Lecture set 4

Communication Networks

Sanjay K. Bose

Lecture Set IV

LANs and Media Access Control (MAC)

Multiple Access Communications

• Shared media basis for broadcast networks – Inexpensive: radio over air; copper or coaxial cable

– M users communicate by broadcasting into medium

• Key issue: How to share the medium?

1 2

3

4

5 M

Shared multiple access medium

Static channelization

Dynamic medium access control

Scheduling Random Access

Approaches to Media Sharing

Partition medium

Dedicated allocation to users

Used in Satellite Transmissions and

Cellular Telephony

Polling: take turns

Request for slot in transmission schedule

Used in Token Ring

Wireless LANs

Loose coordination

Send, wait, retry if necessary

Used in Aloha and

Ethernet

Random Access Protocols

When node has packet to send transmit at full channel data rate R.

no a priori coordination among nodes

two or more transmitting nodes ➜ “collision”,

random access MAC protocol specifies: how to detect collisions

how to recover from collisions (e.g., via delayed retransmissions)

Examples of random access MAC protocols: slotted ALOHA

ALOHA

CSMA, CSMA/CD, CSMA/CA

Pure ALOHA

When frame first arrives, transmit immediately and wait for ACK

If ACK is received, frame transmission was successful

If ACK is not received, wait for random time and try transmission again; repeat this until frame is successfully transmitted

In example shown, all frames are of unit length. Node i’s frame is successful if no other transmission starts between (t0-1, t0) or

between (t0, t0+1)

ACK and frames can share the same channel or the frames can be sent on a “forward” channel while the ACKs are sent on a “reverse” channel

Poisson Process of rate : Probability of k arrivals in any interval of length T is given by

!

k

TT

ek

Performance Analysis of Pure ALOHA (Infinite User Population Model)

Assume that – (a) Packets of same length (b) Time measured in units of packet length (i.e. packet length=1) (c) ACKs are always successful (d) New packets come at rate of from a Poisson process (e) Unsuccessful packets retransmitted after random rescheduling delay (f) New and Retransmitted packets come at rate from a Poisson process

O

R

Channel + O R

O R SP

(1 )R SP

Pure ALOHA channel in Steady State

Performance Analysis of Pure ALOHA (Infinite User Population Model)

Channel + O R

O R SP

(1 )R SP

2 R

SP e

2 R

O Re

Throughput Throughput Load

Capacity = Maximum Throughput = 1/(2e)≈18% at = 1/2 R

The capacity offered by Pure ALOHA is quite low, i.e. only 18%. As a matter of fact, stability considerations will actually force the

system to be operated at substantially below this level.

Performance Analysis of Pure ALOHA (Finite User Population Model)

t0 t0 + 1

t0 - 1

1

n

For n , P{no arrival in slot Δ}= PS =

1 R

n

Arrival of interest

2

2lim 1 R

n

R

ne

n

Therefore, as obtained earlier

2 R

O Re

Throughput Load

S = packets/unit time G = packets/unit time

OR

Throughput of ALOHA

G

success GeGPS 2

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0

0.00

78125

0.01

5625

0.03

125

0.06

25

0.12

50.

25 0.5 1 2 4

G

S

Collisions are means for coordinating access

Max throughput is Smax= 1/2e (18.4%)

Bimodal behavior:

Small G, S≈G

Large G, S↓0

Collisions can snowball and drop throughput to zero

e-2 = 0.184

Slotted ALOHA

Time is slotted in slots equal to the packet length (say unity) Stations synchronized to frame times Stations transmit frames in first slot after frame arrival Backoff intervals in multiples of slots

t

(k+1) k

t0 +1+2tprop+ B

Vulnerable period

Time-out

Backoff period B

t0 +1+2tprop

Only frames that arrive during the previous slot can collide

t0

Throughput of Slotted ALOHA

Gnn

success

Gen

GGpG

GP

GPGPS

)1()1(

intervals]n in arrivals no[

seconds] Xin arrivals no[

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.0

1…

0.0

3…

0.0

625

0.1

25

0.2

5

0.5 1 2 4 8

Ge-G

Ge-2G

G

S 0.184

0.368

Capacity of Slotted ALOHA is 1/e ≈ 36.8% at G=1

Discrete-Time Markov Chain Analysis for Slotted ALOHA

• Time axis divided into slots with system examined just after each slot boundary • Let m be the total number of users • Let backlogged users be the users who tried unsuccessfully to transmit in an earlier slot. Each such user will attempt to transmit with probability qr in every subsequent slot after its unsuccessful attempt and will continue doing so until it finally succeeds – once that happens it will no longer be considered a backlogged users • Users who are not backlogged can generate new packets for transmission in a given slot with probability qa • Let n be the number of backlogged users at the beginning of a given slot (each of them can try to transmit with probability qr ). • Each of the remaining m-n unbacklogged users can choose to transmit a new packet in a given slot with probability qa . Note that if any such attempt is unsuccessful then the corresponding user joins the backlogged user set.


p14

Qa (i, n) = P{i unbacklogged users transmit in given slot}

Qr (i, n) = P{i backlogged users transmit in given slot}

ia

inma qq

i

nm

)1(

ir

inr qq

i

n

)1(

The state transition probabilities, pij ‘s, of the Markov Chain can then be obtained using Qa (i, n) and Qr (i, n) (see next slide)

m 1 2

p22 p00 p11

p01

p10

p02

p12

p03 p13

p21

p23

p32

p24

p25

0


1),1(),0(

0)],1(1)[,0(),0(),1(

1)],0(1)[,1(

2),(,

inQnQ

inQnQnQnQ

inQnQ

nminiQp

ra

rara

ra

ainn

The state transition probabilities of this Markov Chain will then be -

It can be shown that the steady state probabilities Pn of the Markov Chain (i.e. the probability of the system being in state n) will satisfy -

m

n

n

n

i

inin PpPP

0

1

0

1&

Given the transmission probabilities, qa and qr and the user population m, the system can then be solved to obtain the state probabilities, Pi for i=0,1,…..,m


