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- What does the data link layer do?
Data Link Layer is the layer responsible for moving data in and out across the physical network, and performs some minimal error correction. It takes information from higher up in the OSI model and passes it down to the physical Layer to be transmitted across the wire. It provides framing, flow control, and error correction and detection (Davis, 2002, p. 27-28). IEEE has divided the functions performed by the Data Link Layer into two sub layers. This is because data link layer has two functions to do. The first is to coordinate the physical transfer of data, and the second is to manage the access of the physical medium (Steinke, 2003, p. 11). The two layers are as below:
- The Logical Link Control (LLC) Sub layer – It provides service Access points SAPs, which devices can use to send information. Thus LLC sub layer handles the interface with the network layer and is responsible for assembly and disassembly o f frames, addressing, address recognition, and cyclic redundancy check CRC calculations and validation. LLC is network independent (Ogletree, 2002, p. 1005).
- The Media Access Control (MAC) Sub layer – It takes care of transmitting data and correcting errors. Thus the MAC sub layer deals with interfacing the physical media and is responsible for defining how access is gained to the shared network media. MAC is dependent on the type of network (Ogletree, 2002, p. 1005).
- Under what circumstances is forward error correction desirable?
Forward Error Correction FEC techniques are widely used in control and telecommunication systems. The FEC code bytes are used at the end of a transmitted frame by the receiving system to find and correct errors. The objective of communication channels is to achieve maximum data transfer in a minimum bandwidth while maintaining an acceptable quality of transmission, and FEC technique is considered to be the most desirable solution in this case (Kolawole, 2002, p. 176). This is because FEC is a mathematical signal processing technique that encodes data so that errors can be detected or corrected. Here, the redundant information is passed along with the original information. If some original data is lost or received in error, the redundant data is used to reconstruct them. Since, the amount of the redundant information is small, there is very less requirement of additional bandwidth and hence the technique is efficient (Keiser, 2003, p. 277).
- What does the network layer do?
The main function of the network layer is to transfer all the data from source to destination. This layer forms the interface between the human machines and the actual networks. The layers above the network layer i.e. transport, session, presentation and application usually run on the user’s machine, while the network layer and the two layers below it i.e. the data-link and physical layer control the network directly. Functions implemented at the network level include routing, switching, flow control, data sequencing and error recovery. These functions are concerned with end-to-end connections and might even span multiple network links. The most important function of network is route determination i.e. the path that data will take to travel between a node of one network to a node on another network. Flow control at the network layer is used by communication systems to prevent congestion errors and bottlenecks from occurring by providing efficient access to network links (Davis, 2002, p. 27).
- Compare and contrast bridges, routers and gateways.
Most LANS operate in the context of much larger networks. A LAN tapping into a Wide Area Network is common. Many organizations have different LANs for various departments that must communicate with one another. This communication can be achieved through the use of bridge, router and gateways. In physical terms these devices can be considered as computers with varying degrees of sophistication.
- Bridge – A bridge connects two similar networks together, for instance two Ethernet networks. A bridge can only distinguish between one LAN and another but not the individual nodes. Bridges can handle changes in data formats like the control bits, sequence numbers, hardware addresses, error control procedures, and flow control, as well as changes associated with transmission facilities (Carne, 2004, pp. 81). Bridges can further be classified into two
- Dumb Bridge – It simply repeats all the packets from one LAN to another even if the packet destination is on the same LAN as the source workstation (Khoshafian, Baker, 1996, pp. 514).
- Filtering bridge – It only repeats the packets across the bridge when the source and destination LANs are different. This greatly reduces the traffic on the network (Khoshafian, Baker, 1996, pp. 514).
- Routers – These are more knowledgeable than bridges about network traffic and the nodes on the network. The routers can understand the protocols being used on the network, and has the knowledge of the network addresses of the different devices and nodes on both sides of the network. The router decides how to pass and direct to sub-LANs each packet based on the protocol being used and the device for which the packet is intended. This greatly reduces network traffic. Routers handle changes in routes, forwarding addresses, segment sizes, as well as changes associated with the data stream and transmission facilities (Carne, 2004, pp. 81).
- Gateways – Gateways consist of both software and hardware needed to connect two dissimilar networks e.g. connecting a token ring to an Ethernet. Usually a gateway is used to connect a LAN to a mainframe or a wide area network. It repackages disparate signals across networks and acts as an interpreter. These systems operate at the highest level of the OSI layer, i.e. the application layer. Gateways process the various protocols used on each network so that the information form the sender is intelligible to the receiver despite the differences in technology, network protocols or platforms (Carne, 2004, pp. 81).
- Under what circumstances would you want to use a multiprotocol router?
Routers are used to connect different networks together. Multi-protocol router is a router, which connects networks using different protocols, generally at the lower levels of the stack. Its primary purpose is to switch packets from one network segment to another. A multi-protocol router does not translate one protocol to another; rather all protocols are routed together. In this case the router has a routing table for each of the networks with different protocols.
- Prepare a brief discussion of LAN (local area network) bottlenecks and what can be done to improve LAN performance.
LAN performance depends on physical design, protocols supported, and transmission bandwidth. Some types of performance bottlenecks in LANs are as follows:
- Bandwidth of LAN is very narrow or many devices are connected to the LAN which in turn reduces the LAN speed
- Too many peripheral requirements or enhanced peripheral requirements. For instance the present day printers are much more advances, and hence complete pages can be sent all at once with high-end graphics. Such type of issues when not handled properly may lead to bottlenecks
- Inefficiency of server, operating systems as well as the network workstations can cause performance bottlenecks in LANs. Presence or absence of disk drives too may cause bottlenecks in some cases.
- LAN bottlenecks are also caused due to too many stations and/or messages
- High error rates also contribute to bottlenecks, in addition to message storms, due to frequent re-transmissions
- A large chain of confirmations and reconfirmations may also cause bottlenecks in LAN
(Terplan, 1999, pp. 255, 256)
For improving the performance of LAN, the existing bottleneck, a list of which is given above, should be identified and updated. However, care should be taken that this process is progressive else updating a components might cause another one to become a bottleneck. One popular solution to enhance LAN performance is by using a LAN switch, which can increase the bit rate to alleviate congestion. Using a faster workstation too can improve its performance (Shim, 2000, pp. 98).
- What safeguards do you recommend for a company to control the use of illegal copies of software on the LANs?
Possessing illegal copies of software constitutes software piracy. LAN is a tempting way to succumb to piracy, where one or less number of licensed copies of the software is used to install program on more than the authorized number of systems. To safeguard against this problem, a company can audit systems for illegal copies of unauthorized software. Yet another form of software piracy on LAN occurs when too many employees on the network are using a central copy of program at the same time. When the number of people using the software is more than what the license allows, it constitutes software piracy. A company can safeguard against this form of piracy by making sure that the employees understand the restrictions, and by installing metering software, which ensures that, only the licensed number of users have access to the software. When absolutely necessary another software license can be purchased which covers the number of people needing to use the software (Business Software Alliance, n.d. pp. 9. 10). In addition, the company should the institution should establish set procedures that prevent illegal copying of software.
Business Software Alliance, (n.d.), “Software Management Guide”
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Published: Artech House, Massachusetts
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