Medium Access Sub-Layer
MAC
The MAC sub-layer is the lower portion (protocol stackwise) of the Data-Link Layer, just above the Physical
Layer
Data Link Layer
Medium Access Sub-Layer
Physical Layer
Used specifically for Broadcast (Multi-access/Randomaccess) channels used in LANs
LANs Predominantly use multi-access channels
B227 Data Communications Lecture 5-1
Peter Cole 2001
Channel Allocation
Telephone lines use FDM where the available band
width is chopped up into discrete sub-bands - channels,
one for each user.
Problem: if the line chopped up into 100 channels and only five
users are busy then there is a 95% wastage of bandwidth
Static channel allocation methods do not work well with bursty
traffic therefore multi-user single channel systems become
attractive
B227 Data Communications Lecture 5-2
Peter Cole 2001
Dynamic Channel
Allocation
Five assumptions are made in this area
1. Station Model : each station (host) is independent of each
other, ie. No coordination exists between hosts
2. Single Channel Assumption : a single channel is used for
both transmitting and receiving (unlike Distributed Queue
Dual Bus standard for MANs which has a bus for
transmitting and a bus for receiving, or a token ring where
transmission is strictly controlled by a rota)
3. Collision Assumption : if any part of two signals occupy the
wire at the same time a collision has occurred. Only possible
errors are caused by collisions and all stations are capable of
detecting collisions
4. Time Assumptions :
a) Continuous Time : No master clock and transmission
can begin at any time
b) Slotted Time : time divided into discrete portions slots. Frame transmission begins at the beginning of a
slot. A slot can carry three different loads
0. frames = idle
1. frame = successful transmission
2. frame = collision
B227 Data Communications Lecture 5-3
Peter Cole 2001
5. Carrier Sensing : if a host has the ability to sense if the
line is busy or free so transmission can occur. Two types a) Can sense carrier :
b) Can’t sense carrier :
Assumptions 4a & 4b are mutually exclusive as are conditions
5a & 5b
Any system that has host who run the risk of its transmissions
colliding with a transmission from another host in the same
system is know as a contention system
New problem:
with a common channel between all hosts on a broadcast LAN,
The important issue is - who gets to use the channel?
We require Multiple Access protocols to assist in solving this
problem
B227 Data Communications Lecture 5-4
Peter Cole 2001
Aloha
Originally for ground-based radio communication
systems (early 1970’s)
uses fixed-size frames
It operates on assumptions 1,2,3,4a & 5b
ie. It meets all the common channel assumptions but
cannot sense the carrier and transmission begins at any
time
B227 Data Communications Lecture 5-5
Peter Cole 2001
Each frame is said to be vulnerable until it has been received
by all hosts in the system
If propagation is slow ie. Satellite communication, then the
chances of a collision occurring are exponentially greater.
The frame is said to be vulnerable during this
period
B227 Data Communications Lecture 5-6
Peter Cole 2001
Slotted Aloha
The problem with the Pure Aloha scheme is that at best a
channel utilisation rate of 18% can be obtained. (see
mathematical description in Tanenbaum pg 247-9)
Slotted Aloha on the other hand doubled the utilisation rate by
changing to a timed (synchronous) system (assumption 4b)
Here one station acts to synchronise the system in some fashion
 Hosts cannot transmit on the press of carriage return but must
wait for the next time slot
 Collisions still occur and in this situation both colliding
parties back off for a random time period and retransmit at
the beginning of a new time slot
 37% utilisation of channel, successes, 26% collisions,
balance idle
B227 Data Communications Lecture 5-7
Peter Cole 2001
CSMA Protocols
Carrier Sense Multiple Access
Here we work on the principle that a vacant channel can
be detected
Different from collision detection
basically there is a dichotomy in the methods used under
the CSMA banner
 Persistent
 if channel is busy the sender continually checks to
see when the channel is idle
 as soon as the channel is idle it transmits
probability of 1 therefore known as 1-Persistent
CSMA
 On long propagation delays collisions become more
frequent
 Non-Persistent
 if channel is busy the sender waits a random period
of time before sensing the carrier again
 as soon as it is free the sender transmits a frame
B227 Data Communications Lecture 5-8
Peter Cole 2001
P-Persistent CSMA
 Slotted channels only
 Here the sender waits for more than 1 slot to be idle
before sending
 therefore lower probability of a free channel is
lowered
 if channel is busy the sender waits a random period of
time before sensing the carrier again else transmits
B227 Data Communications Lecture 5-9
Peter Cole 2001
CSMA/CD
…./With Collision Detection
CSMA networks with Collision Detect (CSMA/CD) reduce the
cost of collisions by aborting transmission whenever a collision
is detected
If a collision occurs the detecting host may send out a jamming
frame to indicate to all hosts that a collision has been detected
B227 Data Communications Lecture 5-10
Peter Cole 2001
With ALOHA, a collision wastes an entire frame transmission
time
Reducing the time to detect and recover from collisions will
improve overall efficiency
CSMA/CD networks spans a large geographic distance, ie. the
longer the physical distance, the longer the contention period
Another approach to reducing collisions is to prevent them
from happening in the first place
B227 Data Communications Lecture 5-11
Peter Cole 2001
Basic Bit-Map Method
1. Assume N stations numbered 1-N, and a contention period
of N slots (bits).
