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Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4

Communication Networks

Chapter 3


Multiplexing


Frequency Division Multiplexing (FDM)

� Useful bandwidth of medium exceeds required bandwidth of channel

� Each signal is modulated to a different carrier frequency(channel)

� To prevent interference, carrier frequencies (channels) separated so signals do not overlap (guard bands)

� e.g. broadcast radio (a useful spectrum for voice is 300 to 3400 Hz, and a bandwidth of 4 kHz is enough as a guard

� Standard voice multiplexer is a 12X4 kHz channels from 60 to 108 kHz.

� Channel allocated even if no data


Frequency Division MultiplexingDiagram


FDM System


FDM of Three Voiceband Signals


Analog Carrier Systems

� AT&T (USA)

� Hierarchy of FDM schemes

� Group

� 12 voice channels (4kHz each) = 48kHz

� Range 60kHz to 108kHz

� Supergroup

� 60 channel

� FDM of 5 group signals on carriers between 420kHz and 612 kHz

� Mastergroup

� 10 supergroups


Wavelength Division Multiplexing

� Multiple beams of light at different frequency

� Carried by optical fiber

� A form of FDM

� Each color of light (wavelength) carries separate data channel

� 1997 Bell Labs� 100 beams

� Each at 10 Gbps

� Giving 1 terabit per second (Tbps)

� Commercial systems of 160 channels of 10 Gbps now available

� Lab systems (Alcatel) 256 channels at 39.8 Gbps each� 10.1 Tbps

� Over 100km


WDM Operation

� Same general architecture as other FDM

� Number of sources generating laser beams at different frequencies

� Multiplexer consolidates sources for transmission over single fiber

� Optical amplifiers amplify all wavelengths

� Typically tens of km apart

� Demux separates channels at the destination

� Mostly 1550nm wavelength range

� Was 200MHz per channel

� Now 50GHz


Dense Wavelength Division Multiplexing

� DWDM

� No official or standard definition

� Implies more channels more closely spaced than WDM

� 200GHz or less


Synchronous Time Division Multiplexing

� Data rate of medium exceeds data rate of digital signal to be transmitted

� Multiple digital signals interleaved in time

� May be at bit level of blocks (6 inputs each 9.6 kbps can be carried by a single line capacity of 57.6 kbps)

� Time slots pre-assigned to sources and fixed

� Time slots allocated even if no data

� Time slots do not have to be evenly distributed amongst sources


Time Division Multiplexing


TDM System


TDM Link Control

� No headers and trailers

� Data link control protocols not needed

� Flow control� Data rate of multiplexed line is fixed

� If one channel receiver can not receive data, the others must carry on

� The corresponding source must be quenched

� This leaves empty slots

� Error control� Errors are detected and handled by individual

channel systems


Data Link Control on TDM


Framing

� No flag or SYNC characters bracketing TDM frames

� Must provide synchronizing mechanism

� Added digit framing

� One control bit added to each TDM frame

� Looks like another channel - “control channel”

� Identifiable bit pattern used on control channel

� e.g. alternating 01010101…unlikely on a data channel

� Can compare incoming bit patterns on each channel with sync pattern


Types of Communication Networks

� Traditional

� Traditional local area network (LAN)

� Traditional wide area network (WAN)

� Higher-speed

� High-speed local area network (LAN)

� Metropolitan area network (MAN)

� High-speed wide area network (WAN)

Speed and Distance of Communications Networks


Characteristics of WANs

� Covers large geographical areas

� Circuits provided by a common carrier

� Consists of interconnected switching nodes

� Traditional WANs provide modest capacity� 64000 bps common

� Business subscribers using T-1 service – 1.544 Mbps common

� Higher-speed WANs use optical fiber and transmission technique known as asynchronous transfer mode (ATM)� 10s and 100s of Mbps common


Characteristics of LANs

� Like WAN, LAN interconnects a variety of devices and provides a means for information exchange among them

� Traditional LANs

� Provide data rates of 1 to 20 Mbps

� High-speed LANS

� Provide data rates of 100 Mbps to 1 Gbps


Differences between LANs and WANs

� Scope of a LAN is smaller

� LAN interconnects devices within a single building or cluster of buildings

� LAN usually owned by organization that owns the attached devices

� For WANs, most of network assets are not owned by same organization

� Internal data rate of LAN is much greater


The Need for MANs

� Traditional point-to-point and switched network techniques used in WANs are inadequate for growing needs of organizations

