mes and to decide takes place for transporting the message across the DCE/DTE whether the circuit is in an acceptable condition. interface, the data then moving up each of the seven layers until it reaches the final application at the destination end point device. In-band or software flow control involves special control charac- ters or bit patterns sent by the receiver, such as a terminal or a PAD, 54.4.1 Packet switching at the Physical Layer which are embedded in the normal data stream to tell the transmitter to slow down its data output. The Physical Layer describes the physical connection - the elec- After a receiving device has submitted a clear-to-send signal, it has the option of dropping the command at any time if traffic trical interface and procedures involved in establishing a communi- cation path. The connection outside the X.25 cloud is between an becomes too heavy, a process known as out-of-band or hardware flow control. This method in effect stretches the function of an end point device, such as a user terminal, and a PSE on the network. Within the cloud, connections are between the PSEs themselves as RS-232 command originally intended for modem control. The whole subject of flow control outside of software based controls, they make up the network. though, is the subject of complex debate. Each network link will have different characteristics, chosen Within X.25, much of the responsibility for flow control is given according to many parameters such as the maximum acceptable to layers 2 and 3. It should be remembered, too, that the Physical error rate, minimum rate of transfer and the distance over which the Layer takes no account of line transmission quality. It passes valid circuit must run. and corrupted data with equal enthusiasm along the circuit; its job The most common medium for DTE to DCE connection is copper is only to hide the nature of the physical media from the Data Link via a leased line to a public data network, but fibre links are Layer above. becoming increasingly cost effective. 