Data and Computer Communications

Tenth Edition by William Stallings

Data and Computer Communications, Tenth Edition by William Stallings, (c) Pearson

Education - Prentice Hall, 2013

Local Area Network Overview

CHAPTER 11

“There is some evidence that computer networks will have a large impact on society. Likely areas are the economy, resources, small computers, human-to-human interaction, and computer research.”

—What Can Be Automated? The Computer Science and

Engineering Research Study, MIT Press, 1980

Bus Topology Ø  Topology

l  Refers to the way in which the endpoints, or stations, attached to the network are interconnected

Ø  Bus topology l  All stations attach, through a tap, directly to a linear

transmission medium, or bus l  Full-duplex operation between the station and the tap

allows data to be transmitted onto the bus and received from the bus

l  A transmission from any station propagates the length of the medium in both directions and can be received by all other stations

l  At each end of the bus is a terminator, which absorbs any signal, removing it from the bus

A

A

C transmits frame addressed to A

Frame is not addressed to B; B ignores it

A copies frame as it goes by

A

A

Figure 11.1 Frame Transmission on a Bus LAN

B C

A B C

A B C

Star Topology Ø Each station connects to common central

node l  Usually via two point-to-point links

•  One for transmission and one for reception

•  Operate in broadcast fashion •  Physical star, logical bus •  Only one station can transmit at a time (hub) •  Can act as frame switch

Central node

Central Hub,switch,or repeater

Figure 11.2 Star Topology

Physical

Data Link

Medium

Network

Transport

Session

Presentation

Application

OSI ReferenceModel

Physical

Medium AccessControl

Medium

Logical Link Control( ) ( ) ( )

UpperLayer

ProtocolsLLC ServiceAccess Point

(LSAP)

Scopeof

IEEE 802Standards

Figure 11.3 IEEE 802 Protocol Layers Compared to OSI Model

IEEE 802Reference

Model

IEEE 802 Reference Model

Ø Lowest layer corresponds to the physical layer of the OSI model

Ø  Includes a specification of the transmission medium and the topology

Includes functions such as:

Encoding/decoding of signals

Preamble generation/removal

Bit transmission/reception

IEEE 802 Layers Ø  Logical Link Control

Layer (LLC) l  Provide interface to

higher levels l  Perform flow and error

control

Ø  Media Access Control (MAC) l  On transmit assemble

data into frame l  On reception

disassemble frame, perform address recognition and error detection

l  Govern access to LAN transmission medium

TCP segment

IP datagram

LLC protocol data unit

MAC frame

Application data

TCPheader

IPheader

LLCheader

MACheader

MACtrailer

Figure 11.4 LAN Protocols in Context

Application Layer

TCP Layer

IP Layer

LLC Layer

MAC Layer

Logical Link Control

Ø Transmission of link level PDUs between stations

Ø Must support multi-access, shared medium

Ø Relieved of some details of link access by the MAC layer

Ø Addressing involves specifying source and destination LLC users l  Referred to as service access points (SAPs)

LLC Services Unacknowledged connectionless service • Data-gram style service • Delivery of data is not guaranteed

Connection-mode service •  Logical connection is set up between two users • Flow and error control are provided

Acknowledged connectionless service • Datagrams are to be acknowledged, but no logical

connection is set up

LLC Service Alternatives Unacknowledged connectionless service •  Requires minimum logic •  Avoids duplication of mechanisms •  Preferred option in most cases

Connection-mode service •  Used in simple devices •  Provides flow control and reliability mechanisms

Acknowledged connectionless service •  Large communication channel needed •  Time critical or emergency control signals

