 rigidly in the way that cir- dedicated to channel 24. The 193rd framing bit is used to cuits in the TDM system are. Instead, the packets are synchronize the system. The interleaving process repeats created and transmitted as needed. Therefore, X.25 8000 times each second. (Note that 193 bits per frame service can be priced by the packet or by the byte rather times 8000 frames per second equals 1.544 Mbits/sec, than by connection time or as a full-time circuit; you're which is the throughput rate of a T1 line.) using network capacity only when you're sending or re- Today, TDM phone call switching is circuit oriented, ceiving data. though the devices that perform the switching function Despite this freer form of multiplexing, X.25 on digital circuits bear little resemblance to the networks are still connection oriented, and a session mechanical switches that once selected paths made up of between two nodes still requires a virtual circuit-the solitary electrical circuits. virtual circuit has just been unbundled from a fixed One disadvantage to DM-whether you're making time slot. a call with a TDM system or leasing a full-time digital Each packet in an X.25 network has a Logical Channel line-is that your cost will be the same whether you fill Number, or LCN. When a packet comes into a switch, every time slot with data or transmit nothing (for our the switch looks up the LCN to decide which port to purposes, let's consider voice to be just another form of send the packet out of. The path through the network of data). As we know, the data transmissions required for packet switches is defined in advance for Permanent many applications are bursty, with intervals of high Virtual Circuits (PVCs) and established on the fly for demand and no demand distributed almost randomly. Switched Virtual Circuits (SVCs). With SVCs, call Thus, TDM-based networks (or any circuit-oriented setup is required before data transmission can take place. networks with hard resource allocations) are likely to be (Incidentally, the protocol data 