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LAN & MAC (Medium Access Control) protocols. Two basic types of networks: Switched networks: transmission lines, multiplexers, and switches; routing, hierarchical address for scalability.

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Lan mac medium access control protocols
LAN & MAC (Medium Access Control) protocols

  • Two basic types of networks:

    • Switched networks: transmission lines, multiplexers, and switches; routing, hierarchical address for scalability.

    • Broadcast networks: a single shared medium, simpler, no routing, messages received by all stations, flat address; however, when users try to transmit messages into the medium, potential conflict, so MAC is needed to orchestrate the transmission from various users.

    • LAN is a typical broadcast network.


Peer to peer protocols vs mac
Peer-to-peer protocols VS. MAC

  • Both are to transfer user information despite transmission impairments

  • For peer-to-peer:

    • Main concern: loss, delay, resequencing

    • Using control frames to coordinate their actions

    • Delay-bandwidth is important

    • Involved only two peer processes

  • MAC:

    • Main concern: interference from users

    • Using some mechanisms to coordinate the access of channel

    • Delay-bandwidth is important

    • Need the coordination from all MAC entities, any one does not cooperate, the communication will not take place.


What are going to be discussed
What are going to be discussed

  • Introduction to broadcast networks

  • Overview of LANs: frame format & placement in OSI.

  • Random access: ALOHA & CSMA-CD (Carrier Sensing Multiple Access with Collision Detection ) i.e., Ethernet.

  • Scheduling: token-ring.

  • LAN standards (brief view)

  • LAN bridges: used to connect several LANs.


Multiple access communications

3

2

4

1

Shared Multiple

Access Medium

5

M

  • Any transmission from any station can be heard by any other stations

  • If two or more stations transmit at the same time, collision occurs

Figure 6.1


Satellite communication involves sharing of

uplink and downlink frequency bands

= fin

Satellite Channel

= fout

Figure 6.3


Cellular networks: radio shared by mobile users and require MAC

BSS

BSS

MSC

SS#7

STP

HLR

VLR

wireline

terminal

EIR

PSTN

AC

MSC = mobile switching center

PSTN = public switched telephone network

STP = signal transfer point

VLR = visitor location register

AC = authentication center

BSS = base station subsystem

EIR = equipment identity register

HLR = home location register

Figure 4.52


Multi-drop telephone line requires access control MAC

Multidrop telephone lines

Inbound line

Host

Outbound line

Terminals

Figure 6.4


Ring networks and multi-tapped buses require MAC MAC

Ring networks

Multitapped Bus

Figure 6.5



Approaches to sharing transmission medium MAC

Minimize the

incidence of

collision to

achieve reasonable

usage of medium.

Good for bursty traffic.

Medium Sharing Techniques

Static Channelization

Dynamic Medium Access Control

Partitioned channels

are dedicated to

individual users, so

no collision at all.

Good for steady traffic

and achieve efficient

usage of channels

Scheduling

Random Access

Try and error. if no collision,

that is good, otherwise wait a

random time, try again.

Good for light traffic.

Schedule a

orderly access

of medium.

Good for heavier

traffic.

Figure 6.2


Mac and performance
MAC and performance MAC

  • Shared medium is the only means for stations to communicate

  • Some kind of MAC technique is needed

  • Like ARQs, which use ACK frame to coordinate the transmission and consume certain bandwidth, the MAC will need to transfer some coordination information which will consume certain bandwidth of shared medium.

  • Delay-bandwidth product plays a key role in the performance of MAC (as in ARQs).


Delay-bandwidth product and performance MAC

(suppose two station A and B want to transmit information)

Distance d meters

tprop = d /  seconds

A transmits at t = 0

A

B

B transmits before t = tprop and detectscollision shortly

thereafter

A

B

A detects

collision at

t = 2 tprop

A

B

1.  : the speed of light, 3*108 meters/second

2. Before A begins to transmit, A listens to medium, if busy, wait; otherwise, do it (suppose t=0)

3. If B wants to transmit after t=tprop A’s transmission has reached B, so B waits and A captures medium

successfully and transmits its entire message.

4. If B wants to transmit before t=tprop, it listens and no transmission is going on, so B begins

to transmit, then collision occurs. B detects collision shortly, but A detects collision at t=2tprop

5. Therefore, 2tpropis required to coordinate the access for each packet transmitted.

Figure 6.7


MAC efficiency MAC

  • Suppose bit rate of medium is R, then number

  • of bits “wasted” in access coordination is 2tpropR.

And suppose average length of packets is L.