LLC Protocol

Ø Modeled after HDLC Ø Asynchronous balanced mode

l  Connection mode (type 2) LLC service Ø Unacknowledged connectionless service

l  Using unnumbered information PDUs (type 1) Ø Acknowledged connectionless service

l  Using 2 new unnumbered PDUs (type 3) Ø Permits multiplexing using LSAPs

Figure 11.5 LLC PDU in a Generic MAC Frame Format

MACFrame

LLCAddress FieldsI/G

I/G = Individual/GroupC/R = Command/Response

DSAP value C/R SSAP value

MACControl

DestinationMAC Address

SourceMAC Address LLC PDU CRC

LLCPDU DSAP

1 octet 1 1 or 2 variable

SSAP LLC Control Information

Medium Access Control (MAC) Protocol

Ø Controls access to the transmission medium Ø Key parameters:

l  Where •  Greater control, single point of failure •  More complex, but more redundant

l  How •  Synchronous

l  Capacity dedicated to connection, not optimal •  Asynchronous

l  Response to demand l  Round robin, reservation, contention

Asynchronous Systems

Round robin •  Each station

given turn to transmit data

Reservation •  Divide medium

into slots •  Good for stream

traffic

Contention •  All stations

contend for time •  Good for bursty

traffic •  Simple to

implement •  Tends to collapse

under heavy load

MAC Frame Handling

Ø  MAC layer receives data from LLC layer

Ø  PDU is referred to as a MAC frame

Ø  MAC layer detects errors and discards frames

Ø  LLC optionally retransmits unsuccessful frames

Bridges

Ø  Connects similar LANs with identical physical and link layer protocols

Ø  Minimal processing Ø  Can map between MAC formats Ø  Reasons for use:

l  Reliability l  Performance l  Security l  Geography

LAN A

LAN B

Bridge

Figure 11.6 Bridge Operation

Frames withaddresses 11 through20 are accepted andrepeated on LAN B

Frames withaddresses 1 through10 are accepted andrepeated on LAN A

Station 1 Station 2 Station 10

Station 11 Station 12 Station 20

Bridge Design Aspects

Ø Makes no modification to the content or format of the frames it receives

Ø Should contain enough buffer space to meet peak demands

Ø Must contain routing and addressing intelligence

Ø May connect more than two LANs Ø Bridging is transparent to stations

PhysicalPhysical

MAC

MAC-H LLC-H MAC-TUser Data

LLC-H User Data

User Data

LLC

Usert1

t3, t4, t5, t6

t2, t7

t1, t8

t2t3 t4 t5 t6

t7

t8

Physical

MAC

LLC

User

MAC

(a) Architecture

(b) Operation

Figure 11.7 Connection of Two LANs by a Bridge

PhysicalLAN LAN

LAN A

LAN B LAN C

LAN D LAN E LAN GLAN F

Bridge101 Bridge

107

Bridge102

Bridge103

Bridge104

Bridge105

Bridge106

Station 1

Station 4 Station 5 Station 6 Station 7

Station 2 Station 3

Figure 11.8 Configuration of Bridges and LANs, with Alternate Routes

Fixed Routing Ø  Simplest and most common strategy Ø  Suitable for small internets and internets that are

relatively stable Ø  A fixed route is selected for each pair of LANs

•  Usually least hop route Ø  Only change when topology changes Ø  Widely used but limited flexibility

Spanning Tree Ø Bridge automatically develops routing

table Ø Automatically updates routing table in

response to changing topology

Algorithm consists of three mechanisms:

Frame forwarding Address learning Loop resolution

Frame Forwarding Ø  Maintain forwarding database for each port

attached to a LAN Ø  For a frame arriving on port X:

Search forwarding database to see if MAC address is listed for any port except port X

If destination MAC address is not found, forward frame out all ports except the one from which it was received

If the destination address is in the forwarding database for some port y, check port y for blocking or forwarding state

If port y is not blocked, transmit frame through port y onto the LAN to which that port attaches

Address Learning Ø  Can preload forwarding database Ø  When frame arrives at port X, it has come from

the LAN attached to port X Ø  Use source address to update forwarding

database for port X to include that address Ø  Have a timer on each entry in database Ø  If timer expires, entry is removed Ø  Each time frame arrives, source address

checked against forwarding database l  If present timer is reset and direction recorded l  If not present entry is created and timer set

Spanning Tree Algorithm Ø  Address learning works for tree layout if there

are no alternate routes in the network l  Alternate route means there is a closed loop

Ø  For any connected graph there is a spanning tree maintaining connectivity with no closed loops

