Local area networks
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Local Area Networks. hub. hub. hub. hub. LAN characteristics. confined within geographical area relatively high data rate under single management. stations. stations. LAN Ingredients. Topology bus, star, ring Transmission medium twisted pair, coaxial, fiber optics, wireless

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Local area networks

Local Area Networks


Lan characteristics

hub

hub

hub

hub

LAN characteristics

  • confined within geographical area

  • relatively high data rate

  • under single management

stations

stations

LAN Ingredients...

  • Topology

    • bus, star, ring

  • Transmission medium

    • twisted pair, coaxial, fiber optics, wireless

  • Medium access techniques

    • protocol to coordinate the sharing of media

station

router

server


Topology

Topology

  • Topology defines how nodes/stations are connected

  • LAN typical topology :

    • bus/tree : all nodes connected to a common medium

    • star : all nodes are joined to a central nodes

    • ring : nodes form a ring by point-to-point links to adjacent neighbors

ring

star

bus


Topology bus

Topology : Bus

C

D

B

A

D

B and C

D

Application

Application

Presentation

Presentation

Session

Session

Transport

Transport

Network

Network

Data Link

Data Link

Physical

Physical


Topology ring

A

A

A

A

Topology : Ring

A

A

  • nodes function as a repeater

  • only destination copies frame to it, all other nodes have to discarded the frame

  • Unidirectional link

C

B

C

B

B transmits frame

addressed to A

C ignores frame

A

A

C

B

C

B

A copies frame

C absorbs

returning frame


Topology star

A

A

A

A

Topology : Star

  • Each station attaches to the central node

  • Two point-to-point links for tranmitting and receiving

  • Two alternatives operation

    • broadcasting

    • switching

shared hub broadcast

packet to every ports

but only destination

copies frame

A

C

B

A

B

C

switch hub retransmitts

packet to the destination

only

C transmitts frame addressed to A

A

B

C


Transmission media

Transmission media

  • Twisted pair

    • Cat 3 -- voice grade UTP, data rate up to 10Mbps

    • Cat 5 -- data grade UTP, data rate 100 Mbps

  • Coaxial cable

    • baseband coaxial support data rates 20 Mbps at distance of up to 2 km

  • Fiber optics

    • support gigabit data rate and longer distances


Medium access control

Medium Access Control

  • Protocol for controlling access to transmission medium

  • Defined as part of Data Link layer

  • The protocol performs:

    • perform functions related to medium access (MAC sublayer)

    • concerned with the transmission of a link-level between two nodes (LLC sublayer)

Network

LLC

Logical Link Control sublayer

Data Link

MAC

Medium Access Control sublayer

Physical


Llc sublayer functions

LLC Sublayer Functions

  • Provides a compatibility interface, irrespective of the MAC layer used.

  • Service Access Points (SAPs) interface sublayers to upper-layer functions

  • Optional connection, flow control, and sequencing services


Mac frame format

Destination Address

Source

Address

Type

LLC PDU

CRC

MAC control

DSAP

SSAP

CTRL

Data

MAC frame format

MAC Frame

(802.3)

SAPs define a protocol

for upper layer

LLC PDU

DSAP: Destination Service Access Point

SSAP: Source Service Access Point

Control: Command and response functions

Data: LLC SDU


Mac address

24 bits

24 bits

Vendor Code

Serial Number

MAC Address

0000.0c12. 3456

ROM

RAM

  • MAC address is burned into ROM on a network interface card


Mac strategies

MAC Strategies

  • Where?

    • centralized : a designated station has an authurity to grant access to the network

    • distributed : stations can dynamically determined transmission order

  • How?

    • Sychronous : dedicated specific capacity to a conection

    • Asynchronous : dynamic allocate capacity (3 categories available)

      • Round-robin

      • Reservation

      • Contention


Mac techniques

MAC techniques

  • Contention

    • Medium is free for all

    • A node senses the free medium and occupies it

    • Example: Ethernet, IEEE 802.3

  • Round robin

    • Give every body a turn

    • Inefficient for lightly loaded network

    • Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI

  • Reservation

    • schedule a time slot like TDM

    • Example: DQDB


Overview of lan standard

802.2 LLC

FDDI

802.3

CSMA/CD

802.4

Token Bus

802.5

Token Ring

802.6

DQDB

Overview of LAN Standard

  • 802.1 Overview

  • 802.2 Logical Link Control

  • 802.3 CSMA/CD (Ethernet)

  • 802.4 Token Bus

  • 802.5 Token Ring

  • 802.6 Distributed Queue Dual BUS --MAN standard

  • FDDI Fiber Distributed Data Interface


Ethenet 802 3 operation

A

A

A

Ethenet/802.3 Operation

  • Every node can receive a transmission by all other nodes

    • need addressing scheme to identify a destination

    • only destination copies frame to it, all other nodes have to discarded the frame

C

B

A

C

B

A

terminator

C finds the bus is free

C transmits frame addressed to A

C

C

B

B

A

A

A copies frame

B ignores frame

signal is absorbed by the terminators


Token ring 802 5 operation

Token Ring/802.5 Operation

T

T = 0

T = 0

A

A

T

T = 1

A

T

Data

  • Token Ring LANs continuously pass a token or a Token Ring frame


Local area networks

FDDI

FDDI

Dual Ring


Lan frame format summary

DSAP 1

SSAP 1

CTRL 1

Data >=0

Preamble 16

SD 2

FC 2

DA 12

SA 12

802.2 LLC

FCS 8

ED 1 or 2

FS 1

SD 1

AC 1

FC 1

DA 6

SA 6

802.2 LLC

FCS 4

ED 1

FS 1

Preamble 8

DA 6

SA 6

Length 2

802.2 LLC

FCS 4

Preamble 8

DA 6

SA 6

Type 2

Data

FCS 4

LAN Frame Format Summary

802.2 LLC

FDDI

FDDI uses 4-bits symbols

802.5

802.3

Ethernet


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