Define the “drift in state n”, Dn , of the system as the average change in the number of backlogged users over one slot time, starting in state n.

succan PqnmD )(

Psucc = P{successful transmission in one slot} = Average number of successful transmissions in one slot

),1(),0(),0(),1( nQnQnQnQP rarasucc

Note that, when the system is in state n, the expected number G(n) of transmission attempts made in a slot will be -

ra nqqnmnG )()(

For qa <<1, qr <<1, we can show that Psucc ≈G(n)e-G(n) , as obtained before

Carrier Sensing Multiple Access (CSMA)

A

Station A begins transmission at t = 0

A

Station A captures channel at t = tprop

A station senses the channel before it starts transmission

If busy, either wait or schedule backoff (different options)

If idle, start transmission

Vulnerable period is reduced to tprop (due to channel capture effect)

When collisions occur they involve entire frame transmission times

If tprop >packet length, no gain compared to ALOHA or slotted ALOHA

CSMA collisions

Note that collisions can still occur: propagation delay means two nodes may not hear each other’s transmission

In case of collision: entire packet transmission time wasted

spatial layout of nodes

1-persistent CSMA (most greedy) • Start transmission as soon as the channel becomes idle

• Low delay and low efficiency

Non-persistent CSMA (least greedy) • Wait a backoff period, then sense carrier again

• High delay and high efficiency

p-persistent CSMA (adjustable greedy) • Wait till channel becomes idle, transmit with prob. p; or wait one

mini-slot time & re-sense with probability 1-p

• Delay and efficiency can be balanced

CSMA Variations (Based on transmitter behavior when busy channel is sensed)

p-persistent sensing

Analysis of Non-Persistent CSMA

S = Throughput = Carried Traffic G = Load = Offered Traffic a = Propagation Delay

• Assume time measured in units of packet length (i.e. average transmission time of packet =1) • Arrival process for G assumed to follow a Poisson model

Busy Busy Idle Idle Idle

Transmission

starting the

Busy period

Transmission

ending for the

Busy period

End of the Busy

period

a

Gtk

ek

GttkP

!

)(),(

P{k arrivals from offered traffic in time interval t }


B = Length of Busy Period (random variable with mean ) I = Length of Idle Period (random variable with mean )

BI

0

1)(

GGdtetI Gt

Mean length of an Idle Period

Note that the time interval is divided into Cycles, where each cycle is an Idle Period followed by a corresponding Busy Period. Let U denote the time during a cycle that the channel is used without conflicts (i.e. no collisions). Then as a busy period has a successful transmission with no collisions only if there are no arrivals in the initial time interval of length a of a busy period

aGeU


Throughput, S= IB

U

Since we already know and , we need to get to get the throughput B

IU

Successful

Transmission

Period Idle Idle

a

Unsuccessful

Transmission

Period

a

1

a

Y

Let t = start of the busy period t +Y = time of last arrival in interval (t, t +a) Then

1

aYB 1 We therefore need to get the throughput

Y


)1(1

)(

}{)(

)(

0

)(

)(

aGyaG

a

yaGY

yaGY

eG

adyGeyY

ayGeyf

ayeyYPyF

Therefore aG

aG

eaG

Ge

IB

US

)21(

will be the throughput of a non-persistent CSMA system for given load G and given values of the average packet length and the propagation delay (assumed to be fixed)

Throughput of 1-Persistent CSMA

Similar derivation for 1-persistent CSMA gives the following result -

)1(

)21(

)1()1()21(

211

aGaG

aG

eaGeaG

eaG

GaGGG

S

Similar approach may also be followed to analyze a p-persistent CSMA but the approach and the resultant expression are much more complicated

0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

15

62

5

0.0

31

25

0.0

62

5

0.1

25

0.2

5

0.5 1 2 4 8

16

32

64

0.53

0.45

0.16

S

G

a 0.01

a =0.1

a = 1

Throughput of 1-Persistent CSMA

Better than Aloha & slotted Aloha for small a

Worse than Aloha for a > 1

propta

packet duration

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0

1…

0.0

4…

0.1

25

0.3

5…

1

2.8

2…

8

22

.6…

64

0.81

0.51

0.14

S

G

a = 0.01

Throughput of Non-Persistent CSMA

a = 0.1

a = 1

Higher maximum throughput than 1-persistent for small a

Worse than Aloha for a > 1

propta

packet duration

CSMA with Collision Detection (CSMA/CD)

Monitor for collisions & abort transmission Stations with frames to send, first do carrier sensing

After beginning transmissions, stations continue listening to the medium to detect collisions

If collisions detected, all stations involved stop transmission, reschedule random backoff times, and try again at scheduled times

In CSMA collisions result in wastage of the entire time spent transmitting a complete frame

CSMA-CD reduces this wastage to the time taken to detect a collision and abort the corresponding transmission

Binary Exponential Back off

(used in IEEE 802.3 and Ethernet)

• On detecting a collision, the transmitter aborts and sends a 48-bit jamming signal. It then enters the exponential back off phase where it waits for a random delay before attempting to retransmit.

• The random delay is chosen as K*512 bit times with K random

After the nth collision in a row for the same frame (1n16), choose K randomly from the set {0, 1, ......., 2m-1}, where m=min(n, 10)

Binary Exponential Back off

(used in IEEE 802.3 and Ethernet)

In effect, this means the following

• For the first ten (10) retransmission attempts, the mean value of the random delay is doubled

• Thereafter, the mean value of the random delay remains the same for six (6) additional attempts

What happens after that, i.e. after 16 retransmission attempts?