Ø  Algorithm must be dynamic

•  Each bridge assigned unique identifier •  Cost assigned to each bridge port •  Exchange information between bridges to find spanning tree •  Automatically updated whenever topology changes

IEEE 802.1 Spanning Tree Algorithm:

LAN Y

LAN X

Bridge Bridge

t2t1

t0 t0

Station B

Station A

Figure 11.9 Loop of Bridges

Hubs Ø  Active central element of star layout Ø  Each station connected to hub by two lines Ø  Hub acts as a repeater Ø  Length of a line is limited to about 100m Ø  Optical fiber may be used to about 500m Ø  Physically a star, logically a bus Ø  Transmission from any one station is received by

all other stations Ø  If two stations transmit at the same time there

will be a collision

Station Station Station Station

Station

HHUB

Figure 11.10 Two-Level Star Topology

IHUBIHUB

Two cables(twisted pair or

optical fiber)

Transmit

Receive

Shared Bus - 10 Mbps

10 M

bps

10 M

bps

10 M

bps

10 M

bps

A B C D

(a) Shared medium bus

A B C D

(b) Shared medium hub

10 Mbps 10 Mbps

10 Mbps10 Mbps

Total capacityup to 10 Mbps

A B C D

(c) Layer 2 switch

10 Mbps 10 Mbps

10 Mbps10 Mbps

Total capacityN 10 Mbps

Figure 15.11 LAN Hubs and Switches

Layer 2 Switch Benefits

Ø  No change is required to the software or hardware of the attached devices to convert a bus LAN or a hub LAN to a switched LAN

Ø  Have dedicated capacity equal to original LAN l  Assuming switch has sufficient capacity to keep up

with all devices Ø  Scales easily

l  Additional devices attached to switch by increasing capacity of layer 2

Types of Layer 2 Switches

Ø  Store-and-forward switch l  Accepts frame on input

line, buffers briefly, routes to appropriate output line

l  See delay between sender and receiver

l  Boosts overall integrity

Ø  Cut-through switch l  Use destination

address at beginning of frame

l  Switch begins repeating frame onto output line as soon as destination address is recognized

l  Yields highest possible throughput

l  Risk of propagating bad frames

Layer 2 Switch vs. Bridge Ø  Differences between

switches and bridges: Ø  Layer 2 switch can be viewed as full-duplex hub

Ø  Incorporates logic to function as multiport bridge

Ø  New installations typically include layer 2 switches with bridge functionality rather than bridges

Bridge

Frame handling done in software

Analyzes and forwards one

frame at a time

Uses store-and-forward operation

Switch

Performs frame forwarding in

hardware

Can handle multiple frames

at a time

Can have cut-through operation

Figure 11.12 A LAN Configuration

Inaho for takeout. Love to All Tricia

Z

W

XY

Internet

Server

RouterEthernet

switch

Printer

Workstation

Figure 11.13 A Partitioned LAN

Internet

Server

RouterEthernet

switch

Printer

Workstation

Z

V

W

XY

Figure 11.14 A VLAN Configuration

Internet

VLANE

VLAN C

VLAN A

VLANA

VLANA

VLANA

VLANB

VLANB

VLAND

Server

Ethernetswitch withVLAN andIP routingcapability

Printer

Workstation

Z

W

XY

Defining VLANs

Ø Broadcast domain consisting of a group of end stations not limited by physical location and communicate as if they were on a common LAN

Ø Membership by: l  Port group l  MAC address l  Protocol information

Communicating VLAN Membership

Switches need to know VLAN membership

Ø Configure information manually Ø Network management signaling protocol Ø Frame tagging (IEEE802.1Q)

Summary Ø  Bus and tree topologies

and transmission media l  Topologies l  Choice of topology l  Choice of transmission

medium Ø  LAN protocol

architecture l  IEEE 802 reference

model l  Logical link control l  Medium access control

Ø  Hubs and switches l  Hubs l  Layer 2 switches

Ø  Bridges l  Functions of a bridge l  Bridge protocol

architecture l  Fixed routing l  The spanning tree

approach Ø  Virtual LANs

l  The use of virtual LANs l  Defining VLANs l  Communicating VLAN

membership