CEN 4500 Data Communications

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CEN 4500 Data Communications

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CEN 4500 Data Communications Chapter 4: The Medium Access Control Sublayer Instructor: S. Masoud Sadjadi http://www.cs.fiu.edu/~sadjadi/Teaching/ – PowerPoint PPT presentation

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Title: CEN 4500 Data Communications

1
CEN 4500 Data Communications
Chapter 4 The Medium Access Control Sublayer
Instructor S. Masoud Sadjadi http//www.cs.fiu.ed
u/sadjadi/Teaching/ sadjadi At cs Dot fiu Dot
edu
2
Recap

Networks are divided into two categories
Point-2-point connections (WANs)
Broadcast channels (LANs)
a.k.a Multicast Channels
a.k.a Random Access Channels
Key issue in broadcast channels
Determining who gets to use the channel, when
there is a competition
Medium Access Control (MAC) sublayer
Has the protocol that addresses this issue
Technically is the bottom part of the data link
layer
Usually used in LAN and in satellite networks

3
Agenda

The Channel Allocation Problem
Multiple Access Protocols
Ethernet
Data Link Layer Switching
Summary

4
The Channel Allocation Problem

How to allocate a single broadcast channel among
competing users?
Static Channel Allocation in LANs and MANs
Dynamic Channel Allocation in LANs and MANs

5
Static Channel Allocation

Frequency Division Multiplexing (FDM)
If there are N users, the bandwidth is divided
into N equal-sized portions.
Good for small and constant numbers of users,
each of which has a heavy (buffered) load of
traffic.
Not good for users with bursty traffic
Time Division Multiplexing (TDM)
Each user is statically allocated every Nth time
slot.
The same problem with bursty traffics.

6
Why FDM and TDM have a poor performance?

Mean time delay, T, for a channel of capacity C
bps, with arrival rate of ? frames/sec, each
frame having a length drawn from an exponential
probability density function with mean 1/?
bits/frame.
From queuing theory with Poisson arrival and
service times T 1/(?C - ?)
Ex C 100 Mbps, 1/? 10,000 bits/frames, ?
5000 frames/sec, then T 200 ?sec NOT T 100
?sec
TFDM 1/(? (C/N) (?/N)) N/(?C - ?) NT
Ex 10 networks of 10 Mbps, TFDM NT 2 msec

7
Dynamic Channel Allocation Assumptions

Station Model
N independent stations/terminals
The probability of a frame being generated in an
interval of ??t is ??t, where ? is a constant
(the arrival rate of new frames).
Single Channel Assumption
A single channel is available for all
communications.
All stations can transmit on it and all can
receive from it
Collision Assumption
Collision If two frames are transmitted
simultaneously, they overlap in time and the
resulting signal is garbled.
All stations can detect collisions.
There are no errors other than those generated by
collisions.
(a) Continuous Time No master clock. Frame can
start at any time.(b) Slotted Time Time is
divided into discrete intervals (slots). Frame
transmission always begins at the start of a
slot.
(a) Carrier Sense Stations can tell if the
channel is in use.(b) No Carrier Sense Stations
cannot sense the channel before using it.

8
Agenda

The Channel Allocation Problem
Multiple Access Protocols
Ethernet
Data Link Layer Switching
Summary

9
Multiple Access Protocols

ALOHA
Carrier Sense Multiple Access Protocols
Collision-Free Protocols
Limited-Contention Protocols
Wavelength Division Multiple Access Protocols
Wireless LAN Protocols

10
ALOHA

1970, Norman Abramson, Univ. of Hawaii
Was called the ALOHA system
Used ground-based radio broadcasting
The basic idea is applicable to any system, in
which uncoordinated users are competing for the
use of a single shared channel.
Two versions
Pure ALOHA not global time synchronization
Slotted ALOHA time is divided into discrete slots

11
Pure ALOHA

Basic idea
Let users transmit whenever they have data to be
sent.
There will be collisions, of course, and the
colliding frame will be damaged.
However, due to the feedback property of
broadcasting, a sender can always find out
whether its frame was destroyed by listening to
the channel, the same way the other users do.
If listening at the same time of sending is not
possible, then ack is required.
If a frame is destroyed, the sender just wait a
random amount of time and sends it again.
Contention Systems
Systems in which multiple users share a common
channel in a way that can lead to conflicts.

12
Pure ALOHA Example

In pure ALOHA, frames are transmitted at
completely arbitrary times.
The throughput of ALOHA systems is maximized by
having a uniform frame size.

