Switching TechniquesThink how things would be if you could only use your telephone to talk to just one other person! You would not be very productive. So there are requirements for switching systems to route your calls around the world.

There are a number of ways to perform switching:

Circuit Switching Packet Switching Message Switching Cell Switching

Circuit Switching

This method involves the physical interconnection of two devices. A good example of circuit switching involves the Public phone network. A data example would be the classic A/B switch!

Packet Switching

Packet Switching techniques switch packets of data between destinations. Traditionally, this applied to X.25 techniques, but this also applies to TCP/IP and IPX/SPX routers also. Proprietary Frame Relay switches can switch voice signals.

Message Switching

Message Switching techniques were originally used in data communications. An example would be early "store and forward" paper tape relay systems. E-Mail delivery is another example of message switching. In voice systems, you can find Voice Mail delivery systems on the Internet. The classic "forward voice mail" capability in some voice mail systems is another example.

Cell Switching

Cell Switching is similar to packet switching, except that the switching does not necessarily occur on packet boundaries. This is ideal for an integrated environment and is found within Cell-based networks, such as ATM. Cell-switching can handle both digital voice and data signals.

Circuit Switching vs. Packet Switching

The old telephone system (PSTN) uses circuit switching to transmit voice data whereas VoIP uses packet-switching to do so. The difference in the way these two types of switching work is the thing that made VoIP so different and successful.

To understand switching, you need to realize that the network in place between two communicating persons is a complex field of devices and machines, especially if the network is the Internet. Consider a person in Mauritius having a phone conversation with another person on the other side of the globe, say in the US. There are a large number of routers, switches and other kinds of devices that take the data transmitted during the communication from one end to the other.

The Public Switched Telephone Network

© ANOEK DE GROOT/AFP/Getty Images

Tourists often rely on pay phones to make calls when traveling, and calling codes allow those calls to be made.

The Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS), is the wired phone system over which landline telephone calls are made. The PSTN relies on circuit switching. To connect one phone to another, the phone call is routed through numerous switches operating on a local, regional, national or international level. The connection established between the two phones is called a circuit.

In the early days, phone calls traveled as analog signals across copper wire. Every phone call needed its own dedicated copper wire connecting the two phones. That's why you needed operators' assistance in making calls. The operators sat at a switchboard, literally connecting one piece of copper wire to another so that the call could travel across town or across the country. Long-distance calls were comparatively expensive, because you were renting the use of a very long piece of copper wire every time you made a call.

Beginning in the 1960s, voice calls began to be digitized and manual switching was replaced by automated electronic switching [source: WirelessCenter]. Digital voice signals can share the same wire with many other phone calls. The advent of fiber-optic cables now allows thousand of calls to share the same line. But fiber-optic and other high-

bandwidth cables haven't changed the basic nature of circuit switching, which still requires a connection -- or circuit -- to remain open for the length of the phone call.

Routing calls requires multiple switching offices. The phone number itself is a coded map for routing the call. In the United States, for example, we have 10-digit phone numbers.

• The first three digits are the area code or national destination code (NDC), which helps route the call to the right regional switching station.

• The next three digits are the exchange, which represents the smallest amount of circuits that can be bundled on the same switch. In other words, when you make a call to another user in your same exchange -- maybe a neighbor around the corner -- the call doesn't have to be routed onto another switch.

• The last four digits of the phone number represent the subscriber number, which is tied to your specific address and phone lines.

Within a company or larger organization, each employee or department might have its own extension. Extensions from the main phone number are routed through something called a private branch exchange (PBX) that operates on the premises.

To make an international call requires further instructions. The call needs to be routed through your long-distance phone carrier to another country's long-distance phone carrier. To signal such a switch, you have to dial two separate numbers, your country's exit code (or international access code) and the corresponding country code of the place you're calling.

Almost all exit codes are either 00 or 011, although there are a few exceptions like Cuba (119) and Nigeria (009). Country codes are one- to three-digit prefixes that are assigned to specific countries or groups of countries. For example, the country code for the United States is 1, but the United States shares that country code with Canada and several smaller island nations like Jamaica, Puerto Rico and Guam. The country code for Mexico is 52 and Saudi Arabia is 966. Here's a full list of exit and country codes.

