10 Introduction and Layered Network Architecture Chap.1 user I might send a message to user 2,user 2 might reply with a message to 1,who might then send another message to 2,and so forth until the completion of the overall transaction.The important characteristic of a message is that from the standpoint of the network users,it is a single unit of communication.If a recipient receives only part of a message,it is usually worthless. It is sometimes necessary to make a distinction between a message and the rep- resentation of the message.Both in a subnet and in a computer,a message is usually represented as a string of binary symbols,0 or 1.For brevity,a binary symbol will be referred to as a bit.When a message goes from sender to recipient,there can be several transformations on the string of bits used to represent the message.Such transformations are sometimes desirable for the sake of data compression and sometimes for the sake of facilitating the communication of the message through the network.A brief description of these two purposes follows. The purpose of data compression is to reduce the length of the bit string representing the message.From the standpoint of information theory,a message is regarded as one of a collection of possible messages,with a probability distribution on the likelihood of different messages.Such probabilities can only be crudely estimated,either a priori or adaptively.The idea,then,is to assign shorter bit strings to more probable messages and longer bit strings to less probable messages,thus reducing the expected length of the representation.For example,with text,one can represent common letters in the alphabet (or common words in the dictionary)with a small number of bits and represent unusual letters or words with more bits.As another example,in an airline reservation system,the common messages have a very tightly constrained format (date,flight number,names, etc.)and thus can be very compactly represented,with longer strings for unusual types of situations.Data compression will be discussed more in Chapter 2 in the context of compressing control overhead.Data compression will not be treated in general here, since this topic is separable from that of data networks,and is properly studied in its own right,with applications both to storage and point-to-point communication. Transforming message representations to facilitate communication,on the other hand,is a central topic for data networks.In subsequent chapters,there are many examples in which various kinds of control overhead must be added to messages to ensure reliable communication,to route the message to the correct destination,to control congestion,and so on.It will also be shown that transmitting very long messages as units in a subnet is harmful in several ways,including delay,buffer management,and congestion control.Thus,messages represented by long strings of bits are usually broken into shorter bit strings called packets.These packets can then be transmitted through the subnet as individual entities and reassembled into messages at the destination. The purpose of a subnet,then,is to receive packets at the nodes from sites outside the subnet,then transmit these packets over some path of communication links and other nodes,and finally deliver them to the destination sites.The subnet must somehow obtain information about where the packet is going,but the meaning of the corresponding message is of no concern within the subnet.To the subnet,a packet is simply a string of bits that must be sent through the subnet reliably and quickly.We return to this issue in Section 1.3.10 Introduction and Layered Network Architecture Chap. 1 user I might send a message to user 2, user 2 might reply with a message to I, who might then send another message to 2, and so forth until the completion of the overall transaction. The important characteristic of a message is that from the standpoint of the network users, it is a single unit of communication. If a recipient receives only part of a message, it is usually worthless. It is sometimes necessary to make a distinction between a message and the rep￾resentation of the message. Both in a subnet and in a computer, a message is usually represented as a string of binary symbols, 0 or 1. For brevity, a binary symbol will be referred to as a bit. When a message goes from sender to recipient, there can be several transformations on the string of bits used to represent the message. Such transformations are sometimes desirable for the sake of data compression and sometimes for the sake of facilitating the communication of the message through the network. A brief description of these two purposes follows. The purpose of data compression is to reduce the length of the bit string representing the message. From the standpoint of information theory, a message is regarded as one of a collection of possible messages, with a probability distribution on the likelihood of different messages. Such probabilities can only be crudely estimated, either a priori or adaptively. The idea, then, is to assign shorter bit strings to more probable messages and longer bit strings to less probable messages, thus reducing the expected length of the representation. For example, with text, one can represent common letters in the alphabet (or common words in the dictionary) with a small number of bits and represent unusual letters or words with more bits. As another example, in an airline reservation system, the common messages have a very tightly constrained format (date, flight number, names, etc.) and thus can be very compactly represented, with longer strings for unusual types of situations. Data compression will be discussed more in Chapter 2 in the context of compressing control overhead. Data compression will not be treated in general here, since this topic is separable from that of data networks, and is properly studied in its own right, with applications both to storage and point-to-point communication. Transforming message representations to facilitate communication, on the other hand, is a central topic for data networks. In subsequent chapters, there are many examples in which various kinds of control overhead must be added to messages to ensure reliable communication, to route the message to the correct destination, to control congestion, and so on. It will also be shown that transmitting very long messages as units in a subnet is harmful in several ways, including delay, buffer management, and congestion control. Thus, messages represented by long strings of bits are usually broken into shorter bit strings called packets. These packets can then be transmitted through the subnet as individual entities and reassembled into messages at the destination. The purpose of a subnet, then, is to receive packets at the nodes from sites outside the subnet, then transmit these packets over some path of communication links and other nodes, and finally deliver them to the destination sites. The subnet must somehow obtain information about where the packet is going, but the meaning of the corresponding message is of no concern within the subnet. To the subnet, a packet is simply a string of bits that must be sent through the subnet reliably and quickly. We return to this issue in Section 1.3