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route the messages to their final destinations. Therefore, network providers need a mechanism that allows them to deliver the proper connections when and where a customer requests them. When, as you can imagine, is ideally now or bandwidth on demand. Where has two components: path

calculation, which entails establishing the proper physical or logical connection to the ultimate destination, and forwarding, which is concerned with how to actually guide the traffic across the backbone so that it uses that physical and logical connection to best advantage.

The networking techniques that evolved over time to handle the when and where came about because traditionally, relatively few high-capacity backbone cables existed. Those few backbone cables had to be manipulated to meet the needs of many individual customers, all of whom had varied bandwidth needs. Two networking techniques arose:

• Networking modes— There are two networking modes: connection oriented and

connectionless.

• Switching modes— There are also two switching modes: circuit switching and packet

switching. Both of these switching modes offer forms of bandwidth on demand. (But remember that the connection speed can never be greater than the speed of the customer's access line; the fastest connection you can get into the network is what your access line supports.) As you'll learn later in this chapter, circuit switching and packet switching have different ways of performing path calculations and forwarding functions.

The following sections describe networking modes and switching modes in detail. Networking Modes

When most people are evaluating a network, they concentrate on circuit switching versus packet switching. But it's also very important to consider the networking mode, which can be either

For more learning resources, quizzes, and discussion forums on concepts related to this chapter, see www.telecomessentials.com/learningcenter.

Connection-Oriented Networking

As time-sensitive applications become more important, connection-oriented networks are becoming increasingly desirable. In a connection-oriented network, the connection setup is performed before information transfer occurs. Information about the connections in the networks helps to provide service guarantees and makes it possible to most efficiently use network bandwidth by switching transmissions to appropriate connections as the connections are set up. In other words, the path is conceived at the outset, and after the path is determined, all the subsequent information follows the same path to the destination. In a connection-oriented network, there can be some delay up front while the connection is being set up; but because the path is predetermined, there is no delay at intermediate nodes in this type of network after the connection is set up.

Connection-oriented networks can actually operate in either switching mode: They can be either circuit switched or packet switched. Connection-oriented circuit-switched networks include the PSTN (covered later in this chapter and in detail in Chapter 5, "The PSTN"), SDH/SONET (covered in more detail in Chapter 5), and DWDM (covered in detail in Chapter 12, "Optical Networking") networks. Connection-oriented packet-switched networks (covered later in this chapter and in detail in Chapter 7, "Wide Area Networking") include X.25, Frame Relay, and ATM networks.

Connection-oriented networks can be operated in two modes:

• Provisioned— In provisioned networks, the connections can be set up ahead of time based

on expected traffic. These connections are known as permanent virtual circuits (PVCs).

• Switched— In switched networks, the connections are set up on demand and released after

the data exchange is complete. These connections are known as switched virtual circuits (SVCs).

Connectionless Networking

In a connectionless network, no explicit connection setup is performed before data is transmitted. Instead, each data packet is routed to its destination based on information contained in the header. In other words, there is no preconceived path. Rather, each fragment (that is, packet) of the overall traffic stream is individually addressed and individually routed. In a connectionless network, the delay in the overall transit time is increased because each packet has to be individually routed at each intermediate node. Applications that are time sensitive would suffer on a connectionless network because the path is not guaranteed, and therefore it is impossible to calculate the potential delays or latencies that might be encountered.

Connectionless networks imply the use of packet switches, so only packet-switched networks are connectionless. An example of a connectionless packet-switched network is the public Internet—that wild and woolly place over which absolutely no one has any control. It's a virtual network that consists of more than 150,000 separate subnetworks and some 10,000 Internet service providers (ISPs), so being able to guarantee performance is nearly impossible at this time. One solution is to use private internets (that is, Internet Protocol [IP] backbones), which achieve cost-efficiencies but, because they are private, provide the ability to control their performance and thereby serve business- class services. For example, a large carrier (such as AT&T or British Telecom) might own its own internet infrastructure, over a very wide geographic area. Because it owns and controls those networks end to end, it can provision and engineer the networks so that business customers can get the proper service-level agreements and can guarantee the performance of their virtual private

networks and streaming media networks. The downside in this situation is reliance on one vendor for the entire network.

Let's start our discussion of switching modes by talking about switching and routing. Switching is the process of physically moving bits through a network node, from an input port to an output port. (A network node is any point on the network where communications lines interface. So a network node might be a PBX, a local exchange, a multiplexer, a modem, a host computer, or one of a

number of other devices.) Switching elements are specialized computers that are used to connect two or more transmission lines. The switching process is based on information that's gathered through a routing process. A switching element might consult a table to determine, based on number dialed, the most cost-effective trunk over which to forward a call. This switching process is relatively

straightforward compared to the type of path determination that IP routers in the Internet might use, which can be very complex.

Routing, on the other hand, involves moving information from a source to a destination across an internetwork, which means moving information across networks. In general, routing involves at least one intermediate node along the way, and it usually involves numerous intermediate nodes and networks. Routing involves two basic activities: determining the optimal path and transporting information through an internetwork. Routing algorithms are necessary to initialize and maintain routing tables. Routing algorithms work with a whole slew of information, called metrics, which they use to determine the best path to the destination. Some examples of the metrics that a routing algorithm might use are path length, destination, next-hop associations, reliability, delay, bandwidth, load, and communication cost. A router could use several variables to calculate the best path for a packet, to get it to a node that's one step closer to its destination. The route information varies

depending on the algorithm used, and the algorithms vary depending on the routing protocol chosen. Most manufacturers today support the key standards, including Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Intermediate System to Intermediate System (IS-IS). Network engineers generally decide which of these protocols to use. Routing protocols can also be designed to automatically detect and respond to network changes. (Protocols and metrics are discussed in detail in Chapter 9, "The Internet: Infrastructure and Service Providers.")