2. Each station has one slot time during the contention period
(numbered 1-N).
3. Each station J sends a 1-bit during its slot time if it wants to
transmit a frame.
4. Every station sees all the 1-bits transmitted during the
contention period, so each station knows which stations want
to transmit.
5. After the contention period, each station that asserted its
desire to transmit sends its frame.
Disadvantages:
Higher numbered stations get better service than those with
lower numbers. A higher numbered station has less time to
wait (on average) before its next bit time in the current or next
contention period
At light loads, a station must wait N bit times before it can
begin transmission.
B227 Data Communications Lecture 5-12
Peter Cole 2001
IEEE 802 protocols
The IEEE has produced a set of LAN protocols known as the
IEEE 802 protocols. These protocols have also been adopted
by ANSI and ISO:
* 802.2: Logical link standard (device driver interface).
* 802.3: CSMA/CD.
* 802.4: Token bus.
* 802.5: Token ring.
B227 Data Communications Lecture 5-13
Peter Cole 2001
802.3 - ETHERNET
The 802.3 protocol is described as follows.
1. It is a 1-persistent CSMA/CD LAN. A station begins
transmitting immediately when the channel is idle.
2. It originally used coaxial cable. (there are other varieties)
B227 Data Communications Lecture 5-14
Peter Cole 2001
3. Its development history is:
(a) Started with ALOHA.
(b) Continued at Xerox, where Metcalf & Boggs produced a
3 Mbps LAN version.
(c) Xerox, DEC, and Intel standardised a 10Mbps version,
started selling in early 1980s.
(d) IEEE standardised a 10Mbps version (with slight
differences from the Xerox standard).
4. The maximum allowable cable segment is 500 meters on the
original baseband (Thicknet) system, up to 2000 meters for
optic fibre.
5. Segments can be separated by repeaters. Repeaters are
devices that regenerate or "amplify" bit signals (not frames);
a single repeater can join multiple segments.
6. Maximum distance between two stations 2.5 km, maximum
of four repeaters along any path (Thicknet). Why? To insure
that collisions are detected properly. Longer lengths increase
the contention interval.
B227 Data Communications Lecture 5-15
Peter Cole 2001
Binary exponential backoff Algorithm
802.3 (Ethernet) LANs use a binary exponential back-off
algorithm to reduce collisions:
1. It is 1-persistent. When a station has a frame to send, and
the channel is idle, transmission proceeds immediately.
2. When a collision occurs, the sender generates a noise burst,
this is the Jamming Frame, to insure that all stations
recognise the condition and abort transmissions.
3. After the collision, each station waits 0 or 1 contention
periods before attempting transmission again. Station has 5050 probability of waiting 0 or 1 contention periods.
 The idea is that if two stations collide, one will transmit in
the first interval, while the other one will wait until the
second interval. Once the second interval begins,
however, the station will sense that the channel is already
busy and will not transmit.
4. If another collision occurs: Randomly wait 0, 1, 2, or 3 slot
times before attempting transmission again.
5. In general, wait between 0 and (2r - 1) times, where r is the
number of collisions encountered.
B227 Data Communications Lecture 5-16
Peter Cole 2001
6. Finally, freeze interval at 1023 slot times after 10 attempts,
and give up altogether after 16 attempts.
Result?
Low delay at light loads, yet we avoid collisions under heavy
loads.
Also, note that there are no acknowledgments; a sender has no
way of knowing that a frame was successfully delivered. It is
up to the receiver to check the incoming frames checksum and
if there is an error in the frame request re-transmission from the
sender
B227 Data Communications Lecture 5-17
Peter Cole 2001
Fact or Myth:
Ethernet (802.3) performs poorly under heavy loads. That is,
under heavy loads, do excessive collisions negatively impact
performance?
The following performance numbers were taken from an
experimental configuration of an actual Ethernet.
25 hosts were transmitting packets as fast as they could (eg.,
the offered load >> 1).
Major results:
 The average transmission delay increases linearly with
increasing numbers of transmitters.
 This contradicts the myth that delay increases
substantially as offered load increases.
 25 hosts sending maximum sized packets obtain over 97%
of the bandwidth. That is, less that 3% of the bandwidth is
lost due to collisions.
 25 hosts sending minimum sized packets (eg., 64 bytes)
obtain about 85% of available bandwidth. While not
great, losing 15% of the bandwidth to collisions does not
make the network "unusable".
B227 Data Communications Lecture 5-18
Peter Cole 2001
 For bi-modal distributions (eg., mixtures of both small and
large packets) utilisation increases. One large packet for
every 8 small packets results in a utilisation of about
92.5%.
Conclusion:
Ethernets work well in practice.
How then does one explain reports of poor performance in
actual Ethernet LANs? Poor hardware design.
The first generation of Ethernet hardware was designed at a
time machines were unable to send or receive packets at fast
rates.
Today, a single workstation generating packets can completely
saturate an Ethernet. However, many Ethernet cards cannot
themselves process packets at such high rates. They have
limited internal buffering, and frequently cannot handle more
than 2 or 3 back-to-back packets without filling an internal
buffer and discarding the remaining packets.
Thus, the problems had not to do with the Ethernet per say, but
with the implementation of hardware implementation.
B227 Data Communications Lecture 5-19
Peter Cole 2001