Question

PLEASE READ THE CASE BELOW AND ANSWER THE QUESTION

Chapter 4 – Data Link Layer

Th e NDAS’s board of directors has decided that NDAS will centralize communication along two major data links (Atlanta to Tampa and New Orleans to Tampa), as shown in Case Figure 4-1. You recommended leased T-1 lines from the offices to Atlanta and New Orleans, but the board of directors would not approve further cash outlays at this time. You hear a rumor that President Les Coone suggested to several board members that developments in data communication, such as these additional T-1 lines, might be overkill and that these conservative board members vetoed the funding for the additional leased circuits.

As you continue to work on the network plan, you perceive that you are becoming much more involved with the firm’s strategic planning initiatives for survival. You realize that NDAS will become dependent on its communication facilities for its very existence. In other words, the communication systems at NDAS are the foundation for the firm’s mission-critical applications.

You are burdened with all sorts of thoughts about potential problems and possible errors that can threaten NDAS’s communication capabilities. For example, you worry about recent major circuit and switch outages experienced by some of the leading common carriers. You recognize the fact that even though your operations were affected only slightly by these outages, someday NDAS could undergo a costly disruption of its communication network.

You do not view such a situation as a major competitive threat, because NDAS’s principal competitors have similar networks, and all the parcel delivery services would be paralyzed in the event of a disastrous outage. You do, however, want to examine sources of errors in networks and find reasonable error-control mechanisms that will provide NDAS with a competitive edge. For example, are there specialized techniques that can be used for error detection and correction in NDAS communication networks? You decide to research these possibilities further and add this project to your ever-growing list of things to do.

For now, you have been asked to explore the feasibility of integrating voice, video, and data traffic on to a single transmission circuit. You must determine whether this is a viable option. To make this decision, you want to examine the current transmission circuits and assess their ability to handle the anticipated work load.

Question

1. Prepare a position paper on the types of errors you can expect in the NDAS network and the steps you believe NDAS can take to prevent, detect, and correct these errors.

Answer 1

Data can be corrupted during transmission for reliable communication the error must be detected and corrected. The types of errors which can occur in the NDAS network while integrating voice, video, and data traffic are as follows:

• The cross talk which occurs when the circuit picks up signal in another circuit.
• Echoes are because of poor connection where the signal gets reflected back to the transmission equipment.
• Intermodulation noise is also a type of cross talk when signals from two circuits combine to form a signal which falls into a frequency band saved for another signal.
• Impulse noise is the primary source of error in data communication caused by lightning, fluorescent lights and poor connections.
• White Gaussian noise is the background hiss or static on radios and telephones cause by thermal agitation of electrons.
• Attenuation is the loss of power a signal faces while traveling from transmitted computer to the receiving computer. It makes it hard to interpret if data was 0 or 1.
• A single bit error is when only one bit in the data unit has changed.
• A burst bit error means two or more bit in the data unit has changed.

NDAS can detect Error as follows:

• Parity check: In parity check, a parity bit is added to every data unit so the total number of ones is even. It will be a total number of one’s is odd in odd parity check. Simple parity checks can detect all single bit errors. Burst errors can be detected only if a total number of errors in each data unit is odd.
• In the case of a two-dimensional parity check a block of bits is divided into rows. Next, a redundant row of bits is added to the whole block.
• Cyclical redundancy check: Error is detected by using a method of redundancy. It means we must add the extra bits for detecting the errors at the destination.
• Checksum:

The sender follows the steps as follows:

• The units are divided into k sections each of n units.
• All sections are added to use ones complement to arrive at the sum.
• The sum is complemented and becomes the sum check.
• The checksum is sent with the data.

The receiver will follow the following steps:

• The unit is divided into k sections each of n units.
• All the sections are added using one's complement to get the correct sum
• If the sum is complemented and the result is zero the data is accepted otherwise its rejected.

The methods to correct the errors in data transmission at NDAS are as follows:

1. Flow control is implemented to set the procedure to restrict the amount of data sender can send before waiting for an acknowledgment.
2. The error control in the data link layer is defined by the automatic repeat requests, which means the need for retransmission of data.
3. Then they could introduce controlled access because many computers try to transmit data at the same time. This will help to avoid collision of data and the need to retransmit the data.
4. Installing shielded wires, moving cables from sources of noise and changing multiplexing frequency and guard bands will address issues related to cross talk, intermodulation and impulse noise.