Chapter 9 Circuit Switching Data and Computer Communications Part 3 - Wide area Networks - Concepts Chapter 9 Circuit Switching

Switching Networks Long distance transmission is typically done over a network of switched nodes Nodes are not concerned with content of data End devices are referred to as stations Computer, terminal, phone, etc. A collection of nodes and connections is a communications network Data is routed by being switched from node to node

Switching Networks Communication Network Switching Networks Switching Network (used in WAN) Broadcast Network (used in LAN) Switching Networks Circuit-switching networks Packet-switching networks

Nodes Nodes may connect to other nodes only, or to stations and other nodes Node to node links are usually multiplexed A network is usually partially connected But some redundant connections are desirable for reliability Two different switching technologies exist: Circuit switching Packet switching

Simple Switched Network

Simple switching network Communication Network Node provides switching facility (routing) Network Station end node (source & destination) Communication is achieved by transmitting data from source to destination through a network of switching nodes Data communication takes place between two devices that are directly connected by some form of point-to-point transmission medium. However, it is impractical for two devices to be directly, point-to-point connected because of : - The devices are very far apart. - Some device may require a link to many of the others at various time. The solution is to attach each device to a communication network. The devices are refereed to network stations. The stations may be computer, terminal, telephones, or other communicating devices. Each station attaches to a network node. The set of nodes to which stations attach is a boundary of a communication network that is capable of transferring data between pairs of stations. Communication Network

Switching technology Circuit switching Packet switching need a connection established between end nodes connection is maintained until one of end nodes terminates Connection is dedicated to the communication between two nodes Example : Public Switch Telephone Network (PSTN) Packet switching data are transmitted in short messages called packets a connection between the two end-nodes is not maintained a node-to-node link can be dynamically shared by many packets Example : Public Data Network (PDN) like X.25, Frame Relay The type of data communication facility used is a function of the nature of the application, the number of computers involved, and their physical interface.

Key differences Keys Circuit switching Packet switching data dedicated single route different routes node status both must be ready sender ready to send is enough connection dedicated shared utilization poor good data rate fixed varies Prioritization not supported supported The type of data communication facility used is a function of the nature of the application, the number of computers involved, and their physical interface.

Public Switching Telephone Network end office subscriber loop Long-distance office Intercity trunk subscriber loop (local loop) link between subscriber and network connecting trunk (exchanges) switching center; localized support for subscriber Data communication takes place between two devices that are directly connected by some form of point-to-point transmission medium. However, it is impractical for two devices to be directly, point-to-point connected because of : - The devices are very far apart. - Some device may require a link to many of the others at various time. The solution is to attach each device to a communication network. The devices are referred to network stations. The stations may be computer, terminal, telephones, or other communicating devices. Each station attaches to a network node. The set of nodes to which stations attach is a boundary of a communication network that is capable of transferring data between pairs of stations. Intercity trunk (trunks) branches between exchanges; carry multiple voice circuits connecting trunk

PSDN PSDN is designed specifically for the transmission of data rather than voice Communication is shared PSDN= Public Switched Data Network LAN LAN PSDN Telephone companies normally provide a public data service. Such networks is known as public switch data network (PSDN), like the PSTN, are now interconnected internationally and have been designed specifically for the transmission of data rather than voice. A network is public in the sense that many company share a switching network. A cost of this service is normally lower than a leased line. LAN

Packets Messages are broken up into a series of packets packet packet data header data header data header data packet packet packet To control communication operation, control information, as well as user data, must be transmitted. The sending application generates a block of data and passes this to the transport layer. The transport layer may break this block into smaller pieces to make it more manageable. To each of these pieces, the transport layer appends a transport header, containing protocol information as a header. The overall data (control + data) is then passed to the Network access layer as the same manner. Messages are broken up into a series of packets Header is used to route the packet through the network

PSDN switching approach Virtual Circuit network protocols establish a logical route called virtual circuit packets use the same route; data arrive in order similar to circuit switching ! but the route is not dedicated need call setup Datagram each packet is transmitted independently network protocols route each packet as though it were a separate message packet may not arrive in order; need protocol to ensure ordering call setup is not required The type of data communication facility used is a function of the nature of the application, the number of computers involved, and their physical interface.

Virtual circuit A logical connection known as B 1.3 1.2 1.1 A logical connection known as virtual circuit (VC) is set up between two stations. Packets are labeled with a virtual circuit number and a sequence number 1.3 1.2 1.1 A 2.3 2.2 2.1 2.3 2.2 2.1 C virtual circuit #1 B The OSI Layers 1.Physical - Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristic to access the physical medium. 2.Data link - Provides for the reliable transfer of information across the physical link; send block of data (frames) with the necessary synchronization, error control , and flow control. 3.Network - Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections. 4.Transport - Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control. 5.Session - Provides the control structure for communication between applications; establishes, manages, and terminates connections (session) between cooperating applications. 6.Presentation - Provides independence to the application processes from difference in data representation (syntax). 7.Application - Provide access to the OSI environment for user and also provides distributed information services. 2 4 A 1 3 5 C virtual circuit #2

