B227 Lecture Overheads - Topic 4: Satellites

Medium Access on Satellite Networks

Satellite protocols defines Wide Area Networks (WANs) since they cover large geographical areas.

In the medium access sublayer, we are concerned with how the transponder channels are allocated.

Recall that the transponders are the transmitters on the satellites.

Long propogation delay – can’t use carrier sense protocols like CSMA/CD.

It takes too long for a ground station to wait for the signal to reach the satellite and back before detecting there are collisons – normal satellite system is 270 msec round-trip.

Some multiple access methods for satellites:

Polling

ALOHA

Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

Code Division Multiple Access (CDMA)

Polling

Satellite asks each ground station one at a time whether the station has a frame to transmit

Very inefficient – too many stations, too long propogation time.

ALOHA

Using pure ALOHA, we can get about 18% channel efficiency.

With slotted ALOHA, we can double the efficiency.

Needs to synchronise the slot times

Done by a reference ground station transmitting a special signal at regular intervals.

Can use more than one uplink to improve channel utilization.

Stations pick from multiple uplinks to transmit to the satellite

If satellite gets frames from more than one uplink, it buffers it up for transmits one at a time.

Frequency Division Multiplexing (FDM)

Refer to lecture in Topic 2 on FDM

Each transponders divided into multiple frequency channels.

Some problems:

Needs large guard band between allocated frequency bands to avoid interference

Needs careful power control – to avoid causing interference

Besides statically allocating bands to ground stations, we can dynamically allocate them by using contention algorithms

Eg. the SPADE system

Time Division Multiplexing (TDM)

Refer to lecture in Topic 2 on TDM

As with FDM, we allocated time slots for the transponders to ground stations

Can be static or dynamic

Read the textbook on examples different TDM schemes.

Code Division Multiple Access (CDMA)

Refer to an earlier lecture on a few lines about CDMA

Advantages:

No time synchronisation or channel allocation problems

Decentralised

Fully dynamic

Disadvantages:

With noise and when the stations are not coordinated, the capacity of the channels are less than TDM

Requires expensive and fast transmitters

Complex and the multiplexing theory is less understood

[Back to Lecture Notes page]