• After sixteen (16) retransmission attempts the station gives up the attempt to transmit the packet and reports an error

Collision Detection

Collision is detected by changes in the amplitude and pulse width of the signal (from expected values)

Potential problem as signal will attenuate on the line as it travels. If the colliding stations are too far apart, then the collision may not even be detectable

IEEE 802.3 standards avoid this by restricting the maximum length of the coaxial cable and the minimum frame size; minimum frame must be larger than twice the total propagation delay (including repeater delays) between the two farthest nodes of the network (see subsequent slides)

With a star-topology using a Hub, collision detection may be simplified by using logic at the hub. Activity at more than one input at the hub is declared to be a collision and a special collision presence signal is generated and sent out to the stations connected to the hubs

CSMA/CD Collision Detection

CSMA/CD Slot Time

Tp = propagation delay between S1 and S2 including the processing time at repeaters, switches etc. in between S1 and S2 = tp+tpr

S1 S2

Tp

Tp

CSMA/CD Slot Time

Slot time is the time it will take for S1 to find out about the collision between its packet and that of S2 Note that this depends only on the propagation (and processing delay) between S1 and S2 For the network, this will be the worst case time delay that any node on the network must wait before it can find out that its transmission has collided with that of another node.

S1 S2

Tp

Tp

CSMA/CD Slot Time

= 2Tp

CSMA/CD Slot Time

In CSMA/CD, the amount of wasted capacity is reduced to the time it takes to detect a collision

This makes the slot time, mentioned earlier, important from the viewpoint of system performance

In the worst case, it will be no greater than the twice the end-to-end propagation delay (including the applicable processing delays) in the network

An important rule in IEEE 802.3 also is that “frames should be long enough to allow collision detection before the end of transmission” …. otherwise, collisions may go undetected

CSMA-CD Model

Assumptions

Collisions can be detected and resolved in 2tprop

Time slotted in 2tprop slots during contention periods

Assume n busy stations, and each may transmit with probability p in each contention time slot

Once the contention period is over (a station successfully occupies the channel), it takes X seconds for a frame to be transmitted (Packet Duration=X)

It takes tprop before the next contention period starts.

Busy Contention Busy

Time

Idle Contention Busy

Contention Resolution

How long does it take to resolve contention? Contention is resolved (“success’) if exactly 1 station transmits in

a slot: 1)1( n

success pnpP

Psuccess maximized for p=1/n

ennnnP nn

success

1)

11()

11(

1 11max

On average, 1/Pmax = e = 2.718 time slots to resolve contention

secondsPeriod Contention Average 2 etprop

n stations, each transmitting with probability p

Non-Perisitent CSMA/CD Throughput

At maximum throughput, systems alternates between contention periods and frame transmission times

LRdeaeettX

X

propprop /121

1

121

1

2max

Time

Busy Contention Busy Contention Busy Contention Busy

where:

R bits/sec, L bits/frame, X=L/R seconds/frame

a = tprop/X normalized propagation time

meters/sec. speed of light in medium

d meters is “diameter” of system

2e +1 ≈ 6.44

max

1

1 6.44Rd

vL

Throughput for Random Access MACs

0

0.2

0.4

0.6

0.8

1

0.01 0.1 1

ALOHA

Slotted ALOHA

1-P CSMA

Non-P CSMA

CSMA/CD

a

max

For small a: CSMA-CD has best throughput For larger a: Aloha & slotted Aloha better throughput

ETHERNET (CSMA-CD Application)

First Ethernet LAN standard used CSMA-CD 1-persistent Carrier Sensing

R = 10 Mbps

tprop = 51.2 microseconds • 512 bits = 64 byte slot

• accommodates 2.5 km + 4 repeaters

Truncated Binary Exponential Backoff • After nth collision, select backoff from {0, 1,…, 2k – 1},

where k=min(n, 10)

Ethernet Topology

bus topology popular through mid 90s all nodes in same collision domain (can collide with each

other)

today: star topology prevails active switch in center each “spoke” runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

Bus (e.g. on coaxial cable)

Star (e.g. using point-to-point

twisted wire pairs)

LAN Technologies

LAN Technologies Ethernet

hubs, bridges, switches

MAC Addresses on LANs

LAN (or MAC or physical or Ethernet) address:

used to get datagram from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

This is different from the 32-bit IP address which is a

network-layer address

used to get datagram to destination IP network (recall IP network definition)

In effect, the IP Address ensures that the packet is delivered up to the network (LAN) which has the destination station. The MAC address is

then used to deliver the packet to the destination station

MAC Addresses on LANs

Each adapter on LAN has unique LAN address

MAC Address Features

MAC address allocation administered by IEEE

Manufacturer buys portion of MAC address space (to assure uniqueness)

Analogy:

(a) MAC address: like IC Numbers

(b) IP address: like postal address

MAC flat address => portability can move LAN card from one LAN to another

IP hierarchical address NOT portable depends on IP network to which node is attached

ARP: Address Resolution Protocol

Question: How do you determine MAC address of B if you know only B’s IP address?

Answer: You either know it (it is in your cache) or you ask B ....exactly what ARP does

Broadcast the query on the LAN, B hears it and replies to A telling A the

MAC address of B

Needed so that incoming frame can be delivered to B

How does one send packets to a node in another network?

