Chapter 4 The Medium Access Sublayer

Computer Networks by R.S. Chang, Dept. CSIE, NDHU 1

Chapter 4The Medium Access Sublayer

• Deal with broadcast networks and their protocols.• The key issue is how to determine who gets to use the c

hannel when there is competition for it.• Broadcast channels are sometimes referred to as multia

ccess channels or random access channels.• The protocols used to determine who gets next on a mul

tiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer.


Chapter 4 The Medium Access Sublayer

4.1 The Channel Allocation Problem

4.1.1 Static Channel Allocation in LANs and MANs

FDM: Frequency Division MultiplexingTDM: Time Division Multiplexing

Suitable for fixed number of users with constant traffic

Disadvantage: when the number of users is large and continuously varying, or the traffic is bursty, FDM presents some problems.




4.1.1 Static Channel Allocation in LANs and MANs

A simple queuing theory calculation

For a channel of capacity C bps, with an arrival rate of frames/sec, each frame having a length drawn from an exponential probablity density function with mean 1/ bits/frame, the mean time delay

CT

1

Now let us divide the single channel up into N independent subchannels, each with capacity C/N bps. The mean input rate on each of the subchannel will now be /N. Recomputing T, we get NT

NNCTFDM

)/()/(

1




4.1.2 Dynamic Channel Allocation in LANs and MANs

Five key assumptions:1. Station Model. The model consists of N independent statio

ns, each with a program or user that generates frames for transmission.

The probability of a frame being generated in an interval of length t is t, where is a constant (the arrival rate of new frames).

Once a frame has been generated, the station is blocked and does nothing until the frame has been successfully transmitted.





Five key assumptions:2. Single Channel Assumption. A single channel is available

for all communication. All stations can transmit on it and all can receive from it.

As far as the hardware is concerned, all stations are

equivalent, although some protocol software may assign priorities to them.





Five key assumptions:3. Collision Assumption. If two frames are transmitted

simultaneously, they overlap in time and the resulting signal is garbled. This event is called a collision.

All stations can detect collisions. A collided frame must be transmitted again alter. There are no errors other than those generated by collisions.





Five key assumptions:4a. Continuous Time. Frame transmission can begin at any

instant. There is no master clock dividing time into discrete intervals.

4b. Slotted Time. Time is divided into discrete intervals (slots). Frame transmissions always begin at the start of a slot. A slot may contain 0, 1, or more frames, corresponding to an idle slot, a successful transmission, or a collision, respectively.





Five key assumptions:5a. Carrier Sense. Stations can tell if the channel is in use

before trying to use it. If the channel is sensed as busy, no station will attempt to use it until it goes idle.

5b. No Carrier Sense. Stations cannot sense the channel before trying to use it. They just go ahead and transmit. Only later can they determine whether or not the transmission was successful.



4.2 Multiple Access Protocols

4.2.1 ALOHA

Pure ALOHA

ALOHA of U. of Hawaii

ComputerCenter

413MHz at 9600bps

407MHz at 9600bps

the first multiple-access protocol: a method for sharing a transmission channel by enabling the transmitter to access thechannel at random times




4.2.1 ALOHA

Pure ALOHA Frames are transmitted at completely arbitrary times.




4.2.1 ALOHA

Pure ALOHAprotocol• nodes transmit on a common channel• transmit frame of fixed length• when two transmissions overlap, they garble each other

(collision)• the central node acknowledges the correct frames it receives• when a node does not get an acknowledgment within a

specific timeout, it assumes that its frame collided• when a frame collides, the transmitting node schedules a retransmission after a random delay




4.2.1 ALOHA

new frame

old frame

nodes

SG channel

collision?

No

Yes

S

S: the mean number of new frames generated by the infinite populationG: the mean number of transmission attempts (new and old combined)

0PGS where P0 is the probability that a frame does not suffer a collision




4.2.1 ALOHA

pure ALOHA and slotted ALOHA

time

Nodes can starting transmitting at any time.

pure ALOHA

time

slotted ALOHAslot

Nodes must start their transmissions at the beginning of a time slot.




4.2.1 ALOHA

vulnerability period

pure ALOHA

packet packet

slotted ALOHA

Other nodes that are ready at this period will result in collision.




4.2.1 ALOHA

The probability that k frames are generated during a given frame time is given by the Poisson distribution:

!

)2(]Pr[

2

k

eGk

Gk

So the probability of zero frames in a slot is just e-G.

In an interval two time slots long, the mean number of frames generated is 2G. Therefore, the distribution is:

!]Pr[

k

eGk

Gk

The probability of zero frames is e-2G.




