3
Frames, TCP/IP, and VoIP
Digital signals are transmitted synchronously or asynchronously.
- Synchronous operation: The stations and nodes on a network are disciplined by a common clock. Activities occur at specific times in synchrony with other units in the network.
- Asynchronous operation: Literally, not synchronous operation. Network nodes operate with similar clocks, but their activities are not synchronized. Actions are performed when the node is ready and it will not interfere with any connected node. To alert the receiver that data is being transmitted and to synchronize the receiver with the bit stream, asynchronous operation requires the use of start and stop bits, preambles, flags, or other markers.
In Chapter 2, the plesiochronous digital hierarchy (PDH) and the synchronous digital hierarchy (SONET and SDH) were introduced. All of their members, from the slowest (1.544 Mbps) to the fastest (159.25248 Gbps), operate synchronously. In this chapter we will begin to study asynchronous operation, specifically, how frames are built and used. But before beginning we should become familiar with two other terms: real time and nonreal time.
- Real time: Information is processed the moment it is received so that the only delay between the source and receiver is the delay due to the distance between them and the characteristics of the transmission equipment.
- Nonreal time: Information is stored until it can be transmitted. Discipline may be derived from a software procedure such as ...
