This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: May 2015

The word Spade is a loose acronym for SCPC pulse-code-modulated multiple-access demand-assignment equipment. It is basically a pooling-waiting system where different Earth stations are eternally connected a centre pool which connects two respective stations on demand in an assignment-to-assignment basis. Consider figure 2 for a representation of a Spade communications system.

Operation –

• All the earth stations are permanently connected through the common signalling channel (CSC). A, B, C, D, E, and F are the stations that are connected to the pool
• Each earth station has the facility for generating any one of the 794 carrier frequencies using frequency synthesizers. Furthermore, each earth station has a memory containing a list of the frequencies currently available, and this list is continuously updated through the CSC
• To illustrate the procedure, suppose that a call to station F is initiated from station C in figure 2. Station C will first select a frequency pair at random from those currently available on the list and signal this information to station F through the CSC. Station F must acknowledge, through the CSC, that it can complete the circuit. Once the circuit is established, the other earth stations are instructed, through the CSC, to remove this frequency pair from the list.

Diagrammatic representation of a Spade communications system

• The round-trip time between station C initiating the call and station F acknowledging it is about 600 ms. during this time, the two frequencies chosen at station C may be assigned to another circuit. In this event, station C will receive the information on the CSC update and will immediately choose another pair at random, even before hearing from station F
• Once a call has been completed and the circuit disconnected, the two frequencies are returned to the pool, the information again being trans mitted through the CSC to all the earth stations

Function of CSC –

• Other than establishing the connection through the satellite, the CSC passes signalling information from the calling station to the destination station in the example above from station C to station F
• Signalling information in the Spade system is routed through the CSC rather than being sent over a voice channel. Each earth station has an equipment called the demand assignment signalling and switching (DASS) unit which performs the functions required by the CSC.