Datagram from A to B via R, assuming A only knows B’s IP address

A

R B

Router R would know B’s MAC Address (or can use ARP to find this) and will use this to deliver datagram to B

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble:

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

used to synchronize receiver, sender clock rates

Ethernet Frame Structure (more)

Addresses: 6 bytes if adapter receives frame with matching destination

address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol

otherwise, adapter discards frame

Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC: checked at receiver, if error is detected, the frame is simply dropped

Ethernet: Unreliable, Connectionless operation

Connectionless: No handshaking between sending and receiving adapter

Unreliable: receiving adapter doesn’t send ACKs or NACKs to sending adapter

Ethernet uses CSMA/CD

adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection

Before attempting a retransmission, adapter waits a random time following Binary Exponential Backoff (mentioned earlier)

Ethernet CSMA/CD algorithm

1. Adaptor gets datagram from and creates frame

2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits

3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame !

4. If adapter detects another transmission while transmitting, aborts and sends jam signal

Jam Signal: To make sure all other transmitters are aware of collision; length=48 bits

5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times and returns to Step 2

Ethernet Technologies: 10 Mbps

Bus topology with multiple segments connected by repeaters.

Check out the web for the 5-4-3 rule for setting up a “thin” Ethernet network!

Manchester Encoding

Used in 10BaseT, 10Base2

Each bit has a transition

Allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized, global clock among nodes!


Table 16.2, “Data and Computer Communications” William Stalling

Fast Ethernet

100 Mbps rate is called “fast ethernet”

Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub

Hubs are essentially physical-layer repeaters: bits coming in one link go out all other links no frame buffering no CSMA/CD at hub: adapters detect collisions provides some net management functionality

hub

nodes Hubs may be interconnected through other hierarchically higher hubs


Table 16.3, “Data and Computer Communications” William Stalling

Evolution of the Ethernet Standards

10 Mbps

10Base5 10Base2 10Base-T 10Base-F

1 Gbps

802.3z 1000Base-X 802.3ab 1000Base-T

100Mbps

100Base-TX 100Base-FX 100Base-T4

Fast Ethernet

10 Gbps

802.3ae

Gigabit Ethernet

The Gigabit Ethernet Approach

As in the case of Fast Ethernet, Gigabit Ethernet retains the CSMA/CD Protocol and Ethernet Frame Format of 10/100 Mbps Ethernet while defining new medium and transmission specifications.

Fully compatible formats with 10/100 Mbps Ethernets allow easy internetworking and smooth migration

Initial deployment foreseen as backbones in switching hubs but gradually expected to move towards end-systems as bandwidth requirements of end applications grow

A Gigabit Ethernet Scenario

Gigabit Ethernet Links

Scaling up the speed of CSMA/CD

What is involved if one wants to scale up the speed of a network using Ethernet’s CSMA/CD as the MAC protocol?


Ethernet has a minimum frame size of 64 bytes

The reason for enforcing a minimum frame size is to ensure that collision detection can be done even between nodes that are the farthest from each other.

This must be such that it is longer than twice the maximum propagation delay between the two most distant nodes in the network (adding in all the hub and repeater delays in between). This time is also referred to as the slot time of the Ethernet system

Since the minimum frame size is fixed, this effectively limits the maximum propagation (and repeater/hub) delay that may be allowed in an Ethernet network.........and hence limits the cable lengths


In the original 10 Mbps Ethernet standards, this was ensured by having -

Minimum Frame Size = 64 bytes

Maximum Cable Length of 2.5 Km with a maximum of four repeaters on any path

Minimum frame size duration is comfortably larger than the slot time for the above!

(Incidentally, it may also be noted that Ethernet also prescribes a maximum frame size as one with 1500 bytes of data.)

Going from 10 Mbps to 100 Mbps

Minimum Frame Size still 64 bytes for compatibility

Maximum Cable Length has to be reduced

Maximum cable length from 100 Mbps hub to a node is restricted to 100 m

Note that the designers of 100 Mbps Fast Ethernet could have made the minimum frame size ten times longer but they chose not do that in order to keep the system completely compatible with 10 Mbps Ethernet!

Going from 100 Mbps to 1 Gbps

Not practical to just reduce the distance to 10 m and keep frame size the same .............. and imagine then what will happen for a speed of 10 Gbps!

In the interests of compatibility, one would not want to change the frame format either, i.e. use longer frames which are incompatible with earlier Ethernet specifications!

..........so what can be done to solve this problem?


Solution is to use Carrier Extension which keeps the same frame size (ensuring compatibility) but extend the carrier event for small frames so that collisions can be detected even with long cable lengths (see next slide)

Extension uses special extension symbols that cannot occur in the payload

Maximum Hub to Node Distance for 1 Gbps Ethernet

LA

N

MAN/WAN


Carrier Extension is needed to keep the maximum cable lengths reasonably large enough so that practical systems may be implemented

Unfortunately, padding a frame with extension symbols means lower operating efficiency, especially when a number of short frames have to be sent

Packet Bursting is proposed as an extension of Carrier Extension to mitigate this problem to some extent. It may be viewed as effectively being “carrier extension plus a burst of packets ”

Extension Symbols

Extension symbols on subsequent packets added as required to fill up the inter-packet gaps.


