 optics in PSNs with the most common interface, X.25, is only two, SO that to satellites, beyond that subscriber interface. At the other end, of the maximum number of packets that can remain unacknowledged course, will probably be another conventional telephone. at any one time in each direction is two. Cynics have pointed out The role of X.25 as a standard interface to and from PSNs is that X.25 equipment is usually delivered with the window size set paramount in presenting a common face to a user, to whom the to two and is never altered, but a significant factor is also the storage complexities and richness of the internal PSN are transparent, and buffer needed to support a larger window. to an end-point device from the attached packet switch. At any time, any device may be called upon to buffer the entire window size: all packets sent inside the window of a virtual call. Moreover, that packet switch or end point device may be supporting 54.4.3.2 Network Layer flow control numerous virtual calls simultaneously, putting considerable poten- tial pressure on its buffer space. If the buffer fills, the device has to Clearly, the sequence of information transfer at the Network or resort to emergency action which may mean taking down some calls packet level is similar to that of the Data Link Layer. Error detection to retain others. and correction are by and large left to layer 2, but the Network Layer The Network Layer in some PSNs allows for different window does impose its own flow control mechanisms. Layer 2 flow control sizes depending on the nature of the virtual call, mainly those calls affects all traffic on the link; Network Layer flow control operates that need maximum throughput. It will depend on the delay time only on the information relating to a single virtual call running over imposed by different links and packet switches en route between the a link which may be supporting numerous other virtual calls. end point devices. The optimum window size will vary according to 54/12 Packet switching Device A De