| MESHED VSAT
DAMA SCPC Meshed VSAT Networks
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Meshed VSAT networks offer a means of establishing a switched point to point data network which can have the capability to operate at relatively high data rates (up to 2 Mb/s for example). Links are set up directly between remote terminals, often on a call by call basis.
Meshed VSATs are normally configured to operate without a hub or central large earth station and are intended to carry a mix of data traffic and telephony traffic or only data traffic. Although meshed VSAT networks do not have a hub, they generally have a network control station which controls the allocation of resources across the network.
The network control centre is only involved in the signalling for call set up, and in monitoring the operation of the network. This is in contrast to an interactive VSAT network in which all communications, even between two remote terminals on the network, must pass through the hub.
Most meshed VSAT networks were originally designed for telephony and have been adapted for use with data. As a result of this they still have many of the characteristics associated with telephony rather than data transmission.
Typical applications for meshed VSAT networks are:
- LAN-LAN interconnection;
- connections between PABXs;
- videoconferencing;
- remote printing;
- distributing CAD/CAM;
- back-up for terrestrial circuits.
Meshed VSATs provide the satellite equivalent of bandwidth on demand services. It must, however be emphasised, that nearly all meshed VSATs currently available have limited signalling for call set up and can only, in general set up a fixed bandwidth pipe between the two remote sites.
A meshed VSAT network generally consists of two types of terminals. A number of user terminals, each with an antenna (generally) of diameter 2 to 3.7 m, and a single network control station. Connections are set up between user terminals call by call, on an as required basis.
If it is present, the network control centre is only involved in the call set up and tearing down processes. It is the signalling centre for the network and controls the allocation of satellite resources between users based on the call requests it has received.
The principle characteristics of a meshed VSAT network are:
| (i) | Users at remote sites tend to be relatively heavy users, often with very bursty data requirements, sometimes at high data rates. The meshed VSAT's ability to set up circuits on demand is ideally suited to such users. |
| (ii) | The network is a circuit switched network, with circuits provided as required. Circuits can be made available on request in a "dial-up" manner. |
| (iii) | The amount of data transferred in each transaction can be relatively large. |
| (iv) | A large number of meshed VSAT terminals (10 to a few hundred) share the same communications links using demand assignment of capacity. |
| (v) | Connections between remote terminals are made directly without requiring a double hop through the hub. |
| (vi) | The hub, where it exists, is only used for signalling, capacity assignment and fault monitoring. |
Three main technologies are used in modern meshed VSAT systems:
| DAMA SCPC: | SCPC channel pairs are automatically set up on a call by call basis for the duration of each call. |
| TDM SCPC: | each remote terminal transmits on its own TDM channel; other remote terminals in the mesh receive all transmissions other than their own and process those packets carrying their own address in the headers. |
| TDMA: | each remote terminal transmits in bursts in pre-assigned time slots in the TDMA frame; all terminals in the network transmit on a single frequency. |
DAMA SCPC Meshed VSAT Networks
DAMA SCPC Meshed VSAT Configuration
Signal Types and Characteristics
Most systems have one control channel and two or more request channels - this is because the control channel has higher capacity (being TDM) than the request channels (which are generally slotted Aloha).
Traffic channels are assigned from a pool in pairs (traffic is two-way). The assigned channels are not necessarily adjacent in the pool. In general the data rate in each direction will be the same, though this is not necessarily always the case.
Typical DAMA SCPC Meshed VSAT Network Spectrum
Call set up procedure (from remote):
Call close down procedure (from remote):
Once established, traffic channels are generally transparent and are transmitted continuously for the duration of the call (i.e. duty cycle is 100% during the call). The average, long term occupancy of each channel is determined during network design and is dependent on the blocking characteristics required by the customer.
Signalling channels operate in a similar fashion to the TDM/TDMA transmissions in an interactive hubbed VSAT network.
Typical DAMA SCPC Meshed VSAT Signalling Channel Data Formats
The TDM control channel carries a continuously transmitted bitstream which is divided into frames (duty cycle is 100%).