Packet Bursting: When a station has a number of packets to send, the first packet is padded to the slot time if necessary using carrier extension. Subsequent packets are transmitted back to back with until a Burst Timer (of 1500 bytes) expires

10 Gbps Ethernet

10-Gbps Ethernet (IEEE 802.3ae)

WA

N/M

AN

10-Gbps Ethernet (IEEE 802.3ae)

Full Duplex only technology

Fiber-only technology

Does not need the CSMA/CD protocol of slower speed Ethernet at all

No distance limitation since there is no collision

Retains the frame format of original Ethernet for complete inter-operability

10-Gbps Ethernet Usage

High-speed Metropolitan Area Network (MAN) interconnecting high-speed LANs for faster access

High-speed LANs as backbones for campus networks

Storage area networks interconnecting high-capacity storage devices, servers and access points

Applications in Wide Area Networks (WAN) for use by ISPs and service providers or as the network connecting geographically distributed, high-speed LANs belonging to the same organization

Wireless Networks

IEEE 802.11 WiFi

IEEE 802.16 WiMax

Ad-hoc (Mesh)

Networks

Infrastructure Networks

B D

C A

Ad Hoc Communications

Temporary association of group of stations

Within range of each other Need to exchange information Examples: Presentation in meeting, distributed computer game, war

or disaster scenarios

Nodes not only act as source and destination nodes but also function as intermediate nodes (routers) for others

A2 B2

B1

A1

AP1

AP2

Distribution System Server

Gateway to the Internet Portal

Portal

BSS A BSS B

Infrastructure Network

A transmits data frame

(a)

Data Frame Data Frame

A

B C

C transmits data frame & collides with A at B

(b)

C senses medium, station A is hidden from C

Data Frame

B

C A

Hidden Terminal Problem in WiFi

WiFi Networks need new MAC: CSMA-CA (CSMA with Collision Avoidance)

Collision Detection will not work in a wireless network

RTS

A requests to send

B

C

(a)

CTS CTS

A

B

C

B announces A ok to send

(b)

Data Frame

A sends

B

C remains quiet

(c)

CSMA-CA, CSMA with Collision Avoidance

Exposed Terminal Problem in WiFi (unnecessarily reduces throughput)

A wants to send to B, and C to D A is outside the radio range of D and C is outside the radio range of B, but D is within the radio range of B

A

B

C

D

D hears B’s CTS and will not send back a CTS to C. This is even though A can transmit to B and C can transmit to D simultaneously without causing any collision Loss of Throughput Capability

IEEE 802.11 (WiFi) Definitions

Basic Service Set (BSS) Group of stations that coordinate their access using a given

instance of MAC Located in a Basic Service Area (BSA) Stations in BSS can communicate with each other Distinct collocated BSS’s can coexist

Extended Service Set (ESS) Multiple BSSs interconnected by Distribution System (DS) Each BSS is like a cell and stations in BSS communicate with

an Access Point (AP) Portals attached to DS provide access to Internet

A2 B2

B1 A1

AP1

AP2

Distribution System Server Gateway to

the Internet Portal Portal

BSS A BSS B

Infrastructure Network

Distribution Services

Stations within BSS can communicate directly with each other

DS provides distribution services:

Transfer MAC SDUs between APs in ESS

Transfer MSDUs between portals & BSSs in ESS

Transfer MSDUs between stations in same BSS

Multicast, broadcast, or stations’s preference

ESS looks like single BSS to LLC layer

Infrastructure Services

Select AP and establish association with AP

Can then send/receive frames via AP & DS

Reassociation service to move from one AP to another AP

Dissociation service to terminate association

Authentication service to establish identity of other stations

Privacy service to keep contents secret

IEEE 802.11 MAC

MAC sublayer responsibilities

Channel access

PDU addressing, formatting, error checking

Fragmentation & reassembly of MAC SDUs

MAC security service options

Authentication & privacy

MAC management services

Roaming within ESS

Power management

MAC Services

Contention Service: Best effort Contention-Free Service: time-bounded transfer MAC can alternate between Contention Periods (CPs) & Contention-Free

Periods (CFPs)

Physical

Distribution coordination function (CSMA-CA)

Point coordination function

Contention-free service

Contention service

MAC

MSDUs MSDUs

Distributed Coordination Function (DCF)

DCF provides basic access service

Asynchronous best-effort data transfer

All stations contend for access to medium

CSMA-CA

Ready stations wait for completion of transmission

All stations must wait Interframe Space (IFS)

DIFS

DIFS

PIFS

SIFS

Contention window

Next frame

Defer access Wait for reattempt time

Time

Busy medium

Priorities through Interframe Spacing

High-Priority frames wait Short IFS (SIFS)

Typically to complete exchange in progress

ACKs, CTS, data frames of segmented MSDU, etc.

PCF IFS (PIFS) to initiate Contention-Free Periods

DCF IFS (DIFS) to transmit data & MPDUs

DIFS

DIFS

PIFS

SIFS

Contention window

Next frame

Defer access Wait for reattempt time

Time

Busy medium

Contention & Backoff Behavior

If channel is still idle after DIFS period, ready station can transmit an initial MPDU

If channel becomes busy before DIFS, then station must schedule backoff time for reattempt

Backoff period is integer # of idle contention time slots

Waiting station monitors medium & decrements backoff timer each time an idle contention slot transpires

Station can contend when backoff timer expires

A station that completes a frame transmission is not allowed to transmit immediately

Must first perform a backoff procedure

RTS

CTS CTS

Data Frame

A requests to send

B

C

A

A sends

B

B

C

C remains quiet

B announces A ok to send

(a)

(b)

(c)

ACK B (d)

ACK

B sends ACK

Carrier Sensing in 802.11

Physical Carrier Sensing Analyze all detected frames Monitor relative signal strength from other sources

Virtual Carrier Sensing at MAC sublayer

Source stations informs other stations of transmission time (in msec) for an MPDU

Carried in Duration field of RTS & CTS Stations adjust Network Allocation Vector to indicate when

channel will become idle

Channel busy if either sensing is busy

Data

DIFS

SIFS

Defer Access Wait for Reattempt Time

ACK

DIFS

NAV

Source

Destination

Other Stations

Transmission of MPDU without RTS/CTS

Data

SIFS

Defer access

Ack

DIFS

NAV (RTS)

Source

Destination

Other Nodes

RTS

DIFS

SIFS CTS

SIFS

NAV (CTS)

NAV (Data)