Datagram Each packet is transmitted independently. B B.3 B.2 B.1 Each packet is transmitted independently. Packets are labeled with a destination address and may arrive out of sequence B.3 B.2 B.1 A C.3 C.2 C.1 C.3 C.2 C.1 C B The OSI Layers 1.Physical - Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristic to access the physical medium. 2.Data link - Provides for the reliable transfer of information across the physical link; send block of data (frames) with the necessary synchronization, error control , and flow control. 3.Network - Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections. 4.Transport - Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control. 5.Session - Provides the control structure for communication between applications; establishes, manages, and terminates connections (session) between cooperating applications. 6.Presentation - Provides independence to the application processes from difference in data representation (syntax). 7.Application - Provide access to the OSI environment for user and also provides distributed information services. B.3 B.2 2 4 C.1 A 1 B.1 3 5 C.3 C.2 C

Routing methods in packet switching Fixed routing each packet is transmitted independently Flooding network protocol establishes a logical route called virtual circuit packets use the same route; data arrive in order Random routing the route taken is random Adaptive routing the route reacts to changing conditions within the network The OSI Layers 1.Physical - Concerned with transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristic to access the physical medium. 2.Data link - Provides for the reliable transfer of information across the physical link; send block of data (frames) with the necessary synchronization, error control , and flow control. 3.Network - Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections. 4.Transport - Provides reliable, transparent transfer of data between end points; provides end-to-end error recovery and flow control. 5.Session - Provides the control structure for communication between applications; establishes, manages, and terminates connections (session) between cooperating applications. 6.Presentation - Provides independence to the application processes from difference in data representation (syntax). 7.Application - Provide access to the OSI environment for user and also provides distributed information services.

Circuit-Switching Networks During communication, a dedicated communication path exists between sender and receiver; e.g., telephone system. Communication involves 3 phases: Circuit establishment: “Call request” propagates, hop by hop through the network, to establish a dedicated link (channels in each component link in some path, from sender to receiver) Data transfer: continuous transfer; either analog or digital signal Circuit disconnect: “circuit disconnect” signal to deallocate the links

Circuit-Switching Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes Dominant in both voice and data communications today e.g. PSTN is a circuit-switched network Relatively inefficient (100% dedication even without 100% utilization)

Circuit Switching Dedicated communication path between two stations Three phases Establish Transfer Disconnect Must have switching capacity and channel capacity to establish connection Must have intelligence to work out routing

Circuit-Switching Stages Circuit establishment Transfer of information point-to-point from endpoints to node internal switching/multiplexing among nodes Circuit disconnect

Circuit-Switching Networks Disadvantages: Both stations need to be simultaneously ready Inefficient use of link capacities, especially with bursty intermittent traffic Advantages Low delay once circuit is established

Circuit-Switching Networks Public telephone network Best-known example of a circuit-switching network Four generic architectural components Subscribers - Devices that attach to the network Local loop - The link between the subscriber and the network (end office). Also called subscriber loop Exchanges - Switching centers in the network. End offices, long-distance offices, … Trunks - Links between exchanges. Carry multiple voice channels by using FDM or synchronous TDM

Circuit Switching - Applications Inefficient Channel capacity dedicated for duration of connection If there is no data, capacity wasted Set up (connection) takes time Once connected, transfer is transparent Developed for voice traffic (phone)

Public Circuit Switched Network Connecting Trunk

Circuit-Switching Networks

AT&T telephone hierarchy

Switching Concepts Single circuit-switched node network A collection of stations attached to a central switching unit The central switch establishes a dedicated path between any two devices that wish to communicate

Switching Concepts Digital switch Establishes a dedicated path between any two devices Control unit Establishes, maintains, and tears down the connection Network interface (circuit) switch Blocking Nonblocking

Telecommunication Components Subscriber Devices attached to network Local Loop Subscriber loop Connection to network Exchange Switching centers End office - supports subscribers Trunks Branches between exchanges Multiplexed

Circuit Switching Node's Elements

Circuit Switching Node's Elements Digital Switch Provides transparent signal path between devices Network Interface (functions & hardware needed to connect devices to the network) Control Unit; its function is to: Establish connections Generally on demand Handle and acknowledge requests Determine if destination is free construct path Maintain connection Disconnect

Circuit Switching Node: Digital Switch Provides transparent signal path between any pair of attached devices Typically full-duplex

Circuit-Switching Node: Network Interface Provides hardware and functions to connect digital devices to switch Analog devices can be connected if interface includes CODEC functions Typically full-duplex

Circuit-Switching Node: Control Unit Establishes on-demand connections Maintains connection while needed Breaks down connection on completion

Blocking or Non-blocking An important characteristic of a circuit-switching device is whether it is blocking or non-blocking. Blocking A network is unable to connect stations because all paths are in use Used on voice systems example: Short duration calls Non-blocking Permits all stations to connect (in pairs) at once Used for some data connections

Blocking/Nonblocking Networks Blocking: network is unable to connect two stations because all possible paths are already in use Nonblocking: permits all possible connection requests because any two stations can be connected

There are 2 Types of Switching Techniques Internal to a Single Circuit-Switching Node: 1. Space-Division Switching 2. Time-Division Switching