4.2.1 ALOHA

Using S=GP0, we get

For pure ALOHA: S=Ge-2G

For slotted ALOHA: S=Ge-G

S GedS

dGe Ge

dS

dGG S e

S GedS

dGe Ge

dS

dGG S

e

G G G

G G G

, , ,

, , ,

0 1

2 01

2 2

1

2 2 21

To find the maximum value:




4.2.1 ALOHA




4.2.1 ALOHA

In slotted ALOHA, the best we can hope for is 37% success, 37% slots empty, and 26% collisions. Operating at higher values of G reduces the number of empties but increases the number of collisions exponentially.

Consider the transmission of a test frame: success: e-G, failure: 1-e-G, success for k attempts:

1)1( kGGk eeP

Expected number of transmissions:G

k

kGG

kk eekekPE

1

1

1

)1(

As a result of the exponential dependence of E upon G, small increases in the channel load can drastically reduce its performance.




4.2.2 Carrier Sense Multiple Access Protocols

With slotted ALOHA the best channel utilization that can beachieved is 1/e. This is hardly surprising, since with stations transmitting at will, without paying attention to what other stations are doing, there are bound to be many collisions.

In local area networks, however, it is possible for stations to detect what other stations are doing, and adapt their behavior accordingly.

Protocols in which stations listen for a carrier (i.e. a transmission) and act accordingly are called carrier sense protocols.





1-persistent CSMA: the station transmits with a probability of 1whenever it finds the channel idle, if the channel is busy, it waitsuntil it becomes idle

non-persistent CSMA: the station transmits if the channel is idle,if the channel is busy, it waits a random time and tries again

p-persistent CSMA (slotted): the station transmits with a probability of p whenever it finds the channel idle, with a probability of 1-p, it waits until the next slot. If another station has begun transmitting, it acts as if there had been a collision. It waits a random time and starts again.









CSMA with collision detection (CSMA/CD)

Abort a transmission as soon as they detect a collision. Quickly terminating damaged frames saves time and bandwidth.

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.





A conceptual model for CSMA/CD

(How long should each slot be?)




4.2.2 Carrier Sense Multiple Access Protocolsmaximum collision detection time

A B

t=0

PROP

2PROP

1.A startstransmitting 2.B starts

transmitting3. A reaches B

4.B detects collision,transmits JAM, stops

5.B reaches A

6.A detects collision,transmits JAM, stops

The maximum collision detection time is equal totwice the maximum end-to-end propagation delay.





For this reason we will model the contention interval as a slotted ALOHA system with slot width 2 ( is the end to end delay). On a 1-km long coaxial cable, 5sec.

It is important to realize that collision detection is an analog process. The station’s hardware must listen to the cable while it is transmitting. The signal encoding must allow collisions to be detected (e.g., a collision of two 0-volt signals may well be impossible to detect). For this reason, special encoding is commonly used.




4.2.3 Collision-Free Protocols

Although collisions do not occur with CSMA/CD once a station has unambiguously seized the channel, they can still occur during the contention period. These collisions adversely affect the system performance, especially when the cable is long and the frames are short. As very long, high bandwidth fiber optic networks come into use, the combination of large and short frames will become an increasingly serious problem.

In the protocols to be described, we make the assumption that there are N stations, each with a unique address from 0 to N-1 “wired” into it.





A bit-map protocol

Protocols like this in which the desire to transmit is broadcast before the actual transmission are called reservation protocols.





Performance of bit-map protocol

Assuming contention slot: 1 slot, data slot: d slots

Low-numbered stations must wait on the average 1.5N slots and high-numbered stations must wait on the average 0.5N slots before starting to transmit, the mean for all stations is N slots.

Channel efficiency at low load: d/(N+d)

Channel efficiency at high load: Nd/(N+Nd)=d/(d+1)





Binary Countdown

A dash indicates silence.

Station’s binary address





Binary Countdown

The channel efficiency of this method is d/(d+lnN). If, however, the frame format has been cleverly chosen so that the sender’s address is the first field in the frame, even these lnN bits are not wasted, and the efficiency is 100%.

Variations: Use virtual station numbers. The successful station being circularly permuted after each transmission.

Stations C H D A G B E FPriority 7 6 5 4 3 2 1 0 if D transmitsPriority 7 6 0 5 4 3 2 1 others are promoted




4.2.4 Limited-Contention Protocols

Two important performance measures for channel acquisition strategies: delay at low load and channel efficiency at high load

Contention protocolContention-free protocol

Light load Heavy load

channel delay efficiency

good bad

bad good





Obviously, it would be nice if we could combine the best properties of the contention and collision-free protocols, arriving at a new protocol that used contention at low loads to provide low delay, but used a collision-free technique at high load to provide good channel efficiency.

Such protocols will be called limited contention protocols.





