10-01-K.Steenhaut & J.Tiberghien - VUB 1 Telecommunications Concepts Chapter 3.1 Packet Switched Local Area Networks

10-01-K.Steenhaut & J.Tiberghien - VUB1

TelecommunicationsConcepts

Chapter 3.1

Packet Switched

Local Area Networks


Contents• Original Local Area Networks

– Ethernet– Token Ring, Token bus, Cambridge ring, Liu ring, …

• The IEEE 802 standards– IEEE 802.3, IEEE 802.4, IEEE 802.5, ...– IEEE 802.2 : Logical Link Control– IEEE 802.1 : Learning bridges

• High Performance LAN’s and MAN's– High performance Ethernet– Metropolitan Networks– Wireless LAN's

• Side Tracks– Interconnection of data communication systems.






• Side Tracks– Interconnection of data communication systems.


Shared Media Networks

Need for Medium Access Control Protocols

Commonly used in Local Area Networks

The network itself has practically no storage capacity


The 7 OSI layers

Applications Layer

Transport Layer

Networks Layer

Co

nn

ecti

vity

Inte

rop

erab

ilit

y

Internet Sublayer

1 : Physical

2 : Data Link

3 : Network

4 : Transport

5 : Session

6 : Presentation7 : Application


The 3 lower OSI layers

For Shared Medium Networks

Network

Internet

3

Physical

Medium Access Control2

1

Data Link Control





• High Performance LAN’s and MAN's– High performance Ethernet– Metropolitan Networks – Wireless LAN's

• Side Track– Interconnection of data communication systems.


Ethernet

• Developed in the seventies at Xerox Parc– Invented by Lamport

– Commercially supported by Digital and Intel

– Still has a dominant market position

• Originally based on coaxial bus – Passive broadcast medium

– Length limited by electrical properties of cable

» Thick coaxial : 500 m

» Thin coaxial : 185 m

– Repeaters ( = electronic amplifiers ) allow to

» extend cable length

» replace coaxial bus by twisted pairs star

• Medium access control inspired by Aloha


Aloha(N.Abramson, F.Kuo,

1970)

Communication between terminals and mainframevia geostationary satelliteh = 36 000 KmPropagation delay = 240 mS

REPEAT Transmit data block B; Receive same block Br; ok := B = Br; IF NOT ok THEN Wait Random Delay ENDUNTIL ok


ALOHA Performance

t : Transmission time for one data block.

G : Total proposed traffic ( # of blocks in time t )

S : Throughput ( # of successful blocks in time t )

0 <= S <= 1

p : Probability that a block will avoid collision

S = G . p


ALOHA Collision Window

Station A

Station B

2t

Blocks transmitted according to Poisson Distribution

p = e - 2G

S = G . e - 2G


ALOHA Throughput

0

0.1

0.2

0 0.5 1 1.5

0.184

)S( G

G


CSMA/CD operation

• Carrier Sense– Listen before sending

• Multiple Access–Everybody noticing silence can start sending

• Collision Detect–Stop sending when noticing that others sending

–Collision fragments characterized by insufficient length and therefore discarded by MAC entity.

Graceful degradation due to average repetition delay proportional to number of retransmissions


Carrier Sense

A

A

A

B

B

B

Listen before talking(carrier sense)

Send data

Listen and wait

No carrier sensed,transmission starts


Collision Enforcement

A

A

B

B

Collision

Collided signals propagateB detects collision and transmits jam

A B

Collided signals reach AStation A should detect the collision BEFORE it has send its last databit


Minimum packet size

A B

A BNo carrier sensed,transmission starts

Min. packet time > max. round trip delay

Collided signals reach AStation A should detect the collision BEFORE it has send its last databit


1 2 4 8 16 320 64 128 256

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

1,0

0,9

Size of frames (bytes)

1024

512

256

128

64

Ch

an

nele

ffic

ien

cy

Number of stations trying to send

Efficiency of channel


Ethernet Data Frame

Preamble (7 bytes)Start Frame (1 byte)

Destination Address (6 bytes)

Source Address (6 bytes)

Length payload (2 bytes)

Payload (evt. + Padding)