Network Switching TechniquesJanuary 20th, 2009 | Tutorials

Different types of switching techniques are employed to provide communication between two computers. These are : circuit switching, message switching and packet switching.Circuit SwitchingIn this technique, first the complete physical connection between two computers is established and then data are transmitted from the source computer to the destination computer. That is, when a computer places a telephone call, the switching equipment within the telephone system seeks out a physical copper path all the way from sender telephone to the receiver’s telephone. The important property of this switching technique is to setup an end-to-end path (connection) between computer before any data can be sent.Message SwitchingIn this technique, the source computer sends data or the message to the switching office first, which stores the data in its buffer. It then looks for a free link to another switching office and then sends the data to this office. This process is continued until the data are delivered to the destination computers. Owing to its working principle, it is also known as store and forward.

That is, store first (in switching office), forward later, one jump at a time.Packet SwitchingWith message switching, there is no limit on block size, in contrast, packet switching places a tight upper limit on block size. A fixed size of packet which can be transmitted across the network is specified. Another point of its difference from message switching is that data packets are stored on the disk in message switching whereas in packet switching, all the packets of fixed size are stored in main memory. This improves the performance as the access time (time taken to access a data packet) is reduced, thus, the throughput (measure of performance) of the network is improved.

Switching technology In the next three subsections, we present the three switching techniques used in networks: circuit switching, datagram packet switching and virtual circuit packet switching.

Circuit switching

Figure 1.1: Circuit switching. The two different bitstreams flow on two separate circuits.

Circuit switching is the transmission technology that has been used since the first communication networks in the nineteenth century. In circuit switching, a caller must first establish a connection to a callee before any communication is possible. During the connection establishment, resources are allocated between the caller and the callee. Generally, resources are frequency intervals in a

Frequency Division Multiplexing (FDM) scheme or more recently time slots in a Time Division Multiplexing (TDM) scheme. The set of resources allocated for a connection is called a circuit, as depicted in Figure 1.1. A path is a sequence of links located between nodes called switches. The path taken by data between its source and destination is determined by the circuit on which it is flowing, and does not change during the lifetime of the connection. The circuit is terminated when the connection is closed.

In circuit switching, resources remain allocated during the full length of a communication, after a circuit is established and until the circuit is terminated and the allocated resources are freed. Resources remain allocated even if no data is flowing on a circuit, hereby wasting link capacity when a circuit does not carry as much traffic as the allocation permits. This is a major issue since frequencies (in FDM) or time slots (in TDM) are available in finite quantity on each link, and establishing a circuit consumes one of these frequencies or slots on each link of the circuit. As a result, establishing circuits for communications that carry less traffic than allocation permits can lead to resource exhaustion and network saturation, preventing further connections from being established. If no circuit can be established between a sender and a receiver because of a lack of resources, the connection is blocked.

A second characteristic of circuit switching is the time cost involved when establishing a connection. In a communication network, circuit-switched or not, nodes need to lookup in a forwarding table to determine on which link to send incoming data, and to actually send data from the input link to the output link. Performing a lookup in a forwarding table and sending the data on an incoming link is called forwarding. Building the forwarding tables is called routing. In circuit switching, routing must be performed for each communication, at circuit establishment time. During circuit establishment, the set of switches and links on the path between the sender and the receiver is determined and messages are exchanged on all the links between the two end hosts of the communication in order to make the resource allocation and build the routing tables. In circuit switching, forwarding tables are hardwired or implemented using fast hardware, making data forwarding at each switch almost instantaneous. Therefore, circuit switching is well suited for long-lasting connections where the initial circuit establishment time cost is balanced by the low forwarding time cost.

The circuit identifier (a range of frequencies in FDM or a time slot position in a TDM frame) is changed by each switch at forwarding time so that switches do not need to have a complete knowledge of all circuits established in the network but rather only local knowledge of available identifiers at a link. Using local identifiers instead of global identifiers for circuits also enables networks to handle a larger number of circuits.

Traffic engineering (TE) consists in optimizing resource utilization in a network by choosing appropriate paths followed by flows of data, according to static or dynamic constraints [39]. A main goal of traffic engineering is to balance the load in the network, i.e., to avoid congestion on links on a network while other links are under-utilized. To achieve such goals, traffic engineering methods can vary from offline capacity planning algorithms to automatic, dynamic changes. Since circuit switching allocates a fixed path for each flow, circuits can be established according to traffic engineering algorithms.