Up until now the only contention protocols we have studied have been symmetric, that is, each station attempts to acquire the channel with some probability p, will all stations using the same p.

Performance of the symmetric case: suppose that k stations are contending for channel access, each has a probability p of transmitting during each slot

The probability that some station successfully acquire the channel is kp(1-p)k-1

Maximum occurs at p=1/k with Pr[success with optimal p]=1

1

k

k

k




4.2.4 Limited-Contentionn Protocols

As soon as the number of stations reaches even 5, the probability has dropped close to it asymptotic value of 1/e.





Limited contention protocols decrease the amount of competition by dividing the stations up into (not necessarily disjoint) groups. Only the members of group 0 are permitted to compete for slot 0. If one of then succeeds, it acquires the channel and transmits its frame.

If the slot lies fallow or if there is a collision, the members of group 1 contend for slot 1, etc. by making an appropriate division of stations into groups, the amount of contention for each slot can be reduced, thus operating each slot near the left end of Fig. 4-8.





The adaptive tree walk protocol

Slot 0

Slot 1 (if collision)

Depth first search for all ready stations





When the load on the system is heavy, it is hardly worth the effort to dedicate slot 0 to node 1, because that makes sense only in the unlikely event that precisely one station has a frame to send. Similarly, one could argue that nodes 2 and 3 should be skipped as well for the same reason.

Put in more general terms, at what level in the tree should the search begin?





Assume that each station has a good estimate of the number of ready stations, q, for example, from monitoring recent traffic.

Assume the root (node 1) is at level 0. Each node at level i has a fraction 2-i of the stations below it.

If the q ready stations are uniformly distributed, the expected number of them below a specific node at level i is just 2-iq. Intuitively, we would expect the optimal level to begin searching the tree as the one at which the mean number of contending stations per slot is 1, that is, the level at which 2-iq=1. Solving this equation we find that i=log2q.





Improvements:

For example, consider the case of stations G and H being the only ones waiting to transmit. Probe node 1: collisionProbe node 2: idleProbe node 6: idleProbe GProbe H

Node 3 and node 7 can be skipped. Why?




4.2.5 Wavelength Division Multiple Access Protocols





M: number of slots in the control channeln+1: number of slots in the data channel, where n of these

are for data and the last one is used by the station to report on its status

On both channels, the sequence of slots repeats endlessly, with slot 0 being marked in a special way so latecomers can detect it. All channels are synchronized by a single global clock.





The protocol supports three classes of traffic:(1) constant data rate connection-oriented traffic(2) variable data rate connection-oriented traffic(3) datagram traffic

For the two connection-oriented protocols, the idea is that for A to communicate with B, it must first insert a CONNECTION REQUEST frame in a free slot on B’s control channel. If B accepts, communication can take place on A’s data channel.





Each station has two transmitters and two receivers, asfollows:1. A fixed-wavelength receiver for listening to its own control

channel.2. A tunable transmitter for sending on other station’s control

channel.3. A fixed-wavelength transmitter for outputting data frames.4. A tunable receiver for selecting a data transmitter to listen

to.

In other words, every station listens to its own control channel for incoming requests but has to tune to the transmitter’s wavelength to get the data.




4.2.6 Wireless LAN Protocols

The hidden terminal problem

When A transmits to B and C also transmits to B simultaneously, the frames will be collided at B. Since A and C can not see each other.





The exposed terminal problem

When C hears B’s transmission intended for A, it may falsely conclude that it can not send to D now.





MACA (Multiple Access with Collision Avoidance)

MACA: basis for IEEE802.11 wireless LAN standard

The basic idea behind it is for the sender to simulate the receiver into outputting a short frame, so stations nearby can detect this transmission and avoid transmitting themselves fir the during of upcoming (large) data frame.






RTS (30 bytes) and CTS contains the data length that will eventually follow.






Any station hearing the RTS is clearly close to A and must remain silent long enough for the CTS to be transmitted back to A without conflict.

Any station hearing the CTS is clearly close to B and must remain silent during the upcoming data transmission, whose length it can tell by examining the CTS frame.






Despite these precautions, collisions can still occur. For example, B and C could both send RTS frames to A at the same time. In the event of a collision, an unsuccessful transmitter (I.e., one that does not hear a CTS within the expected time interval) waits a random amount of time and tries again later. The algorithm used is binary exponential backoff.





MACAW (MACA for Wireless)

1. Introducing an ACK frame after each successful data transmission

2. Adding carrier sense (keeping a station from transmitting an RTS at the same time another nearby station is also doing so to the same destination

3. Running the backoff algorithm separately for each data stream rather than for each station (improving fairness)

4. Exchanging information about congestion between stations and making backoff algorithm react less violently

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Chapter 4 The Medium Access Sublayer