AT&T DS1/DMi/ISDN-PRI manual Circuit-Switched Versus Packet-Switched Channels, Framing Formats

1-10INTRODUCTION

Circuit-Switched Versus Packet-Switched Channels

A circuit-switched channel provides the full bandwidth of a channel to the single terminating application on an end-to-end basis. For example, the full 64K-bps B-channel bandwidth is continuously available for both calling and called users.

As a contrast, a single packet-switched channel divides the bandwidth of a channel into multiple logical channels. The logical channels use a channel's bandwidth on an as-required and multiplexed basis. The order in which the logical channels are multiplexed is controlled by a packet switching protocol, such as X.25. A System 85 R2V4, Generic 2, and Generic 1 ISDN-PRI provide only circuit switched B-channels. External hardware may be used for providing packet switched channels and thus provide access to a variety of packet networks.

Framing Formats

A frame is a set of 24 8-bit time slots grouped as a single transmission unit. Each DS1 frame has 192 bits (24 x 8), plus 1 bit (called a framing bit) that is inserted at the beginning of each frame. Since each frame repeats time slots in the same sequence as previous frames, time slots representing a single conversation or data stream form a channel. DS1 frames repeat the 24-channel sequence in the same order as previous frames at an 8,000 frames per second rate.

There are two methods or formats for providing framing. Either format may be chosen, depending on the equipment and application. The type of framing used at both equipment ends of a DS1 transmission facility must be identical. The framing format does not place any requirement on the type of signaling or line coding to be used.

D4 Framing

The D4 framing format uses a synchronization scheme that relies on a continuously-repeated 12-bit fixed pattern. This 12-bit synchronization pattern is formed by the framing bit from 12 consecutive frames. The receiving port finds the pattern across frames by identifying the beginning and end of each frame. The 12-frame unit which contains the synchronization pattern (one D4 cycle) is called the D4 s u p e r f r a m e . Figure 1-4,D 4 F r a m i n g , shows the D4 superframe format for a DS1/DMI/ISDN-PRI signal.