Assignment On Wireline And Wireless Communications

Assignment – 1 Difference between wire line and wireless communications Communication involves a network for its realization. A network in Electronics Communication is a group of interconnected computers, communications stations, or other facilities, often organized for simultaneous operation and data transfer. A network can operate in two ways- it may be a wire line network or a wireless network. These are defined as under: Wire line:- A type of network in which one or more wires or cables are deployed for conducting currents for communication, control, or measuring purposes. Wireless:- A type of network pertaining to data communications and control systems that operate without wires (e.g. Bluetooth link between a notebook and a desktop computer). The above two types of networks are different from each other not only in terms of the use or disuse of cables but also in terms of nature and scope of their respective operation, bandwidth , type of device, protocol, time varying quality of the two-way physical links, type of services and consumer expectations. The Distinction between Wire line and Wireless Communications Some basic aspects which distinguish wire line from wireless communications are as follows. Limited Spectrum The radio spectrum, and therefore the capacity available for wireless access service, is generally limited by regulation. Thus, unlike wire line communications wherein an increasing user population can easily be served by deploying additional wire (or fiber) facilities to connect those users to the network (thereby increasing the total capacity available to serve that increasing population), the available radio spectrum cannot arbitrarily be expanded. The cellular approach resolves this dilemma by dividing the service area into radio cells, each equipped with a base station, and reusing the allocated spectrum as often as possible among the radio cells, subject to constraints imposed by co-channel interference among the cells. Then, as the user population grows, or users individually demand greater capacity, the cells must be divided such that a greater number of geographically smaller cells are available to serve the demand; again, frequency reuse among the greater number of smaller cells is mandatory. Limited spectrum also drives the need for spectrally efficient modulation and source compression coding to remove signal information redundancy.

Time-Varying Quality of the Two-Way Physical Links Between Mobile and Base Station In a wire line network the physical link between a remote user terminal and the end-office switch or remote concentrator are of time-invariant high quality. In a wireless network the radio link is subject to several time-varying impairments arising from inherent user mobility and unavoidable changes caused by motion of the surrounding environment (closing doors and passing trucks are two examples of the latter). These impairments are manifested in a time-varying bit error rate (BER) performance of the radio link, with the BER often too low to meet the needs of the application. Root causes of these time-varying impairments are multipath propagation, shadow fading, distance-dependent signal power path loss, and co-channel interference arising in surrounding cells. Multipath propagation, caused by the superposition of radio waves reflected from surrounding objects, gives rise to frequency-selective fading which, in turn, causes frequent and deep reduction in received power for narrowband signals and rapidly changing time dispersion of broadband signals. Shadow fading, caused by the presence of large physical objects (buildings, walls, etc.) which preclude direct line of sight between a radio transmitter and receiver, also gives rise to strong signal power attenuation, but varies at a much lower rate than does multipath fading. Path loss refers to the reduction in the fraction of transmitted power which is actually received as the distance between transmitter and receiver increases, and also produces a slowly time-varying effect. These propagation-related phenomena (multipath, shadow fading, and path loss) reduce immunity to radio receiver noise and, perhaps more important, increase the susceptibility to co-channel interference arising in other cells which happen to be using the same frequency. In addition, the level of co-channel interference encountered by a given base-mobile link is strongly influenced by the time-varying number of currently active users in any other cells sharing a common frequency. An Unknown Time-Varying Access Point Unlike a wire line connection, wherein users attach via fixed network ports, wireless access implies that the user's point of attachment is unknown at the time of connection establishment and may change throughout the duration of the connection. Corollaries are as follows. First, some extraordinary means must be created to allow users to signal connection requests (and/or to accept a connection) prior to the actual establishment of a connection. In a wire line setting, the same facilities that will subsequently carry the user's application signals are also used to carry the user's control signals. In a wireless setting, a user's connection can be used to carry control signals only after it has been established, and some extraordinary means must be provided to enable reconnections establishment signaling. Second, to establish a connection to a called wireless user, the network must locate that user. This can readily be done by paging if the called wireless user is known to be somewhere within some small cluster of cells. Otherwise, the usermobile equipment must advise a database, known as the home location register, of that user's current location. Then, any calls to that user will prompt a database inquiry to determine the user's location; calls can then be forwarded to the user. Third, throughout a connection, the position of the user must continually be tracked so that the connection can be maintained as the user travels among different cells.