Issues in Linking ATM networks via Satellite
This page lists some of the issues which need to be considered when transporting an ATM cell stream using
a satellite link.
Which cell transport method should be used?
Various schemes are possible. Most demonstrator networks have chosen to use terrestrial frame formats (e.g. PLCP). These may be poorly suited to the satellite user needs.
Some of the poposed schemes include:
- PDH (plesiochronous digital heirarchy)
- SONET SDH (synchronous digital heirarchy)
- PLPC (cell framing)
- Raw access to the synchornous bit stream (no framing)
How does noise effect the cell stream?
satellite links typically offer lower rate connections than tereestrial systems, and although they may provide a reasonably low BER during normal operation, are subject to effects whihc produce occasional reduction in the link quality of service. These may lead to a loss of signal (outage) or prolonged burst of errors.
Some of the areas which need further investigation include:
- Cell Acquisition
- Cell In-Sync Maintainance
- Cell Discard Rate
- Effect of discard on OAM cells
- Need for error control
How do corrupted cells effect the user?
The ATM cell layer provides only limited guarentees on the cell header (suing an 8-bit HEC) and no protection on the cell payload. This requires either an error-free channel or an application which is insensitive to errors. In practice, AAL-5 may be used to layer a packet-oriented service over the
basic cell-service. This should provide full error detection.
Some of the issues include:
- Effect of discard on AAL
- Effect of error cell insertion on AAL
- Effect of discard on transport protocol
- Effect of discard on application (see below)
How does delay effect the user?
Satellite delay will be an issue for many services.
Some of the issues include:
- Effect of delay on ATM signalling protocol
- Effect of delay on AAL flow control and congestion control
- Effect of delay on application (see below)
How is the satellite bandwidth to be managed?
Many tests are being considered with fixed SCPC assigned
circuits. These tests address the needs of the ATM applications
but may not necessarily be representative of a future network
of users who need to share the satellite capacity. Given the
high data rates needed for many services, a dynamic assignment
scheme seems very desirable in a "production system".
Some of the issues include:
- Fixed assignement
- Demand assignment
- Support for broadcast / multicast
- Support for deterministic QoS
- Need for pre-emption and service availability
- Use of on-board processing or on-board switching
How will applications perform?
Some of the types of applications which require study include:
- Interactive Applications (e.g. remote login)
- Bulk Applications (e.g. file transfer, mail)
- Semi-Interactive Applications (e.g. WWW, nfs, etc)
- Internet Stream Applications (e.g. video, audio)
- Native ATM Stream Applications (e.g. video, audio)
- Circuit Emulation (e.g. E1,T1)
Gorry Fairhurst - ERG home page