Transmission of MPDU with RTS/CTS

Collisions, Losses & Errors

Collision Avoidance When station senses channel busy, it waits until channel becomes

idle for DIFS period & then begins random backoff time (in units of idle slots)

Station transmits frame when backoff timer expires If collision occurs, recompute backoff over interval that is twice as

long

Receiving stations of error-free frames send ACK Sending station interprets non-arrival of ACK as loss Executes backoff and then retransmits Receiving stations use sequence numbers to identify duplicate

frames

Point Coordination Function

PCF provides connection-oriented, contention-free service through polling

Point coordinator (PC) in AP performs PCF

Polling table up to implementor

CFP repetition interval Determines frequency with which CFP occurs Initiated by beacon frame transmitted by PC in AP Contains CFP and CP During CFP stations may only transmit to respond to a poll from PC

or to send ACK

CF

End

NAV

PIFS

B D1 +

Poll

SIFS

U 1 +

ACK

D2+Ack+

Poll

SIFS SIFS

U 2 +

ACK

SIFS SIFS

Contention-free repetition interval

Contention period

CF_Max_duration

Reset NAV

D1, D2 = frame sent by point coordinator U1, U2 = frame sent by polled station TBTT = target beacon transmission time B = beacon frame

TBTT

PCF Frame Transfer

IEEE 802.11 MAC Layer

MAC Layer Acknowledgement for the transmitted fragment (Multicast packets are not acknowledged in this way)

If the fragment being acknowledged did not suffer from a collision, then its ACK will also not have to undergo a collision. This is ensured by the NAV values of the RTS and CTS frames and by the fact that the DIFS interval is longer than the SIFS interval Each fragment of a multi-fragment packet PDU is separately acknowledged

IEEE 802.11 Fragmentation & Reassembly

LAN protocols, like Ethernet, use large packets (LANPDUs), e.g. Ethernet packets may have upto 1518 bytes of data In a Wireless LAN, smaller packet sizes may be preferred (higher probability of packet error with large packets, smaller packets incur smaller overheads due to packet loss) Fragmentation may therefore happen earlier in an IEEE 802.11 LAN than in an IEEE 802.3 LAN


802.11 MAC receives a MAC Service Data Unit (MSDU) of length up to 2304 bytes from its higher layer and can optionally divide each MSDU into several smaller MAC Protocol Data Units (MPDU). This is the Fragmentation process. (Each MPDU has its own header and CRC.) Fragments are sent to the destination within the same RTS/CTS exchange using a Stop-&-Wait protocol. The destination node does Reassembly of the fragments to get the original MSDU and pass it on to its higher layer.

IC0101, LAN Infrastructure 97


MSDU

H CRC H CRC

Fragment 1 Fragment 2

ACK

S

I

F

S

S

I

F

S

Timing Between Fragments of a MSDU

IEEE 802.11: Procedure for a Station to Join an Existing Cell (BSS)

To join a cell (after power-up, sleep or entering a new cell), the station needs to get synchronization information from the AP of that cell. This can be done in one of the two following ways -

Passive Scanning: Station waits to receive a Beacon Frame from the AP. This is sent out periodically with the synchronization information

Active Scanning: Station tries to locate a reachable AP by sending a Probe Request Frame and waits for the Probe Response Frame from the AP for synchronization.

Both methods are valid and any one of them may be chosen. The choice is decided by considerations like power consumption and/or performance.

IEEE 802.11: Authentication Process

Once a station locates an AP and decides to join its BSS, it has to go through an Authentication Process to identify itself as a valid user of the Wireless LAN facility and also verify the identity of the AP.

This is done by the station and AP exchanging information with appropriate passwords.

IEEE 802.11: Association Process

This is started after the Authentication Process has been successfully completed

This involves exchange of information about the station and the BSS and their respective capabilities This allows the overall network (i.e. the set of APs) to know about the current location of the station and the AP it is currently associated with. A station can actually transmit/receive data frames only after the Association Process is over

IEEE 802.11: Roaming

This is the process of a station moving from one BSS to another without losing the connection. The transition from one BSS to another is performed between packet transmissions

Temporary disconnection during roaming will have a bad effect on overall performance as it would require retransmissions to be done by the higher layers

IEEE 802.11: Roaming

The 802.11 protocol does not define how roaming is to be done but does define the basic tools required such as active/passive scanning and a re-association process to change a station from the AP of one BSS to another.

IEEE 802.11: Security Issues

Intruders should not be able to get unauthorized access to the resources of the Wireless LAN

The Authentication Process is expected to prevent this from happening Potential eavesdroppers should not be able to capture and interpret the traffic on the Wireless LAN Prevented by 802.11’s WEP algorithm (Wired Equivalent Privacy) which encrypts the transmissions on the wireless medium. WEP is a simple algorithm based on RSA which is reasonably strong.

IC0101, LAN Infrastructure 104

IEEE 802.11: Power Saving Issues

Power saving is important at the mobile stations as battery power is a scarce resource 802.11 standards directly address this issue and provide the mechanism for stations to go into sleep mode without losing data The AP keeps a continually updated record of the stations currently in Power Saving mode. Data intended for these stations is buffered at the AP until either the station sends a polling request for it or it changes its operational mode. Apart from synchronization information, Beacon Frames from the AP also send information about which Power Saving Stations have frames buffered at the AP. The indicated stations can then download their data as per their convenience.