There are two types of routers that you should be familiar with: static routers and dynamic routers. A static router knows only its own table; it has no idea what the routing tables of its upstream

neighbors look like, and it does not have the capability of communicating with its upstream

neighbors. If a link goes down in a network that uses static routers, the network administrator has to manually reconfigure the static routers' routing tables to take the downed trunk out of service. This reconfiguration would not affect any change in the upstream routers, so technicians at those locations would then also have to include or accommodate the change. A dynamic router, on the other hand, can communicate with its upstream neighbors, so if a change occurred to its routing table, it would forward that change so that the upstream routers could also adjust their routing tables. Furthermore, a dynamic router not only has a view of its own routing table, but it can also see those of its neighbors, or the entire network or routing area, depending on the protocol. It therefore works much better in addressing the dynamic traffic patterns that are common in today's networks.

As noted earlier in the chapter, there are two switching modes: circuit switching and packet switching. Circuit switches are position based; that is, bits arrive in a certain position and are switched to a different position. The position to which bits are switched is determined by a

combination of one or more of three dimensions: space (that is, the interface or port number), time, and wavelength. Packet switching is based on labels; addressing information in the packet headers, or labels, helps to determine how to switch or forward a packet through the network node.

Circuit Switching

Circuit switching has been the basis of voice networks worldwide for many years. You can apply three terms to the nature of a circuit-switched call to help remember what this is: continuous, exclusive, and temporary. One of the key attributes of a circuit-switched connection is that it is a reserved network resource that is yours and only yours for the full duration of a conversation. But

when that conversation is over, the connection is released. A circuit-switched environment requires that an end-to-end circuit be set up before a call can begin. A fixed share of network resources is reserved for the call, and no other call can use those resources until the original connection is closed. A call request signal must travel to the destination and be acknowledged before any transmission can actually begin. As Figure 4.1 illustrates, you can trace the path from one end of the call to the other end; that path would not vary for the full duration of the call, and the capacity provisioned on that path would be yours and yours alone.

Figure 4.1. A circuit-switched call

Advantages and Disadvantages of Circuit Switching Circuit switching uses smany lines to

economize on switching and routing computation. When a call is set up, a line is dedicated to it, so no further routing calculations are needed.

Since they were introduced in the mid-1980s, digital cross-connect systems (DCSs) have greatly eased the process of reconfiguring circuit-switched networks and responding to conditions such as congestion and failure. DCSs create predefined circuit capacity, and then voice switches are used to route calls over circuits that are set up by these DCSs. DCSs are analogous to the old patch panels. You may have seen a main distribution frame (MDF) on which twisted-pair wiring is terminated. The MDF is a manual patch panel, and before DCSs were introduced, when it was necessary to reconfigure a network based on outage, congestion, or customer demand as a result of shifting traffic patterns, technicians had to spend days or even weeks, manually making changes at the MDF. The DCS is a software patch panel, and within the software are databases that define alternate routes— alternate connections that can be activated in the event that the network encounters a condition that requires some form of manipulation. DACSs are one of the elements of the PSTN that contribute to its reliability: When network conditions change, in a matter of minutes, a DCS can reconfigure the network around those changes. With such tools, the PSTN is able to offer five 9s reliability—in other words, 99.999% guaranteed uptime. (DCSs are discussed in more detail in Chapter 5.)

Circuit switching offers the benefits of low latency and minimal delays because the routing

calculation on the path is made only once, at the beginning of the call, and there are no more delays incurred subsequently in calculating the next hop that should be taken. Traditionally, this was sometimes seen as a disadvantage because it meant that the circuits might not be used as efficiently as possible. Around half of most voice calls is silence. Most people breathe and occasionally pause in their speech. So, when voice communications are conducted over a circuit that's being

continuously held, and half the time nothing is being transmitted, the circuit is not being used very efficiently. But remember that this is an issue that is important when bandwidth is constrained. And

as mentioned earlier in the book, through the optical revolution, bandwidth is being released at an astounding rate, so the efficient use of circuits because of bandwidth constraints will not present the same sort of issue in the future that it once did. Hence, the low latencies or delays that circuit switching guarantees are more important than its potential drawbacks in bandwidth efficiency. Circuit switching has been optimized for real-time voice traffic for which Quality of Service (QoS) is needed. Because it involves path calculation at the front end, you know how many switches and cables you're going to go through, so you can use a pricing mechanism that's based on distance and time. The more resources you use, either over time or over distance, the greater the cost. Again, developments in fiber economics are changing some of the old rules, and distance is no longer necessarily an added cost element. (QoS is discussed in more detail in Chapter 10, "Next-Generation Networks.")

Generations of Circuit Switches Circuit switches have been around for quite some time. We've already been through three basic generations, and we're beginning to see a fourth generation.

The first generation of circuit switches was introduced in 1888. It was referred to as the step relay switch, the step-by-step switch, or the Strowger switch, in honor of the man who invented it (see

Figure 4.2).

Figure 4.2. A step relay switch