detects eventual shifts. meaning of the different items will become clear throughout the rest This necessary time reference consists of a particular pattern of of this chapter. several bits, called the framing pattern. When the receiver has lost Before discussing the T1 and E1 technologies in detail, consider frame alignment, it searches for this pattern in order to realign itself first the general structure of a digital time division multiplex. with as short a delay as possible. The framing pattern is periodically The digital hierarchy levels 21/5 1 frame b.321b.321b.321 b.321b.321b.321 channel A channel Z channel Z-1 channel C channel B channel A 1 frame Z.CBAZ.CBA bit b-1 bit 4 bit 2 bit 1 Figure 21.3 Digital TDM frame structures carried by the frame according to one of the following organisa- Aside from analog signalling by sinusoidal carriers (in-band or tions: out-band) used in analog systems and translatable by PCM modula- tion, digital systems lend themselves by nature much better to direct 1. Grouped framing pattern, which consists of a number of conse- digital transmission of signalling information. Several solutions can cutive bits at the beginning of a frame. be considered: 2. Distributed framing pattern which, as the name suggests, is spread over the frame on a bit by bit basis, or over several 1. In-octet signalling, which is also called bit robbing. In this the frames at one bit per frame. least significant bit (LSB) among the 8 bits that represent a coded sample of speech is periodically (typically every six There is of course a danger of simulating the framing pattern by frames) assigned to signalling. This results in an imperceptible chance combination of other information carrying bits. There are degradation of the telephone transmission. However, when data different ways to protect the system from this, such as: are transferred over the same network, the impact of losing the LSB every sixth byte is not acceptable. 2. Out-of-octet signalling, also referred to as channel by channel 1. One can ch