1. CSC4220/6220 Computer Networks
Dr. WenZhan Song
Associate Professor, Computer Science

2. Course roadmap Introduction
Application Layer: WWW, FTP, , DNS, multimedia
Transport Layer: reliable end-end data transfer principles, UDP, TCP
Network Layer: IP addressing, routing and other issues
Data Link Layer: framing, error control, flow control
Medium Access Control (MAC) Layer: multiple-access, channel allocation
Physical Layer: wired, wireless, satellite
Other Topics: network security, social issues, hot topics, research directions

3. Data Link Layer Road Map
Data link layer design issues
Framing
Error Control
Reliable data transfer and flow control
Example data link protocols
HDLC and PPP
Multiple Access Protocols
Static channel allocation
Dynamic channel allocation
LAN technologies and their MAC protocols
Ethernet
WiFi and WiMax

4. Multiple Access Links and Protocols
Two types of “links”:
point-to-point
PPP for dial-up access
point-to-point link between switch and host
broadcast (shared wire or medium)
traditional Ethernet
upstream HFC
wireless LAN

5. Multiple Access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes: interference
collision if node receives two or more signals at the same time
multiple access protocol
distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself!
no out-of-band channel for coordination

6. Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at rate R.
2. When M nodes want to transmit, each can send at average rate R/M
3. Fully decentralized:
no special node to coordinate transmissions
no synchronization of clocks, slots
4. Simple

7. MAC Protocols: a taxonomy
Static Channel Allocation
divide channel into smaller “pieces” (time slots, frequency, code)
allocate piece to node for exclusive use
Dynamic Channel Allocation
channel not divided, allow collisions
“recover” from collisions

8. Static Channel Allocation: TDMA
TDMA: time division multiple access
access to channel in "rounds"
each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

9. Static Channel Allocation: FDMA
FDMA: frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
time
frequency bands

10. Dynamic Channel Allocation
ALOHA, slotted ALOHA
CSMA, CSMA/CD, CSMA/CA – in future slides
Collision free protocols
Bit-map protocol
Binary countdown
Limited contention protocols
Adaptive tree walk
Wavelength division multiple access protocol
Not currently used in major system yet

11. Pure (unslotted) ALOHA
unslotted Aloha: simpler, no synchronization
when frame first arrives
transmit immediately, if collision, then retransmit with p, or waits for another frame time with 1-p
collision probability increases:
frame sent at t0 collides with other frames sent in [t0-1,t0+1]

12. Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0,1+t0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting N -> infty ...
= 1/(2e) = .18
Even worse !

13. Slotted ALOHA Assumptions all frames same size
time is divided into equal size slots, time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot, all nodes detect collision
Operation
when node obtains fresh frame, it transmits in next slot
no collision, node can send new frame in next slot
if collision, node retransmits frame in each subsequent slot with prob. p until success

14. Slotted ALOHA
C – collision E – empty S - successful
single active node can continuously transmit at full rate of channel
highly decentralized: only slots in nodes need to be in sync
simple
collisions, wasting slots
idle slots (even with data)
nodes may be able to detect collision in less than time to transmit packet
clock synchronization

15. Slotted Aloha efficiency
Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send
For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1
For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37
Suppose N nodes with many frames to send, each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that an arbitrary node has a success = Np(1-p)N-1
At best: channel
used for useful
transmissions 37%
of time!

16. Throughput versus offered traffic for ALOHA systems.

17. CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: may transmit entire frame
If channel sensed busy, defer transmission
Human analogy: don’t interrupt others!

18. Nonpersistent CSMA
Station wishing to transmit listens and obeys following:
If medium is idle, transmit; otherwise, go to 2
If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1
Consequences:
Random delays reduces probability of collisions
Consider two stations become ready to transmit at same time
While another transmission is in progress
If both stations delay same time before retrying, both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission
Even if one or more stations waiting

19. 1-persistent CSMA
Station wishing to transmit listens and obeys following: 
If medium idle, transmit; otherwise, go to step 2
If medium busy, listen until idle; then transmit immediately
Consequences:
avoid idle channel time
1-persistent stations selfish
If two or more stations waiting, collision guaranteed
Gets sorted out after collision

20. P-persistent CSMA
Station wishing to transmit listens and obeys following: 
If medium idle, transmit with probability p, and delay one time unit with probability (1 – p)
If medium busy, listen until idle and repeat step 1
If transmission is delayed one time unit, repeat step 1
Consequences:
Compromise that attempts to reduce collisions
Like nonpersistent
And reduce idle time
Like 1-persistent
What is an effective value of p? depend on load

21. Persistent and Nonpersistent CSMA

22. Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access protocols.

23. Collision-Free Protocols
The basic bit-map protocol.
Contention period, consists N slots
Station j want to transmit frame, insert a 1 bit into slot j
Each station has complete knowledge after listen contention period
Transmit frame in numerical order
Never has collisions
Overhead is 1 bit per station

24. Collision-Free Protocols (2)
The binary countdown protocol
Station want to transmit, broadcasts its address as a binary string, starting with the highest bit
Bits in each address position from different stations are Boolean ORed together
The result address is the winner of the bidding, it may now transmit a frame
Higher address has a higher priority (good or bad)

25. Limited-contention protocols
Adaptive tree walk protocol

26. Wavelength Division Multiple Access Protocols

27. Wireless LAN Protocols
(a) Hidden station problem (b) Exposed station problem

28. MACA protocol (Multiple Access with Collision Avoidance )
(a) A sending a RTS (Request to Send) to B. (b) B responding with a CTS (Clear to Send) to A.

29. Wireless LAN Protocols
Multiple Access with Collision Avoidance (MACA)
The sender stimulate the receiver into outputting a short frame, so stations nearby can detect this transmission and avoid transmitting for the duration of the upcoming data frame
Multiple Access with Collision Avoidance for Wireless (MACAW)
Introduce ACK to enable retransmit
Add carrier sense to avoid conflict with RTS
Run backoff algorithm for each pair, instead of each station
Exchange congestion info between neighbors

30. Summary of MAC protocols
What do you do with a shared media?
Channel Partitioning, by time, frequency or code
Time Division, Frequency Division
Random partitioning (dynamic),
ALOHA, S-ALOHA
CSMA
carrier sensing: easy in some technologies (wire), hard in others (wireless)
CSMA/CD used in Ethernet
Collision-free and limited contention protocols
WDMA
MACA, MACAW – wireless
CSMA/CA used in

31. Reading Assignment
Chapter 5.3 and 5.5
Chapter 6.3