Texts 3-4

Text 3. Parity Bit 

In the PC environment, 7- or 8-bit characters are often used to read, process, store, and transmit information. Seven bits are enough to encode all upper and lowercase characters, symbols, and function keys, which number 128, in conformance with the American Standard Code for Information Interchange (ASCII). An option all eighth bit, called the “parity” bit, is used to check data integrity. When used, it is inserted between the last bit of a character and the first “stop” bit. 

The parity bit is included as a simple means of error checking. There is even and odd parity. The devices at each end of the connection must have the same parity setting. The idea is that parity is agreed upon before the start of transmission. The actual configuration is done from within an operating environment such as Windows when setting up the connection preferences of the modem.  

 Suppose the parity chosen is odd. The transmitter will then set the parity bit in such a way as to make an odd number of 1s among the data bits and the parity bit. For example, if there are five 1s among the data bits, already an odd number, the parity bit will be set to 0. If errors are detected at the receiving device, a notification is sent in the header of the return packet, so that only corrupt bytes need to be retransmitted. 

While asynchronous communication is a relatively simple and, therefore, inexpensive method of serial data transmission, it is very inefficient. This is because asynchronous transmissions include high overhead in that each byte carries at least two extra bits for the start-stop functions, which results in a 20 percent loss of useful bandwidth (2/10 = 0.20 or 20 percent). For large amounts of data, this adds up quickly. For example, to transmit 1000 characters, or 8000 bits, 2000 extra bits must be transmitted for the start and stop functions, bringing the total number of bits sent to 10,000. The 2000 extra bits is equivalent to sending 250 more characters over the link.  

Text 4.Wireless technologies

Terrestrial microwave – Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low-gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately 48 km (30 mi) apart.

Communications satellites – The satellites communicate via microwave radio waves, which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically in geosynchronous orbit 35,400 km (22,000 mi) above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.

Cellular and PCS systems use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.

Radio and spread spectrum technologies – Wireless local area network use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. IEEE 802.11 defines a common flavor of open-standards wireless radio-wave technology.

Infrared communication can transmit signals for small distances, typically no more than 10 meters. In most cases, line-of-sight propagation is used, which limits the physical positioning of communicating devices.

A global area network (GAN) is a network used for supporting mobile across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial wireless LANs. 

* 10дугаар 7 хоногийн 2p бие даалтын сорилд орох тул англиар цээжлэн бэлдэж, дэвтэр дээрээ монгол руу орчуулснаа лаб дээр  үзүүлээрэй.