Telephony

Circuit Switch - The circuit switching is based on FDMA or TDMA technologies. In FDMA the radio spectrum is divided into narrow carrier frequencies, and in TDMA the channel bandwidth is divided into timeslots (a set of consecutive bits). The granularity of the bandwidth division determines the number of subscribers that can be supported simultaneously. The carrier frequency or the timeslot are allocated for the duration of the call. After the call ends the resource is returned to the free pool.

In a circuit-switch PSTN call the CPE to local exchange media (twisted copper-wire or fibre local loop) is dedicated for the subscriber call and remains idle when not in use. The trunks that connect PSTN switches are the shared media, the timeslots or carrier frequencies are allocated on demand for the duration of the call. The connection of the subscriber line with the trunk is made by the Time-Switch. The end-to-end call is established by the time-space-time switching. A Time-Switch is a memory mapping device that cross-connects the two time-slots. The time-switch has a cross-connect table for the subscriber lines and the trunk lines outgoing from the exchange. The Space-Switch connects the incoming & outgoing trunk lines, the space switch does not switch in time, and the times-slot is switched in space i.e. the incoming time-slot is mapped to the same identification number outgoing timeslot. E.g. NY X₁ to FL X₁ or NY X₁ to FL Y₁. To make a circuit-switch connection the allocated trunk timeslot identification number is written into the subscriber line entry in the time-switch cross-connect table and the subscriber line identification number is written into the trunk timeslot entry in the cross-connect table. The trunk time-slot is allocated by the space-switch.

PSTN New York - TS                        PSTN TAX - SS                      PSTN Florida - TS

Cross-Connect Table for the PSTN switch shown in the Figure.

The cross-connect mappings in the table will remain for the duration of the call. Thus the analog voice signal received on the subscriber line of John is converted to a digital signal and sent out on trunk NY X₁ to PSTN TAX. At TAX the signal received on NY X₁ is sent out on FL Y₁ to PSTN Florida. At PSTN Florida the signal received on FL Y₁ is sent to S_george. The reverse mapping is used for George to John communication. The ITU-T Q.931 signaling protocol is used to setup the time-slot cross-connect and to allocate the space slot.

            When John calls George, the PSTN New York Exchange sends a space slot allocation request for the call after the caller and the called party verification. The PSTN TAX allocates and reserves the forward path timeslots NY X₁ & FL Y₁. PSTN NY communicates with PSTN FL to connect the call to George on the timeslot FL Y_1. After the call is complete the cross-connect table mappings are removed and the space-slot and time-slots are returned to the free pool.

            The call from John to George is a circuit-switched call because an end-to-end transmission media circuit is established for the duration of the call. The resources allocated for this circuit are not used for any other purpose for this duration. GSM is TDMA circuit-switch technology. ATM/Frame Relay Virtual Circuits (PVC/SVC) and MPLS Label Switch Paths (LSP) are circuit switch technologies that carry data packets for multiple applications. The PSTN local-loop line when connected to the data modem carries data packets, the connection is circuit-switched only upto the media gateway.

Dedicated/Leased Line - Is a permanent end-to-end transmission media circuit that is established by the system administrator when the subscriber subscribes for the dedicated/leased/private line service. The circuit is neither established on start of the call nor dismantled at the end of the call. The resources allocated for the dedicated circuit are not returned to the pool at the end of the call. The resources can be freed only when the subscriber unsubscribe the dedicated/leased line service. The end-to-end subscribers can talk by going off-hook without any dynamic call-setup at the exchange. E.g. If John wants a private/leased line connection to George, the cross-connect mappings in the table will remain for the duration of the leased line service subscription. The dedicated line service is generally used as hotline and inter-office communication (PBX, intranet). If there is an ATM/Frame Relay or Gigabit Ethernet switch in the end-to-end path a Permanent Virtual Circuit (PVC) or LSP is configured to reserve the bandwidth for the duration of the service subscription.

Packet-Switch - The Ethernet, ATM, and CDMA are packet switch technologies. In packet switching the physical/virtual transmission media bandwidth is not reserved for a connection. The information (data/voice/video) is encoded into the Layer 2 frames of maximum fixed size and transmitted on the media by the scheduler based on the frame arrival time and the frame priority. The frames from the multiple end users are multiplexed onto the transmission media for the better bandwidth utilization. The capacity of the system: the processing power, the transmission media bandwidth and the load from the users determines the maximum number of subscribers that can be connected simultaneously.

The ONT to OLT single physical transmission media the Ethernet fibre cable is used to carry both voice & data. The IP packets multiplexed on this fibre link are:

Ÿ         The call from PSTN subscriber 234-567-8901 to subscriber 123-458-7890, the analog voice is converted to digital data and encoded in IP packets at ONT.

Ÿ         The calls from IP phone 123-555-7890 to John at 234-666-8901, the VOIP packets.

Ÿ         The e-mail or internet access by John@office.com, the IP data packets.

The IP packets have different destination, source IP address & application protocol identification to differentiate between the packets. The packets are routed and distributed by the media & IP gateways to PSTN & DSLAM respectively.

            The call from John’s PSTN phone to other PSTN phone has phone to ONT line as a dedicated resource, no other network resource is dedicated for the call, the voice data is packet switched on the shared media. The VOIP call from John’s IP phone to the other IP phone has no dedicated resource for the call.

Billing: In a circuit switch call the subscriber is billed for the duration of the call irrespective of whether or not the voice/data is being transmitted on the circuit, i.e. the idle/silent moments are also billed. In the dedicated line the subscriber is billed for the duration of the leased line service, the rent/lease amount for the dedicated line is payable monthly/annually. In a packet-switch connection the subscriber is billed for the amount of data downloaded or uploaded on the connection, i.e. the idle/silent moments are not billed.

Bibliography

1.      http://www.eventhelix.com/ThoughtProjects/Xenon/HardwareArchitecture.htm

2.      http://en.wikipedia.org/wiki/Time_division_switch

3.      http://www.utstar.com/Solutions/Broadband/GEPON/

4.      http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=01Bfq661619904