Then efficiency in use of the medium is:

1

L

1

Efficiency =

=

=

tpropR

  • L + 2tpropR

1+2a

1 + 2

L

a=tpropR / L i.e., the ratio of (one-way) delay-bandwidth product

to the average packet length.

Suppose a = 0.01, then efficiency = 1/1.02 = 0.98

a = 0.1, then efficiency = 1 / 2 = 0.50


Examples of efficiency
Examples of efficiency MAC

  • Ethernet (CSMA-CD):

    • Efficiency = 1/(1+6.44a) where a = tpropR/L.

  • Token-ring networks:

    • Efficiency = 1/(1+a’ ) where a’ = ring-latency in bits/L where ring-latency contains:

      • The sum of bit delays introduced at each ring adapter.

      • Delay-bandwidth product where delay is the time required for a bit to circulate around the ring.


Typical LAN structure and network interface card MAC

RAM

RAM

(a)

A LAN connects servers, workstations,

Printers, etc., together to achieve sharing

  • NIC is parallel with memory

  • but serial with network

  • 2. ROM stores the implementation of MAC

  • 3. Unique physical address burn into ROM

  • 4. A hardware in NIC recognizes physical,

  • broadcast & multicast addresses.

  • 5. NIC can be Set to “promiscuous” mode

  • to catch all transmissions.

(b)

Ethernet Processor

ROM

Figure 6.10


IEEE 802 LAN standards MAC

Network Layer

Network Layer

LLC

802.2 Logical Link Control

Data Link

Layer

802.11

Wireless

LAN

Other

LANs

802.3

CSMA-CD

802.5

Token Ring

MAC

Physical

Layer

Various Physical Layers

Physical

Layer

OSI

IEEE 802

One LLC and several MACs, each MAC has an associated set of physical layers.

MAC provides connectionless transfer. Generally no error control because of relatively error free.

MAC protocol is to direct when they should transmit frames into shared medium.

Figure 6.11


The MAC sublayer provides unreliable datagram service MAC

MAC

MAC

MAC

PHY

PHY

PHY

Unreliable Datagram Service

Important: all three MAC entities must cooperate to provide datagram service, I.e., the interaction

between MAC entities is not between pairs of peers, but rather all entities must monitor all frames.

Figure 6.12


MAC MAC

MAC

MAC

PHY

PHY

PHY

LLC provides three HDLC services: 1. Unacknowledged connectionless service, recall HDLC has

unnumbered frames; 2. Reliable connection-oriented service in the form of HDLC ABM mode;

3. Acknowledged connectionless service, need to add two unnumbered frames to HDLC frame set.

C

A

A

C

Reliable Packet Service

LLC

LLC

LLC

LLC can provide reliable packet transfer service

Figure 6.13


  • LLC provides additional addressing, i.e., SAP (Service Access Point). Like PPP, LLC can

  • support several different network connections with different protocols at the same time.

  • Typical SAPs: IP: 06, IPX: E0, OSI packets: FE etc.

  • In practice, LLC SAP specifies in which buffer the NIC places the frame, thus allowing the

  • appropriate network protocol to retrieve the data.

1 byte

1

1 or 2

Destination

SAP Address

Source

SAP Address

Control

Information

Source SAP Address

Destination SAP Address

C/R

I/G

1

7 bits

7 bits

1

I/G = Individual or group address

C/R = Command or response frame

LLC PDU structure and its support for several SAPs

Figure 6.14


Header overhead: TCP, IP: >=20 Access Point). Like PPP, LLC can

LLC: 3 or 4

MAC: 26

TCP

segments

Reliable

IP Header

Unreliable

IP Packet

IP

LLC PDU

LLC Header

Data

Orderly

MAC Header

FCS

Chaos

MAC frame

LLC PDU and MAC frame

Figure 6.15


Random access
Random Access Access Point). Like PPP, LLC can

  • Why random access?

    • Reaction time (i.e. 2 times of propagation delay) is very important for performance, e.g. in Stop-and-Wait, when reaction time is small (i.e. the ACK will arrive soon) the performance is very good, however, if reaction time is large, then performance is very bad.

    • Therefore, proceed the transmission without waiting for ACK and deal with collision/error after the fact, i.e. random access.

  • Three types of random accesses:

    • ALOHA, slotted ALOHA, and CSMA-CD


Aloha
ALOHA Access Point). Like PPP, LLC can

  • Basic idea:

    • let users transmit whenever they have data to be sent.

    • When collision occurs, wait a random time ( why? ) and retransmit again.

  • Differences between regular errors &collision

    • Regular errors only affect a single station

    • Collision affects more than one

    • The retransmission may collide again

    • Even the first bit of a frame overlaps with the last bit of a frame almost finished, then two frames are totally destroyed.


