Time-Division Multiple Access (TDMA)
TDMA divides the channels into distinct time slots. Each time slot is designated for a particular networks node, as if it were a dedicated line. The WAN switching device rotates from time slot to time slot for each channel. This is similar to a 24-hour television programming, where the time has been specified for a particular program. TDMA does not guarantee the most efficient use of the network medium since transmission occurs only via one channel at a time. The timing of node transmission is also important, since a node may transmit at an interval that is out of synchronization with its time slot.
  Frequency Division Multiple Access (FDMA)
FDMA divides the channels into frequencies instead of time slots. Each channel has its own broadcast frequency and bandwidth. The switching device switches from frequency to frequency as it sends data. This is similar to four listners with headsets sharing a radio modified to have four channels. The first listener might be listening to a classical station, the second to a talk show, the third to a base ball game and the fourth to the news. Each listener is at a different frequency. The radio inputs to each channel so quickly that none can tell it is quickly switching from channel to channel as it receives the signal on each frequency.
  Statistical Multiple Access (SMA)
Statistical multiple access or statistical multiplexing, is used by many WAN technologies, such as X.25, ISDN and frame relay. This method is more efficient than TDMA and FDMA, because the physical medium bandwidth is dynamically allocated according to the application need. The switching device continuously monitors each channel to determine the communication requirements. For example, at one moment a channel may need to transmit a large graphics file, and then be quiet. Algorithms on the switch determine the bandwidth needed to transmit the file. After the file is transmitted, the switch reallocates bandwidth to another channel. This might be compared to the way in which a workstation operating system automatically decides how much memory to give to three applications running at the same time. It might give 15 KB for an active word-processing file, 7 MB for an image from a scanner and 1.2 MB for printing a graphic.

  Circuit Switching
Circuit switching involves creating a dedicated physical circuit between the sending and receiving nodes. This acts as a straight channel on which to send data back and forth without interruption, similar to a telephone call between two parties. The transmission channel remains in the service until the two nodes disconnect. Communication via circuit switching implies that there is a dedicated communication path between two stations. The path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection. Communication via circuit switching involves three phases, as illustrated in Figure.

Phase I: Circuit establishment. Before any signals can be transmitted, an end­to-end (station to station) circuit must be established. For example, Station A (see Figure) sends a request to node 4, requesting a connection to Station E. Typically, the link from A to 4 is a dedicated line, so that part of the connection already exists. Node 4 must find the next leg in a route leading to node 6 based on routing information and measures of availability and perhaps cost. Node 4 selects the link to node 5, allocates a free channel (using FDM or TDM) on that link and sends a message requesting connection to E. So far, a dedicated path has been established from A through 4 to 5. Because a number of stations may attach to 4, it must be able to establish internal paths from multiple stations to multiple nodes. The remainder of the process proceeds similarly. Node 5 dedicates a channel to node 6 and internally ties that channel to the channel from node 4. Node 6 completes the connection to E. In completing the connection, a test is made to determine if E is busy or is prepared to accept the connection.
Phase II: Data transfer. Information can now be transmitted from A through the network to E. The data may be analog or digital, depending on the nature of the network. As the carriers evolve the fully integrated digital networks, the use of digital (binary) transmission for both voice and data is becoming the dominant method. The path is: A-4 link, internal switching through 4; 4-5 channel, internal switching through 5; 5-6 channel, internal switching through 6; 6-E link. Generally, the connection is full duplex.
Phase III: Circuit disconnect.  After some period of data transfer, the connection is terminated, usually by the action of one of the two stations. Signals must be propagated to nodes 4, 5 and 6 to de-allocate the dedicated resources.
Note that the connection path is established before data transmission begins. Thus, channel capacity must be reserved between each pair of nodes in the path and each node must have available internal switching capacity to handle the request connection. The switches must have the intelligence to make these allocations and to devise a route through the network.
Circuit switching can be rather inefficient. Channel capacity is dedicated for the duration of a connection, even if no data are being transferred. For a voice connection, utilization may be rather high, but it still does not approach 100 per cent. For a terminal-to-computer connection, the capacity may be ideal during most of the time of the connection. In terms of performance, there is delay prior to signal transfer for call establishment. However, once the circuit is established, the network is effectively transparent to the users. Information is transmitted at a fixed data rate with no delay other than the propagation delay through the transmission links. The delay at each node is negligible.
Circuit switching was developed to handle voice traffic but is now also used for data traffic. The best-known example of a circuit-switching network is the public telephone network. This is actually a collection of national networks intern-connected to form the international service. Although originally designed and implemented to service analog telephone subscribers, it handles substantial data traffic via modem and is gradually being converted to a digital network. Another well-known application of circuit switching is the private branch exchange (PBX) used to interconnect telephones within a building or office.
  Message Switching
Message switching uses a store-and-forward communication method to transmit data from the sending to the receiving node. The data is sent from one node to another, which stores it temporarily until a route towards the data's final destination becomes available. Several nodes along the route store and forward the data until it reaches the destination node. Message switching is used, for example, when you send an e-mail message on an enterprise network with file servers acting as "post offices". The message goes from one post office to the next until it reaches the intended recipient. 7.1.6 Packet Switching
Circuit switching was designed for voice communication. In a telephone conversation, for example, once a circuit is established it remains connected for the duration of the session. Circuit switching creates temporary (dialed) or permanent (leased) dedicated links that are well suited to this type of communication.
A key characteristic of circuit-switching networks is that resources within the network are dedicated to a particular call. For voice connections, the resulting circuit will enjoy a high percentage of utilization because most of the time, one party or the other is talking. However, as the circuit-switching network began to be used increasingly for data connections, two shortcomings became apparent:
(1) In a typical user/host data connection (for example, a personal computer user logged on to a database server), much of the time the line is idle. Thus, with data connections, a circuit-switching approach is inefficient.
(2) In a circuit-switching network, the connection provides for transmission at a constant data rate. Thus, each of the two devices that are connected must transmit and receive at the same data rate as the other. This limits the utility of the network in interconnecting a variety of the host computers and workstations.
To understand how packet switching addresses these problems, let us briefly summarize the packet-switching operation. Data are transmitted in short packets. A typical upper bound on packet length is 1000 octets (bytes). If the source has no longer message to send, the message is broken up into a series of packets as shown in Figure 7.2. Each packet contains a portion (or all for a short message) of the user's data, plus some control information that the network requires to be able to route the packet through the network and deliver it to the intended destination. At each node en route, the packet is received, stored briefly and passed on to the next node.

