Design issues for the different layers

NAME  : CH M.ZIA UL HAQ

COURSE:   CN

EXAM QUIZ:

Q.1   What are the key design issues for the different layers in computer networks?

Ans. 

ADDRESSING :  A Network normally has many computers,some of which have

                              multiple processes, a means is needed for a process on one

                              machine to specify with whom it wants to talk.As a result

                              of having multiple destinations,some form of addressing is

                              required to specify a specific destination.

ERROR CONTROL: Physical communication circuits are not perfect.Many

                                   error-detecting and error-correcting codes are known.

                                   However,at both the ends of the connection,there must be

                                   an agreement on which  method is used.In addition,

                                   the receiver must have some way of telling the sender

                                   which messages have been correctly received and

                                   which have not.

FLOW CONTROL: It is necessary to keep a fast sender from swamping a slow

                                  receiver with data.Various solutions have been proposed

                                  for this problem.Some of them involve some kind of

                                  feedback from the receiver to the sender,either directly or

                                  indirectly,about the receiver’s current situation.Others limit

                                  the sender to an agreed -on transmission rate.This subject

                                  is called flow control.

MULTIPLEXING: Mutiplexing is necessary when it is expensive or inconvenient

                                 to set up separate connection for each pair of communicating

                                 processes,and the underlying layer may decide to use the

                                 same connection for multiple,unrelated conversations.

                                 Multiplexing is needed in the physical layer,for example,is

                                 needed ,when all  connections has to be sent over at most a

                                 few physical circuits.

       ROUTING:     When  there are multiple paths between source and destination

                                 a route must be chosen.For this a low-level decision must be made

                                 to select one of the available circuits based on the current traffic 

                                 load.For this,routing is necessary.

Q.2

Discuss the flow of information at the correct layers in a connection from

C1 to C2.

                          C1              S1              R1            R2             C2

 

 

                                       

                        HOST   ETHERNET      ROUTERS            HOST

                                        SWITCH

                   C1 ,C2 ->  Hosts

                         S1  ->  Ethernet switch

                     R1,R2 -> Routers

Sol:

       ----------------------------------------------------------------------------------------

                                                                                                                           Application

      -----------------------------------------------------------------------------------------

                                                                                                                           Transport

      ------------------------------------------------------------------------------------------

                                                                                                                            Network

     -------------------------------------------------------------------------------------------

                                                                                                                             Link/Lan  

    --------------------------------------------------------------------------------------------

                                                                                                                             Physical

   ---------------------------------------------------------------------------------------------

     

Q.3

Explain the Bellman-Ford Alogithm.

Sol:

 The Bellman -ford algorithm solves the single source shortest  path problem

 in which edge weights may be negative.The Bellman-Ford  algorithm

 returns a Boolean value indicating whether or not there is a negative

 weight cycle that is reachable from the source.If there is such a cycle,the

 algorithm  indicates that no solution exists.If there is no such cycle,the

 algorithm produces the shortest paths and their weights.

Algorithm:

1.Initialise all the vertices ,set their distance from source to a high value,

   say infinity.Set the value of distance for source to 0.

2.Repeat the following for  no. of vertices - 1 times:

   for each edge(u,v) that belongs to the graph,

      if  distance[v] > distance[u] +weight(u,v)

     then  distance[v] = distance[u] +weight(u,v)

3.Repeat the following steps for each edge(u,v) that belongs to graph.:

    If distance[v] > distance[u] +weight(u,v)

    then “No negative -weight cycle exists between source and destination” 

 4.Return the distance values for all the nodes after all the iterations .

Q.4:

Measurements of slotted ALOHA with many number of usersshow that

10% of the slots were idle.

a)      What is the channel load ?

b)      What is the throughput?

c)      Is the channel underloaded or overloaded?

Ans:

        We know that

a)      10% of the slots are idle.

So,  let P  = probability that a frame does not suffer a collision.

So,P = 0.1

But,P = e^-G

->   0.1 =  e^-G

->  G = 2.3

Channel load =2.3

b)Throughput  =  S = Ge^-G = 0.23

     

    c) Since, G >1, the channel is overloaded.

Q.5

10,000  airline reservation are competing for the use of a single SLOTTED ALOHA

channel.The average station makes 18 requesrs/hr.A slot is of 125 microseconds.

 What is the approx channel load?

Ans:

Total number of requests per second for 10000 stations =10000* 18 requests/hr

                                                                                          = 50 requests/second.

Channel load G =No of requests/sec * time for 1 slot

                          = 50 * 125/1000000

                          =   6.25 * 10^-3

.