Data Communication can mean many things to different people, but when industry with computers and their communication with them. So, people are usually discussing equipment that wan designed to provide or provide or gather information needs to communicate. Data Communication provides the tools, product and equipment to make it. The distance over which data moves within a computer may vary from a few thousandths of an inch. The amount of Data Communications builds from this point on, because there are many factors such as distance, topology, protocol, signaling, and security. Data Communications will continue to develop and change considerably for the probable future. In the companies unlike manufacturers are not biased toward solution and the applications.
Question 1
A group of business employees would like to set up a small networking office. Explain the meaning of topology. Discuss and draw the different types of network topology that are available.
Answer of Question 1
Nowadays, networkings are very popular. So, a network consists of multiple computers connected with some type of interface, each have one or more interface devices such as a Network Interface Card (NIC). Each computer is support by network software that provides the server or client functionality.
Network can be of any of the following three types such as Local Area Network (LAN), Metropolitan Area Network (MAN) and Wide Area Network (WAN). Hence, the local area networks (LANs) interconnect data processing devices that serve communities of users. In the first industrial in the context of the minicomputer world, than the LAN come into it’s to a personal computers (PCs) develop into the norm in the most networking environments. Seen the LAN networks were as the solution to the research problem. Then the computer could be fixed with a new I/O device and directly linked to one another. In the beginning, LAN is organizing to support shared printer access and to assist the movement of files between systems. The system would be taken long to harness the platform to support other application, such as email. The components of LAN are requiring the incorporation of many different components that determine how the devices are connected. Second is what the format of the data will be transmitted in. Third is how to ensure that multiple stations can transmit at the same time. By the way, this is the major elements of the LAN and some options commonly available to network designers. Metropolitan Area Network is the connection of devices that covers a geographical area of region that is larger than Local Area Network (LAN) but smaller than Wide Area Network. It implies the interconnection of network in the city into a larger network. Then the Wide Area Network (WAN) is the interconnection if devices across a geographical are. The connection spans from country to country.
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What is a topology? In a LAN, the organization can be described by the physical topology and the logical topology. The physical topology of network refers to the arrangement of cable, computers and other peripherals. Therefore, physical topology should not exist confused with logical topology which is the method how data actually transfers in a network as different to its design. So, the logical topology of a LAN is attaching devices and the flow of signals between attache’s devices. Topology can be measured as a virtual shape or arrangement of a network. However, the shape actually does not match to the actual physical design of the devices on the computer network. The different topologies available to LAN, while discuss in the following. There are five types of topology network including star, bus, ring, tree, and mesh.
Bus Topology
Figure 1: Bus topology
(Florida Center for Instructional Technology College of Education)
(University of South Florida, 2009)
Bus topology is the simplest ways a network can be organize. In bus topology, all computers are connecting to the same transmission line by using a cable. It is coaxial. Bus topology is easy to handle and put into action and is best suit for small networks. So, the advantages of bus topology are easy to use and understand. Second is requires least quantity of cable to connect the computers together. Therefore, it is less expensive than extra cabling arrangements.
Ring Topology
Figure 2: Ring Topology
( Network topology, kioskea’s Creative Commons License Deed V2.0, 2007 ) (http://en.kioskea.net/contents/initiation/topologi.php3)
In this type of ring topology, each computer is connect to the next computer with the last one connect to first. Consequently, each retransmits what it receives from the previous computer. Then the message flows around the ring in one direction. Ring topology does not subject to signal loss problem as a bus network experiences. By the way, there is no execution because there is no end to the ring. Ring topology advantages are each node has equal access and capable of high speed data transfer.
Mesh Topology
Figure 3: Mesh topology
Mesh topology is a unique network design in each computer on the network connects to other. It is creating a point-to-point connection between each device on the network. The function of mesh design is to provide a high level of redundancy. If one network cable fails, the data always have an alternative path to get its destination. The advantages of mesh topology are provides redundant paths between devices and the network can expand without interruption to current users.
Star Topology
Figure 1: Star topology
(Florida Center for Instructional Technology College of Education)
(University of South Florida, 2009)
A star topology is design with each file server, workstation, and peripherals. All of them are connect to a central network hub, switch, or concentrator. So, data on a star network passes through hub, switch or concentrator before ongoing to its destination. The common arrangement is use twisted pair cable. It also is use coaxial cable or fiber optic cable. The advantages of star topology including easy to add a new computer system to the network, crash of one workstation does not affect the entire network, uses a single access protocols and very fast
Tree Topology
Figure 5: Tree Topology
(Florida Center for Instructional Technology College of Education)
(University of South Florida, 2009)
At the last tree topology is also known as a hierarchical topology and a central root node that is connecting to one or more nodes of a lower hierarchy. In each node the network has a exact fix number of nodes connect to a lower level. A tree topology combines individuality of linear bus and star topology. Hence, it consists of groups of star-configure workstations connect to a linear bus backbone cable in figure 5. Tree topology allow for the increase of an existing network, and enables schools to configure a network to meet their needs. Finally of advantages of a tree topology is point-to-point cabling for individual segments and support by numerous hardware and software venders.
