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VCEL vertical cavity surface emitting laser
VCI virtual channel identifier
VDSL very high data rate DSL
VoIP voice over IP
VoMPLS voice over MPLS
VPI virtual path identifier
VPN virtual private networks
VPC virtual path connections
VTG virtual tributary group
WAN wide area networks
WDM wavelength division multiplexing
WF wildcard-filter style
xDSL x-type digital subscriber line
This book deals with several different circuit-switching networks and connection-oriented packet-switching networks. These networks, although seemingly different, have all been built around the notion of a connection. That is, a connection has to first be set up between two users before they can communicate. Such a connection is set up by allocating network resources to it. The nature of these resources, as will be seen in this book, depends on the type of the network.
The notion of a connection is also prevalent in the IP network and IP-related protocols. For instance, a TCP connection has to be set up before two TCP users can communicate. This type of connection, however, is not the same as the connection in circuit-switching networks and connection-oriented packet-switching networks. For instance, let us consider an IP network that runs over Ethernet. In this case, when two peer TCP protocols set up a connection, the IP routers and the Ethernet switches are not aware of this connection and so do not allocate any resources to it.
In this chapter, we first describe the concept of a connection as used in this book, and then give examples of connections from the circuit-switching and connection-oriented packet-switching networks described in this book. Subsequently, we describe the organization of this book and the scope and objectives of each chapter. Finally, we present some of the well-known national and international standards committees involved with the standardization process of networking architectures and protocols.
1.1 COMMUNICATION NETWORKS
Communication networks can be classified into the following two broad categories: switched communication networks and broadcast communication networks. As shown in Figure 1.1, switched communication networks are further classified into circuit-switching networks and packet-switching networks. Circuit switching and packet switching are two different technologies that evolved over a long time. Examples of circuit-switching networks are the telephone network and the wavelength routing optical network. Examples of packet-switching networks are the IP network, ATM, frame relay, and MPLS networks. Examples of broadcast communication networks are packet radio networks, satellite networks, and multi-access local networks (such as the Ethernet).
Packet-switching networks are further classified as connection-oriented networks and connectionless networks. Examples of connection-oriented networks are: X.25, ATM, frame relay, and MPLS. The prime example of a connectionless network is the ubiquitous IP network.
Connection-oriented Networks Harry Perros © 2005 John Wiley & Sons, Ltd ISBN: 0-470-02163-2
• X.25 • IP network
• Frame relay
Figure 1.1 A classification of communication networks.
In a circuit-switching network, in order for two users to communicate, a circuit or a connection has to be first established by the network. Specifically, three phases are involved: circuit establishment, data transfer, and circuit disconnect. These three phases take place, for instance, when we make a phone call. Circuit establishment takes place when we dial a number. At that moment, the telephone network attempts to establish a connection to the phone of the called party. This involves finding a path to the called party, allocating a channel on each transmission link along the path, and alerting the called party. The data transfer phase follows, during which we converse with the person we called. Finally, the circuit disconnect phase takes place when we hang up. At that moment, the network tears down the connection and releases the allocated channel on each link on the path. The connection is dedicated to the two users for the duration of the call, even when no data is being sent. That is, the channel allocated on each transmission link along the path from our phone to the one we called is not shared with any other phone calls. Also, in order for the call to be established, both stations must be available at the same time.
Circuit switching is a good solution for voice, since it involves exchanging a relatively continuous flow of data. However, it is not a good solution for the transmission of bursty data; that is, data that continuously alternates between an active period and a silent period. Transmission of data only takes place when the source is in the active period. Such intermittent data transmission is typical in high-speed networks, and leads to low utilization of the circuit-switching connection.