Section 1.2. Types of Packet-Switched Networks

1.2. Types of Packet-Switched Networks

Packet-switched networks are classified into datagram or connectionless networks and virtual-circuit or connection-oriented networks, depending on the technique used for transferring information. The simplest form of a network service is based on the connectionless protocol. In this type of network, a user can transmit a packet anytime , without notifying the network layer. Packets are encapsulated into a certain "formatted" header, resulting in the basic Internet transmission unit of data, or datagram . A datagram is then sent over the network, with each router receiving the datagram forwarding it to the best router it knows , until the datagram reaches the destination. In this scheme, packets may be routed independently over different paths. However, the packets may arrive out of sequence. In this case, a certain network function (to be discussed later) takes care of the error control, flow control, and resequencing packets.

A related , though more complex, service is the connection-oriented protocol. Packets are transferred through an established virtual circuit between a source and a destination. When a connection is initially set up, network resources are reserved for the call duration. After the communication is finished, the connection is terminated , using a connection-termination procedure. During the call setup, the network can offer a selection of options, such as best-effort service, reliable service, guaranteed delay service, and guaranteed bandwidth service, as explained in Chapters 8 and 12.

1.2.1. Connectionless Networks

Connectionless packet switching achieves high throughput at the cost of queuing delay. In this approach, large packets are normally fragmented into smaller packets. Packets from a source are routed independently of one another. The connectionless-switching approach does not require a call setup to transfer packets, but it has error-detection capability. The main advantage of this scheme is its capability to route packets through an alternative path in case a fault is present on the desired transmission link. On the flip side, this may result in the packet's arriving out of order and requiring sequencing at the destination.

Figure 1.6 (a) shows the routing of four packets in a connectionless network from point A to point B. The packets traverse the intermediate nodes in a store-and-forward fashion, whereby packets are received and stored at a node on a route; when the desired output of the node is free for that packet, the output is forwarded to its next node. In other words, on receipt of a packet at a node, the packet must wait in a queue for its turn to be transmitted. Nevertheless, packet loss may still occur if a node's buffer becomes full. The node determines the next hop read from the packet header. In this figure, the first three packets are moving along the path A, D, C, and B, whereas the fourth packet moves on a separate path, owing to congestion on path AD.

Figure 1.6. Two models of data networks: (a) a connectionless network and (b) a connection-oriented network

The delay model of the first three packets discussed earlier is shown in Figure 1.7. The total transmission delay for a message three packets long traversing from the source node A to the destination node B can be approximately determined. Let t _p be the propagation delay between each two nodes, and let t _f be the packet-transfer time from one node to the next one. A packet is processed once it is received at a node with a processing time t _r . The total transmission delay, D _p for n _h nodes and n _p packets, in general is

Equation 1.1

Figure 1.7. Signaling delay in a connectionless environment

In this formula, D _p gives only the transmission delay. As discussed in Chapter 11, another delay component, known as the packet-queueing delay , can be added to D _p .At this point, we focus only on the transmission delay and will discuss the queueing delay later.

Example.

Figure 1.7 shows a timing diagram for the transmission of three packets on path A, D, C, B in Figure 1.6. Determine the total delay for transferring these three packets from node A to node B.

Solution.

Assume that the first packet is transmitted from the source, node A, to the next hop, node D. The total delay for this transfer is t p + t f + t r . Next, the packet is similarly transferred from node D to the next node to ultimately reach node B. The delay for each of these jumps is also t p + t f + t r . However, when all three packets are released from node A , multiple and simultaneous transmissions of packets become possible. Thus, the total delay for all three packets to traverse the source and destination via two intermediate nodes is D p = 3 t p + 5 t f + 4 t r .

Connectionless networks demonstrate the efficiency of transmitting a large message as a whole, especially in noisy environments, where the error rate is high. It is obvious that the large message should be split into packets. Doing so also helps reduce the maximum delay imposed by a single packet on other packets. In fact, this realization in fact resulted in the advent of connectionless packet switching.

1.2.2. Connection-Oriented Networks

In connection-oriented , or virtual-circuit , networks , a route set up between a source and a destination is required prior to data transfer, as in the case of conventional telephone networks. Figure 1.6 (b) shows a connection-oriented packet-switched network. The connection set-up procedure shown in this figure requires three packets to move along path A, D, C, and B with a prior connection establishment. During the connection setup process, a virtual path is dedicated, and the forwarding routing tables are updated at each node in the route.

Virtual-circuit packet switching typically reserves the network resources, such as the buffer capacity and the link bandwidth, to provide guaranteed quality of service and delay. The main disadvantage in connection-oriented packet-switched networks is that in case of a link or switch failure, the call set-up process has to be repeated for all the affected routes. Also, each switch needs to store information about all the flows routed through the switch.

The total delay in transmitting a packet in connection-oriented packet switching is the sum of the connection set-up time and the data-transfer time. The data-transfer time is the same as the delay obtained in connectionless packet switching. Figure 1.8 shows the overall delay for the three packets presented in the previous example. The transmission of the three packets starts with connection request packets and then connection accept packets. At this point, a circuit is established, and a partial path bandwidth is reserved for this connection. Then, the three packets are transmitted. At the end, a connection release packet clears and removes the established path.

Figure 1.8. Signaling delay in a connection-oriented packet-switched environment

The estimation of total delay time, D _t , to transmit n _h packets is similar to the one presented for connectionless networks. For connection-oriented networks, the total time consists of two components : D _p , which represents the time to transmit packets, and D _c , which represents the time for the control packets. Control-packets' time includes the transmission delay for the connection-request packet, the connection-accept packet, and the connection-release packet:

Equation 1.2

Another feature, called cut-through switching , can significantly reduce the delay. In this scheme, the packet is forwarded to the next hop as soon as the header is received and the destination is parsed. We see that the delay is reduced to the aggregate of the propagation times for each hop and the transfer time of one hop. This scheme is used in applications in which retransmissions are not necessary. Optical fiber transmission has a very low loss rate and hence uses cut-through switching to reduce the delay in transmitting a packet.