamming after a collision
Transceiver sees voltage sum
Outgoing signal + Incoming signal
Looks for voltages impossible for local alone
Attenuation can prevent detection
Limits segment length

19. Ethernet Collision Detection
Hub topology Ethernets
Controller/card handles carrier detection
Hub handles
Collision detection
Jamming after a collision
Detect transmission activity on each line
If more than 1 line (host) active
Assert collision to all lines
Continue until no lines active
Attenuation is less critical

20. Experience With Ethernet
Number of hosts limited to 200 in practice, standard allows 1024
Range much shorter than 2.5 km limit in standard
Round-trip time is typically 5 or 10 μs, not 50μs

21. Experience With Ethernet
Higher-level flow control (i.e., TCP) limits load
30% maximum load recommended
Large packets recommended
Star topologies easier to administer than Bus
Failure isolation, node addition w/o complex topology
(Backwards) Compatibility more important than elegance or raw performance (1 > 0)

22. Token Ring

23. Token Ring Overview
Token Ring network “was” a candidate to replace Ethernet; used in some MAN backbones
16Mbps IEEE (based on earlier 4Mbps IBM ring)
100Mbps Fiber Distributed Data Interface (FDDI)

24. Token Ring Rationale
Why emulate a shared medium with point-to-point links?
Why a shared medium?
convenient broadcast capabilities
switches are costly
Why emulation?
simpler MAC algorithm: only have 2 wires
fairer access arbitration
fully digital (802.3 collision detection requires analog)

25. Data flows in a particular direction
IBM Token Ring – IEEE 802.5
Data flows in a particular direction
a node receives frames from its upstream neighbor
a node forwards frames to its downstream neighbor

26. IBM Token Ring – IEEE 802.5 Ring is viewed as a single shared medium
each node is allowed to transmit according to some distributed algorithm for medium access
all nodes see all frames; destination saves a copy of frame as it flows past
The term “token” indicates the way the access to shared channel is managed

27. Token Ring State Diagram
No token
(pass frames along)
get token
release token
Token
(insert new frames)

28. Token in a Token Ring
Token is a special bit pattern that rotates around the ring
A node must capture token before transmitting
A node releases token after done transmitting
Immediate release- token follows last frame (FDDI)
Delayed release – after last frame returns to sender

29. Token Release oken T T oken Frame Frame (a) immediate release
(b) delayed release

30. Remove your frame when it comes back around
Token in a Token Ring
Remove your frame when it comes back around
Transmit another frame or re-insert the token
Stations get round-robin service as the token circulates around the ring

31. Review Lecture 10 CSMA/CD MAC algorithm
Constraints on collision detection
Min frame size – RTT 51.2us
Retries after collision
Binary exp backoff: Min value unique
Frame reception: acceptable frames
Bus and Hub topology: CD handle
Exp: nodes, dist, RTT, 30% load, largeframe

32. Review Lecture 10
Token Ring
MAN backbone
IBM, token ring, FDDI
Rationale: emulate shared med p2p links
Directional data flow
Shared med: see all frames, distr MAC algo
Token – special bit pattern
Capture and release – round robin service
When a node receives a claim frame, it compares the bid with its own bid
If its bid is higher, it updates TTRT & forward the frame
If its bid is lower, it replaces with its own claim frame
If bids are equal, higher address node wins