� Need for high capacity and low costs over large area

� MAN provides:� Service to customers in metropolitan areas

� Required capacity

� Lower cost and greater efficiency than equivalent service from telephone company


Switching Terms

� Switching Nodes:

� Intermediate switching device that moves data

� Not concerned with content of data

� Stations:

� End devices that wish to communicate

� Each station is connected to a switching node

� Communications Network:

� A collection of switching nodes


What is it all about?

� How do we move traffic from one part of the network to another?

� Connect end-systems to switches, and switches to each other

� Data arriving to an input port of a switch have to be moved to one or more of the output ports

Switched Network


Observations of Figure 3.3

� Some nodes connect only to other nodes (e.g., 5 and 7)

� Some nodes connect to one or more stations

� Node-station links usually dedicated point-to-point links

� Node-node links usually multiplexed links� Frequency-division multiplexing (FDM)

� Time-division multiplexing (TDM)

� Not a direct link between every node pair


Techniques Used in Switched Networks

� Circuit switching

� Dedicated communications path between two stations

� E.g., public telephone network

� Packet switching

� Message is broken into a series of packets

� Each node determines next leg of transmission for each packet


Switching

(a) Circuit switching. (b) Packet switching.


A generic switch


Phases of Circuit Switching

� Circuit establishment� An end to end circuit is established through

switching nodes

� Information Transfer� Information transmitted through the network

� Data may be analog voice, digitized voice, or binary data

� Circuit disconnect� Circuit is terminated

� Each node deallocates dedicated resources


Characteristics of Circuit Switching

� Can be inefficient

� Channel capacity dedicated for duration of connection

� Utilization not 100%

� Delay prior to signal transfer for establishment

� Once established, network is transparent to users

� Information transmitted at fixed data rate with only propagation delay


A circuit switch

� A switch that can handle N calls has N logical inputs and N

logical outputs N up to 200,000

� In practice, input trunks are multiplexed

� example: DS3 trunk carries 672 simultaneous calls

� Multiplexed trunks carry frames = set of samples

� Goal: extract samples from frame, and depending on position in frame, switch to output

� each incoming sample has to get to the right output line and the right slot in the output frame

� demultiplex, switch, multiplex


Components of Public Telecommunications Network

� Subscribers - devices that attach to the network; mostly telephones

� Subscriber line - link between subscriber and network� Also called subscriber loop or local loop

� Exchanges - switching centers in the network� A switching centers that support subscribers is an

end office

� Trunks - branches between exchanges


How Packet Switching Works

� Data is transmitted in blocks, called packets

� Before sending, the message is broken into a series of packets

� Typical packet length is 1000 octets (bytes)

� Packets consists of a portion of data plus a packet header that includes control information

� At each node en route, packet is received, stored briefly and passed to the next node

Packet Switching


Packet Switching Advantages� Line efficiency is greater

� Many packets over time can dynamically share the same node to node link

� Packet-switching networks can carry out data-rate conversion� Two stations with different data rates can

exchange information

� Unlike circuit-switching networks that block calls when traffic is heavy, packet-switching still accepts packets, but with increased delivery delay

� Priorities can be used


Disadvantages of Packet Switching

� Each packet switching node introduces a delay

� Overall packet delay can vary substantially� This is referred to as jitter

� Caused by differing packet sizes, routes taken and varying delay in the switches

� Each packet requires overhead information� Includes destination and sequencing information

� Reduces communication capacity

� More processing required at each node


Packet Switching Networks -Datagram

� Each packet treated independently, without reference to previous packets

� Each node chooses next node on packet’s path

� Packets don’t necessarily follow same route and may arrive out of sequence

� Exit node restores packets to original order

� Responsibility of exit node or destination to detect loss of packet and how to recover


Packet Switching Networks –Datagram

� Advantages:

