Chapter 12: Circuit Switching Technology

Overview

A circuit switch connects the output of one circuit to the input of another allowing information to be passed. In a circuit-switched network, terminals have full real-time access to each other up to the bandwidth of the circuit. Prior to 1891, circuits ran from point to point, and were always connected over a physical facility such as a pair of wires. That was until Almon Strowger invented an electromechanical replacement for the telephone operator, creating the first automatic switching system. This chapter examines the technology of circuit switching and the evolution that has taken place over the years.

Telecommunications Networks Value Effective Switching

The feasibility of direct connectivity between network subscribers decreases as the number of nodes on network increases. An effective switched network employs three forms of switching: circuit switching, packet switching, and message switching.

Attributes of an effective switched network:
·        Connectivity
·        Ease of addressing
·        Interconnectability
·        Robustness
·        Capacity

Network Architecture

A primary challenge for network design is determining how to assemble the most economical configuration of circuits and equipment based on peak and average traffic load, grade of service required, and switching, circuit, and administrative costs.

How Networks can be Connected - Five Basic Topologies:
·        Ring
·        Bus
·        Branching Tree
·        Mesh
·        Star

Characteristics of Switching Equipment Technology

Network switching equipment is implemented to support end-user requirements while maintaining an acceptable level of interoperability with the existing (and sometimes antiquated) network infrastructure. Despite these variables, there are basic elements that comprise most switching systems in use today.

The History of Switching

·        Manual switchboard - operator made connections using system of “plugs” and “jacks”.
·        Lighted lamps on switchboard indicated line “on-hook”, operator provided manual supervision of line usage.
·        Strowger electromechanical switch came about when number of subscribers exceeded practical limits of manual switchboard.
·        Followed pulses from dial to switch calls to terminating node- problematic due to permanent signal (phone off hook) and calling party hold (calling party fails to hang up) conditions.
·        Crossbar analog; reed-relay analog; pulse amplitude modulated analog.
·        Blocking network – no simultaneous service.

Attributes of Modern-day Switching

·        Non-blocking network – connections can be established to handle high traffic volume.

• The requirement for non-blocking service must be evaluated by the telecom manager – traditional, blocking service may suffice.
• Subject to call-processing delays.
• Incoming matching loss (IML) percentage (incoming call fails to connect due to switch blockage) should be considered in network evaluation.

·        Pulse code modulated digital network.

• Encoded signal connected over 8-bit parallel paths.
• Signal assigned to time-slot or space division – usually a hybrid.

·        Modern switching networks are wired in grids.

• Each stage of grid consists of a switching matrix, connecting input links to output links.
• Links or junctors are wired between switching stages to provide path from any input port to any output port.
• Capacity related directly to number of switching stages on the network.

Switching System Architecture

·        Switching network
·        Controller
·        Database
·        Line ports
·        Trunk ports
·        Service circuits
·        Common equipment

Switching System Control

·        Direct-control system – calls routed in response to dialed digits.

• Switching path established in response to dial pulses.
• Example: Telex network.
• Step-by-step switches.

·        Common-control system – circuitry enables optimum call routing.

• Use electromechanical logic circuits.
• Vulnerable to total central office failure.
• Flexibility to handle call overloads.
• Use system of markers, registers, and translators to establish path through network.
• Crossbar, electronic switches.

Service Circuits

Line – Seven Functions

• Battery feeds from office to line
• Over-voltage protection
• Ringing
• Supervision
• Coding
• Hybrids for digital line circuits
• Testing access

Trunk

• Interface with signaling protocols of interoffice trunks to switching system protocols.

Service

• Ringing and call progress tones
• Recorded announcements
• Permanent signal tones
• Testing circuits

Effects of Changing Policy/Standards on Network Switching

The telephone network was originally designed to be owned and operated by a single entity. Changes in telecommunications policy have negated this concept, and have impacted circuit switching on the network. Standards changes have also driven changes in current switching techniques.

Access to Local Exchange/Long Distance Networks

·        AT&T had a monopoly on long distance service until the mid 1970s.
·        Network divided into 5 classes of switching systems and their interconnecting trunks.
·        Class 5 central offices (lowest), owned and operated by LECS, direct service to subscribers.
·        Class 4 and higher offices used for toll call switching.
·        Competition in long distance, (1969).
·        Divestiture – 1984, of Bell Operating Companies (BOCs).
·        Allowed for other carriers to access network.
·        Exchange Network Facilities Interconnecting Arrangement (ENFIA).
·        7-digit access number, PIN number to access long distance net.
·        Riddled with problems.
·        Equal Access – 1986, agreement with DOJ, required BOCs to provide access.
·        All IXCs gain access to trunk side of local switching systems.
·        Provided through tandem switching systems.

North American Numbering Plan

·        1995, new numbering plan was established as supply of available area codes was exhausted.
·        Prior to 1995, 2^nd digit in area code had to be a “0” or “1”, changed to any digit 0 to 9.
·        Switching equipment had to be re-programmed to identify the change in numbering.

International Dialing

·        International dialing requires countries to conform to dialing plan standardized by the ITU.
·        ITU standard requires a specified country code up to 3 digits.
·        NSN standard was 11 digits, and was increased to 14 digits in 1997.
·        ITU expanded total number of digits in dialing plan from 12 to 15 digits in 1998.