Exam 2

 

Your exam will comprise 3 questions taken (with minor modifications) from the following set of 10 questions, and 3 additional 'short answer' questions:

 

  1. Two stations communicate via a 1 Mbps satellite link with a propagation delay of 270 ms. The satellite serves merely to retransmit data received from one station to another, with negligible switching delay.  Using HDLC frames of 1024 bits with 3-bit sequence numbers, what is the maximum possible data throughput (not counting overhead bits)?

 

  1. An upper layer message is split into 10 frames, each of which has an 80% chance of  arriving undamaged.  If no error control is done by the Data Link layer protocol, on average how many times must the message be sent to get the whole thing through?

 

  1. A group of N stations share a 56-Kbps pure ALOHA channel (pure ALOHA is almost identical to CSMA/CD).  Each station transmits a 1000-bit frame on an average of once every 100 seconds, even if the previous one has not been sent (e.g., the previous frames may be buffered).  What is the maximum value of N.

 

  1. A 1-km-long, 10-Mbps CSMA/CD LAN has a propagation speed of 200 meters/microsecond.  Data frames are 256 bits long, including header, checksum and other overhead.  The first bit slot after a successful transmission is reserved for the receiver to capture the channel to send a 32-bit acknowledgement frame.  What is the effective data rate, excluding overhead, assuming there are no collisions.

 

  1. A very heavily loaded 1-km-long, 10 Mbps token ring has a propagation speed of 200 meters/microsecond.  Fifty stations are uniformly spaced around the ring.  Data frames are 256 bits, including 32 bits of overhead.  Acknowledgements are piggybacked onto the data frames and are thus included as spare bits within data frames and are effectively free.  The token is bits.  Is the effective data rate of this ring higher or lower than the effective data rate of a 10-Mbps CSMA/CD network?

 

  1. A Datagram subnet allows routers to drop packets whenever they need to.  The probability of a router discarding a packet is p.  Consider the case of a source host connected to the source router, which is connected to the destination router, and then to the destination host.  If either of the routers discards a packet, the source eventually times out and retransmits.  If both host-router, and router-router links are counted as hops, what is the mean number of:
    1. Hops a packet makes per transmission
    2. Transmissions a packet makes?
    3. Hops required per received packet?

 

  1. Ten 9600-bps lines are to be multiplexed using TDM.  Ignoring overheads, what is the total capacity required for synchronous TDM?  Assuming you wish to limit average line utilization to 80%, and assuming that each line is busy 50% of the time, what is the capacity required for statistical TDM?

 

  1. Consider a broadband tree with a number of equally spaced stations with a data rate of 10 Mbps and a tree consisting of 10 cables, each of length 100 meters emanating from a headend.  Propagation speed is 200 meters/microsecond.
    1. What is the average time to send a 1000-bit frame to another station measured from the beginning of transmission to the end of reception?
    2. If two stations transmit at exactly the same time, how long will it take for the transmitters to detect the collision, in seconds and in bit time?

 

  1. A group of N users located in the same building are all using the same remote computer via an ATM network.  The average user generates L lines of data traffic (input + output) per hour, on average, with the mean line length being P bytes, excluding the ATM headers.  The packet carrier charges C cents per byte of user data transported plus X cents per hour for each open ATM virtual channel.  Under what conditions is it cost effective to multiplex all N transport connections onto the same ATM virtual channel, if such multiplexing adds 2 bytes of data to each cell?  Assume a single ATM virtual channel has sufficient capacity to support all N users.

 

  1. Consider an Internet node with an IP address of 196.128.220.0.
    1. How many addressable (static IP addresses) hosts can this node support?
    2. If subnetting were adopted using a subnet mask of 255.255.255.140, how many subnets will this support?
    3. What will be the size of each subnet router’s routing table when compared to the routing table of the router if subnetting were not adopted.