At the lowest (logical) level, network links look like serial lines. In this chapter we address how packet structures are built on top of serial lines, via encoding and framing. Encoding determines how bits and bytes are represented on a serial line; framing allows the receiver to identify the beginnings and endings of packets. We then conclude with the high-speed serial lines offered by the telecommunications industry, T-carrier and SONET, upon which almost all long-haul point-to-point links that tie the Internet together are based.
- 4.1: Encoding and Framing
- We generally have to avoid transmitting long runs of identical bits, because the receiver may simply lose count; this is the clock synchronization problem. This means that, one way or another, we cannot always just send the desired bits sequentially. Exactly how we do this is the encoding mechanism. Once we have settled the transmission of bits, the next step is to determine how the receiver identifies the start of each new packet, which is the framing problem.
- 4.2: Time-Division Multiplexing
- But perhaps the most pervasive alternative to packets is the voice telephone system’s time division multiplexing, or TDM, sometimes prefixed with the adjective synchronous. The idea is that we decide on a number of channels, N, and the length of a timeslice, T, and allow each sender to send over the channel for time T, with the senders taking turns in round-robin style. Each sender gets to send for time T at regular intervals of NT, thus receiving 1/N of the total bandwidth.