Sec.1.2 Messages and Switching 11 1.2.2 Sessions Messages between two users usually occur as a sequence in some larger transaction; such a message sequence (or,equivalently,the larger transaction)is called a session. For example,updating a data base usually requires an interchange of several messages. Writing a program at a terminal for a remote computer usually requires many messages over a considerable time period.Typically,a setup procedure (similar to setting up a call in a voice network)is required to initiate a session between two users,and in this case a session is frequently called a connection.In other networks,no such setup is required and each message is treated independently;this is called a connectionless service.The reasons for these alternatives are discussed later. From the standpoint of network users,the messages within a session are typically triggered by particular events.From the standpoint of the subnet,however,these message initiation times are somewhat arbitrary and unpredictable.It is often reasonable,for subnet purposes,to model the sequence of times at which messages or packets arrive for a given session as a random process.For simplicity,these arrivals will usually be modeled as occurring at random points in time,independently of each other and of the arrivals for other sessions.This type of arrival process is called a Poisson process and is defined and discussed in Section 3.3.This model is not entirely realistic for many types of sessions and ignores the interaction between the messages flowing in the two directions for a session.However,such simple models provide insight into the major trade-offs involved in network design,and these trade-offs are often obscured in more realistic and complex models. Sometimes it will be more convenient to model message arrivals within a ses- sion by an on/off flow model.In such a model,a message is characterized by a se- quence of bits flowing into the subnet at a given rate.Successive message arrivals are separated by random durations in which no flow enters the network.Such a model is appropriate,for example,for voice sessions and for real-time monitoring types of applications.When voice is digitized (see Section 2.2).there is no need to trans- mit when the voice is silent,so these silence periods correspond to the gaps in an on/off flow model.One might think that there is little fundamental difference between a model using point arrivals for messages and a model using on/off flow.The out- put from point message arrivals,followed by an access line of fixed rate,looks very much like an on/off flow (except for the possibilitity that one message might arrive while another is still being sent on the access line).The major difference between these models,however,is in the question of delay.For sessions naturally modeled by point message arrivals (e.g.,data base queries),one is usually interested in delay from message arrival to the delivery of the entire message (since the recipient will process the entire message as a unit).For sessions naturally modeled by flow (such as digitized voice),the concept of a message is somewhat artificial and one is usu- ally interested in the delay experienced by the individual bits within the flow.It ap- pears that the on/off flow model is growing in importance and is particularly appro- priate for ISDN and broadband ISDN networks.Part of the reason for this growth is the prevalence of voice in ISDN and voice and video in broadband ISDN.An-Sec. 1.2 Messages and Switching 11 1.2.2 Sessions Messages between two users usually occur as a sequence in some larger transaction; such a message sequence (or, equivalently, the larger transaction) is called a session. For example, updating a data base usually requires an interchange of several messages. Writing a program at a terminal for a remote computer usually requires many messages over a considerable time period. Typically, a setup procedure (similar to setting up a call in a voice network) is required to initiate a session between two users, and in this case a session is frequently called a connection. In other networks, no such setup is required and each message is treated independently; this is called a connectionless service. The reasons for these alternatives are discussed later. From the standpoint of network users, the messages within a session are typically triggered by particular events. From the standpoint of the subnet, however, these message initiation times are somewhat arbitrary and unpredictable. It is often reasonable, for subnet purposes, to model the sequence of times at which messages or packets arrive for a given session as a random process. For simplicity, these arrivals will usually be modeled as occurring at random points in time, independently of each other and of the arrivals for other sessions. This type of arrival process is called a Poisson process and is defined and discussed in Section 3.3. This model is not entirely realistic for many types of sessions and ignores the interaction between the messages flowing in the two directions for a session. However, such simple models provide insight into the major trade-offs involved in network design, and these trade-offs are often obscured in more realistic and complex models. Sometimes it will be more convenient to model message arrivals within a ses￾sion by an on/off flow model. In such a model, a message is characterized by a se￾quence of bits flowing into the subnet at a given rate. Successive message arrivals are separated by random durations in which no flow enters the network. Such a model is appropriate, for example, for voice sessions and for real-time monitoring types of applications. When voice is digitized (see Section 2.2), there is no need to trans￾mit when the voice is silent, so these silence periods correspond to the gaps in an on/off flow model. One might think that there is little fundamental difference between a model using point arrivals for messages and a model using on/off flow. The out￾put from point message arrivals, followed by an access line of fixed rate, looks very much like an on/off flow (except for the possibilitity that one message might arrive while another is still being sent on the access line). The major difference between these models, however, is in the question of delay. For sessions naturally modeled by point message arrivals (e.g., data base queries), one is usually interested in delay from message arrival to the delivery of the entire message (since the recipient will process the entire message as a unit). For sessions naturally modeled by flow (such as digitized voice), the concept of a message is somewhat artificial and one is usu￾ally interested in the delay experienced by the individual bits within the flow. It ap￾pears that the on/off flow model is growing in importance and is particularly appro￾priate for ISDN and broadband ISDN networks. Part of the reason for this growth is the prevalence of voice in ISDN and voice and video in broadband ISDN. An-