MAC Layer
4/16/2008
Guest lecture: Antonis Stampoulis
Outline
 Recap
 Non-partitioning MAC protocols
 Random access
 Taking turns
2
MAC Protocols: a Taxonomy
Goals
 efficient, fair, decentralized, simple
Three broad classes:
 channel partitioning

divide channel into smaller “pieces” (time slot, frequency,
code)
 Non-partitioning
 random access
• allow collisions
 “taking-turns”
• a token coordinates shared access to avoid collisions
3
Channel Partitioning: CDMA
CDMA (Code Division Multiple Access)
 Used mostly in wireless broadcast channels
(cellular, satellite, etc)
 Unique “code” assigned to each user; i.e., code
set partitioning
 All users share same frequency, but each
user m has its own “chipping” sequence (i.e.,
code) cm to encode data

e.g. cm = 1 1 1 -1 1 -1 -1 -1
Example: Sprint
4
CDMA: Encoding and Decoding
 Assume original data are represented by 1 and -1
 Encoded signal = (original data) modulated by
(chipping sequence)


assume
cm = 1 1 1 -1 1 -1 -1 -1
if data is d, send d cm,
• if data d is 1, send cm
• if data d is -1 send -cm
 Decoding: inner-product (summation of bit-by-bit
product) of encoded signal and chipping sequence

if inner-product > 0, the data is 1; else -1
 If codes are orthogonal, multiple users can “coexist”
and transmit simultaneously with minimal
interference
5
CDMA: Two-Sender Interference
Code 1: 1 1 1 -1 1 -1 -1 -1
Code 2: 1 -1 1 1 1 -1 1 1
6
Outline
 Recap
 Non-partitioning MAC protocols
 Random access
 Taking turns
7
Random Access Protocols
 When a node has packets to send
 transmit at full channel data rate R.
 no a priori coordination among nodes
 Two or more transmitting nodes -> “collision”
 Random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed
retransmissions)
 Examples of random access MAC protocols:
 slotted ALOHA and pure ALOHA
 CSMA and CSMA/CD, CSMA/CA
8
Slotted Aloha
[Norm Abramson]
 Time is divided into equal size slots (= pkt trans.
time)
 Node with new arriving pkt: transmit at beginning of
next slot
 If collision: retransmit pkt in future slots with
probability p, until successful.
Success (S), Collision (C), Empty (E) slots
9
Slotted Aloha Efficiency
Q: What is the fraction of successful
slots?
suppose n stations have packets to send
suppose each transmits in a slot with probability p
- prob. of succ. by a specific node:
p (1-p)(n-1)
- prob. of succ. by any one of the N nodes
S(p) = n * Prob (only one transmits)
= n p (1-p)(n-1)
10
Goodput vs. Offered Load
Slotted Aloha
0.5
1.0
1.5
2.0
G = offered load = np
 when p n < 1, as p (or n) increases
 probability of empty slots reduces
 probability of collision is still low, thus goodput increases
 when p n > 1, as p (or n) increases,
 probability of empty slots does not reduce much, but
 probability of collision increases, thus goodput decreases
 goodput is optimal when p n = 1
11
Maximum Efficiency vs. n
0.4
1/e = 0.37
maximum efficiency
0.35
0.3
0.25
0.2
At best: channel
use for useful
transmissions 37%
of time!
0.15
0.1
0.05
0
2
7
12
17
n
12
Pure (unslotted) Aloha
 Unslotted Aloha: simpler, no clock synchronization
 Whenever pkt needs transmission:
 send without awaiting for the beginning of slot
 Collision probability increases:
 pkt sent at t0 collide with other pkts sent in [t0-1, t0+1]
13
Pure Aloha (cont.)
Assume a node transmit with probability p in one unit of time
P(success by a given node) = P(node transmits)
* P(no other node transmits in [t0-1,t0]
* P(no other node transmits in [t0, t0+1]
= p . (1-p)n-1 . (1-p)n-1
= p . (1-p)2(n-1)
P(success by any of N nodes) = n p . (1-p)2(n-1)
- Bound: 1/(2e) = .18
14
Goodput vs. Offered Load
protocol constrains
effective channel
throughput!
0.4
0.3
Slotted Aloha
0.2
0.1
Pure Aloha
0.5
1.0
1.5
2.0
G = offered load = Np
15
Dynamics of (Slotted) Aloha
 In reality, the number of stations backlogged
is changing

