CS610 Computer Network Assignment No 1 Solution Fall 2012

Question 1 [5 Marks]

Carrier Sense Multiple Access/Collision Detection (CSMA/CD) is an access method used with the Ethernet.
Discuss how CSMA/CD will work with an Ethernet switch when 10baseT is implemented in some network. How
collisions are managed by an Ethernet switch?


10-Mbps Ethernet-10Base-T

10Base-T provides Manchester-encoded 10-Mbps bit-serial communication over two unshielded twisted-pair cables. Although the standard was designed to support transmission over common telephone cable, the more typical link configuration is to use two pair of a four-pair Category 3 or 5 cable, terminated at each NIC with an 8-pin RJ-45 connector (the MDI), as shown in Figure: The Typical 10Base-T Link Is a Four-Pair UTP Cable in Which Two Pairs Are Not Used pair is configured as a simplex link where transmission is in one direction only, the 10Base-T physical layers can support either half-duplex or full-duplex operation.

Figure: The Typical 10Base-T Link Is a Four-Pair UTP Cable in Which Two Pairs Are Not Used

Although 10Base-T may be considered essentially obsolete in some circles, it is included here because there are still many 10Base-T Ethernet networks, and because full-duplex operation has given 10BaseT an extended life.

10Base-T was also the first Ethernet version to include a link integrity test to determine the health of the link. Immediately after powerup, the PMA transmits a normal link pulse (NLP) to tell the NIC at the other end of the link that this NIC wants to establish an active link connection:

  • If the NIC at the other end of the link is also powered up, it responds with its own NLP.
  • If the NIC at the other end of the link is not powered up, this NIC continues sending an NLP about once every 16 ms until it receives a response.

The link is activated only after both NICs are capable of exchanging valid NLPs.

Ethernet Collision Management

In 10BASE-T networks, typically the central point of the network segment was a hub. This created a shared media. Because the media is shared, only one station could successfully transmit at a time. This type of connection is described as a half-duplex communication. As more devices were added to an Ethernet network, the amount of frame collisions increased significantly. During periods of low communications activity, the few collisions that occur are managed by CSMA/CD, with little or no impact on performance. As the number of devices and subsequent data traffic increase, however, the rise in collisions can have a significant impact on the user’s experience. A good analogy is when we leave for work or school early in the morning, the roads are relatively clear and not congested. Later when more cars are on the roads, there can be collisions and traffic slows down.


Question 2 [10 Marks]

Given the two polynomials x9+x7+x4+x3+x2+x and x3+x+1. Consider 1st polynomial as dividend and 2nd as
divisor. You are required to perform the following tasks:

· Convert both polynomials into binary equivalents.
· CRC will consist of how many bits, mention.
· Find the CRC value of these binary equivalents with each step written in your solution.


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