1. Chap 4 Multiaccess Communication (Part 1)
Ling-Jyh Chen

2. Overview Ethernet and Wi-Fi are both “multi-access” technologies
Broadcast medium, shared by many hosts
Simultaneous transmissions will result in collisions
Media Access Control (MAC) protocol required
Rules on how to share medium

3. Media Access Control Protocols
Channel partitioning
Divide channel into smaller “pieces” (e.g., time slots, frequency)
Allocate a piece to node for exclusive use
E.g. Time-Division-Multi-Access (TDMA) cellular network
Taking-turns
Tightly coordinate shared access to avoid collisions
E.g. Token ring network
Contention
Allow collisions
“recover” from collisions
E.g. Ethernet, Wi-Fi

4. Contention Media Access Control Goals
Share medium
If two users send at the same time, collision results in no packet being received (interference)
If no users send, channel goes idle
Thus, want to have only one user send at a time
Want high network utilization
TDMA doesn’t give high utilization
Want simple distributed algorithm
no fancy token-passing schemes that avoid collisions

5. Evolution of Contention Protocols
Aloha
Developed in the 1970s for a packet radio network
Slotted Aloha
Improvement: Start transmission only at fixed times (slots)
CSMA = Carrier Sense Multiple Access
Improvement: Start transmission only if no transmission is ongoing
CSMA
CD = Collision Detection
Improvement: Stop ongoing transmission if a collision is detected (e.g. Ethernet)
CSMA/CD

6. 4.2 Idealized slotted multiaccess model
m transmitting nodes and one receiver
Slotted system
packets are of the same length
each packet requires one time unit for transmission
the reception of each packet starts at an integer time and ends before the next integer time

7. Collision or Perfect Reception
Poisson Arrivals overall arrival rate of the system: λ individual rate of each node: λ/m
Collision or Perfect Reception
If just one node sends a packet in a given slot, the packet is correctly received.
If two or more nodes send a packet in a given time slot, then there is a collision and the receiver obtains no information about the contents or the source of the transmitted packets.

8. Retransmission of Collisions
0,1,e Immediate Feedback
Assuming each node obtains feedback from the receiver at the end of each slot
Retransmission of Collisions
Assuming each packet involved in a collision must be retransmitted in some later slot.
A node with a packet that must be retransmitted is said to be backlogged.

9. Two addition assumptions
No buffering
If one packet at a node is currently waiting for transmission or colliding with another packet during transmission, new arrivals at that node are discarded and never transmitted.
This assumption provides the lower bound to the delay for systems with buffering and flow control!
Infinite set of nodes (m=∞):
This assumption provides the upper bound!

10. Slotted ALOHA The basic idea:
Each unbacklogged node simply transmit a newly arriving packet in the first slot after packet arrival.
Slotted ALOHA risks occasional collisions but achieves very small delay if collisions are rare.
Contrast to TDM systems, which avoids collisions at the expense of large delays.

11. Collisions in S-ALOHA

12. Slotted ALOHA (cont.)
When a collision occurs, each node sending one of the colliding packets discovers the collision at the end of the slot and becomes backlogged.
Such nodes wait for some random number of slots before retransmitting.

13. Slotted ALOHA (cont.)
Using infinite-node assumption, the total number of retx and tx in a given slot is a Poisson random variable with parameter G, where G> λ.
The prob. of a successful transmission in a slot is
In equilibrium, the arrival rate, λ, should be the same as the departure rate, Ge-G.

14. Slotted ALOHA (cont.)
Using GNUPlot set xr [0:5] plot x*exp(-x)

15. Slotted ALOHA (cont.)
The MAX departure rate occurs at G=1 and is 1/e ≈
If G<1, too many idle slots are generated.
If G>1, too many collisions are generated.

16. Slotted ALOHA (cont.) Markov Chain for Slotted ALOHA
State: the number of backlogged packets
Increases by the number of new arrivals transmitted by unbacklogged nodes
Decreases by one each time if a packet is transmitted successfully.

17. Slotted ALOHA (cont.)
qr: the prob. of a backlogged node retx in the next slot
i.e., the number of slots from a collision until a given node involved in the collision retx is a geometric R.V. having value i>1 with prob. qr(1-qr)i-1
qa: the prob. of an unbacklogged node transmits a packet in the given slot
i.e. qa=1-e-λ/m

18. Slotted ALOHA (cont.)
Qa(i, n): the prob. that i unbacklogged nodes transmit packets in a given slot
Qr(i, n): the prob. that i backlogged nodes transmit.

19. Slotted ALOHA (cont.)

20. Slotted ALOHA (cont.)
Dn: “drift” in state n, i.e. the expected change in backlog over one slot time
G(n): the expected number of attempted transmissions in a slot
If qa and qr are small,

21. Slotted ALOHA (cont.)
The “drift” is the difference between the throughput curve (Ge-G) and the straight line:

22. Slotted ALOHA (cont.) Using infinite-node assumption:
Using no-buffering assumption:
4.2.3 (optional)

23. Unslotted ALOHA
Unslotted ALOHA (a.k.a. Pure ALOHA) was the precursor to slotted ALOHA.
In Pure ALOHA, each node transmits a new packet immediately upon receiving, rather than waiting for a slot boundary.
If a packet is involved in a collision, it is retransmitted after a random delay.

24. Collisions in (Pure) ALOHA

25. Unslotted ALOHA (cont.)
A frame (red frame) will be in a collision if and only if another transmission begins in the vulnerable period of the frame
Vulnerable period has the length of 2 frame times

26. Unslotted ALOHA (cont.)
Since arrivals are independent, Psucc=e-2G
Since attempted transmissions occur at rate G(n), the throughput = Ge-2G
The MAX throughput of a Pure ALOHA system = 1/(2e), achieved when G=0.5.
If λ is very small and the mean retx time is very large, the system can be expected to run for long periods w/o major backlog buildup.
The main adv. of pure ALOHA is that it can be used with variable-length packets.

27. Comparison of ALOHA and S-ALOHA