TANDBERG D11624 - Manual (Page 7)

Tandberg Video on Frame Relay
D11624 rev.01 7
2. The second method is to establish a traffic prioritisation scheme for any channels
carrying video through a FRAD (Frame Relay Access Devices) on a defined DLCI.
This ensures that video frames are first out. Therefore, intelligent buffer management
ensures video frames, which are less tolerant of delay, have priority over data frames
which can usually tolerate some delay.
4. Equipment for Video over Frame Relay
There are many H.320 compliant systems on the market which users may wish to place
on a frame relay network. This may be accomplished by an external H.320 to frame relay
conversion unit such as the VFX-250S.
Using an external converter
This is a simple solution,
which maintains existing investment in video equipment, while providing the benefits of
operating over a frame relay network.
Note: Some video codec manufactures claim their product to be frame relay compatible
by outputting HDLC, which can then be carried transparently by a FRAD. This is not a
'true' frame relay solution and certain limitations apply.
4.1 Framing the Picture
Preparing video for the frame network requires a framer that packetises output from the
codec: Suggested equipment could be a video framer from Memotec working in front of
its CX900e, and an AVI-2000 from ACT to complement the ACT SDM-9300 FRAD.
Both FRAD manufacturers use the Science Dynamics VFX-250S to become "video-
capable."
The video framer interacts with the RS449 output of Data Terminal Equipment (DTE)->
the codec. The VFX-250S provides clock to the DTE at 64, 96, 128, 192, 256, 384, 512,
768, 1.024 or 1.536 Kbps. It supports continuous full-duplex data transfers at up to 2.048
Kbps.
Normally, the VFX-250S's selected clock is locked to a submultiple of the network
clock. When running 384 Kbps with the Memotec CX900e, the ACT's SDM-9300
FRAD requires to drop to 256 Kbps. Another option allows the VFX-250S automatically
to vary the output clock rate to the DTE by slight increments as required to maintain
slip-free data transfer.
The VFX-250S creates standard frame relay frames with 6-byte headers and user-
selected packet lengths. Short packets (for example, 256 bytes) add frame overhead but
have less impact on the audio/video stream if they get lost. Long packets (up to 1.580
bytes) are more efficient but increase the delay. It is possible to run the VFX-250S