tiplex) or E1 (or higher: E2, E3) frames each 125 usec in duration. Inside the switch was another matter. For instance, with Nortel's DMS-100 the incoming 8-bit time slot was mapped into a 10-bit time slot, as shown in Figure 6.22. 16 The example used in the figure is DS1. Note in Figure 6.22 that one bit is a parity bit (bit 0) and the other appended bit (bit 1) carries the supervisory signaling information, telling the switch whether the time slot is or busy. 17 Bits 2 through 9 are the bits of the original 8-bit time slot. Because Nortel in their DMS-100 wanted a switch that was simple to convert from E1 to DS1, they built up their internal bit rate to 2.560 Mbps as follows: 10 bits per time slot, 32 time slots X 8000 frame rate) or 2.560 Mbps. 18 This now can accommodate E1, all 32 channels. As mentioned, 5 DS1s are easily mapped into 4 E1s and vice versa (Ref. 17). 15 Junctor is a path connecting switching networks internal to a switch. 16 Nortel was previously called Northern Telecom. 17 Parity bit is used for error detection. It is a redundant bit appended to an array of bits to make the sum of all the 1 bits (marks) (in the array) always odd or always even. 18 The 8000 frames per second or frame rate is common on all conventional PCM systems. As the reader will recall from Section 6.2.1, this is the Nyquist sampling rate for the 4-kHz analog voice channel on converting it to a PCM equivalent. DIGITAL NETWORKS Typical voice channel Bit number 12345678 Every 6th frame = A signaling bit Every 12th frame = B signaling bit 9876543210 Parity Channel supervision MSB sent first Figure 6.22 Bit mapping in the DMS-100, DS1 to DMS. DMS is the internal bit rate/structure. MSB stands for most significant bit; LSB stands for least significant bit. Busy/idle TMS Buffer PCM data Signaling Framing Parity Figure 6.23 The composition of the AT&T 5ESS internal 16-bit time slot. Another popular digital switch is AT&T's 5ESS, which maps each 8-bit time slot into a 16-bit internal PCM word. It actually appends eight additional 