Frame Types

Management frames

Station association & disassociation with AP

Timing & synchronization

Authentication & deauthentication

Control frames

Handshaking

ACKs during data transfer

Data frames

Data transfer

Address

2

Frame

Control

Duration/

ID

Address

1

Address

3

Sequence

control

Address

4

Frame

body CRC

2 2 6 6 6 2 6 0-2312 4

MAC header (bytes)

Frame Structure

MAC Header: 30 bytes

Frame Body: 0-2312 bytes

CRC: CCITT-32, 4 bytes over CRC over MAC header & frame body

Address

2

Frame

Control

Duration/

ID

Address

1

Address

3

Sequence

control

Address

4

Frame

body CRC

Protocol

version Type Subtype

To

DS

From

DS

More

frag Retry

Pwr

mgt

More

data WEP Rsvd

2 2 6 6 6 2 6 0-2312 4

2 2

MAC header (bytes)

4 1 1 1 1 1 1 1 1

Frame Control (1)

Protocol version = 0 Type: Management (00), Control (01), Data (10) Subtype within frame type Type=00, subtype=association; Type=01, subtype=ACK MoreFrag=1 if another fragment of MSDU to follow

To

DS

From

DS

Address

1

Address

2

Address

3

Address

4

0 0 Destination

address

Source

address BSSID N/A

0 1 Destination

address BSSID

Source

address N/A

1 0 BSSID Source

address

Destination

address N/A

1 1 Receiver

address

Transmitter

address

Destination

address

Source

address

Meaning

Data frame from station to

station within a BSS

Data frame exiting the DS

Data frame destined for the

DS

WDS frame being distributed

from AP to AP

Address

2

Frame

Control

Duration/

ID

Address

1

Address

3

Sequence

control

Address

4

Frame

body CRC

Protocol


To

DS

From

DS

More

frag Retry

Pwr

mgt

More

data WEP Rsvd

2 2 6 6 6 2 6 0-2312 4

2 2 4 1 1 1 1 1 1 1 1

To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS

Frame Control (2)

Address

2

Frame

Control

Duration/

ID

Address

1

Address

3

Sequence

control

Address

4

Frame

body CRC

Protocol


To

DS

From

DS

More

frag Retry

Pwr

mgt

More

data WEP Rsvd

2 2 6 6 6 2 6 0-2312 4

2 2

MAC header (bytes)

4 1 1 1 1 1 1 1 1

Frame Control (3)

Retry=1 if mgmt/control frame is a retransmission Power Management used to put station in/out of sleep mode More Data =1 to tell station in power-save mode more data

buffered for it at AP WEP=1 if frame body encrypted

Physic

al

layer

LLC

Physical layer

convergence

procedure

Physical medium

dependent

MAC

layer

PLCP

preamble

LLC PDU

MAC SDU MAC

header CRC

PLCP

header PLCP PDU

Physical Layers

802.11 designed to Support LLC Operate over many physical layers

IEEE 802.11 Physical Layer Options

Frequency Band

Bit Rate Modulation Scheme

802.11 2.4 GHz 1-2 Mbps Frequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum

802.11b 2.4 GHz 11 Mbps Complementary Code Keying & QPSK

802.11g 2.4 GHz 54 Mbps Orthogonal Frequency Division Multiplexing

& CCK for backward compatibility with 802.11b

802.11a 5-6 GHz 54 Mbps Orthogonal Frequency Division Multiplexing

The LLC and MAC Sublayer Structure

Data link

layer

802.3

CSMA-CD

802.5

Token Ring

802.2 Logical link control

Physical layer

MAC

LLC

802.11

Wireless

LAN

Network layer Network layer

Physical

layer

OSI IEEE 802

Various physical layers

Other

LANs

Logical Link Control Layer

PHY

MAC

PHY

MAC

PHY

MAC

Unreliable Datagram Service

PHY

MAC

PHY

MAC

PHY

MAC

Reliable frame service

LLC LLC LLC

A C

A C

IEEE 802.2: LLC enhances service provided by MAC

LLC PDU Structure

1 Source

SAP Address Information

1 byte

Control

1 or 2 bytes

Destination SAP Address Source SAP Address

I/G

7 bits 1

C/R

7 bits 1

I/G = Individual or group address C/R = Command or response frame

Destination SAP Address

1 byte

Examples of SAP Addresses: 06 IP packet E0 Novell IPX FE OSI packet AA SubNetwork Access protocol (SNAP)

Encapsulation of MAC frames

IP

LLC Header

Data

MAC Header

FCS

LLC PDU

IP Packet

Interconnecting LAN Segments

Hubs

Bridges

Switches Remark: switches are essentially multi-port

bridges.

What we say about bridges also holds for switches!

Interconnecting with Hubs

Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one

large collision domain if a node in CS and a node EE transmit at same time: collision

Can’t interconnect 10BaseT & 100BaseT

Bridges

Link layer device

stores and forwards Ethernet frames

examines frame header and selectively forwards frame based on MAC destination address

when frame is to be forwarded on segment, uses CSMA/CD to access segment

transparent

hosts are unaware of presence of bridges

plug-and-play, self-learning

bridges do not need to be configured

Tra

nspa

rent

Sel

f Lea

rnin

g

Bridges offer Traffic Isolation

Bridge installation breaks LAN into LAN segments

Bridges filter packets: same-LAN-segment frames not usually

forwarded onto other LAN segments

segments become separate collision domains

bridge collision domain

collision domain

= hub

= host

LAN (IP network)

LAN segment LAN segment

Forwarding Packets through Bridges

How to determine the LAN segment to which to forward frame?

