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NET NET 456 456 –– High Speed NetworksHigh Speed Networks
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Reformatted slides from textbookData and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011 - Original Slides from Dr. Lawrie Brown
12-Nov-11
Lecture 02Chapter 10: Circuit Switching and Packet Switching
Dr. Anis Koubaa
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Circuit Switching and Packet Switching
He got into a District Line train at Wimbledon Park, changed on to the Victoria Line at Victoria and on to the Jubilee Line at Green Park for West Hampstead. It was a long and awkward journey but he enjoyed it.
NET 456: High Speed Networks, by Dr. Anis KoubaaTextbook: Data and Computer Communications, 9/EBy Williams Stalling – Original Slides from Dr. Lawrie Brown2
long and awkward journey but he enjoyed it.
—King Solomon's Carpet,
Barbara Vine (Ruth Rendell)
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Outline
� Switched Communications Networks
� Circuit-Switching Networks
� Circuit-Swicting Concepts
� Space Division Swicting (Crossbar)
� Time Division Switching
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� Time Division Switching
� Softswitch architecture
� Packet Switching Principle
� Switching Technique
� Packet Size
� External Network Interface
� Comparison of Circuit Switching and Packet Switching
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Switched Communications Networks
� switching nodes provide a switching facility that move data between nodes
� stations – devices attached to the network
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� stations – devices attached to the network
� nodes – switching devices that provide communication
� connected by transmission links
� dedicated point-to-point
� usually multiplexed using either FDM or TDM
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Switched Network
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Communication Networks
� communications network – a collection of nodes
� in addition to switching functions, some nodes also deliver data to attached stations
� network is not fully connected so there is not a direct link between every possible pair of nodes
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direct link between every possible pair of nodes
� redundant connections increase network reliability
� switching technologies:
� circuit switching
� packet switching
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Circuit Switching Circuit Switching
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Circuit Switching
� uses a dedicated path between two stations
� can be inefficient
� channel capacity dedicated for duration of connection
if no data, capacity wasted
� has three phases
establish
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� if no data, capacity wasted
� set up (connection) takes time
� once connected, transfer is transparent
transfer
disconnect
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Public Telecommunications Network
� examples of circuit switching network:
Public telephone network
subscribers –
devices that attach to the network
Data switch
Interconnects digital data
Private branch exchange (PBX)
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the network
subscriber line –
link subscriber and network
exchanges –
switching centers in network
trunks –
branches between exchanges
digital data processing devices such as terminals and computers
interconnects telephones within a building
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Public Circuit Switched Network
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Circuit Establishment
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Circuit-Switching Technology
� Driven by applications that handle voice traffic� Key requirement is no transmission delay and no variation in delay
� A constant signal transmission rate must be maintained
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maintained� transmission and reception occur at the same signal rate.
� Efficient for analog transmission of voice signals ☺
� Inefficient for digital transmission �
� Transparent� once a circuit is established it appears as a direct connection; no special logic is needed
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Circuit-Switching ConceptsCircuit-Switching Concepts
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Circuit-Switching Concepts
• provides a transparent signal path• must allow full-duplex transmission
digital switch
network interface
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• functions and hardware needed to connect digital devices
• establishes, maintains, and tears down the connection
control unit
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Blocking or Non-blocking
blocking network non-blocking network
� may be unable to connect stations because all paths are in use
� used on voice systems
� permits all stations to connect at once
� grants all possible connection requests as long
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� used on voice systems because it is expected for phone calls to be of short duration and that only a fraction of the phones will be engaged at any one time
connection requests as long as the called party is free
� when using data connections terminals can be continuously connected for long periods of time so nonblocking configurations are required
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Space Division Switching (Crossbar)
� originally developed for analog, space division switching has been carried over into the digital realm
� signal paths are physically separate from one another
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separate from one another
� path is dedicated solely to transfer signals
� basic building block of switch is a metallic crosspoint (or semiconductor gate)
Crossbar Matrix10 full-duplex I/O lines
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Space Division Switching: Limitations
� The crossbar switch has a number of limitations:� The number of crosspoints grows
with the square of the number of attached stations. This is costly for a large switch.
� The loss of a crosspoint prevents
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� The loss of a crosspoint prevents connection between the two devices whose lines intersect at that crosspoint.
