Switching Systems

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Chapter 4

• Switching Systems

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Chapters Outline

• Switching Concepts
• Circuit Switching space division
• Circuit Switching time division
• Time Slot Interchanger
• Circuit Switching Systems
• Circuit Switching Applications
• Customer Switching
• Architecture of the Definity PBX System
• Message Switching
• Packet Switching datagram implementation
• Packet Switching virtual circuit implementation

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Switching Concepts

• Earlier telephone switching was done manually by
  having operators negotiate with each others until
  a connection is established
• Switching systems provide economic benefits,
  considering that we have more users than
  communication links
• Centralized switching reduces the number of
  interconnecting links

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Switching

• A method of establishing connections and sending
  information between nodes on a network.
• Circuit switching - establishing a connection
  between two devices on a network, before they
  begin transmitting data.
• The advantage of circuit switching is that it
  guarantees two devices exclusive use of a
  communications channel.

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Switching
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Message Switching
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Packet Switching
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Switching Systems

• The combined collection of hardware and software
  that establishes connections between lines and
  trunks in order to complete calls.
• Although makes and models differ between
  manufacturers, all modern switches share four
  essential elements
• Switching matrix
• Line or trunk circuits
• Central control computer
• Common equipment

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Local Switching Systems

• A local switch performs the following functions
• Provides dial tone to a local subscriber
• Accepts and interprets signals (including
  off-hook notification, dial tones, and so on)
  from the local subscriber
• Receives signals from the destinations local
  switch about when to terminate the call
• Records local subscriber billing information
• Stores information about subscribers (such as
  what type of service they have chosen) in a
  subscriber database
• Tests and maintains the subscribers local loop

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Tandem Switching

• A tandem switch may perform the following
  functions
• Provide termination for trunks at Class 1, 2, 3,
  and 4 central offices
• Gather and transmit information about telephone
  network traffic and congestion
• Determine the fastest path over the PSTN for
  long-distance calls
• Carry data and voice signals between central
  offices
• Test and maintain trunks
• Assist in trunk configuration

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Lucents ESS Switch
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Nortels DMS Switch
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Nortels DMS Switch
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Switch Signaling

• The exchange of information between the
  components of a telephone network or system for
  the purposes of establishing, monitoring, or
  releasing phone circuits as well as controlling
  system operations.
• Switch-signaling functions include
• Transmitting Address Information
• Supervising
• Transmitting Information

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Switching Equipment

• Major functions of switching equipment at a
  central office
• Dial tone
• Customer and phone number identification
• Call setup
• Call routing
• Call supervision
• Line testing and maintenance

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Central Office Hierarchy

• Serving area (of a local office) - the
  geographical boundary that includes all its
  subscribers. It extends roughly three miles in
  all directions from the central office (CO).
• Trunk - a transmission route between switches
  that typically has a great deal more capacity
  than a feeder.
• Regional offices - Class 1 central offices.

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Central Office Hierarchy
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The Telecommunications Act of 1996
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The Telecommunications Act of 1996

• The Act codified requirements for the
  interconnection of all local exchange carriers.
  These policies included
• Interconnecting with other service providers and
  not imposing any barriers to interconnection
• Enabling nondiscriminatory resale of their
  services to competitors
• Providing number portability, or the ability of
  telecommunications service users to retain their
  same telephone number without hampering the
  quality, reliability, or convenience of their
  phone service
• Allowing competitors to access and connect to
  their facilities

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The Telecommunications Act of 1996

• To increase competition in local phone service,
  the Act placed the following requirements on all
  ILECs
• Negotiating interconnection agreements in good
  faith
• Providing competitors with the same type and
  quality of access to their facilities that they
  themselves could obtain at their cost
• Providing competitors with access to subscriber
  information, such as telephone numbers and
  billing data
• Offering nondiscriminatory, wholesale prices for
  telecommunications services to all competitors

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Switching Concepts
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Type of switching

• Circuit Switching a physical path is provided
  for the duration of the call
• Message Switching customer exchange information
  using store-and-forward techniques. A message is
  delivered as a whole - telegraph service, email,
  etc..
• Packet Switching A message is fragmented into
  packets where each packet may take a different
  rout along the network -- Internet

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Type of Switching
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Functions of a Switching System

• Attending
• Path selection
• rout selection
• busy testing
• path establishment
• network interconnection
• signal transmission
• alerting
• supervision

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Switching Concepts
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Circuit Switching Space Division Switching

• Old technology
• replaced by digital switched network
• its characteristics include
• establishment of spatial path through the network
• links are maintained for the duration of the call
• signals can be digital or analog
• uses metallic devices or semiconductor elements
• networks may be made of several stages

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Circuit Switching Space-Division
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Circuit Switching Time Division

• It builds on the concepts of time-division
  multiplexing
• multiplexed information is transferred from input
  points to output points
• Uses a chip set known as the time-slot-interchange
  r to assist in the connecting input multiplexed
  channels to output multiplexed channels

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Circuit Switching Time-Division Switching
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TSI -- Time Slot IntechangerEX Mitel, ATT,
Motorola Chip Set
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Functions of a Circuit Switching System

• Battery Feed
• Overvoltage Protection
• Ringing Signals
• Supervision
• CODEC
• Hybrid circuits separate the incoming and
  outgoing signals going to coming from receivers
  and transmitters two-to-four-wire conversion
• Testing detection of faults on lines and trunks
• Known as BORSCHIT circuit (traceable to Iwerson
  of ATT Bell Labs.