ALOHA random access scheme Access Point). Like PPP, LLC can

Suppose L: the average frame length, R: rate, X=L/R: frame time

1. Transmit a frame at t=t0 (and finish transmission of the frame at t0+X )

2. If ACK does not come after t0+X+2tprop or hear collision, wait for random time: B

3. Retransmit the frame at t0+X+2tprop+B

Two modes: collide only from time to time and snowball effect collision

First transmission

Retransmission

t

t0

t0+X

t0-X

t0+X+2tprop

t0+X+2tprop

Vulnerable

period

Backoff period: B

Time-out

Retransmission

if necessary

When collision occurs?

Vulnerable period: t0-X to t0+X, (2X seconds) if any other frames are transmitted during

the period, the collision will occur.

Therefore the probability of a successful transmission is the probability that there is no

additional transmissions in the vulnerable period.

Figure 6.16


The performance of ALOHA Access Point). Like PPP, LLC can

  • Let S be the arrival rate of new frames in units of frames/X seconds,

  • S is also the throughput of the system.

  • Let G be the total arrival rate in units of frames/X seconds, G

  • contains the new and retransmissions and is the total load.

  • Assume that aggregate arrival process resulting from new and

  • retransmitted frames has a Poisson distribution with an average

  • number of arrivals of 2G frames/2X seconds, i.e.,

(2G)k

P[k transmissions in2X seconds] =

e-2G

, k=0,1,2,…

k!

Therefore, the throughput of the system is:

S=G*P[no collision] =G*P[0 transmission in 2X seconds]

(2G)0

e-2G =G e-2G

=G*

0!


What results can be obtained from the graph? Access Point). Like PPP, LLC can

1.peak value at G=0.5 with S=0.184

2.for any given S, there are two values of G, corresponding to

the two modes: occasional collision mode with S G and

frequent collision mode with G >> S

0.184

Ge-2G

Throughput S versus load G for ALOHA

Figure 6.17


Slotted ALOHA Access Point). Like PPP, LLC can

  • Synchronize the transmissions of stations

    • All stations keep track of transmission time slots and are allowed to initiate transmissions only at the beginning of a time slot.

  • Suppose a packet occupies one time slot

    • Vulnerable period is from t0-X to t0, i.e., X seconds long.

Therefore, the throughput of the system is:

S=GP[no collision] =GP[0 transmission in X seconds]

(G)0

=G

e-G =G e-G

0!


Slotted ALOHA random access scheme Access Point). Like PPP, LLC can

t0=(k+1)X

First transmission

Retransmission

t

(k+1)X

=nX

kX

t0+X+2tprop

t0+X+2tprop

Backoff period: B

Time-out

Retransmission

if necessary

Vulnerable period: t0-X to t0 , i.e., X seconds long

Figure 6.16


Peak value at Access Point). Like PPP, LLC can G=1 with S=0.368 for slotted ALOHA, double compared with ALOHA.

In LAN, propagation delay may be negligible and uncoordinated access of shared medium

is possible but at the expense of significant wastage due to collisions and at very low throughput.

Throughput of ALOHAs is not sensitive to the reaction time because stations act independently.

0.368

Ge-G

S

0.184

Ge-2G

G

Throughput S versus load G for ALOHA and slotted ALOHA

Figure 6.17


Csma carrier sensing multiple access
CSMA (Carrier sensing multiple access) Access Point). Like PPP, LLC can

  • Problem with ALOHAs: low throughput because the collision wastes transmission bandwidth.

  • Solution: avoid transmission that are certain to cause collision, that is CSMA. Any station listens to the medium, if there is some transmission going on the medium, it will postpone its transmission.


Suppose Access Point). Like PPP, LLC can tprop is propagation delay from one extreme end to the other extreme end of the medium.

Station A begins transmission at t=0

A

Station A captures

channel

at t=tprop

A

When transmission is going on, a station can listen to the medium anddetect it.

After tprop, A’s transmission will arrive the other end; every station will hear it and refrain

from the transmission, so A captures the medium and can finish its transmission.

But in ALOHAs, it is X or 2X

Vulnerable period = tprop

In LAN,generally, tprop < X

sense

sense

sense

sense

CSMA random access scheme

Figure 6.19


Three different csma schemes
Three different CSMA schemes Access Point). Like PPP, LLC can

  • Based on how to do when medium is busy:

    • 1-persistent CSMA

    • Non-persistent CSMA

    • p-persistent CSMA


1-persistent CSMA Access Point). Like PPP, LLC can

sense channel when want to transmit a packet, if channel is busy, then

sense continuously, until the channel is idle, at this time, transmit the

frame immediately.