On the other hand, circuit switching networks are not reactive when a network topology change occurs. For instance, on a link failure, all circuits on a failed link are cut and communication is interrupted. Special mechanisms that handle such topological changes have been be devised. Traffic engineering can alleviate the consequences of a link failure by pre-planning failure recovery. A backup circuit can be established at the same time or after the primary circuit used for a communication is set up, and traffic can be rerouted from the failed circuit to the backup circuit if a link of the primary circuit fails. Circuit switching networks are intrinsically sensitive to link failures and rerouting must be performed by additional traffic engineering mechanisms.

Datagram packet switching

Conceived in the 1960's, packet switching is a more recent technology than circuit switching which addresses a disadvantage of circuit switching: the need to allocate resources for a circuit, thus incurring link capacity wastes when no data flows on a circuit. Packet switching introduces the idea of cutting data on a flow into packets which are transmitted over a network without any resource being allocated. If no data is available at the sender at some point during a communication, then no packet is transmitted over the network and no resources are wasted. Packet switching is the generic name for a set of two different techniques: datagram packet switching and virtual circuit packet switching. Here, we give an overview of datagram packet switching.

Figure 1.2: Datagram Packet Switching. Packets from a given flow are independent and a router can forward two packets from the same flow on two

different links.

Different from circuit switching, datagram packet switching does not require to establish circuits prior to transmission of data and terminate circuits after the transmission of data. The switches, called routers, have to make a lookup in the forwarding table, called routing table, for each incoming packet. A routing table

contains a mapping between the possible final destinations of packets and the outgoing link on their path to the destination. Routing tables can be very large because they are indexed by possible destinations, making lookups and routing decisions computationally expensive, and the full forwarding process relatively slow compared to circuit switching. In datagram packet switching networks, each packet must carry the address of the destination host and use the destination address to make a forwarding decision. Consequently, routers do not need to modify the destination addresses of packets when forwarding packets.

Since each packet is processed individually by a router, all packets sent by a host to another host are not guaranteed to use the same physical links. If the routing algorithm decides to change the routing tables of the network between the instants two packets are sent, then these packets will take different paths and can even arrive out of order. In Figure 1.2 for instance, packets use two different paths to go from User 1 to User 5. Second, on a network topology change such as a link failure, the routing protocol will automatically recompute routing tables so as to take the new topology into account and avoid the failed link. As opposed to circuit switching, no additional traffic engineering algorithm is required to reroute traffic.

Since routers make routing decisions locally for each packet, independently of the flow to which a packet belongs. Therefore, traffic engineering techniques, which heavily rely on controlling the route of traffic, are more difficult to implement with datagram packet switching than with circuit switching.

Virtual circuit packet switching

Figure 1.3: Virtual circuit packet switching. All packets from the same flow use the same virtual circuit.

Virtual circuit packet switching (VC-switching) is a packet switching technique which merges datagram packet switching and circuit switching to extract both of their advantages. VC-switching is a variation of datagram packet switching where packets flow on so-called logical

circuits for which no physical resources like frequencies or time slots are allocated (see Figure 1.3). Each packet carries a circuit identifier which is local to a link and updated by each switch on the path of the packet from its source to its destination. A virtual circuit is defined by the sequence of the mappings between a link taken by packets and the circuit identifier packets carry on this link. This sequence is set up at connection establishment time and identifiers are reclaimed during the circuit termination.

We have seen the trade-off between connection establishment and forwarding time costs that exists in circuit switching and datagram packet switching. In VC-switching, routing is performed at circuit establishment time to keep packet forwarding fast. Other advantages of VC-switching include the traffic engineering capability of circuit switching, and the resources usage efficiency of datagram packet switching. Nevertheless, a main issue of VC-Switched networks is the behavior on a topology change. As opposed to Datagram Packet Switched networks which automatically recompute routing tables on a topology change like a link failure, in VC-switching all virtual circuits that pass through a failed link are interrupted. Hence, rerouting in VC-switching relies on traffic engineering techniques.

In practice, major implementations of VC-switching are X.25 [70], Asynchronous Transfer Mode (ATM [6]) and Multiprotocol Label Switching (MPLS [50]). The Internet, today's most used computer network, is entirely built around the Internet Protocol (IP), which is responsible for routing packets from one host to another. Because of the central role of IP in the Internet, we now discuss how ATM and MPLS interact with IP.

Circuit switching

In telecommunications, a circuit switching network is one that establishes a circuit (or channel) between nodes and terminals before the users may communicate, as if the nodes were physically connected with an electrical circuit.

