There are three basic switching paradigms: circuit, message, and packet switching. Circuit switching packet switching are transmission technologies while message switching is a service technology. In circuit switching, a call connection between two terminating equipments corresponds to the allocation of a prescribed set of physical facilities that provide a transmission path of a certain bandwidth or transmission capacity. These facilities are dedicated to the users for the duration of the call. The primary performance issues, other than ose related to quality of transmission, are related to whether or not a transmission path is available at call set-up time and how calls are handled if facilities are not available. Message switching is similar in concept to the postal system. When a user wants to send a message to one or more recipients, the user forms the message and addresses it. The message switching system reads the address and forwards the complete message to the next switch in the path. The message moves asynchronously through the network on a message switch-to-message switch basis until it reaches its destination. Message switching systems offer services such as mail boxes, multiple destination delivery, automatic verification of message delivery, and bulletin board. Communication links between the message switches may be established using circuit or packet switching networks as is the case with most other networking applications. Examples of message switching protocols that have been used to build message switching systems Mail Transfer Protocol (SMTP)and the International Telegraph and Telephone Consultative C (CCITT)X400 series. The former is much more widely deployed, while the latter has significantly capabilities, but its deployment is plagued by having two incompatible versions(1984 and 1988) problems. Many commercial vendors offer message switching services based on either one of the above protocols or a proprietary protocol In the circuit switching case, there is a one-to-one correspondence between the number of trunks between nodes and the number of simultaneous calls that can be carried. That is, a trunk is a facility between two switches that can service exactly one call, and it does not matter how this transmission facility is derived. Major design issues include the specification of the number of trunks between node pairs and the routing strategy used to determine the path through a network in order to achieve a given call blocking probability. When blocked calls are queued, the number of calls that may be queued is also a design question. a packet-switched communication system exchanges messages among users by transmitting sequences of packets which comprise the messages. That is, the sending terminal equipment partitions a message into a sequence of packets, the packets are transmitted across the network, and the receiving terminal equipment reassembles the packets into messages. The transmission facility interconnecting a given node pair is viewed a single trunk, and the transmission capacity of this trunk is shared among all users whose packets traverse both nodes. While the trunk capacity is specified in bits per second, the packet handling capacity of a node pair depends both upon the trunk capacity and the nodal processing power. In many packet-switched networks, the path traversed by a packet through the network is established during a call set-up procedure, and the network is referred to as a virtual circuit packet switching network. Other networks provide datagram service, a service that allows users to transmit individually addressed packets without the need for call set-up Datagram networks have the advantage of not having to establish connections before communication takes place, but they have the disadvantage that every packet must contain complete addressing information. Virtual circuit networks have the advantage that addressing information is not required in each packet, but have the disadvantage that a call set-up must take place before communication can occur. Datagram is an example of connectionless service while virtual circuit is an example of connection-oriented service. Prior to the late 1970s, signaling for circuit establishment was in-band. That is, in order to set up a call through the network, the call set-up information was sent sequentially from switch to switch using the actual circuit that would eventually become the circuit used to connect the end users. In an extreme case, this amounted to trying to find a path through a maze, sometimes having to retrace one's steps before finally emerging at the destination or just simply giving up when no path could be found. This had two negative characteristics: first, the rate of signaling information transfer was limited to the circuit speed, and second, the circuits that could have been used for accomplishing the end objective were being consumed simply to find a path between the end-points. This resulted in tremendous bottlenecks on major holidays, which were solved by virtually disal- lowing alternate routes through the toll switching network. An alternate out-of-band signaling system, usually called common-channel interoffice signaling(CCIS), was developed primarily to solve this problem. Signaling now takes place over a signaling network that is e 2000 by CRC Press LLC© 2000 by CRC Press LLC There are three basic switching paradigms: circuit, message, and packet switching. Circuit switching and packet switching are transmission technologies while message switching is a service technology. In circuit switching, a call connection between two terminating equipments corresponds to the allocation of a prescribed set of physical facilities that provide a transmission path of a certain bandwidth or transmission capacity. These facilities are dedicated to the users for the duration of the call. The primary performance issues, other than those related to quality of transmission, are related to whether or not a transmission path is available at call set-up time and how calls are handled if facilities are not available. Message switching is similar in concept to the postal system. When a user wants to send a message to one or more recipients, the user forms the message and addresses it. The message switching system reads the address and forwards the complete message to the next switch in the path. The message moves asynchronously through the network on a message switch-to-message switch basis until it reaches its destination. Message switching systems offer services such as mail boxes, multiple destination delivery, automatic verification of message delivery, and bulletin board. Communication links between the message switches may be established using circuit or packet switching networks as is the case with most other networking applications. Examples of message switching protocols that have been used to build message switching systems are Simple Mail Transfer Protocol (SMTP) and the International Telegraph and Telephone Consultative Committee (CCITT) X.400 series. The former is much more widely deployed, while the latter has significantly broader capabilities, but its deployment is plagued by having two incompatible versions (1984 and 1988) and other problems. Many commercial vendors offer message switching services based on either one of the above protocols or a proprietary protocol. In the circuit switching case, there is a one-to-one correspondence between the number of trunks between nodes and the number of simultaneous calls that can be carried. That is, a trunk is a facility between two switches that can service exactly one call, and it does not matter how this transmission facility is derived. Major design issues include the specification of the number of trunks between node pairs and the routing strategy used to determine the path through a network in order to achieve a given call blocking probability. When blocked calls are queued, the number of calls that may be queued is also a design question. A packet-switched communication system exchanges messages among users by transmitting sequences of packets which comprise the messages. That is, the sending terminal equipment partitions a message into a sequence of packets, the packets are transmitted across the network, and the receiving terminal equipment reassembles the packets into messages. The transmission facility interconnecting a given node pair is viewed as a single trunk, and the transmission capacity of this trunk is shared among all users whose packets traverse both nodes. While the trunk capacity is specified in bits per second, the packet handling capacity of a node pair depends both upon the trunk capacity and the nodal processing power. In many packet-switched networks, the path traversed by a packet through the network is established during a call set-up procedure, and the network is referred to as a virtual circuit packet switching network. Other networks provide datagram service, a service that allows users to transmit individually addressed packets without the need for call set-up. Datagram networks have the advantage of not having to establish connections before communication takes place, but they have the disadvantage that every packet must contain complete addressing information. Virtual circuit networks have the advantage that addressing information is not required in each packet, but have the disadvantage that a call set-up must take place before communication can occur. Datagram is an example of connectionless service while virtual circuit is an example of connection-oriented service. Prior to the late 1970s, signaling for circuit establishment was in-band. That is, in order to set up a call through the network, the call set-up information was sent sequentially from switch to switch using the actual circuit that would eventually become the circuit used to connect the end users. In an extreme case, this amounted to trying to find a path through a maze, sometimes having to retrace one’s steps before finally emerging at the destination or just simply giving up when no path could be found. This had two negative characteristics: first, the rate of signaling information transfer was limited to the circuit speed, and second, the circuits that could have been used for accomplishing the end objective were being consumed simply to find a path between the end-points. This resulted in tremendous bottlenecks on major holidays, which were solved by virtually disal￾lowing alternate routes through the toll switching network. An alternate out-of-band signaling system, usually called common-channel interoffice signaling (CCIS), was developed primarily to solve this problem. Signaling now takes place over a signaling network that is