we need to study the dynamics when using a fixed
transmission probability p
 Assume we have a total of m stations (the
machines on a LAN):
n of them are currently backlogged, each tries
with a (fixed) probability p
 the remaining m-n stations are not backlogged.
They may start to generate packets with a
probability pa, where pa is much smaller than p

16
Model
n backlogged
each transmits with prob. p
m-n: unbacklogged
each transmits with prob. pa
17
Dynamics of Aloha: Effects of Fixed
Probability
desirable stable point
dep.
and
arrival
rate
of
backlogged
stations
0
successful
transmission rate at
offered load
np + (m-n)pa
- assume a total of
m stations
- pa << p
- success rate is the
departure rate, the
rate the backlog is
reducing
new arrival rate:
(m-n) pa
undesirable
stable point
offered load = 1
Lesson: if we fix p, but n varies, we may have an
undesirable stable point
m
n: number of
backlogged
stations
18
Summary of Aloha Protocols
 Problems


slotted Aloha has better efficiency but clock
synchronization is hard to achieve
low efficiency due to waste of collision or empty slots
• when offered load is optimal (p = 1/N), the goodput is about
37%
• when the offered load is not optimal, the goodput is even
lower

undesirable steady state at a fixed transmission rate,
when the number of backlogged stations varies
 Thus problems to be addressed:
 approximate slotted Aloha without clock synchronization
 reduce the penalty of collision or empty slots
 infer optimal transmission rate
19
CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit
Objective: approximate slotted Aloha without clock
synchronization
 If backlogged, wait until channel sensed idle, then
transmit pkt with prob. p
 human analogy: don’t interrupt others !
20
CSMA Collisions
propagation delay means
two nodes may not
hear each other’s
transmission
A
B
C
D
t0
time
collisions can still
occur:
spatial layout of nodes along Ethernet
Collision:
entire packet transmission
time wasted; still not very
efficient!
21
CSMA/CD (Collision Detection)
 Human analogy: the polite conversationalist
CSMA/CD:

observations:
• collisions can be detected within short time
• if colliding transmissions are aborted, we can reduce
channel wastage
carrier sensing, deferral as in CSMA
 collision detection:

• easy in wired LANs: measure signal strengths, compare
transmitted, received signals
• difficult in wireless LANs: receiver shut off while
transmitting
22
CSMA/CD: Collision Detection
spatial layout of nodes along Ethernet
spatial layout of nodes along Ethernet
C
D
A
t0
t0
time
B
time
A
B
C
B detects
collision,
aborts
D
D detects
collision,
aborts
instead of wasting the whole packet
transmission time, abort after detection.
23
Efficiency of CSMA/CD
 Given collision detection, instead of wasting the
whole packet transmission time (a slot), we waste
only the time needed to detect collision.
P/C
P: packet size, e.g. 1000 bits
C: link capacity, e.g. 10Mbps
 Use a contention slot of 2 T, where T is one-way
propagation delay (why 2 T ?)
 When the transmission probability p is approximately
optimal (p = 1/N), we try approximately e times
before each successful transmission
24
Efficiency of CSMA/CD
 The efficiency (the percentage of useful time) is
approximately
P
C
P  e 2T
C