Now assume that Figure 7.2 depicts a simple packet-switching network. Consider a packet to be sent from Station A to Station E. The packet includes control information that indicates that the intended destination is E. The packet is sent from A to node 4. Node 4 stores the packet, determines the next leg of the route (say 5) and queues the packet to go-out on that link (the 4-5 link). When the link is available, the packet is transmitted to node 5, which forwards the packet to node 6 and finally to E. This approach has a number of advantages over circuit switching:
s Line efficiency is greater, because a single node-to-node link can be dynamically shared by many packets over time. The packets are queued up and transmitted as rapidly as possible over the link. By contrast, with circuit switching, time on a node-to-node link is pre-allocated using synchronous time-division multiplexing. Much of the time, such a link may be ideal because a portion of its time is dedicated to a connection that is ideal.
•        A packet-switching network can perform data-rate conversion. Two stations of different data rates can exchange packets because each connects to its node as its proper data rate.
•    When traffic becomes heavy on a circuit-switching network, some calls are blocked; that is, the network refuses to accept additional connection requests until the load on the network decreases. On a packet-switching network, packets are still accepted, but delivery delay increases.
•    Priorities can be used. Thus, if a node has a number of packets queued for transmission, it can transmit the higher-priority packets first. These packets will therefore experience less delay than lower-priority packets.
If the station has a message to send through a packet-switching network that is of length greater than the maximum packet size, it breaks the message up into packets and sends these packets, one at a time, to the network. A question arises as to how the network will handle this stream of packets as it attempts to route them through the network and deliver them to the intended destination. There are two approaches that are used in contemporary networks: datagram and virtual circuit.
In the datagram approach to packet switching, each packet is treated independently from all others. Even when one packet represents just a piece of a multipacket transmission, the network (and network layer functions) treats it as though it existed alone. Packets in this technology are referred to as datagrams. Figure 7.1 shows how the datagram approach can be used to deliver three packets from Station A to Station E. In this example, all the three packets (or datagrams) belong to the same message, but may go by different paths to reach their destination. This approach can cause the datagrams of a transmission to arrive at their destination out of order. It is the responsibility of the transport layer in most protocols to reorder the datagrams before passing them on to the destination port. The link joining each pair of nodes can contain multiple channels. Each of these channels is capable, in turn, of carrying datagrams either from several different sources or from one source. Multiplexing can be done using TDM or FDM.
In the virtual circuit approach to packet switching, the relationship between all packets belonging to a message or session is preserved. A single route is chosen between the sender and receiver at the beginning of the session. When the data are sent, all packets of the transmission travel one after another along that route. The difference from the datagram approach is that, with virtual circuits, the node need not make a routing decision for each packet. It is made only once for all packets using that virtual circuit.

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