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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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Multiplexing
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Frequency Division MultiplexingDiagram
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
FDM System
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
FDM of Three Voiceband Signals
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Dense Wavelength Division Multiplexing
� DWDM
� No official or standard definition
� Implies more channels more closely spaced than WDM
� 200GHz or less
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Time Division Multiplexing
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
TDM System
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Data Link Control on TDM
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Switching
(a) Circuit switching. (b) Packet switching.
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
A generic switch
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Packet Switching Networks –Datagram
� Advantages:
� Call setup phase is avoided
� Because it’s more primitive, it’s more flexible
� Datagram delivery is more reliable
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Effect of Packet Size on Transmission
� 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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Asynchronous Transfer Mode (ATM)
� 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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Asynchronous Transfer Mode (ATM)
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Advantages of Virtual Paths
� Simplified network architecture
� Increased network performance and reliability
� Reduced processing and short connection setup time
� Enhanced network services
Call Establishment
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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)
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
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)
Stallings, Wireless Communications & Networks, Second Edition, © 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Examples of UBR applications
� Text/data/image transfer, messaging, distribution, retrieval
� Remote terminal (e.g., telecommuting)