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INTRODUCTION
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 superframe. Figure 1-4, D4 Framing, shows the D4 superframe format for a
DS1/DMI/ISDN-PRI signal.