CS 313 Introduction to
Computer Networking &
Telecommunication
Medium Access Control Sublayer
Chi-Cheng Lin, Winona State University
Topics

Introduction

Channel Allocation Problem

Multiple Access Protocols

CDMA
2
Introduction

Broadcast networks
Key issue: who gets to use the channel
when there is competition
Referred to as
Multiaccess channels
Random access channels

MAC (Medium Access Control) sublayer
LANs
Wireless networks
Satellite networks
3
Channel Allocation Problem

Channel Allocation
Static
Dynamic

Performance factors
Medium access delay
Time between a frame is ready and the frame
can be transmitted
Throughput
#frames can be transmitted in unit time
interval
4

FDM
Static Channel Allocation
Bandwidth divided into N equal sized
portions for N users
Problems
#senders large
#senders continuously varies
bursty traffic
Discussion: #users > N  ?
<N?
=N?
N times worse than all frames queued in
one big queue
5
Static Channel Allocation

TDM
Each user is statically allocated every Nth
time slot
Same problems as FDM

Under what circumstances are static
channel allocation schemes efficient?
6
Dynamic Channel Allocation

Key assumptions
1. Station model
Independent
Work is generated constantly
One program per station
Station is blocked once a frame has been
generated until the frame has been successfully
transmitted
2. Single channel assumption
7
Dynamic Channel Allocation

Key assumptions
3. Collision Assumption
Collision:
Two frames are transmitted simultaneously,
overlapped in time and resulting signal garbled
Can be detected by all stations
No other errors
8
Dynamic Channel Allocation

Key assumptions
4. Time: either continuous or discrete (slotted)
Continuous
 Frame transmission can begin at any instant
 No "master clock" needed
Slotted
 Time divided into discrete intervals (slots)
 Frame transmissions begin at the start of a slot
 #frames contained in a slot: 0  ?
1?
>1  ?
9
Dynamic Channel Allocation

Carrier sense ("carrier" refers to
electrical signal): either Y or N
Yes
A station can check channel before transmission
If busy, station idle
Wired LANs
No
“Just do it"
Can tell if transmission successful later
Wireless networks, cable modems
10
Multiple Access Protocols
ALOHA
 Carrier sense multiple access protocols
(CSMA)
 CSMA w/ collision detection (CSMA/CD)
 Collision-free protocols
 Limited-contention protocols

11
ALOHA

Applicable to any contention system
System in which uncoordinated users are
competing for the use of a single shared
channel

Two versions
Pure ALOHA
Slotted ALOHA
12
Pure ALOHA
Let users transmit whenever they have data
to be sent
 Colliding frames are destroyed
 Sender can always find out destroyed or not

Feedback (property of broadcasting) or ACK
LANs: immediately
Satellites: propagation delay (e.g., 270msec)
By listening to the channel
If frame is destroyed
wait a random amount of time and retransmit
(why "random"?)
13
Pure ALOHA
Where are
the collisions?
14
Slotted ALOHA
Discrete time
 Agreed slot boundaries
 Synchronization needed
 Performance

Which ALOHA has a shorter medium access
delay?
Which ALOHA has a higher throughput?
15
Performance of ALOHA

Slotted ALOHA can double the
throughput of pure ALOHA
Throughput versus offered traffic for ALOHA systems.
16
Carrier Sense Multiple Access (CSMA)
Protocols
Stations can listen to the channel (i.e.,
sense a carrier in the channel)
 Types

1-persistent CSMA
Nonpersistent CSMA
p-persistent CSMA
17
Performance of MAC Protocols
Comparison of the channel utilization versus load for various
random access protocols.
18
CSMA w/ Collision Detection
(CSMA/CD)

Can listen to the channel and detect
collision
Stop transmitting as soon as collision
detected
Widely used on LANs (e.g., Ethernet)
 Collision detection

Analog process
Special encoding is used
19
CSMA w/ Collision Detection
(CSMA/CD)

Conceptual model
3 states
Contention
Transmission
Idle

Minimum time to detect collision
determines time slot
Depends on propagation delay of medium
20
CSMA/CD Model
21
CSMA/CD Algorithm
Source: http://www.10gea.org/gigabit-ethernet/
22
Collision-Free Protocols

Model
N Stations: 0,1, ..., (N-1)

Question
Which station gets the channel after a
successful transmission?

Protocols
Bit-map (i.e., reservation) protocol
Token passing protocol
Example: Token ring
23
Collision-Free Protocol
Token
Station
Direction of
transmission
Token ring
Performance of Contention and
Collision-Free Protocols

Contention
Low load => low medium access delay :)
High load => low channel efficiency :(

Collision-Free
Low load => high medium access delay :(
High load => high channel efficiency :)
25
Summary of Channel Allocation
Methods/Systems
*
*
*
*
*
*
*
*
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* | Token Passing
| Contention-free protocol
|