What is the fundamental purpose of a communication system? The fundamental purpose of a communication system is the exchange of data between two parties. Discuss the communication model s elements. The communication model s elements are: Source: This device generates data to be transmitted; examples are telephones and personal computers. Transmitter: This device accepts the data transmitted from the source and transforms and encodes this information in such a way as to produces electromagnetic signals that can be transmitted across some source of transmission system. Transmission System: This can be a single transmission line or a complex network connecting source and destination. Receiver: The receiver accepts the signal from the transmission system and converts it into a form that can be handled by the destination device. Destination: This takes the incoming data from receiver. Discuss the key tasks that must be performed in a data communications system. The key tasks that must be performed in a data communications system: Transmission system utilization: refers to the need to make efficient use of transmission facilities that are typically shared among of communicating devices. 1
Interface: to communicate, a device must interface with the other transmission systems. Signal generating: once interface is established, signal generation is required for communication. These signals must be: - Capable of being propagated through the transmission system. - Interpretable as data at the receiver. Synchronization: there must be synchronization between transmitter and receiver to know when a signal begins to arrive and when it ends. Exchange management: There is a variety of requirements for communication between two parties that might be collected under the term exchange management. Error detection and correction: error detection and correction are required in circumstances where errors cannot be tolerated. Flow control: is required to assure that source does not overwhelm the destination by sending data faster than they can be processed and absorbed. Addressing and Routing: addressing and routing are required so a secure system can indicate the identity of the intended destination, and can choose a specific route through this network. Recovery: allows an interrupted transaction to resume activity at the point of interruption or to condition prior to the beginning of the exchange. Message formatting: has to do with an agreement between two parties as to the form of the data to be exchanged or transmitted. Security: Frequently need to provide some measure of security in data communication systems. Network management: capabilities are needed to configure the system, monitor its status, react to failures and overloads, and plan intelligently for future growth. 2
Discuss the data communication model s operation by an example. Assume a PC user wants to send an email message m to another user. The process is modeled as follows: User keys in message m comprising bits g buffered in source PC memory. Input data is transferred to I/O device (transmitter) as sequence of bits g(t) using voltage shifts. Transmitter converts these into a signal s(t) suitable for transmission media being used. During transmission through a specific media, signal may be impaired so received signal r(t) may differ from s(t). The receiver decodes signal recovering g (t) as estimate of original g(t). Which is buffered in destination PC memory as bits g being the received message m. What are the main categories of communications networks? The main categories of communications networks are: Local Area Network (LAN). Metropolitan Area Network (MAN). Wide Area Network (WAN). 3
Discuss the characteristics of each communications networks category. LAN characteristics: It has a small scope (Building or small campus). Usually owned by same organization as attached devices. It has high data rates. It may be Switched LAN (Ethernet), or Wireless LAN. MAN characteristics: Middle ground between LAN and WAN. Span a larger area than LAN (city area). Private or public network. It has a high speed. WAN characteristics: Span a large geographical area. Rely in part on common carrier circuits. Can be implemented using - Circuit switching. - Packet switching. - Frame relay. - Asynchronous Transfer Mode (ATM). Discuss the different technologies used in WAN. There are different technologies used in WAN: Circuit switching: - In which a dedicated communication path is established and used between two stations through the nodes of the network. - This path is a connected sequence of physical links between nodes. - On each link, a logical channel is dedicated to the connection. - Data generated by the source are transmitted along the dedicated path as rapidly as possible. 4
- At each node, incoming data are routed or switched to the appropriate destination without delay. - Ex: the telephone network. Packet switching: - In which data are sent out in terms of small chunks of data called packets at a time unit. - Each packet is passed through the network from node to node along some path leading from the source to destination. - At each node, the entire packet is received, stored briefly, and then transmitted to the next node. - Packet switching systems have large overheads to compensate for errors at all intermediate nodes. - Ex: terminal to computer and computer to computer communications. Frame relay: - Modern systems are more reliable and error rate has been dramatically lowered and remaining errors can be caught at end system. - It provides higher speed and less error control overhead than packet switching. ATM: - It is a culmination of development in circuit switching and packet switching (cell relay). - It uses fixed length packet, unlike frame relay which uses variablelength packets. - It provides little overhead for error control. - It provides higher speed from 10Mbps to Gbps range. - It allows the definition of multiple virtual channels with data rates that are dynamically at the time of virtual channels creation, those it can provide constant data rate channel. 5
Define the Internet. The Internet: it is the first operational packet switching network, which evolved from the ARPANET (Advanced Research Project Agency) of the US Department of Defense in 1969. What are the key elements of the internet? The key elements of the internet Hosts: End systems to be interconnected, and can be PCs, workstations, servers, mainframes, and so on. Network: Most hosts that use the Internet are connected to a network, such as a local area network (LAN) or a wide area network (WAN) Routers: Networks are in turn connected by routers. How the internet operates? The Internet operates as following: - A host may send data to another host anywhere on the internet. - The source host breaks the data to be sent into a sequence of packets called IP datagrams or IP packets. 6
- Each packet includes a unique numeric address to the destination called IP address. - Based on this address, each packet travels through a series of routers and networks from source to destination. - Each router receives a packet and makes a routing decision and forwards the packets along its way to the destination. Discuss the Internet Architecture? The Internet Architecture: Individual hosts and LANs are connected to Internet Service Provider (ISP) through a Point of Presence (PoP). The connection is made of a series of steps starting with the Customer Premises Equipment (CPE). CPE is the communication equipment located onsite with the host. Define: Internet Service Provider (ISP), Point of Presence (PoP), Customer Premises Equipment (CPE), Network Access Point (NAP), and Network Service Provider (NSP). Internet Service Provider (ISP): A company that provides other companies or individuals with access to, or presence on, the Internet. Point of Presence (PoP): An ISP POP is the edge of the ISP s network; connections from users are accepted and authenticated here. Customer Presence Equipment (CPE): Telecommunications equipment that is located on the customer s premises (physical location) rather than on the provider s premises or in between. Network Access Point: It a physical facility that provide the infrastructure to move data between connected network. Network Service Provider (NSP): A company that provides backbone services to an Internet service provider (ISP). 7
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