Frame Check Sequence (4 bytes)

Min

: 6

4 b

ytes

Max

: 1

518

byt

es


Thick Ethernet

Thick cableTotal Length

<= 500 m


Ethernets with repeaters

RepeatersDistance <= 3000 m

Half RepeatersOptical Link


Thin Ethernets

Thin cableTotal Length

<= 185 m


Thin Ethernets

Repeaters

Thin cableDistance <= 3000 m

Segment Length<= 185 m


Repeaters for thin Ethernet


Ethernet over twisted pairs

One or two pairs per direction


Twisted pair Ethernet(10 Mb/s)

Repeaters (= hubs)

Twisted pairsDistance <= 3000 m

Segment Length<= 100 m

Class 3 utp


Cascade of Ethernet hubs

A

B

HUB = Set of repeaters > all frames broadcasted

B

B

B

B

B B B


High-performance Ethernet(100 Mb/s)

Repeaters

Twisted pairsDistance <= 400mSegment Length

<= 100 m

4 twisted pairs (class 3)2 twisted pairs (class 5)


Ethernet in office buildings









– Ethernet– Token Ring





IEEE 802 Standards

in the OSI model

802

.7B

roa

db

and

802

.3~

Eth

ern

et

802

.4~

To

ke

n B

us

802

.5~

To

ke

n R

ing

802

.6M

etr

op

olit

an

802

.??

??

IEEE 802.2Logical Link Control

IEEE 802.1Management, interconnection , Upper layers interface

2

1

MAC

LLC

Physical

3














IEEE 802.2Logical Link

Control• Purpose :– To fit LAN’s into the general OSI framework

– Correct transmission errors

• Services offered : – Data link multiplexing by means of LLC addresses

– Data link error correction and flow control

» Class 1 : Connectionless, unacknowledged service

» Class 2 :

• Connection oriented reliable link control

• Sliding window protocol

• Window size = 128 frames.

» Class 3 : Connectionless, acknowledged.


LLC Frame Format

• Flag : Unique bit pattern (01111110)• Address (2*16 bit) : allows to multiplex a single

MAC address for different data links.• Control (16 bit) : Controls the sliding windows• Payload (variable length) : Data from layer 3• CRC (16 bit) : Redundant bits obtained by dividing

the address, control and payload fields by a predefined polynomial.

Flag CRCAddress Control FlagPayload


LLC bit stuffing

• The bit pattern 01111110 should never occur in the address, control, payload and CRC fields.

• After 5 consecutive 1’s a 0 is inserted by the TX

• A 0 preceded by 5 1’s is removed by the RX.

0100110111110101101111111110111111000110000

0100110111110010110111110111101111101000110000

Message to be transmitted :

Message effectively transmitted :


LLC Frame Types

Unnumberedused to set up connections

used to transfer data in connectionless mode

Supervisoryused for managing the sliding window

in connection oriented mode

Informationused to transfer data in connection oriented

mode








Interconnection of LAN’s

• Layer 7 : Application level Gateways

• Layer 3 : ROUTERS– Independent networks (lan or wan) interconnected by means

of an internet protocol.

• Layer 2 : BRIDGES– Independent MAC protocols on interconnected lan’s.

– Distance restrictions apply to each lan individually

– Half bridges can be interconnected by any lan or wan.

– Traffic between lan’s can be filtered according to MAC level addresses.

• Layer 1 : REPEATERS– No influence on MAC protocol

– Same traffic in all interconnected lan’s


Repeaters

Layer 1

Layer 2a

Layer 2b

Layer 3b

Layer 3a

Layer 1Layer 1

Repeater

Layer 2a

Layer 2b

Layer 3b

Layer 3a

Layer 1

Internet

Data Link

Medium Access Control

Network


Practical Ethernets

RepeatersDistance <= 3000 m

Half RepeatersOptical Link













Application Level GatewaysApplication gateway

Layer 2

Layer 3

Layer 1

Layer 5

Layer 4

Layer 7

Layer 6

Layer 2

Layer 3

Layer 1

Layer 5

Layer 4

Layer 7

Layer 6

Layer 2

Layer 3

Layer 1

Layer 5

Layer 4

Layer 7

Layer 6

Layer 2

Layer 3

Layer 1

Layer 5

Layer 4

Layer 7

Layer 6


Gateway exampleX400 - SMTP mail gateway

X400 users SMTP usersmailgateway


Firewalls

An application gateway between the Internet and an intranet is a fairly secure firewall.

