Telecommunications architectures

2001: Home assignment 10 - Model answer

Notice! The role of this model answer is instructional only. Everything that is asked in the assignment may not be covered in detail. The model answer gives you the content of the answer and an idea of what it should be. The things that are important in the grading of the homework have been emphasized. Please, inform the course staff if you think that there is a mistake in the model answer!

1. SS7 (4 points)

1. Your friend from Tampere calls you. After a nice little conversation you hang up the phone. What ISUP messages move through the network? List all the messages in the correct order and briefly explain the purpose of each. Assume that the switch hierarchy has only one level and there is a direct trunk link between Espoo and Tampere.
   
   1. IAM (initial address message from Tampere switch (T) to Espoo switch (E). IAM is the basic message necessary to initiate a call and it contains the information about the initiating switch, the destination switch, the trunk selected, the calling and called numbers etc.
   2. ACM (address complete message) from E to T. Indicates that IAM has received its proper destination. Contains information about the receiving switch, the sending switch and the selected trunk.
   3. ANM (answer message) from E to T. Indicates that the called subscriber has picked up the phone. Contains information about the recipient switch, the sending switch and the selected trunk.
   4. REL (release message) from E to T. Indicates that you have hung up the phone. Identifies the trunk associated with the call.
   5. RLC (release complete message) from T to E. Idicates that the trunk used has been disconnected and is now idle. Contains the infromation about the trunk used.

2. During your conversation, someone else tries to call you from Lappeenranta. Explain the messages.
   
   1. IAM from Lappeenranta switch (L) to Espoo switch (E)
   2. REL from E to L (called party busy)
   3. RLC from L to E (the one from Lappeenranta hangs up)
      This was not specified in the assignment, so it's not necessary to mention RLC here. However, it's quite unusual that someone just goes on listening to the busy tone :)

2. Switching (4 points)

1. Is a non-blocking three-stage switch generally less expensive (using less crosspoints) than a crossbar switch? If not, how many inputs/outputs must there be to make crossbar switch more expensive (order of magnitude is enough)? (assume a symmetric switch)
   

   about 100

2. A three-stage non-blocking space division switch has 12 input and 12 output lines. The first stage has four crossbars. How many crossbars are needed in the second stage for the switch to be non-blocking?
   

   5 crossbars is enough in the second stage.
   Clos' formula for wide-sence non-blocking switch: k=2n-1
   Here, n=12/4=3 (and not 4 as many of you thought)
   => k=3*2-1=5
   For rearrangeably non-blocking switch, 3 crossbars is enough.

3. Batcher-Banyan switch (4 points)

1. 4 cells (input lines 2, 4, 6, 7; destinations 7, 1, 0 and 6) are going through a 8x8 Batcher-Banyan switch. For each of the 6 stages in the Batcher switch and the four steps in the banyan switch (including the input and output), list which cells are there as an eight-tuple (cell on line 0, cell on line 1 and so on). Indicate lines with no cell by -.
   

    ---7710000
    --7-10--11
    7----711--
    -7-6-6----
    1---6-6---
    -0610--6--
    010---7-66
    6610---777
   

2. Two cells having same destination are on input lines of a 8x8 Batcher-banyan switch (assume there's no trap network).
   1. Will they always collide?
      

      Yes, because cells should be ordered on the last stage of the Batcher switch -> they will collide on last stage of Batcher or even earlier.

   2. Does the colliding stage depend of their starting points?
      

      Yes. For example: (X = the cell's destination, which is not important here:
      

      XXX collision
      --X here (stage 3)
      ----------
      XX--------
      ----------
      ----------
      ----------
      ----------
      

      but:
      

      XXXXXX collision
      -----X here (stage 6)
      ----------
      ----------
      ----------
      -X--X-----
      --X-------
      X--X------
      
      

   3. Does it depend of their destination?
      

      No, if there's no other cells moving in the switch at the same time. The cells' destination can be marked with X as in previous example. The value of X is not important until the Banyan swith, but the cells will collide in the Batcher switch already.

   4. Does it depend of other cells moving in the switch at same time?
      

      Yes, because the other cells can change cells' routes.

4. Explain these terms briefly (4 points)

1. time slot interchanger
   

   - a part of time division switch, reorders input frames and produces output frames

2. crosspoint switch
   

   - same as crossbar switch, a switch where all input lines and all output lines are connected to each other directly (has nxn crosspoints)

3. out-of-band signaling
   

   - signaling that does not take place over the same path as the conversation

4. ISUP
   

   - ISDN User Part, defines the messages and protocol used in the establishment and tear down of voice and data calls over the public switched network and to manage the trunk network on which they rely.

5. signal unit
   

   - messages, in which the signaling information is passed over the signaling link

6. knockout switch
   

   - an ATM switch design that uses output queueing. Has a concentrator, which selects out n cells for queueing on each line.

7. switching fabric
   

   - the hardware that makes data coming into a network node come out by the correct port.

8. rearrangeably non-blocking switch
   

   - any connection request from a free input link to a free output link never blocks, but may require modifying the paths of ongoing connections.