Sec.1.2 Messages and Switching 13 probability density between some minimum and maximum,and fixed length.As mentioned above,long messages are becoming much more common because of graphics and long file transfers. 4.Allowable delay.The allowable expected delay varies from about 10 msec for some real-time control applications to 1 sec or less for interactive terminal to computer applications,to several minutes or more for some file transfer applications.In other applications,there is a maximum allowable delay (in contrast to expected delay). For example,with packetized voice,fixed-length segments of the incoming voice waveform are encoded into packets at the source.At the destination,these packets must be reconverted into waveform segments with some fixed overall delay;any packet not received by this time is simply discarded.As described above,delay is sometimes of interest on a message basis and sometimes,in the flow model,on a bit basis. 5.Reliability.For some applications,all messages must be delivered error-free.For example,in banking applications.in transmission of computer programs,or in file transfers,a single bit error in a message can have serious consequences.In other applications,such as electronic mail,all messages must be delivered,but an occasional bit error in a message can usually be visually corrected by the reader. Finally,in other applications,both occasional bit errors and occasional loss of entire packets or messages are allowable.For example,in distributed sensor systems, messages are sometimes noisy when transmitted,and occasional lost messages are soon replaced with more up-to-date messages.For packetized voice,the occasional loss (or late delivery)of a packet or an occasional bit error simply increases the noisiness of the received voice signal.It should be noted,however,that the use of data compression for packetized voice and other applications greatly increases the need for error-free communication. 6.Message and packet ordering.The packets within a message must either be main- tained in the correct order going through the network or restored to the correct order at some point.For many applications(such as updating data bases),messages must also be delivered in the correct order,whereas for other applications,message order is unimportant.The question of where to handle reliability and message ordering (i.e.,at the external sites or within the subnet or both)is an important design issue. This is discussed in Section 2.8. In keeping all these characteristics in mind,it is often helpful to focus on four types of applications which lie somewhat at the extreme points and which do not interact very well together in subnets.One is interactive terminal to computer sessions,in which messages are short,the message rate is low,the delay requirement is moderately stringent, and the need for reliability is high.Another is file transfer sessions,in which the messages are very long,the message arrival rate is typically low,the delay requirement is very relaxed,and the need for reliability is very high.The third is high-resolution graphics in which the messages are again long,sometimes up to 10%bits.the delay requirement is stringent,and the arrival rate is low.The fourth is packetized voice.Here the concept of a message is not very useful,but the packets are short,the packet arrival rate is high,Sec. 1.2 Messages and Switching 13 probability density between some minimum and maximum, and fixed length. As mentioned above, long messages are becoming much more common because of graphics and long file transfers. 4. Allowable delay. The allowable expected delay varies from about 10 msec for some real-time control applications to 1 sec or less for interactive terminal to computer applications, to several minutes or more for some file transfer applications. In other applications, there is a maximum allowable delay (in contrast to expected delay). For example, with packetized voice, fixed-length segments of the incoming voice waveform are encoded into packets at the source. At the destination, these packets must be reconverted into waveform segments with some fixed overall delay; any packet not received by this time is simply discarded. As described above, delay is sometimes of interest on a message basis and sometimes, in the flow model, on a bit basis. 5. Reliability. For some applications, all messages must be delivered error-free. For example, in banking applications, in transmission of computer programs, or in file transfers, a single bit error in a message can have serious consequences. In other applications, such as electronic mail, all messages must be delivered, but an occasional bit error in a message can usually be visually corrected by the reader. Finally, in other applications, both occasional bit errors and occasional loss of entire packets or messages are allowable. For example, in distributed sensor systems, messages are sometimes noisy when transmitted, and occasional lost messages are soon replaced with more up-to-date messages. For packetized voice, the occasional loss (or late delivery) of a packet or an occasional bit error simply increases the noisiness of the received voice signal. It should be noted, however, that the use of data compression for packetized voice and other applications greatly increases the need for error-free communication. 6. Message and packet ordering. The packets within a message must either be main￾tained in the correct order going through the network or restored to the correct order at some point. For many applications (such as updating data bases), messages must also be delivered in the correct order, whereas for other applications, message order is unimportant. The question of where to handle reliability and message ordering (i.e., at the external sites or within the subnet or both) is an important design issue. This is discussed in Section 2.8. In keeping all these characteristics in mind, it is often helpful to focus on four types of applications which lie somewhat at the extreme points and which do not interact very well together in subnets. One is interactive terminal to computer sessions, in which messages are short, the message rate is low, the delay requirement is moderately stringent, and the need for reliability is high. Another is file transfer sessions, in which the messages are very long, the message arrival rate is typically low, the delay requirement is very relaxed, and the need for reliability is very high. The third is high-resolution graphics, in which the messages are again long, sometimes up to 109 bits, the delay requirement is stringent, and the arrival rate is low. The fourth is packetized voice. Here the concept of a message is not very useful, but the packets are short, the packet arrival rate is high