IntranetInternetFirewall













Routers

Layer 1

Layer 2a

Layer 2b

Layer 3b

Layer 3a

Layer 1Layer 1

Layer 2a

Layer 2b

Layer 3b

Layer 3a

Layer 1

Layer 2aLayer 2a

Layer 3b

Layer 2b

Layer 3a

Layer 3b

Layer 2b

Layer 3a

Router













Bridges

Layer 1

Layer 2a

Layer 2b

Layer 3b

Layer 3a

Layer 1Layer 1

BRIDGE

Layer 2a

Layer 2b

Layer 3b

Layer 3a

Layer 1

Internet

Data Link

Layer 2aLayer 2a

Network


Bridges

Bridge


Bridges

Bridge

WiFi


Bridges and multiple Network Protocols

BRIDGE

IP

IPX








Filtering Bridges

Lan 1 Lan 2

RX

RX

Filter

FilterBuffer

Buffer

TX

TX

Addresses Database

123536234

831556246456


Learning Bridges

• Frames with unknown destination are always forwarded (promiscuous mode)

• Source addresses of all frames are monitored and added to the database, with a time stamp.

• Frames with a destination address known to be on the same side as their source are not forwarded.

• Addresses which have not been confirmed since some time are removed from the database.

• The learning algorithm requires that there are no loops through the interconnected lan’s.


Spanning Tree Algorithm

Root

(Lowest serial Nr)

Disabled


Half Bridges

Half Bridges

Any Data Link

Ethernet frames are tunneled through the network interconnecting the half bridges


Backbones

Half Bridges

FDDIMANATMISDNF.R.X25


Level 2a tunneling

via level 2a intermediate network

2a

2b

4

3

1

2a

1

2a

1

2a

2b

4

3

1

2a

1

2a

1

Specific labels can be added

to the frames to identify data flow


Level 2a tunneling

via level 2b intermediate network

2a

2b

4

3

1

2a

1

2a

2b

1

2a

2b

4

3

1

2a

1

2a

2b

1


Level 2a tunneling

via level 3 intermediate network

2a

2b

4

3

1

2a

1

2a

2b

3

1

2a

2b

4

3

1

2a

1

2a

2b

3

1


Virtual LAN’s








High performance Ethernet

less collisions

Data rate

10 MB/s

100 MB/s

1 GB/s

Single segment (broadcast)

One station per segment = no collisions (switching)

Throughput


Switched LAN’s








Cable TV Networks

Head-endTrunk cables

Distribution cables


Cable TV Networks

frequency

VTM RTBF VRT RTLTF1

7-8 MHz


Data over Cable TV

frequency

VTM RTBF BRTN RTLReturn

7-8 MHz

Data

Cablemodem

Cablemodem


Gigabit Ethernet as MAN

Head-end Distribution cables(optical fibers)

Backbone (WDM optical fibers)








IEEE 802.1Management, interconnection, upper layers interface

802

.3~

Eth

ern

et

802

.4~

To

ke

n B

us

802

.5~

To

ke

n R

ing

802

.11

Wir

ele

ss

IEEE 802.2Logical Link Control

2

1

MAC

LLC

Physical

3

IEEE 802 Standards

in the OSI model


IEEE 802.11

Fre

qu

en

cy

ho

pp

ing

s

pre

ad

sp

ec

tru

m

??

?

Infr

ared

IEEE 802.11Medium Access Control

Dir

ec

t S

eq

uen

ce

sp

rea

d s

pe

ctr

um

1 or 2 Mb/s(provisions for migrationto higher speeds)

One common MAC layer

Provisions for a variety

of physical layers


802.11 Hidden Station

A

B

C

RTS

CTSCTS

Documents

10-01-K.Steenhaut & J.Tiberghien - VUB 1 Telecommunications Concepts Chapter 3.1 Packet Switched Local Area Networks