Use Forwarding Data Base (FDB) with Self Learning

Bridge

Bridge

Bridge

Bridge

Segment/LAN

Segment/LAN

Segment/LAN

Segment/LAN

Segment/LAN

Self Learning

A bridge has a bridge table

entry in bridge table:

(Node LAN Address, Bridge Interface, Time Stamp)

stale entries in table dropped (TTL can be 60 min)

bridges learn which hosts can be reached through which interfaces

when frame received, bridge “learns” location of sender: incoming LAN segment

records sender/location pair in bridge table

Forwarding Data Base (FDB)

7 8 9

Port 1 Port 2 Port N

Segment 1

Segment 2

Segment N

3

4 5 6

1 2

Forwarding Database

FDB

Station # Port #

1 1 2 1 3 1 4 2 5 2 6 2 7 N 8 N 9 N

Forwarding Database

(Time stamp not shown)

Bridge FDBs in Networks with Multiple Bridges

Bridge Port Port

Bridge Port Port

1 1 1

2 2 2

1 2 3 4 5

Station # Port # Station # Port #

1 2 3 4 5

1 2 3 4 5

1 1 2 2 2

1 1 1 2 2

FDB FDB

Filtering/Forwarding

When a bridge receives a frame:

Examine FDB entry for the frame’s destination address

if entry found for destination then{

if destination on segment from which frame arrived then drop the frame

else forward the frame on interface indicated

}

else flood

forward on all interfaces other than the one on which the frame arrived

Spanning Tree Algorithm (STA)

A Spanning Tree is a path list of one and only one path between all the nodes in an extended LAN (i.e. multiple LANs connected by bridges)

If the network is represented by a graph, then the spanning tree maintains the connectivity of all the nodes in the graph but removes all possible loops

STA is needed because logical loops can lead to packets circulating endlessly in the network

Following the STA, the network automatically disables certain bridges/ports. Note that these bridges are not removed since they may be needed if the topology changes and the spanning tree is reconfigured dynamically.

Spanning Tree Algorithm (STA)

To implement the STA in a network -

each bridge must have a unique Bridge ID

each port in the bridge must have a unique Port ID

all the bridges on the LAN must recognize a unique MAC group address

B1

P1 P2 P3

B1: Bridge ID

P1, P2, P3: Port IDs Example

Sample Topology and Spanning Tree

B1

P1

P2

Segment 1

Segment

2

Segment

3

Segment 4

B2

P1

P2

B4

P1

P2

B5

P1

P2

B3

P1

P2

P3

A Sample Topology of a Bridged LAN

All LANs assumed to have equal cost


B1

P1

P2

Segment 1

Segment

2

Segment

3

Segment 4

B2

P1

P2

B4

P1

P2

B5

P1

P2

B3

P1

P2

P3

B1 Root Bridge

R Root Port

D Designated Port/Bridge

R

R

R

R D

D

D D



B1

P1

P2

Segment 1

Segment

2

Segment

3

Segment 4

B3

P1

P2

P3

R

D

D

D D

Even though bridges B2, B4 and B5 have not been shown, they are still present and may be used if the topology changes or if bridges or links fail.


Spanning Tree Algorithm

Bridges participating in the Spanning Tree Algorithm do the following together -

Select a root bridge in the LAN - this is selected as the bridge with the lowest bridge ID. Bridge priorities ignored for simplicity

Determine the root port for each bridge except the root bridge. The root port is the port with the least-cost path to the root bridge. If there is a tie, the root port is chosen as the port with the lowest port ID.

Select a designated bridge for each LAN. This is the bridge that offers the least-cost path from the LAN to the root bridge. If there is a tie, the designated bridge is chosen as the one with the lowest bridge ID. The port connecting the LAN to its designated bridge is called a designated port. Note that all the ports of the root bridge must be chosen as designated ports and that the root port of a bridge cannot be chosen as a designated port.

For the given root bridge, the resulting network would be a minimum spanning tree.

Bridges vs. Routers

both store-and-forward devices routers: network layer devices (examine network layer

headers)

bridges are link layer devices

routers maintain routing tables, implement routing algorithms

bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

Routers vs. Bridges

Bridges + and -

+ Bridge operation is simpler requiring less packet processing

+ Bridge tables are self learning

- All traffic confined to spanning tree, even when alternative bandwidth is available

- Bridges do not offer protection from broadcast storms

Routers vs. Bridges

Routers + and -

+ arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)

+ provide protection against broadcast storms

- require IP address configuration (not plug and play)

- require higher packet processing

bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

Ethernet Switches

Essentially a multi-interface bridge

layer 2 (frame) forwarding, filtering using LAN addresses

Switching: A-to-A’ and B-to-B’ simultaneously, no collisions

large number of interfaces

often: individual hosts, star-connected into switch

Ethernet, but no collisions!

Ethernet Switches

cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame slight reduction in latency

combinations of shared/dedicated, 10/100/1000 Mbps interfaces

Summary Comparison

hubs bridges routers switches

traffic

isolation

no yes yes yes

plug & play yes yes no yes

optimal

routing

no no yes no

cut

through

yes no no yes

Physical

partition

Logical partition

Bridge

or

switch

VLAN 1 VLAN 2 VLAN 3

S1 7

2 3 4 5 6 1

8

9 Floor n – 1

Floor n

Floor n + 1

S2

S3

S4

S5

S6

S7

S8

S9

Virtual LAN (VLAN)

Logical partition

Bridge

or

switch

VLAN 1 VLAN 2 VLAN 3

S1 7

2 3 4 5 6 1

8

9 Floor n – 1

Floor n

Floor n + 1

S2

S3

S4

S5

S6

S7

S8

S9

Per-Port VLANs

Bridge only forwards frames to outgoing ports associated with same VLAN

Tagged VLANs

More flexible than Port-based VLANs

Insert VLAN tag after source MAC address in each frame VLAN protocol ID + tag

VLAN-aware bridge forwards frames to outgoing ports

according to VLAN ID

VLAN ID can be associated with a port statically through configuration or dynamically through bridge learning

IEEE 802.1q

Lecture set 4

Documents

r sp pure aloha channel

slotted aloha aloha

slotted aloha time

csmaca pure aloha

random time

access max throughput

length b time

frame transmission