Specifications

166119 8/31/2006

2.1.1.1 Timeslots and Channel Rates

The number of timeslots assigned to a channel determine the bandwidth of the channel,

Since the frame rate is constant, the greater the number of channel bits in the frame, the

greater the port rate that can be supported.

The basic frame rate is 400 frames-per-second, and each channel bit, or timeslot in the frame

provides 400 Hz of bandwidth available for allocation to a channel. This allows port rates that

are multiples of this rate to be supported. For example, 19.2KHz is 48 x 400 Hz, and

therefore requires 48 timeslots per frame.

Timeslots are assigned in a manner that distributes their placement throughout the frame.

The purpose of this method is to insure that the rate of arrival or departure of channel bits on

the composite port is approximately matched to the bit rate of the channel port, and therefore

minimizes the requirement for an elastic storage buffer for each channel.

In addition, by not requiring a pre-determined timeslot map for each channel or channel rate,

channels may be allocated and de-allocated from time to time without disrupting the

operation or data flow of other channels. In other words, newly allocated channels are fit into

the timeslot map wherever there are available timeslots, but do not affect those already

assigned.

The above concepts are illustrated in the diagram of Figure 4. In this simple case, the

composite frame is 20 bits, corresponding to an 8KHz link. The frame size could be chosen

as large as 320 bits (128KHz), but for simplicity 8KHz is chosen. Given that the frame bit

occupies 1 bit (400Hz), and the management channel 3 bits (1200Hz), 16 bits are left in

which to assign channel bandwidth.

The first channel, “a”, is 2400Bps and is assigned available timeslot positions 5, 6, 9, 12, 15,

and 18. (The pattern is unimportant, but noteworthy that the timeslots are distributed

throughout the frame, and their location in the timeslot map allocates them to channel “a”.

When channel “b” is allocated, also 2400Bps, the timeslots are chosen from among the

remaining available timeslots, but in a different placement and pattern. None of the timeslots

of channel “a” are disturbed in the process. When complete, there are still 4 timeslots

remaining corresponding to 1600Hz of bandwidth, enough for a 1200Bps channel, for

example.

Although this is a very simple example (8KHz composite is usually insufficient), higher link

rate examples may be configured and are assigned in the same way. All 16 ports may be

time-division multiplexed on the composite link given sufficient bandwidth. The 1600Hz

required for the framing bit and the management channel is fixed however, and must be

considered when planning channel allocation.