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makes phones work- twisted pair loop
- 48 volts dc
- signal is electrical analog of sound wave patterns
networking principles- mesh network: too many wires
- wire sharing topology and mac to reduce wires
- public network uses a tree topology: hierarchy of stars
- the network is circuit switched
- analog electromechanical switches
- digital switches: don't wear out, more reliable and more features
system signals (meta data)- on hook: available for incoming calls
- off hook: wishes to make a circuit request
- tone: ready to provide circuit, what is your request?
- ringback: the circuit you requested has been completed
- ringing voltage: incoming call
- tones or pulses: code to describe destination address
- busy signal: destination off hook
- call waiting tones: another incoming call was requested
- flash: on hook+off hook sequence: switch between two calls
- howler: off hook with no circuit request
circuit request: destination node identification- lily tomlin switch: use human voice communication to tell the human switch what destination node you want and, as lonnie points out, hope that s/he is not having a bad day
- pulse dialing: when you dial '5' it opens and closes the 48v loop 5 times. the rotary register at the central office causes an electromechanical device to select the requested circuit.
- dtmf: dual tone multifrequency or touch-tone dialing sends a pair of sound frequencies corresponding to each digit. a computer at the central office decodes the tones and uses digital switching to set up the requested circuit.
post divestiture organization of the public network- network divided into local service and long distance service
- each local service area is divided into several local access and transport areas (lata)
- intra lata calls are handled by the local phone company
- inter lata calls are routed to the pop of the long distance provider
- pop = point of presence. each lata contains pops from at least one long distance service
- the subscriber selects a long distance provider with the 10xxx code where xxx is the long distance provider's id
- if no long distance carrier code is punched, the call is sent to the pop of the default carrier
call switching examples- caller 1 to caller 2: same lata same central office: central office conncets local loops of callers 1 and 2 using circuit switching. the circuit is held for the session.
- caller 1 to caller 2: same lata different central office: the office code or 'prefix' is used to identify the destination central office. a central office may have many prefixes but a prefix may only belong to one central office. caller 1 switched to trunk connecting the two central offices and the receiving central office switches the call to caller 2 and this entire circuit is held for the session.
- caller 1 to caller 3: same lata but different central office without a direct connection between the two central offices: call switched to tandem office within the lata.
- caller 1 to caller 4: inter lata call: call switched to designated pop within the lata. the long distance carrier delivers the signal to the destination central office.
- area codes designate geographical areas only and not a switching hierarchy
- terminology: connections between switches are called trunks and connections between switches and subscribers are called local loops
- recent regulatory weirdness: local providers may provide long distance service and long distance carriers may also provide local service and both local and long distance providers may offer cable television service and on-line movie rental ('on demand' service). cable companies may offer local or long distance phone service. multiple cable tv providers may operate in the same geographical area. all of these companies may also provide internet and other data services.
- the technology and the regulatory structure are now in place for an an integrated nationwide voice-data digital service (possibly using atm) that would be fast enough to achieve the goals of the information superhighway.
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