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backlogged nodes might discover an “idle” medium at the same time; if they both send
data, a collision ensues. For both these reasons, CSMA does not achieve 100% utilization,
and needs a backoff scheme, though it usually achives higher utilization than stabilized
slotted Aloha over a single shared medium. You will investigate this protocol in the lab. 10.8 A Note on Implementation: Contention Windows In the protocols described so far, each backlogged node sends a packet with probability p,
and the job of the protocol is to adapt p in the best possible way. With CSMA, the idea is to
send with this probability but only when the medium is idle. In practice, many contention
protocols such as the IEEE 802.3 (Ethernet) and 802.11 (WiFi) standards do something a
little different: rather than each node transmitting with a probability in each time slot,
they use the concept of a contention window.
A contention window scheme works as follows. Each node maintains its own current
value of the window, which we call CW. CW can vary between CWmin and CWmax;
CWmin may be 1 and CWmax may be a number like 1024. When a node decides to transmit, it does so by picking a random number r uniformly in [1, CW] and sends in time slot
C + r, where C is the current time slot. If a collision occurs, the node doubles CW; on
a successful transmission, a node halves CW (or, as is often the case in practice, directly
resets it to CWmin). SECTION 10.9. SUMMARY 15 You should note that this scheme is similar to the one we studied and analyzed above.
The doubling of CW is analogous to halving the transmission probability, and the halving
of CW is analogous to doubling the probability (CW has a lower bound; the transmission
probability has an upper bound). But there are two crucial differences:
1. Transmissions with a contention window are done according to a uniform probability distribution and not a geometrically distributed one. In the previous case, the a
priori probability that the ﬁrst transmission occurs t slots from now is geometrically
distributed; it is p(1 − p)t−1 , while with a contention window, it is equal to 1/CW
for t ∈ [1, CW] and 0 otherwise. This means that each node is guaranteed to attempt
a transmission within CW slots, while that is not the case in the previous scheme,
where there is always a chance, though exponentially decreasing, that a node may
not transmit within any ﬁxed number of slots.
2. The second difference is more minor: each node can avoid generating a random
number in each slot; instead, it can generate a random number once per packet transmission attempt.
In the lab, you will implement the key parts of the contention window protocol and
experiment with it in conjunction with CSMA. There is one important subtlety to keep in
mind while doing this implementation. The issue has to do with how to count the slots
before a node decides to transmit. Suppose a node decides that it will transmit x slots from
now as long as the medium is idle after x slots; if x includes the busy slots when another
node transmits, then multiple nodes may end up trying to transmit in the same time slot
after the ending of a long packet transmission from another node, leading to excessive
collisions. So it is important to only count down the idle slots; i.e., x should be the number
of idle slots before the node attempts to transmit its packet (and of course, a node should
try to send a packet in a slot only if it believes the medium to be idle in that slot). 10.9 Summary This lecture discussed the issues involved in sharing a communication medium amongst
multiple nodes. We focused on contention protocols, developing ways to make them provide reasonable utilization and fairness. This is what we learned:
1. Good MAC protocols optimize utilization (throughput) and fairness, but must be
able to solve the problem in a distributed way. In most cases, the overhead of a
central controller node knowing which nodes have packets to send is too high. These
protocols must also provide good utilization and fairness under dynamic load.
2. TDMA provides high throughput when all (or most of) the nodes are backlogged
and the offered loads is evenly distributed amongst the nodes. When per-node loads
are bursty or when different nodes send different amounts of data, TDMA is a poor
3. Slotted Aloha has surprisingly high utilization for such a simple protocol, if one can
pick the transmission probability correctly. The probability that maximizes throughput is 1/N , where N is the number of backlogged nodes, the resulting utilization LECTURE 10. SHARING A COMMON MEDIUM: 16 MEDIA ACCESS PROTOCOLS tends toward 1/e ≈ 37%, and the fairness is close to 1 if all nodes present the same
load. The utilization does remains high even when the nodes present different loads,
in contrast to TDMA.
It is also worth calculating (and noting) how many slots are left idle and how many
slots have more than one node transmitting at the same time...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.
- Fall '13
- The Land