The start of a frame is denoted by a framing packet contain a unique word (UW) and a control word (CNTRL) which, together, provide framing, timing and control information.
The rest of the frame is filled by fixed length data packets which each contain:
Each TDMA request channel contains frames which are synchronised to the control channel frames. Each request channel frame is divided into slots. Individual IDUs transmit in these slots generally using Slotted Aloha as and when they require to set up and close down calls. Transmissions are intermittent with a peak duty cycle generally in the range 25 to 30 % for slotted Aloha.
Each request channel packet consists of:
A typical DAMA SCPC meshed VSAT system consists of the following terminals:
Remote site terminals, which consists of the following:
An NCS (Network Control System) site, which consists of the following:
The NCS typically costs in the range of 0.25 to 0.5 million Euro (excluding RFT, antenna, civil works, etc). Remote site costs are typically in the range 5,000 to 50,000 Euro installed depending on configuration and network capacity.
TDM SCPC Meshed VSAT Configuration
Signal Types and Characteristics
Each remote site transmits a single TDM carrier which is received by all other remote sites in the network. The data packets in the received transmissions are continually monitored and those carrying the remote site address are selected for processing; all other packets are ignored.
Typical TDM SCPC Meshed VSAT Network Spectrum
TDM SCPC Meshed VSAT Frame and Packet Structure
The TDM carrier is modulated by a continuously transmitted bitstream which is divided into frames (duty cycle of 100%).
The start of a frame is denoted by a framing packet contain a unique word (UW) and a control word (CNTRL) which, together, provide framing, timing and control information.
The rest of the frame is filled by fixed length data packets which each contain:
The data packet is often an encapsulated frame relay, ATM or Ethernet packet.
Call set up is not required because permanent virtual circuits are established between each remote site and all of the others as part of the network configuration procedure when the network is installed or modified.
A typical TDM SCPC meshed VSAT system consists of several remote sites, each of which consists of the following:
No network control site is required.
Remote site costs are typically in the range 10,000 to 25,000 Euro installed, depending on configuration and network capacity.
TDMA Meshed VSAT Configuration
Signal Types and Characteristics
All terminals in the network transmit bursts of signalling and traffic data in time slots in a single TDMA channel. The burst time plan is defined by the NCS based on call requests from the remote terminals by following predetermined algorithms.
Typical TDMA Meshed VSAT Network Spectrum
Transmission is not continuous, because some time slots may not be used. However, at peak traffic levels, time slot utilisation, and hence duty cycle, may approach 100%.
TDMA Meshed VSAT Frame and Packet Structure
The start of a TDMA frame is denoted by a framing packet containing a unique word (UW) and a control word (CNTRL) which, together, provide framing, timing and control information. The framing packet is generated by the NCS and defines the timing for the whole network. Signalling from the NCS to remotes is carried in the control word.
Data packets, transmitted by remotes in time slots allocated by the NCS, each contain:
Signalling from remotes to NCS is carried in the header.
In addition, it is common practice to assign one or more time slots to each remote terminal for signalling when there is no traffic from that terminal.
Network signalling, including requests for channels from remotes as well as time slot assignments from the NCS are carried in the headers of data packets.
MF TDMA Meshed Network Transmissions
A variant on TDMA is multi-frequency TDMA, in which several parallel TDMA carriers are used for network traffic. This has the advantage that network traffic capacity can be high but each terminal only requires a relatively low data rate transmit and receive capability (hence minimising costs).
Operation is the same as for conventional TDMA except each remote station is assigned a channel and time slot for its transmissions, rather than just a time slot.
The NCS monitors all transmissions for signalling information.
A typical TDMA meshed VSAT system consists a number of remote sites and an NCS site.
The remote sites each consist of the following:
The NCS (network control system) site consists of:
The NCS typically costs 0.25 million to 0.5 million Euro (excluding RFT, antenna, civil works, etc), but may be much cheaper in some systems.
Remote site cost are typically in the range 20,000 to 500,000 Euro installed.