13
Pure ALOHA Channel Efficiency?

Let the frame size denote the amount of time
needed to transmit the standard, fixed-length
frame.
Assume that infinite population of users
generates new frames according to a Poisson
distribution with mean N frames per frame time.
If N gt 1, the user community is generating more
frames than the channel can handle, so nearly
every frame will suffer a collision.
For reasonable throughput, we expect 0 lt N lt 1.
In addition to the new frame, the stations also
generate retransmissions of garbled frames.
Assume that the probability of k transmission
attempts per frame time, old and new combined, is
also Poisson, with mean G per frame time.
Clearly G gt N
S GP0, where S is throughput and P0 is the
probability that a frame does not suffer
collision.

14
Pure ALOHA Vulnerable Period

A frame will not suffer a collision if not other
frames are sent within one frame time of its
start.
Vulnerable period for the shaded frame.

15
Pure ALOHA Throughput

The probability that k frames are generated
during a given frame time is given by the Poisson
distribution
Prk Gk e-G / k!
Pr0 e-G
In an interval of two frame time, the mean number
of frames generated is 2G.
Then, the probability of no other traffic being
initiated during the entire vulnerable period is
P0 e-2G
Using S GP0 , we get S G e-2G

16
Pure ALOHA Throughput

Maximum throughput occurs at G 0.5, which is
about 0.184, or 18. Not encouraging!

Throughput versus offered traffic for ALOHA
systems.
17
Slotted ALOHA

1972, Roberts, doubling the capacity of ALOHA
Basic idea
Users need to agree on slot boundaries
One special station emit a pip at the start of
each interval, like a clock.
The users need to wait until the beginning of the
next slot.
Throughput
The vulnerable area is halved
So, P0 e-G, and S G e-G
Probability of collision is 1- P0 or 1- e-G
The probability of a transmission requiring
exactly k attempts Pk e-G (1- e-G)k-1
The expected number of transmissions
E ??k1kPk eG

18
Carrier Sense Multiple Access Protocols

ALOHA and Slotted ALOHA are bound to have many
collisions as the stations start transmitting at
will.
In LANs, it is possible for stations to detect
what other stations are doing and adapt their
behavior accordingly.
Carrier Sense Protocols
Protocols in which stations listen for a carrier
(i.e., a transmission) and act accordingly
Persistent and Nonpersistent CSMA
CSMA with Collision Detection

19
Persistent CSMA

When a station has data to send, it first listens
to the channel ro see if anyone else is
transmitting
If the channel is busy, the station waits until
it becomes idle.
When the station detects an idle channel, it
transmits a frame with the probability of one,
hence the name 1-persistent CSMA.
The propagation delay has an important effect on
the performance of this protocol
The longer the propagation delay, the more chance
of collision.
With propagation delay of zero, there will still
be collisions.

20
Nonpersistent CSMA

A conscious attempt is made to be less greedy.
Before sending, a station senses the channel.
If the channel is already in use, the station
does not continually sense it for the purpose of
seizing it immediately upon detection the end of
the previous submission.
Instead, it waits a random period of time and
then repeats the algorithm.
Consequently, this algorithm leads to a better
channel utilization, but longer delays.

21
P-Persistent CSMA

It applies to slotted channels
When a station becomes ready to send, it senses
the channel.
If it is busy, it waits until the next slot.
If it is idle, it transmits with a probability p
With probability q1-p, it defers until the next
slot.
If that slot is also idle, it either transmits or
defers again, with probability p and q.
This process is repeated until either the frame
has been transmitted or another station has begun
transmitting.
In the latter case, it waits a random time and
starts again

22
Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load
for various random access protocols.

23
CSMA with Collision Detection

Persistent and nonpersistent CSMA protocols are
clearly an improvement over ALOHA
No station will start transmission if it senses
that the channel is busy!
Another improvement
Abort transmission as soon as a collision is
detected.
This saves time and bandwidth
CSMA/CD
Is widely used in LANs in the MAC sublayer
It is the base for the popular Ethernet LAN

24
CSMA/CD Conceptual Model

Lets assume that at time t0 a station has
finished transmitting its frame.
Any other station having a frame to send may now
attempt to do so.
If two or more stations decide to transmit
simultaneously, there will be a collision.
Collisions can be detected by looking at the
power or pulse width of the received signal and
comparing it to the transmitted signal.
After a station detects a collision, it aborts
its transmission, waits a random period of time,
and then tries again, assuming that no other
station has started transmitting in the meantime.
Therefore, our model for CSMA/CD will consists of
alternating contention and transmission periods,
with idle periods occurring when all stations are
quiet.

25
CSMA with Collision Detection

CSMA/CD can be in one of three states
contention, transmission, or idle.