A group of business employees would like to set up a small networking office. Before set up the topology, office must consider when selecting a topology such as cost, flexibility and reliability. Cost of selecting that is selected for a Local Area Network has be install and perhaps a lengthy process including the installation cables and raceways. Another way for a network to be cost effective one would attempt to minimize installation cast. This may be achieved by using the suitable hardware linking cables, good modems, cost effective computers to reduce cost. Second is flexibility is one of the main advantage of a Local Area Network. It is ability to have the data processing and peripheral nodes distributes around a given area. Next be consistency is select for the network can help through allowing the location of the responsibility to be detect and to present come means of isolating the responsibility.
The best of topology for small networking office is bus topology. It is the easy method of networking computers. So, this consists of a single cable as a trunk, backbone or segment that connects all the computers in the network. However, each system is directly attached to be common communication channel. Then signal is transmitter over the channel makes up the messages. While each message is passes along the guide each system receives it. After receiving the message each system scan the destination address contain in the message. On a bus topology signals are sending to all the computers in the network to keep the signal from active back and forth along the cable a terminator is place at the end of the cable. A bus topology only can one computer send data at a time, therefore the more computers in the bus slower data transmission in the network. Usually, bus topology is easy for small office use (example in figure 1).
Question 2
The purpose of data link control is to provide functions like flow control, error detection and error control. Explain each function in detail.
Answer of Question 2
The data communications have much more needed to control and manage to substitute. So, the list some of the requirements and objectives for effective of data communication between two directly connected transmitting receiving station such as frame synchronization, flow control, error control, addressing, error detection and recovery, control and data on same link and link management. So, line access controls determine which station can pass on next. This is easy for two stations on a full-duplex link. When more than two stations are in use on a full-duplex link such as multipoint or any number of stations is in use on a half-duplex line. However, transmission needs to be not acceptable suspiciously. The service of frame synchronization is the data link layer is responsible for providing synchronization at the frame level. This is determines the beginning and end of each frame. Therefore, the physical layer is usually responsible for maintaining bit synchronization. Flow control is sometimes of the receiving station must be able to cut off the transmitter, so the receiver may be too “busy” to accept of new frame. By the way, at the data link layer, flow control enables the receiver to tell the transmitter it is not ready, and to later identify its motivation to accept more frames. Another is error control is bits errors introduce by the transmission system should be correct. In the addressing on two station connections, addresses can be use to separate commands from responses. Then, addresses are necessary on multipoint links with more than two stations to denote the future receiver and sometimes to classify the sender as well. Error detection and recovery is using a grouping of order numbers and an error detecting or correcting code, so the Data Link layer protocol ensure that frame with error are accepted and not deliver to higher layers. Then, the recovery is by means of retransmission for error-detecting codes. Timers are use to ensure with the purpose of all transmit frames are receive. Maintain of control and data on same link. It is usually not attractive to have a physically divide communications pathway for control information. As a result, the receiver must be able to distinguish control information from the data being transmitted. The link management of initiation, maintenance, and termination of a sustained data exchange requires a fair amount of coordination and assistance among station. It actions for the management of this exchange are involve. This requirement is content by the physical interfacing techniques. A data link protocol that satisfies these requirements is a rather complex of issue. Begin to looking at three key of mechanisms that are part of data link control such as flow control, error detection, and error control.
2.1 Flow Control
First part of data control link is flow control. Flow control is mechanisms are necessary in order to stop the transmitter form overwhelming a receiving entity with data. So, this can be achieved by letting the receiver control all data flow from the sender. Accordingly, the popular of flow control mechanisms allow the receiver to dens credit to the sender in terms of how much data can be transmitted. That function credit exist at the receiver call the window size. Flow control might be needed due to many reasons. At first is link the capacity. However, if the link is share to many transmitter-receiver pairs, the total amount of data on the link may exceed its capacity at some time. By the way, in second reason can be unavailability of sufficient memory resources at the receiver station. The link is possible is not busy and enough memory available to process or store the usual the data and still can congestion. So, congestion means a condition in which packets queue to be process increase a line above a certain threshold. The queue can simply be because of the receiving station having to forward in each packet on a slower link. At the same time, flow control also can be necessary and implement at all layers. When implement on DLC layers, the results in rule of data flow across a single link. Thus, of implementation scheme is when the widow size is in one packet. Finally, flow controls have two examples to processes. There are stop-and-wait (SnW) flow control and sliding window flow control.
2.1.1 Stop-and-Wait (SnW) Flow Control
In this system, stop-and-wait is an entity transmits a packet. After, the destination entity receives the packet, and then it indicates its motivation to accept another packet by sending back an acknowledgement to the packet just receive. So, this small packet is call as Acknowledgement Packet (ACK). At figure 6 illustrates a timing diagram.
