Solutions to questions on the second midterm

1. What is multiplexing? Why is it considered cost effective?

   Multiplexing is a technology that enables information sources, each of which each require a given transmission capacity to share a larger transmission capacity. The cost effectiveness of multiplexing is due to economies of scale possible with facilities that have large transmission capacities. Specifically, the cost per Kbps declines with the increase in the data rate of the transmission facility.

2. Name the types of multiplexing of which you are aware. In each case, define it. Indicate also the type of transmission technology with each of these types of multiplexing can be used.

   Frequency division multiplexing: FDM is used with analog transmission technology. FDM is the partitioning of the available bandwith of a transmission medium into lower bandwidth channels that are used to carry information. It is possible when the useful bandwidth of a transmission medium exceeds the required bandwidth of the signals to be transmitted.

   Synchronous Time Division Multiplexing: STDM is used with digital transmission technology usually. It enables the interleaving of transmission signals in time and is possible when the data rate of the medium exceeds the data rate of the signals to be transmitted. In STDM, data are organized into frames which consist of time slots which are statically preassigned to each information source. Asynchrnous Time Divisiion Multiplexing: ATDM is used with digital transmission usually. It is similar to STDM except that the time slots in a frame are not preassigned. This permits ATDM to save "wasted" time slots when there is no traffic and enables it to make better use of available transmission capacity.

3. What is the type of multiplexing used with DOV modems? What is the principal advantage of using DOV technology? DOV modems use FDM. They are cost effective since they enable the use of voice line to make concurrent data and voice calls using traditional analog transmission technology.

4. What is fractional T-1 service? What motivates the need for this service?

   Fractional T-1 service is a tarriffed service that allows a customer to by data transmission capacity less than the T-1 rate of 1.544 Mbps. Organizations that do not need the data rates supplied by T-1 lines can purchase just what they need.

5. LATA's were created after the divestitute of At & T in 1984.

   Who are the carriers that can complete

   • intra-lata calls

     Local exchange carriers, regulated by the state PUC.

   • inter-late,interstate calls

     IXC's, regulated by the state PUC.

   • inter-lata, inter-state calls

     IXC's, regulated by the FCC

   • What was the provision for equal access in the MFJ that broke up AT & T in 1984.

     All IXC's were to be given equal access to the cutomers and the CO facilities of the LEC. The impact was competition that has led to lower long distance prices.

6. What is common channel signalling?

   CCS is the use of separate packet switched network used by switching service points to communicate with switching control points. The control signalling on this separate network makes several services possible.

   800 service, caller ID, 900 service

7. Question on software defined networks
   • CMU uses the AT & T software defined network. What is a software defined network? How does it differ from a private voice network.

     A software defined network is bulk rate service. It is a virtual network defined on the PSTN. It is made possible through the use of CCS. Users make calls as if on a private network (e.g., reduced digit calling) but the calls are completed using the PSTN infrastructure.

   • I place a five digit call to the CMU branch office in New York city. How does the SDN complete the call?

     The five digits are communicated from the telephone to the local SDN central office (a SSP). CCS is used to "translate" the five digit number to a 12 digit number by looking up a database maintained at a service control point. This 12 digit number is then used to complete the call.

8. What type of switching does the PSTN use?

   circuit switching

9. What type of switching does a public data network use?

   Packet switching

10. Why are these different?

    Circuit switching works well with stream traffic such as voice. Packet switching works well with bursty traffic such as data.

11. Why would an organization consider using an alternative access provider to connect to an IXC POP?

    To save on access charges.

12. Solution to the cost/benefit analysis problem

    • If the cost of a leased line is $650/month, which will be more economical: A leased line or a PDN?

      Site connection charges: $200/month

      Packet charge: $1.50 per 1000 packets

      Number of characters per packet: 128

      Total site connection charges: $400 (2 * 200)

      Traffic in packets/day: 600,000/128 = 4687.5 packets/day

      Packet charge/day: 4687.5 *1.50/1000 = $7.03/day

      Packet charge per month (assuming a 30 day month) = $ 210

      Total charges for PDN solution = $610 (210 + 400)

      PDN is cheaper than a leased line

    • How many characters must be exchanged for the leased line to cost the same as a PDN?

      Leased line charges = $650/month

      Fixed connection charges for a PDN = $400/month

      What is the traffic that generates $250/month is packet charges?

      Traffic must be 711,111 characters/day for the PDN solution to cost the same as the leased line solution.

    • Type of equipment needed to work with following types of lines or service.

      Analog lines: modems

      digital leased lines: CSU/DSU

      dial up access to a X.25 network: modems

    • What is the difference in the strategy used to price the public switched telephone network, leased lines and the public data network?

      PSTN: connect time sensitive (also distance sensitive as a second order effect)

      PDN: usage sensitive (price per packet)

      leased line: distance sensitive

13. What is the difference between virtual circuits and datagram service? When is one preferred over another?

    Virtual circuits is one mode of packet switching in which a path (not a dedicated channel) through the network is set up for session between a sending and receiving device. All packets between these devices follow the same path through the network.

    In datagram service each packet follows its own path through the packet switched network.

    Virtual circuits are preferred to datagram service when a large amount of data needs to be transferred between two devices. Datagram service is prefferred due it lack of overhead on set up and its ability to deal flexibly with problems in the network.

14. The X.25 protocol has three layers: packet, link and physical. What is the difference between a packet layer protocol and a the link layer protocol in terms of the service that they provide.

    The packet layer provides reliable connection-oriented end to end data transport. The link layer provides reliable transport over a link, not over a network as does the packet layer.

15. What are the two common ways in which telecommunication services are provided and managed in an organization?

    Provision of service: Make vs. buy. An example is the choice between PBX vs. centrex to provide intra organziational service.

    Management: Telecommunication services are either managed as a utility or employ billback in which consumers of the service are billed. provided as a utility

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