Public Switched Telephone Network (PSTN)
Traffic over a major “highway” system. PSTN constructed by AT&T’s research division, Bell Labs, by standardized transfer methods and central office switches.
(definition) “The establishment on demand, of an individual connection from a desired inlet to a desired outlet within a set of inlets and outlets for as long as is required for the transfer of information.”
“The establishment on demand, of an individual connection from a desired inlet to a desired outlet within a set of inlets and outlets for as long as is required for the transfer of information.”
(definition) What goes in, comes out, for as long as desired from within the network until one party hangs up.
Intro to Switching
A switch routes a call based on a number system
Essential Switch Functions
- Information receiving
- Information sending
- Busy Testing
Functions of Switching Systems
- monitor line activity
- send incoming information to control function
- send control signals to outgoing lines
- process signaling and set-up/knock down connections
- make connections between input and output lines
Basic Switch Requirements
A switch must be able to connect any incoming call to one of a multitude of outgoing calls. Switch must have the ability to hold and terminate calls. A switch has to prevent new calls from intruding into circuits already in use.
General Switch Requirements
- Speed of call setup should be kept short relative to the call holding time
- Grade of service should be high
- .99 overall
- .95 busy-hour
- HIGH availability!
- Full: any input to any output
- Blocking: Possibility exists that call setup may fail due to insufficient switching resources
- Non-blocking: If any input Ij and output Oj are free, they can be connected
Time Division Switching
The Time mapping of inputs and outputs:
Space Division Switching
Spatial mapping of inputs and outputs. Used primarily in analog switching systems.
Single Stage Switches
- Crosspoint switches
- Complex – many crosspoints ( i × j )
- Poor utilization of crosspoints
- Not fault tolerant
Multiple Stage Switches
Input connected to output via two or more smaller switches. Crosspoints shared by several possible connections (potential for blocking). Possible to provide multiple paths between any 2 ports.
Three Stage Switch Matrix (Multiple Stage Switch example)
Reducing Cross-point Complexity
- Increase number of stages
- Allow some blocking
- Switch in more than one dimension
Number of Cross-points Non-blocking Switch
|Nbr of Lines||3-Stage||1-Stage|
- Most of the switching in US is digital, why?
- Transmission is digital
- Maintenance of signal quality
- Leverage improvements in density of Integrated Circuits
- Advanced features and services through software
- Integration of voice/data services
The End of The Word
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