Figure 6: Stop-and-Wait Flow Control
(Data Communication Principles for Fixed and Wireless Networks, 2002)
The transmission time is due to a limited capacity of a link. So, propagation time is due to a limited velocity of signal propagation. Then, the processing time is due to limited processing capacity of the receiving station. It depends on a number of factors, including but not limited to, processor type, queue size and protocol type use after receiving a data packet.
2.1.2 Sliding-window (SW) Flow Control
Figure 7: Sliding-window (SW) Flow Control
¼ˆData Communication Principles for Fixed and Wireless Networks.¼‰
Sliding window is generality of Stop and Wait to more than one packet. This for system to receiving allows the sender to send up a sure maximum number of packets without getting further of ACK. Therefore allow to be transmitter without receiving an ACK is to be maximum window size. Usually each ACK allows to expanding the number of packets to the maximum window size.
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In the example, the receiver specifies a maximum window size of 4. The packets can be sequence number from 0 through 7 a maximum of four packets are allows to be transmit without getting further credit. Thus, suppose that packets numbers 0,1,2 and 3 have been transmit and then the receiver has not accept them. After the processing of receive packets is success then the receiver an ACK for the entire four packets. Finally, on receiving the ACK packet, the transmitter is acceptable to send packet numbers 4,5,6 and 7.
2.2 Error Detection
Error detection is about communication impairments and the effect of data rate and signal to noise ratio on bit error rate. This system will be error, resulting in the change of one or more bits in transmitted packet. At example, there have two copies of data. The receiver compares copies equal then no error. So, the probability of same bits corrupted low. The parity is value bit. That character has even or odd number ones. The even number bit error goes understand.
2.2.1 Cyclic Redundancy Check (CRC)
CRC is an extension of the parity block concept. It is nothing degree polynomial. The value of each bit is a coefficient. It is given a k bit block of bits, or message, the transmitter generates an n bit sequence. So, the resulting packet consisting transmit k+n bits which is exactly divisible by some number. The receivers then divide the incoming packet by that number and if no remainder, assume was no error.
2.3 Error Control
Error controls are transmission impairments random and affect bits at random locations. So, describe a link with respect to its impairment effect is by probability of error. That call variously link error probability, bit error probability, bit error rate, or frame error rate. In addition, there have two type of errors are lost frame and damaged frame. Lost frame is a frame fails to arrive at the other side and damaged frame is a recognizable frame does arrive, but some of the bits are in error. In the most common technique for error control are bases on some or the entire following element. There are error detection, positive acknowledgment, retransmission after timeout, and negative acknowledgement and retransmission.
2.3.1 Stop and Wait ARQ
This is sending station keeps a copy of every packet transmitted. Once transmission it waits for an ACK for each packet before sending the next packet. If an ACK received before the timeout, the stored copy of transmitted packet is discarded. So, an expiring the timer then the transmitter sends the copy of the packet again. If ACK damaged, transmitter will not recognize it. The transmitter will retransmit the same packet on timer expiry. There will start from sequence number; the receiver will know that this was duplicated packet. Example in figure 8.
Figure 8: Stop and Wait ARQ
2.3.2 Go back N ARQ
Go and back ARQ are the transmitter sends packets as allowed by current window size. If there are no errors in the packets, then the normal flow control operation continues as discussed above. In the way of error, the receiver discards the packet and does not increment its receiver window pointer. When it receives the packet with next sequence number, it may send a negative acknowledge (NAK). Usually, NAK implemented by sending the ACK packet asking for the discarded packet. If transmitter is already expecting an ACK for this packet, it will know that the packet in question was never received. Then go back by resetting its window passion at the discarded packet number and restart transmission of this packet.
2.3.3 Selective reject ARQ
The only packets retransmitted are those that receive a negative acknowledge. It calls as SREJ, means time out. Protocols provide for a selective reject mechanism in which retransmission is sought for only the packet that was in error instead of a while black of packets. This can be accomplished by sending a NAK as soon as a packet is received in error. After the packet in error is successfully receive, all the packets can be marked as received and processes by the receiver. This performance measure of the ARQ schemes, then the selective reject tops. Finally, the performance of go back N ARQ is much better than stop and wait ARQ.
Conclusion
Finally I have to finish this assignment. What I have know about networking and Data link control. In first question is easy for me to do. Networkings have three types such as Local Area Network (LAN), Metropolitan Area Network (MAN) and Wide Area Network (WAN). The main for question asking is local area networking. Local Area Networking for personal computers and back end network and storage area network. Then, spare out five type topology. There are bus, star, ring, mesh and tree. All of them have advantages and disadvantages. Because, it can give users to choose which want is better for their home or company.
In question two, I have explained all of them like flow control, error detection and error control. Three type of them are discussing transmit or retransmit. In conclusion, I have learned about networking how to set up their system and the mechanism how to process the packets.
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