The bit delay is constant during a connection, as opposed to packet switching, where packet queues may cause varying packet transfer delay. Each circuit cannot be used by other callers until the circuit is released and a new connection is set up. Even if no actual communication is taking place in a dedicated circuit that channel remains unavailable to other users. Channels that are available for new calls to be set up are said to be idle.

Virtual circuit switching is a packet switching technology that may emulate circuit switching, in the sense that the connection is established before any packets are transferred, and that packets are delivered in order.

There is a common misunderstanding that circuit switching is used only for connecting voice circuits (analog or digital). The concept of a dedicated path persisting between two communicating parties or nodes can be extended to signal content other than voice. Its advantage is that it provides for non-stop transfer without requiring packets and without most of the overhead traffic usually needed, making maximal and optimal use of available bandwidth for that communication. The disadvantage of inflexibility tends to reserve it for specialized applications, particularly with the overwhelming proliferation of internet-related technology.

Contents

[hide]

• 1 The call • 2 Compared to datagram packet

switching • 3 Examples of circuit switched

networks • 4 See also

• 5 External links

[edit] The call

For call setup and control (and other administrative purposes), it is possible to use a separate dedicated signalling channel from the end node to the network. ISDN is one such service that uses a separate signalling channel while Plain Old Telephone Service (POTS) does not.

The method of establishing the connection and monitoring its progress and termination through the network may also utilize a separate control channel as in the case of links between telephone exchanges which use CCS7 packet-switched signalling protocol to communicate the call setup and control information and use TDM to transport the actual circuit data.

Early telephone exchanges are a suitable example of circuit switching. The subscriber would ask the operator to connect to another subscriber, whether on the same exchange or via an inter-exchange link and another operator. In any case, the end result was a physical electrical connection between the two subscribers' telephones for the duration of the call. The copper wire used for the connection could not be used to carry other calls at the same time, even if the subscribers were in fact not talking and the line was silent.

[edit] Compared to datagram packet switching

Since the first days of the telegraph it has been possible to multiplex multiple connections over the same physical conductor, but nonetheless each channel on the multiplexed link was either dedicated to one call at a time, or it was idle between calls.

With circuit switching, and virtual circuit switching, a route is reserved from source to destination. The entire message is sent in order so that it does not have to be reassembled at the destination. Circuit switching can be relatively inefficient because capacity is wasted on connections which are set up but are not in continuous use (however momentarily). On the other hand, the connection is immediately available and capacity is guaranteed until the call is disconnected.

Circuit switching contrasts with packet switching which splits traffic data (for instance, digital representation of sound, or computer data) into chunks, called packets, that are routed over a shared network.

Packet switching is the process of segmenting a message/data to be transmitted into several smaller packets. Each packet is labeled with its destination and the number of the packet, precluding the need for a dedicated path to help the packet find its way to its destination. Each is dispatched and many may go via different routes. At the destination, the original message is reassembled in the correct order, based on the packet number. Datagram packet switching networks do not require a circuit to be established and allow many pairs of nodes to communicate almost simultaneously over the same channel.

[edit] Examples of circuit switched networks

• Public Switched Telephone Network (PSTN) • ISDN B-channel • Circuit Switched Data (CSD) and High-Speed Circuit-Switched Data (HSCSD)

service in cellular systems such as GSM • Datakit • X.21 (Used in the German DATEX-L and Scandinavian DATEX circuit switched

data network)

Introduction to WAN Technologies

This chapter introduces the various protocols and technologies used in wide-area network (WAN) environments. Topics summarized here include point-to-point links, circuit switching, packet switching, virtual circuits, dialup services, and WAN devices. Chapters in Part III, "WAN Protocols," address specific technologies in more detail.

What Is a WAN?

A WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. Figure 3-1 illustrates the relationship between the common WAN technologies and the OSI model.

Figure 3-1 WAN Technologies Operate at the Lowest Levels of the OSI Model

Point-to-Point Links

A point-to-point link provides a single, pre-established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. Point-to-point lines are usually leased from a carrier and thus are often called leased lines. For a point-to-point line, the carrier allocates pairs of wire and facility hardware to your line only. These circuits are generally priced based on bandwidth required and distance between the two connected points. Point-to-point links are generally more expensive than shared services such as Frame Relay. Figure 3-2 illustrates a typical point-to-point link through a WAN.