� The crosspoints are inefficiently utilized; even when all of the attached devices are active, only a small fraction of the crosspointsare engaged. Crossbar Matrix
10 full-duplex I/O lines
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3 Stage Space Division Switch
� multiple-stage switches are employed to overcome the limitations
� This type of arrangement has two advantages over a single-stage crossbar matrix:
� The number of crosspoints is
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� The number of crosspoints is reduced, increasing crossbar utilization (for the total number of crosspoints for 10 stations is reduced from 100 to 48)
� There is more than one path through the network to connect two endpoints, increasing reliability. three-stage switch
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Time Division Switching
� modern digital systems use intelligent control of space & time division elements
� modern circuit switches use digital time division techniques to set up and maintain virtual circuits
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techniques to set up and maintain virtual circuits
� TDM: partition low speed bit stream into pieces that share higher speed stream
� individual pieces manipulated by control logic to flow from input to output
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Softswitch ArchitectureSoftswitch Architecture
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Softswitch Architecture
� latest trend in circuit-switching technology
� computer running specialized software that turns it into a smart phone switch
� costs less and provides more functionality
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� costs less and provides more functionality
� In addition to handling the traditional circuit-switching functions, a softswitch can convert a stream of digitized voice bits into packets (e.g. Voice over IP).
� Media gateway (MG) – physical switching
� Media gateway controller (MGC) – call processing logic
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Traditional Circuit Switching
� the MG and MGC are distinct entities and may be provided by different vendors
� ITU-T has issued a standard for a media gateway control protocol between the MG and MGC
� H.248.1 (Gateway Control Protocol,
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� H.248.1 (Gateway Control Protocol, Version 3, 2005).
� RFC 2805 (Media Gateway Control Protocol Architecture and Requirements, 2000) provides an overview of media gateway concepts
the architecture of a traditional telephone network circuit switch with the softswitch architecture
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Softswitch
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Packet Switching Packet Switching
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Packet Switching
� circuit switching was designed for voice
� packet switching was designed for data
� transmitted in small packets (max around 1 KByte).
packets contains user data and control info
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� packets contains user data and control info
� user data may be part of a larger message
� control information includes routing (addressing)
� packets are received, stored briefly (buffered) and passed on to the next node
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Packet Switching
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Figure 10.8 illustrates how a longer message is broken up into a series of packets, which are transferred over the packet-switched network.
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Advantages
� link efficiency
� single link shared by many packets over time
� packets queued and transmitted as fast as possible
� data rate conversion
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data rate conversion
� stations connects to local node at own speed
� nodes buffer data if required to equalize rates
� packets accepted even when network is busy (delay will just increase)
� priorities can be used
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Switching Techniques
� station breaks long message into packets
� packets sent one at a time to the network
� packets can be handled in two ways:
� datagram
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� datagram
� each packet is treated independently with no reference to previous packets
� virtual circuit
� a preplanned route is established before any packets are sent
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Datagram Diagram
� In the datagram approach,each packet is treatedindependently, with noreference to packets thathave gone before.
� The packets, each with the
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� The packets, each with thesame destination address, donot all follow the sameroute, and they may arriveout of sequence at the exitpoint
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Virtual Circuit Diagram� In the virtual circuit approach, a
preplanned route is established before any packets are sent. Once the route is established, all the packets between a pair of communicating parties follow this same route through the network.
� it is somewhat similar to a circuit in a circuit-switching network and is
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it is somewhat similar to a circuit in a circuit-switching network and is referred to as a virtual circuit.
� Each packet contains a virtual circuit identifier as well as data.
� Each node on the pre-established route knows where to direct such packets; no routing decisions are required.
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Virtual Circuits vs. Datagram
� virtual circuits
� network can provide sequencing and error control
� packets are forwarded more quickly
� less reliable
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� less reliable
� datagram
� no call setup phase
� more flexible
� more reliable (node failure does not impact network operation)
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There is a significant relationship between
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between
Packet Size
and transmission time.
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Event Timing
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Comparison of Communication Switching
Techniques
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External Network Interface
� ITU-T standard for interface between host and packet switched network
� almost universal on packet switched networks and packet switching in ISDN
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switching in ISDN
� defines three layers
� Physical
� Link
� Packet
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X.25 Use of Virtual Circuits
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Circuit vs. Packet Switching
� performance depends on various delays� propagation delay
� time it takes a signal to propagate between nodes
� transmission time� time it takes for a transmitter to send a block of data
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time it takes for a transmitter to send a block of data
� node delay� time it takes for a node to perform processing as it switches data
� range of other characteristics, including:� transparency
� amount of overhead
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Summary
� switched communications networks
� stations / nodes
� circuit switching networks
� circuit switching concepts
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� digital switch, network interfacing, control unit
� softswitch architecture
� packet switching principles