1
1 5PT

1
15 a
, where a 
TC
P
C
 The value of a plays a fundamental role in the
efficiency of CSMA/CD protocols.
 Question: you want to increase the capacity of a link
layer technology (e.g., , 10 Mbps Ethernet to 100
Mbps, but still want to maintain the same efficiency,
what do you do?
25
Outline
 Recap
 Non-partitioning MAC protocols
 Random access
 Taking turns
26
“Taking Turns” MAC Protocols: Motivation
 Channel partitioning MAC protocols
share channel efficiently and fairly at high load
 inefficient at low load: delay in channel access,
1/N bandwidth allocated even if only 1 active
node !
 can bound worst case performance

 Random access MAC protocols
efficient at low load: single node can fully utilize
channel
 high load: collision overhead
 cannot bound worst case performance

27
“Taking Turns” MAC Protocols: Polling
 Basic idea: master node
“invites” slave nodes to
transmit in turn
 Example: Bluetooth uses
a polling method, where
M is Master; s1, s2 are
two slaves
Concerns:

polling overhead and latency

single point of failure (master)
28
Polling Example: Distributed Polling
 Time divided into slots
 Begins with N short reservation slots
reservation slot time equal to channel end-end propagation
delay
 station with message to send posts reservation
 reservation seen by all stations
 After reservation slots, message transmissions ordered by

known priority
29
Token Passing
 Basic idea: control token passed
from one node to next
sequentially
 Example: Token Ring
 a token rotates around a ring to
each node in turn
 all nodes (computers, routers,
etc.) copy all data and tokens,
and repeat them along the link
of the stations
 when a node wishes to transmit
packet(s), it grabs the token as
it passes
 it holds the token while it
transmits
 when it is done, it releases the
token and sends it on its way
Concerns:

token overhead and latency

single point of failure (token)
30
Token Passing: Illustration
Listen:
Talk:
data
l4
l1
l3
l2
token/data
31
Token Ring: Two Variants
 Release After Reception
(RAR)
 Example: IEEE 802.5
Token Rings (4Mbps)
station 1 station 2 station N station 1
Packet 1
token
 Release After Transmissions
(RAT)

Example: Fiber Distributed
Data Interface (FDDI)
(100Mbps)
station 1 station 2 station N station 1
Packet 1
Packet 2
Packet 2
32
Efficiency of Token Ring: RAR
 Release After Reception (RAR)
 maximum efficiency RAR achieved when a station
continuously transmits
1
2
N
1
a
Packet 1
1
 RAR 
Packet 2

1
1 a
where
a:
E - 2 - E propagatio n delay
Packet Transmissi on time
if the sender always releases its token after a data
transmission
1
2
n
1
Packet 1
a/N
Packet 2
a
1
 RAR 
1
1  NN1 a
1

1 a
Assume token very small, stations
equally spaced
where
a:
E - 2 - E propagatio n delay
Packet Transmissi on time
N: number of hosts
33
Efficiency of Token Ring: RAT
 Release After Transmission (RAT)
 maximum efficiency achieved when a station continuously
transmits
1
2
n
Packet 1
1
a
1
Packet 2

the efficiency is approaching 1
assume a station always releases its token after a packet
transmission
Assume token very small, stations
1
Time 0
2
3 n
1
a/N
a
Packet 1
Time 1+a/N
Time 2+2a/N
Packet 2
Packet 3
equally spaced
 RAT
1

1 a / N
where
a:
E - 2 - E propagatio n delay
Packet Transmissi on time
N: number of hosts
34
Comparison of Efficiency
1
 CSMA / CD 
1  5a
1
 RAR 
1 a
1
 RAT 
1 a / N
where
a:
E - 2 - E propagatio n delay
Packet Transmissi on time
N: number of hosts
35
Summary of MAC Protocols
 How do you access a shared media?
Channel Partitioning, by time, frequency or code
 Random access,

• ALOHA, S-ALOHA, CSMA, CSMA/CD
 “Taking-turns”
• polling
• token passing
 In real protocols, they are often combined to
achieve given objective
36