Switching Techniques Space-Division Switching Time-Division Switching Developed for analog environment, but has been carried over into digital communication Requires separate physical paths for each signal connection Uses metallic or semiconductor “gates” Time-Division Switching Used in digital transmission Utilizes multiplexing to place all signals onto a common transmission path Bus must have higher data rate than individual I/O lines

Space Division Switch Developed for the analog environment (but is now used for both analog & digital signals) Separate physical paths for every channel (signal path) Its basic device is the Crossbar switch Number of crosspoints grows as square of number of stations Loss of crosspoint prevents connection Inefficient use of crosspoints All stations connected, only a few crosspoints in use Non-blocking

Crossbar Matrix (switch)

Multistage Switch Reduced number of crosspoints More than one path through network Results in increased reliability More complex control May be blocking

Three Stage Switch

Time Division Switching Partition low speed bit stream into pieces that share higher speed stream e.g. TDM bus switching based on synchronous time division multiplexing Each station connects through controlled gates to high speed bus Time slot allows small amount of data onto bus Another line’s gate is enabled for output at the same time

Routing in Circuit-Switched Networks Traditional circuit-switched model is hierarchical, sometimes supplemented with peer-to-peer trunks Newer circuit-switched networks are dynamically routed: all nodes are peer-to-peer, making routing more complex

Routing in Circuit-Switching Networks Many connections will need paths through more than one switch Therefore, the network needs to find an appropriate route. Important factors are: Efficiency Resilience Public telephone switches are a tree structure Static routing uses the same approach all the time Dynamic routing allows for changes in routing depending on traffic

Alternate Routing Possible routes between two end offices are predefined Originating switch selects the best route for each call Routing paths can be fixed (1 route) or dynamic (multiple routes, selected based on current and historical traffic)

Alternate Routing Diagram

Control Signaling Control unit manages the establishment, maintenance, and termination of signal paths Includes signaling from subscriber to network, and signals within network In-channel signaling uses the same channel for control signals and calls Common-channel signaling uses independent channels for controls (e.g. SS7)

Control Signaling Functions The means by which the network is managed, calls are established, maintained, and terminated Audible communication with subscriber Transmission of dialed number Call cannot be completed indication Call ended indication Signal to ring phone Billing info Equipment and trunk status info Diagnostic info Control of special equipment

Control Signal Sequence Both phones on hook Subscriber lifts receiver (off hook) End office switch signaled Switch responds with dial tone Caller dials number If target is not busy, ringer signal is sent to target subscriber Feedback to caller Ringing tone, engaged tone, unobtainable (disconnected line, etc.) Target accepts call by lifting receiver Switch terminates ringing signal and ringing tone Switch establishes connection Connection is released when Source subscriber hangs up

Switch to Switch Signaling Subscribers connected to different switches Originating switch seizes inter-switch trunk Off hook signal is sent on trunk, requesting a digit register at target switch (so that address may be communicated) Terminating switch sends off hook followed by on hook (known as wink) to show register-ready status Originating switch sends address

Control Signals through a Circuit-Switching Telephone Network

Location of Signaling Subscriber to network Within network Depends on subscriber device and switch Within network is concerned with management of subscriber calls and network More complex Two types of control signaling are used in circuit switching networks: In-channel signaling Common channel signaling

In-Channel Signaling Use same channel for signaling and call Inband Requires no additional transmission facilities Inband Uses same frequencies as voice signal Can go anywhere a voice signal can Impossible to set up a call on a faulty speech path (because the control signals used to set up the path have to follow same path) Out of band Voice signals do not use full 4kHz bandwidth Narrow signal band within 4kHz used for control Can be sent whether or not voice signals are present Requires extra electronics Slower signal rate (narrow bandwidth)

Drawbacks of In Channel Signaling Limited transfer rate Delay between entering address (dialing) and connection This is overcome by use of common channel signaling

Common Channel Signaling Control signals carried over paths independent of voice channel One control signal channel (path) can carry signals for a number of subscriber channels and is therefore the common control channel for these subscriber lines (channels) Two modes of operation are used in Common channel signaling: Associated Mode Common channel closely tracks inter-switch trunks Disassociated Mode Additional nodes (signal transfer points) Effectively two separate networks

Common v. In Channel Signaling

Common Channel Signaling Modes

Signaling System Number 7 SS7 It is the most widely used common channel signaling scheme Used in ISDN Optimized for 64kbps digital channel network Call control, remote control, management and maintenance Reliable means of transfer of info in sequence Will operate over analog and below 64k Point to point terrestrial and satellite links Although the network being controlled is a circuit switched network, the control signaling is implemented using packet switching technology

SS7 Signaling Network Elements Signaling point (SP) Any point in the network capable of handling SS7 control messages Signal transfer point (STP) A signaling point capable of routing control messages Control plane Responsible for establishing and managing connections Information plane Once a connection is set up, info is transferred in the information plane

Transfer Points

Signaling Network Structures STP capacities Number of signaling links that can be handled Message transfer time Throughput capacity Network performance Number of SPs Signaling delays Availability and reliability Ability of network to provide services in the face of STP failures