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Interfaces in a Circuit Switching System

• Station Line Interface detect off-hook, CODEC
  functions, detect on Hook, communicate with the
  switching control subsystem, detect faults
• Trunk Interfaces provides interfacing to various
  transmission links in the overall hierarchical
  network
• CO trunks
• DID
• Digital/Analog
• Auxiliary
• Local Area Network Interfaces
• ISDN PRI

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Interfaces in a Circuit Switching Switching
System

• Service Circuits
• tone detection circuits
• tone clock circuits
• pooled modem circuits
• recorded announcement circuits
• recording for traffic measurement and billing
  functions, used to devise a particular traffic
  pattern depending on BUSY-HOUR call rate

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Circuit Switching Software

• Plays a very important role in circuit switching
  systems
• call processing software
• origination -- detection of call request
• digit collection
• digit translation
• routing
• alerting
• answer
• drop
• System Administration
• System Maintenance
• Billing and record keeping

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Circuit Switching Software

• Testing
• Traffic engineering
• Services changes and rearrangement
• Feature Selection
• Data Base Management
• Maintenance
• fault recognition
• fault recovery
• system integrity
• auditing
• system recovery

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Circuit Switching Applications

• local exchange switching designed to serve the
  general public
• trunk switching tandem, toll, and transien
  switches
• gateway switching rout traffic between national
  and international calls, must accommodate
  different signaling format
• customer switching PBX, KEY systems

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Call translation in a Local Exchange Switch
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Key Telephone Systems

• A system that gives all telephone users within an
  organization centralized access to one of many
  incoming PSTN lines.
• A KTS typically consists of a wall-mounted
  control console that connects to the
  organizations internal phones and to the
  telephone companys end office.
• This console, called a key service unit (KSU),
  signals the telephone attendant about incoming
  calls, controls busy indica-tor lights on line
  buttons, and tracks call information, among other
  tasks.

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Key Telephone Systems
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Key Telephone Systems
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Key Telephone Systems

• Features of electronic KTSs include
• Call forwarding
• Caller identification
• Direct inward dialing
• Do not disturb
• Message waiting
• Music on hold
• Station restriction
• Voice mail

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Private Branch Exchange (PBX)

• A switch owned and operated by a business or
  other private organization that connects multiple
  telephone sets to one or more of the telephone
  companys central offices.
• Provides call setup and routing within an
  organization.
• Whereas a KTS depends on a CO for dial tone, a
  PBX actually provides its connected telephones
  with dial tone.
• A PBX can connect an organization to the PSTN
  through a trunk, a connection with significantly
  more bandwidth than a local loop which serves to
  directly connect switches.

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Private Branch Exchange (PBX)
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Components of a PBX

• A PBX consists of four significant elements
• Station lines - the lines that connect individual
  telephone sets to the switch.
• Central processor - the computer that oversees
  all PBX operations, including processing incoming
  calls, outgoing calls, call setup, routing,
  supervision, and data gathering
• Switching module - The equipment that
  accomplishes call setup, routing, and
  supervision, just as a switch in a central office
  does.
• Trunks - The lines that connect the switch to COs.

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Components of a PBX
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Features Offered by a PBX

• Automatic call distribution (ACD)
• Automated route selection (ARS)
• Computer-telephony integration (CTI)
• Power failure provisions
• Station message detail recording (SMDR)
• System management
• Trunk queuing
• Unified messaging
• Wireless capabilities

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Private Branch Exchange (PBX)

• Owned or leased and operated on customer premises
• station users are connected to the PBX and the
  PBX is connected to the local exchange switch via
  trunks, or digital transmission facility such as
  T1
• In the network hierarchical, a PBX is classified
  as a customer premises equipment

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PBX Core Functions

• Station to station calling capability
• Concentration of customer stations to central
  office capability
• Attendant handling of incoming calls from the
  central office (manually or automated)

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PBX Enhanced Functions

• Automatic call distribution (intercept operators,
  department store catalog divisions, directory
  assistance, customer services
• calls are distributed among attendants (agents)
  to maximize efficiency
• calls are queued, if all agents are busy
• management information data are collected to
  administer switching facilities and the size of
  agent groups