� Call setup phase is avoided

� Because it’s more primitive, it’s more flexible

� Datagram delivery is more reliable


Packet Switching Networks –Virtual Circuit

� Preplanned route established before packets sent

� All packets between source and destination follow this route

� Routing decision not required by nodes for each packet

� Emulates a circuit in a circuit switching network but is not a dedicated path� Packets still buffered at each node and queued for

output over a line


Packet Switching Networks –Virtual Circuit

� Advantages:

� Packets arrive in original order

� Packets arrive correctly

� Packets transmitted more rapidly without routing decisions made at each node

Effect of Packet Size on Transmission



� Breaking up packets decreases transmission time because transmission is allowed to overlap

� Figure 3.9a� Entire message (40 octets) + header information

(3 octets) sent at once

� Transmission time: 129 octet-times (An octet is a unit of digital information that consists of eight bits)

� Figure 3.9b� Message broken into 2 packets (20 octets) +

header (3 octets)

� Transmission time: 92 octet-times



� Figure 3.9c

� Message broken into 5 packets (8 octets) + header (3 octets)

� Transmission time: 77 octet-times

� Figure 3.9d

� Making the packets too small, transmission time starts increases

� Each packet requires a fixed header; the more packets, the more headers


Asynchronous Transfer Mode (ATM)

� The word Asynchronous in ATM is incontrast to Synchronous Transfer Mode(STM) that was proposed earlier on, whichwas based on the SONET/SDH hierarchy.� ATM was standardized by ITU-T (old CCITT) in 1988 as the

transfer mode of B-ISDN� It can carry a variety of different types of traffic,

such as– Voice– Video– Data

� At speeds varying from fractional T1 (1.544 Mbit/s line rate)to 2.4 Gbps



� Also known as cell relay

� Operates at high data rates

� Resembles packet switching� Involves transfer of data in discrete chunks, like

packet switching

� Allows multiple logical connections to be multiplexed over a single physical interface

� Minimal error and flow control capabilities reduces overhead processing and size

� Fixed-size cells simplify processing at ATM nodes


ATM Terminology

� Virtual channel connection (VCC)� Logical connection in ATM

� Basic unit of switching in ATM network

� Analogous to a virtual circuit in packet switching networks

� Exchanges variable-rate, full-duplex flow of fixed-size cells

� Virtual path connection (VPC)� Bundle of VCCs that have the same end points


Advantages of Virtual Paths

� Simplified network architecture

� Increased network performance and reliability

� Reduced processing and short connection setup time

� Enhanced network services

Call Establishment


Virtual Channel Connection Uses

� Between end users

� Can carry end-to-end user data or control signaling between two users

� Between an end user and a network entity

� Used for user-to-network control signaling

� Between two network entities

� Used for network traffic management and routing functions


Virtual Path/Virtual Channel Characteristics

� Quality of service� Specified by parameters such as cell loss ratio and

cell delay variation

� Switched and semipermanent virtual channel connections

� Cell sequence integrity

� Traffic parameter negotiation and usage monitoring

� Virtual channel identifier restriction within a VPC


ATM Cell Header Format

� Generic flow control (GFC) – 4 bits, used only in user-network interface� Used to alleviate short-term overload conditions in

network

� Virtual path identifier (VPI) – 8 bits at the user-network interface, 12 bits at network-network interface� Routing field

� Virtual channel identifier (VCI) – 8 bits� Used for routing to and from end user


ATM Cell Header Format

� Payload type (PT) – 3 bits� Indicates type of information in information

field

� Cell loss priority (CLP) – 1 bit� Provides guidance to network in the event

of congestion

� Header error control (HEC) – 8 bit� Error code


ATM Service Categories

� Real-time service

� Constant bit rate (CBR)

� Real-time variable bit rate (rt-VBR)

� Non-real-time service

� Non-real-time variable bit rate (nrt-VBR)

� Available bit rate (ABR)

� Unspecified bit rate (UBR)


Examples of CBR Applications

� Videoconferencing

� Interactive audio (e.g., telephony)

� Audio/video distribution (e.g., television, distance learning, pay-per-view)

� Audio/video retrieval (e.g., video-on-demand, audio library)


Examples of UBR applications

� Text/data/image transfer, messaging, distribution, retrieval

� Remote terminal (e.g., telecommuting)

chapter 3 - ieu.edu.trhomes.ieu.edu.tr/hozcan/ce360/lect3-communication...stallings, wireless...

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