If more than one station are sensing, then they will begin transmission

the same time when channel becomes idle, so collision. At this time,

each station executes a backoff algorithm to wait for a random time, and

then re-senses the channel again.

Problem with 1-persistent CSMA is “high collision rate”.


Non-persistent CSMA Access Point). Like PPP, LLC can

sense channel when want to transmit a packet, if channel is

idle, then transmit the packet immediately. If busy, run backoff

algorithm immediately to wait a random time and then

re-sense the channel again.

Problem with non-persistent CSMA is that when the channel

becomes idle from busy, there may be no one of waiting

stations beginning the transmission, thus waste channel

bandwidth,


p Access Point). Like PPP, LLC can -persistent CSMA

sense channel when want to transmit a packet, if channel is

busy, then persist sensing the channel until the channel

becomes idle. If the channel is idle, transmit the packet

with probability of p, and wait, with probability of 1-p,

additional propagation delay tprop and then re-sense again


Throughput versus load Access Point). Like PPP, LLC can G for 1-persistent

(three different a=tprop/X )

S

0.53

1-Persistent

CSMA

0.01

0.45

0.16

0.1

G

1

Figure 6.21 - Part 2


Throughput versus load Access Point). Like PPP, LLC can G for non-persistent

(three different a=tprop/X )

S

0.81

Non-Persistent

CSMA

0.01

0.51

0.14

0.1

G

1

1-persistent is sharper than non-persistent.

a=tprop/X has import impact on the throughput.

When a approaches 1, both 1-persistent and non-persistent is

worse than ALOHAs.

Figure 6.21 - Part 1


Csma cd
CSMA-CD Access Point). Like PPP, LLC can

  • When the transmitting station detects a collision, it stops its transmission immediately, Not transmit the entire frame which is already in collision.

  • The time for transmitting station to detect a collision is 2tprop.

  • In detail: when a station wants to transmit a packet, it senses

    channel, if it is busy, use one of above three algorithms (i.e., 1-persistent, non-persistent, and p-persistent schemes). The transmitter senses the channel during transmission. If a collision occurred and was sensed, transmitter stops its left transmission of the current frame; moreover, a short jamming signal is transmitted to ensure other stations that a collision has occurred and backoff algorithm is used to schedule a future re-sensing time.

  • The implication: frame time X >= 2tprop, , since X=L/R, which means that there is a minimum limitation for frame length.


A begins to Access Point). Like PPP, LLC can

transmit at

t=0

B

A

B begins to

transmit at

t= tprop-

B detects

collision at

t= tprop

B

A

A

B

A detects

collision at

t= 2 tprop-

The reaction time in CSMA-CD is 2tprop

It takes 2 tprop to find out if channel has been captured

Figure 6.22


  • When Access Point). Like PPP, LLC can a is small, i.e, tprop << X, the CSMA-CD is best and all CSMAs are

    • better than ALOHAs.

  • When a is approaching 1, CSMAs become worse than ALOHA.

  • ALOHAs are not sensitive to a because they do not depend on reaction time.

CSMA/CD

1-P CSMA

Non-P CSMA

max

Slotted Aloha

Aloha

a

= tprop /X

Maximum achievable throughput of random access schemes

Figure 6.24


Summary of random access schemes Access Point). Like PPP, LLC can

(Continuous) ALOHA: try to send a frame anytime, if collision, wait random time, resend.

Vulnerable period: 2X , maximum throughput: 0.184.

Slotted ALOHA: send a frame at the beginning of a time slot. If collision, wait a random

time to a new time slot, and resend again.Vulnerable period: X , maximum throughput: 0.368.

1-persistent CSMA: listen before transmission, if busy, continuously listen until channel

become idle, then transmit immediately. If collision, wait a random time, re-listen.

Vulnerable period: tprop , throughput: 0.53 for a=tprop/X=0.01

Non-persistent CSMA: listen before transmission, if busy, wait a random time, re-listen.

if idle, transmit. If collision, wait a random time, re-listen.

Vulnerable period: tprop , throughput: 0.81 for a=tprop/X=0.01

p-persistent CSMA: persist listening to the channel until idle. At this time, with probability p,

transmit the packet, and with probability of 1-p, do not transmit but wait additional

tprop and then re-listen. If collision, wait random time, re-listen.

Vulnerable period: tpropthroughput dependent on p.

CSMA-CD: using any one of above three.Listen during transmission. If collision,

stop its transmission immediately.Vulnerable period: tprop.throughput: >0.90 for a=tprop/X=0.01

Important: a=tprop/X affects performance. Requirement: frame lengthcan not below

certain value for given tprop, (the distance of LAN).


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