bits onto the 8-bit PCM word, as shown in Figure 6.23. 6.11.3.2 Higher-Level Multiplex Structures Internal to a Digital Switch. We pictured a simple time-slot interchanger switch with 24 eight-bit time slots to satisfy DS1 requirements. It would meet the needs of 24 subscribers without blocking. There is no reason why we could build a TSI with a DS3 rate. The basic TSI then could handle 672 subscribers (i.e., 672 time slots). If we added a concentrator ahead of it for 4:1 concentration, then the time switch could handle 4 X 672, or 2688 subscribers. An example of this new thinking is the AT&T 5ESS, which is a TST switch. It has a capacity for 100,000 or more lines. They are able to accomplish this simpler architecture by using larger capacity time-slot interchangers (TSIs) and accordingly with higher bit rates in the space stage. A 5ESS TSI handles 512 time slots.19 However, each TSI port has an incoming/outgoing time-slot rate of 256 time slots. Two ports are required (in one direction) to handle the 512 time slots: one for odd-numbered channels and one for even- numbered channels. Thus the bit rate at a TSI port is 256 X 16 X 8000 = 32.768 Mbps. This odd-channel, even-channel arrangement carries through the entire switching fabric, with each port handling 256 time slots or 32.768 Mbps. Another example of a widely implemented modern digital switch is the Northern Tele- com DMS-100 with supernode/ENET. They modified the older DMS100 conventional switch, which had a TSTS-folded architecture. Like the 5ESS, they also moved into the 19 Remember that a 5ESS time slot has 16 bits (see Figure 6.23). 6.12 DIGITAL NETWORK 2048-time-slot domain in the ENET (extended network). But their time slot is 10 bits, and the ENET uses a 10-bit parallel format, SO each line (i.e., there are 10 lines) has 2048 X 8000 or 16.384 Mbps. 6.12 DIGITAL NETWORK 6.12.1 Introduction The North American public switched telecommunications network (PSTN) is 100% dig- ital. The interexchange carrier (IXC) portion has been 100% digital for many year