Figure 3-2 A Typical Point-to-Point Link Operates Through a WAN to a Remote Network

Circuit Switching

Switched circuits allow data connections that can be initiated when needed and terminated when communication is complete. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the two networks are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated. Figure 3-3 illustrates an example of this type of circuit.

Figure 3-3 A Circuit-Switched WAN Undergoes a Process Similar to That Used for a Telephone Call

Packet Switching

Packet switching is a WAN technology in which users share common carrier resources. Because this allows the carrier to make more efficient use of its infrastructure, the cost to the customer is generally much better than with point-to-point lines. In a packet switching setup, networks have connections into the carrier's network, and many customers share the carrier's network. The carrier can then create virtual circuits between customers' sites by which packets of data are delivered from one to the other through the network. The section of the carrier's network that is shared is often referred to as a cloud.

Some examples of packet-switching networks include Asynchronous Transfer Mode (ATM), Frame Relay, Switched Multimegabit Data Services (SMDS), and X.25. Figure 3-4 shows an example packet-switched circuit.

The virtual connections between customer sites are often referred to as a virtual circuit.

Figure 3-4 Packet Switching Transfers Packets Across a Carrier Network

WAN Virtual Circuits

A virtual circuit is a logical circuit created within a shared network between two network devices. Two types of virtual circuits exist: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

SVCs are virtual circuits that are dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the virtual circuit between the source and destination devices. Data transfer involves transmitting data between the devices over the virtual circuit, and the circuit termination phase involves tearing down the virtual circuit between the source and destination devices. SVCs are used in situations in which data transmission between devices is sporadic, largely because SVCs increase bandwidth used due to the circuit establishment and termination phases, but they decrease the cost associated with constant virtual circuit availability.

PVC is a permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with the establishment and termination of virtual circuits, but they increase costs due to constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.

WAN Dialup Services

Dialup services offer cost-effective methods for connectivity across WANs. Two popular dialup implementations are dial-on-demand routing (DDR) and dial backup.

DDR is a technique whereby a router can dynamically initiate a call on a switched circuit when it needs to send data. In a DDR setup, the router is configured to initiate the call when certain criteria are met, such as a particular type of network traffic needing to be transmitted. When the connection is made, traffic passes over the line. The router configuration specifies an idle timer that tells the router to drop the connection when the circuit has remained idle for a certain period.

Dial backup is another way of configuring DDR. However, in dial backup, the switched circuit is used to provide backup service for another type of circuit, such as point-to-point or packet switching. The router is configured so that when a failure is detected on the primary circuit, the dial backup line is initiated. The dial backup line then supports the WAN connection until the primary circuit is restored. When this occurs, the dial backup connection is terminated.

WAN Devices

WANs use numerous types of devices that are specific to WAN environments. WAN switches, access servers, modems, CSU/DSUs, and ISDN terminal adapters are discussed in the following sections. Other devices found in WAN environments that are used in WAN implementations include routers, ATM switches, and multiplexers.

WAN Switch

A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model. Figure 3-5 illustrates two routers at remote ends of a WAN that are connected by WAN switches.

Figure 3-5 Two Routers at Remote Ends of a WAN Can Be Connected by WAN Switches

Access Server

An access server acts as a concentration point for dial-in and dial-out connections. Figure 3-6 illustrates an access server concentrating dial-out connections into a WAN.

Figure 3-6 An Access Server Concentrates Dial-Out Connections into a WAN

Modem

A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form. Figure 3-7 illustrates a simple modem-to-modem connection through a WAN.

Figure 3-7 A Modem Connection Through a WAN Handles Analog and Digital Signals

CSU/DSU

A channel service unit/digital service unit (CSU/DSU) is a digital-interface device used to connect a router to a digital circuit like a T1. The CSU/DSU also provides signal timing for communication between these devices. Figure 3-8 illustrates the placement of the CSU/DSU in a WAN implementation.

Figure 3-8 The CSU/DSU Stands Between the Switch and the Terminal

ISDN Terminal Adapter

An ISDN terminal adapter is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232 on a router. A terminal adapter is essentially an ISDN modem, although it is called a terminal adapter because it does not actually convert analog to digital signals. Figure 3-9 illustrates the placement of the terminal adapter in an ISDN environment.

Figure 3-9 The Terminal Adapter Connects the ISDN Terminal Adapter to Other Interfaces