26
CSMA/CD Modeling the Contention Period

Worst case scenario
Assume ? is the channel propagation time
If station A starts transmission at t0 and B at
the farthest in the channel start transmission at
t0 ? - ??, then A will not know about the
collision until t0 2? - ??
Therefore, we model the contention period as a
slotted ALOHA system with slot width 2?
Collision detection is an analog process
so the signal encoding must allow collisions to
be detected (two 0 volts will be 0 volt).
A sending station must continuously monitor the
channel, listening for noise bursts that might
indicate a collision.
So, CSMA/CD with a single channel is a
half-duplex system inherently, as the receiving
logic is in use.
No MAC-sublayer protocol guarantees reliable
delivery (the receiving side may not correctly
copy the frame!).

27
Collision-Free Protocols

In CSMA/CD still collisions can happen during the
contention period
Adversely affecting the system performance.
Especially when the cable is long and frames are
short
Collision-Free Protocols
Do not have any collisions
Not widely used yet
Examples
A Bit-Map Protocol
Binary Countdown Protocol

28
A Bit-Map Protocol

Efficiency
Low loads With the overhead per frame, N bits,
and data d bits, the efficiency is d / (N d)
High loads With the overhead per frame, 1 bit,
the efficiency is d / (1 d)
Problem The overhead is one bit per station, so
it does not scale well

The basic bit-map protocol.
29
Binary Countdown Protocol

All addresses are assume to be the same length.
The bits in each address position from different
stations are BOOLEAN ORed together.
The channel efficiency
d / (d log2 N)

The binary countdown protocol. A dash indicates
silence.
30
Limited-Contention Protocols

Performance measures
Low delay at low loads
Contention protocols (e.g., pure of slotted
ALOHA)
High channel efficiency at high loads
Collision-free protocols
It would be best if we could combine the best
properties of the contention and collision-free
protocols.
Limited-Contention Protocol
Uses a contention protocol at low load
Uses collision-free protocol at high load

31
Limited-Contention Protocols

Acquisition probability for a symmetric
contention channel.

32
Adaptive Tree Walk Protocol

The tree for eight stations.

33
Wavelength Division Multiple Access Protocols

A different approach to channel allocation is to
divide the channel into sub-channels using FDM,
TDM, or both, and dynamically allocate them as
needed.

Wavelength division multiple access.
34
Wireless LAN Protocols

Portal and Mobile computers may not be the same!
CSMA may not be appropriate, because what matters
is interference at the receiver, and not at the
sender side.
Hidden Station Problem a station not being able
to detect a potential competitor for the medium
because the competitor is too far away.
Exposes Station Problem a station falsely avoid
transmission, because it senses activity on the
network that does not affect the intended
receiver.

35
Wireless LAN Protocols

A wireless LAN
Hidden Station Problem A is transmitting and if
C transmits too, then there will be collision at
B.
B is transmitting and for that, C is avoiding
transmission to D.

36
Wireless LAN Protocols (2)

The Multiple Access with Collision Avoidance
(MACA) protocol.
(a) A sending an RTS to B.
(b) B responding with a CTS to A.

37
Agenda

The Channel Allocation Problem
Multiple Access Protocols
Ethernet
Wireless LANs
Broadband Wireless
Bluetooth
Data Link Layer Switching
Summary

38
Ethernet

Ethernet Cabling
Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Ethernet Performance
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
IEEE 802.2 Logical Link Control
Retrospective on Ethernet

39
IEEE 802

The IEEE has standardized a number of LANs and
MANs under the name IEEE 802.
802.3 is Ethernet (based on the original
Ethernet)
802.11 is for Wireless LAN
802.15 is for Bluetooth
802.16 is for Wireless MAN
802.2 is for logical link control for both 802.3
and 802.11

40
Ethernet Cabling

10Base5 10Base2
10 Mbps, Base is for baseband signaling, 500
185 meters
10Base-T and 10Base-F
T for Twisted Pair and F for Fiber

The most common kinds of Ethernet cabling.
41
Ethernet Cabling (2)

Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.

42
Ethernet Cabling (3)

Cable topologies. (a) Linear, (b) Spine, (c)
Tree, (d) Segmented.

43
Ethernet Cabling (4)

(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.

44
Ethernet MAC Sublayer Protocol

Preamble (10101010 pattern) The Manchester
encoding will produce a 10 MHz square wave for
6.4 ??sec to allow the receiver clock to
synchronize
Type multiple network protocols Which process
to give the frame to.
Pad Frame size at least 64 bytes frames must
take more than 2?.
For 10 Mbps, max length of 2500 m, and four
repeaters, 2? is 50 ?sec
So, 500 bits is the smallest frame that can work
SoF Start of Frame delimiter for compatibility
with 802.4 and 802.5