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PBX
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Example of a PBX System Lucent Technologies
Definity PBX

• Features
• call conferencing 6 parties
• call forward
• hold
• transfer
• hunt groups
• attendant services, primary and night service
  attendant.
• digital, analog, and ISDN station types support
• system administration terminal (SAT)

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Basic Architecture of the Definity PBX

• Uses time division switching to handle switching
  functions
• uses 256 time slot TDM bus
• 5 time slots are used for communication among the
  control complex and the peripherals of the system
  
• The bus provides means of communications among
  two types of circuit boards angels and the
  archangel.
• Port boards are referred to by the angels and
  they include analog, digital, ISDN, DS1, ton
  detector, CO trunks, DID(direct inward dialing)
  trunks,

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Lucent Technologies Definity PBX Architecture
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Intra-system communication

• Carried over 5 dedicated time slots
• uplink and down link messages
• scenario of a user going off hook
• the angel detects the off-hook condition
• waits for a downlink message asking it if it has
  anything for the control complex
• it report the off-hook condition using an uplink
  message defined for its type
• the archangel sends a series of downlink messages
  to connect tone generator circuits to the user
  (dial tone)
• the angel collects the dialed digits, sends them
  in an uplink message

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Store and Forward Switching

• Message Switching
• Packet Switching
• datagram implementation
• virtual circuit implementation

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Message Switching

• the message is sent as a whole
• email
• telegraphy
• effective
• no delay associated with setting the call. A rout
  does not have to be pre-established.
• Message switching networking may provide protocol
  conversion and error checking functions
• disadvantages
• delay in message delivery no real-time
  communication

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Packet Switching

• History
• Defense Department looking for ways to secure
  communications in case of war
• In the hierarchical network, if you take a node
  (regional switch), you knock out communication in
  a large geographical part of the country
• In packet switching networks, weight is
  distributed among nodes a number of nodes can be
  taken out while communications are maintained.
• Hard to tap conversations in packet switched
  networks

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Packet-Switching Networks

• Includes X.25, ISDN, ATM and frame-relay
  technologies
• Data is broken into packets, each of which can be
  routed separately
• Advantages better line efficiency, signals can
  always be routed, prioritization option
• Disadvantages transmission delay in nodes,
  variable delays can cause jitter, extra overhead
  for packet addresses

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Packet-Switching Techniques

• Datagram
• each packet treated independently and referred to
  as a datagram
• packets may take different routes, arrive out of
  sequence
• Virtual Circuit
• preplanned route established for all packets
• similar to circuit switching, but the circuit is
  not dedicated

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Packet-Switched Routing

• Adaptive routing changes based on network
  conditions
• Factors influencing routing are failure and
  congestion
• Nodes must exchange information on network status
• Tradeoff between quality and amount of overhead

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Packet-Switched Congestion Control

• When line utilization is gt80, queue length grows
  too quickly
• Congestion control limits queue length to avoid
  througput problems
• Status information exchanged among nodes
• Control signals regulate data flow using
  interface protocols (usually X.25)

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Packet Switching

• Does not was resources like circuit switching
• being in use in many WAN application
• classified by two implementations
• datagram implementation
• virtual circuit implementation

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Packet Switching Datagram Implementation

• sequential delivery of packets is not guaranteed
• packets may not be delivered at all or may be
  duplicated
• all packets must contain the full destination
  address

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Packet Switching Datagram Implementation
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Switching at the Node
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Packet Switching Virtual Circuit Implementation

• A DTE places a call to another DTE by sending a
  special packet into the network indicating a
  desire to setup a connection
• The network then sets up a rout for subsequent
  packet transfer
• The rout stay in place for the duration of the
  connection
• Virtual circuit connection is end-to-end
  connection between two users over a
  store-and-forward network
• In virtual circuit connections
• the user sees the packet delivered in sequence
• no routing decisions are needed

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Computer Telephony Integration

• Significant call control functions that CTI
  enables, include
• Automatic dialing from a PC interface
• Call screening based on incoming phone number
  identification
• Call forwarding programmed through a simple PC
  interface
• Automated attendant services
• Call logging and the collection of call
  accounting data

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Computer Telephony Integration

• Some of the significant media processing features
  that CTI enables include
• Voice message recording and playback
• Fax storing and sending
• Speech recognition and text-to-speech translation
  (for instance, your CTI-enabled computer could
  read an e-mail message to you)
• Online call recording

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Computer Telephony Integration

• Some of the customer data management features
  that CTI enables include
• Access to personal phone books saved on the
  network
• Caller database records that appear when a number
  is selected from the phone book
• Retrieval of customer calling patterns and
  billing information
• Access to customer schedules

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Computer Telephony Integration
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Power Equipment
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