Figure 2-23 A switched network. in Software

Generator UPC - 13 in Software Figure 2-23 A switched network.

Figure 2-23 A switched network.
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uses to uniquely identify the incoming data. It does this by creating a unique combination of the unique logical address with the shared physical port to create an entirely unique virtual circuit identifier. When the message arrives at the first switch, the switch enters the logical channel information in a routing table that it uses to manage incoming and outgoing data. There can be other information in the routing table as well such as QoS indicators. These details will be covered later when we discuss the Network layer (layer three). The technology that a customer uses in a switched network is clearly not dedicated, but it gives the appearance that it is. This is called virtual circuit service, because it gives the appearance of being there when in fact it isn t. Virtual Private Networks (VPNs), for example, give a customer the appearance that they are buying private network service. In a sense, they are; they do have a dedicated logical facility. The difference is that they share the physical facilities with many other users, which enables the service provider to offer the transport service for a lower cost. Furthermore, secure protocols protect each customer s traffic from interception. VPNs are illustrated in Figure 2-24. As you may have intuited by now, the degree of involvement that the Transport layer has varies with the type of network. For example, if the network consists of a single, dedicated, point-to-point circuit, then very little could happen to the data during the transmission because the data would consist of an uninterrupted, single-hop stream. No switches along the way could cause pieces of the message to go awry. The Transport layer therefore would have little to do to guarantee the delivery of the message. However, what if the architecture of the network is not as robust as a private line circuit What if this is a packet network, in which case the message is broken into segments by the Transport layer which are independently routed through the fabric of the network Furthermore, what
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Figure 2-24 VPN.
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if there is no guarantee that all of the packets will take the same route through the network wilderness In that case, the route actually consists of a series of routes between the switches, like a string of sausage links. In this situation, the components of the message are not guaranteed to arrive in sequence. In fact, there is no guarantee that they will arrive at all! The Transport layer therefore has a major responsibility to ensure that all of the message components arrive, and that they carry enough additional information in the form of yet another header, this time on each packet, to enable them to be properly resequenced at the destination. The header, for example, contains sequence numbers that the receiving Transport layer can use to reassemble the original message from the stream of random packets. Consider the following scenario. A transmitter fires a message into the network, where it passes through each of the upper layers until it reaches the originating Transport layer, which segments the message into a series of five packets, labeled one of five, two of five, three of five, and so on. The packets enter the network and proceed to make their way across the wilderness of the network fabric. Packets one, two, three, and five arrive without incident, although they do arrive out of order. Packet four unfortunately gets caught in a routing loop in New Mexico. The receive Transport layer, tasked with delivering a complete, correct message to the layers above, puts everything on hold while it awaits the arrival of the errant packet. The layer, however, will only wait so long. It has no idea where the packet is. It does, however, know where it is not. After some predetermined period of time, the receive Transport layer assumes that the packet isn t going to make it and initiates recovery procedures that result in the retransmission of the missing packet. Meanwhile, the lost packet has finally stopped and asked for directions, extricated itself from the traffic jams of Albuquerque, and made its way to the destination. It arrives, covered with dust, an I ve Seen Crystal Caverns bumper sticker on its trailer, expecting to be incorporated into the original message. By this time, however, the packet has been replaced with the resent packet. Clearly, some kind of process must be in place to handle duplicate packet situations, which happen rather frequently. The Transport layer then becomes the center point of message integrity. Transport-layer standards are diverse and numerous. ISO, the ITU-T, and the IETF publish recommendations for layer four. The ITU-T publishes X.224 and X.234, which detail the functions of both connectionoriented and connectionless networks. ISO publishes ISO 8073, which defines a Transport protocol with five layers of functionality ranging from TP0 through TP4:
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