f information in packets rather request packet requests a communications path be established to the than in their Data Link equivalent, frames. A Network Layer for an called device. It contains the address of that called device and often X.25 PSN is connection oriented. A connection is established be- the address of the calling device, too, so that the recipient knows tween two end point devices via a virtual circuit, and making that where the call has originated. connection is a major function of this layer. Packet switch 4 looks at its routeing table to decide over which Much of layer 3's work is transparent to end users, such as link the call request should be passed en route to its destination. It multiplexing simultaneous calls over a single physical connection. decides to pass the request to packet switch 2 which carries out the Packet switching and the OSI model 54/11 same process and sends it to switch 1 which passes the request to If the Data Link Layer is flow controlled and data is temporarily the destination device. blocked from running over the circuit, then no Network Layer If B decides to accept the call request, it returns a call accepted packets can be transferred. If the Data Link Layer is not flow packet along the route set up by A's request. The communications controlled, but an individual virtual call is being blocked, other path is now successfully established, the virtual call created, be- virtual calls can operate normally, unaffected by the temporary tween devices A and B and end-to-end information can now be blocking of that single virtual call. transferred. Flow control at layer 3 is governed, as at layer 2, by the window Should B reject the call, it sends a clear request packet to tell A N system, typified by the X.25 level 3 protocol. The standard that the call is unacceptable. At the end of the virtual call, a clear sequence number for data transfer in each direction takes up three request packet is also sent by one of the parties and acknowledged bits in the packet, with possible se