User guide

 5.7 Jitter

Following is an explanation of packet jitter:

◦ The sent packet sequence has a constant time interval between adjacent

packets.

◦ The packet sequence arrives at the other TMUX with a variable time interval

between the same adjacent packets.

◦ This effect, termed “packet jitter”, is introduced by the statistical nature of packet

transport, in which packets are queued onto an outgoing link before being

processed.

For the usual data application this is of no concern, but management of jitter is essential

for good circuit emulation.

A TMUX device resolves “packet jitter” by storing milliseconds of circuit payload, typically

20ms, in its “playout buffer.” Sometimes this is called the “de-jitter buffer.” Thus, the

circuit emulation application deliberately inserts delay such that, should the subsequent

time interval between adjacent packets be lengthened to almost 20ms, the TMUX

“playout buffer” absorbs this variation, and the application continues successfully.

5.8 Canopy Passive Reflectors

In radio, jitter is a measure of the quality of the signal (whereas dBM is a measure of the

power of the signal). Interference, either from multipathing, other Canopy modules, or

external interference will all increase jitter.

Passive reflectors provide an additional 18dBi gain on top of the usual integrated antenna

(for the 2.4GHz BH20 the gain is only 8dBi because of its increased wavelength). Not

only does this increase the effective range, it also narrows the radio beam width from

about 60° to about 6°. Consequently, while alignment requires more precision, the

passive reflector complex effectively “rejects” much multipath signal reception, reducing

Jitter.

Motorola strongly recommends that passive reflectors be employed at both ends of any

backhaul context, and in particular for the circuit emulation application context.

5.9 Ramifications of Enabling AES Channel Encryption

AES Channel Encryption is significant to the TMUX circuit emulation application. AES

mandates an encryption key update once every 24 hours. Thus, enabling AES in the

circuit emulation application context will necessarily cause a circuit interruption as the

Canopy units exchange the protocol handshake necessary for placing a new encryption

key in place.

5.10 Enabling Canopy BH20 Hardware Scheduling

Starting in Release 7.2 the Canopy BH supports the Hardware Scheduling feature.

Laboratory testing indicates that HW Scheduling improves the circuit emulation

application measurably. Motorola recommends that BH20 units be updated to Release

7.2 and HW Scheduling enabled.

The reason HW Scheduling operation is superior to the original SW Scheduling is that a

fragment ARQ retransmission is accomplished much quicker, thereby reducing the

interpacket jitter and so not stressing the TMUX playout algorithm as much.

5.11 T1/E1 Cabling

T1/E1 Cable nomenclature is not uniformly observed, and the differences from Ethernet

cable often not appreciated. In that spirit a review is provided.