Cellular Telephone Networks
A cellular telephone network comprises mobile transceivers, called cellular telephones, and a network of fixed transceivers, called base stations, that are strategically positioned along the terrain (Figure 2-10). Base stations are used to connect cellular telephones to the ground-based telephone system.
There are two kinds of cellular networks: analog and digital. Cellular telephones began in the 1970s with the expansion of AT&T into the mobile telephone service market.
Cellular telephones used analog technology at that time. This changed in mid-1995 when IBM developed technology that digitized information transmitted over the cellular telephone network. Cellular telephone networks then became capable of transmitting both voice and data.
The transmission range of a cellular telephone is determined by the strength of the battery powering the phone and the location of the nearest base station. Transmission range drops as power is drained from the cellular telephone and the telephone is taken farther from a base station.
Engineers can provide reliable cellular telephone transmissions by strategically positioning many base stations around the country so that a cellular telephone is always within the vicinity of a base station. Cellular transceivers are also designed to minimize the power drain that occurs from transmitting and receiving signals.
A cellular telephone is in continuous communication with base stations as it moves throughout the cellular network. Transmission from a cellular telephone is broadcast 360 degrees and is received by a base station closest to the cellular telephone. Cellular telephone networks are designed so that the signal is automatically transferred to the next closest base station using a technique called a hand-off: the connection between the cellular telephone and the cellular telephone network is dropped for a fraction of a second, the cellular telephone moves between base stations, and the next base station reestablishes the signal.
The area covered by a base station is called a cell. The split-second gap during the hand-off goes unnoticed most times, as long as cells are near each other. The hand-off doesn't have a negative effect on voice communications because persons on the call adjust for the slight break in communication. However, the communications drop has a dramatic effect on data communication over a cellular telephone network. Analog cellular telephone networks lose data during transmissions when a hand-off occurs, which is unacceptable for data communications. Digital cellular telephone networks also lose connection during hand-off, but a digital cellular telephone network uses software to recover lost data by requesting that the transceiver resend the data.
Digital cellular telephone networks trap and correct errors. Analog cellular telephone networks lack error-control capability. Analog networks transmit one long burst of information over a communications channel that can either be used for transmitting or receiving information but not both simultaneously, which is called half-duplex. In contrast, digital cellular telephone networks transmit information in small packets, called frames or cells, as described previously in this chapter. Pauses between transmissions give the receiver an opportunity to notify the transmitter if an error occurred in receiving a packet.
Cellular Digital Packet Data
IBM pioneered digital cellular telephone networks with the introduction of their Cellular Digital Packet Data (CDPD) protocol, commonly known as IP wireless. IP wireless requires that an Internet protocol (IP) address be assigned to each cellular transceiver in the cellular telephone network. An IP address uniquely identifies each device within the cellular telephone network and is used to reestablish a connection if communication is lost during a hand-off.
Base stations have multiple entry points called ports, each of which is identified by a unique port number. A transceiver is assigned to a base station port in the cellular telephone network. A transceiver continues to transmit to the port number until a handoff occurs, at which time the transceiver is assigned another port number associated with the next base station.
IBM developed a special modem called a CDPD modem for transmitting digital information over an analog cellular telephone network. The CDPD modem transmits small bursts of encrypted data, which frees the communication channel between bursts to transmit error messages.
Speed is the major stumbling block in using a cellular telephone network to transmit data. The standard analog transmission rate of a cellular telephone network is 9,600 bits per second, which is increased to 14,400 bits per second using CDPD. These speeds are sufficient to transmit delivery information, inquire about the status of an order, or provide remote access to email, but are insufficient for full Internet access.
Digital Wireless Transmissions
A digital cellular telephone network can transmit both voice and data simultaneously using multiplex transmission. There are three multiplex transmission methods used on a digital cellular telephone network: Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), and a third-generation wireless standard called 3G.
CDMA uses spread-spectrum transmission to use multiple communications channels for transmission, which dramatically increases data throughput over the network. The cellular telephone temporarily uses on-board memory in transceivers to store data to keep transmissions flowing during a hand-off. This is called a soft hand-off. TDMA uses one communications channel shared among transmissions by using time slots. Transmission time is divided into time slots, and then each packet is assigned to a time slot. The 3G multiplexing technique uses either CDMA or TDMA to increase the throughput to 56 kilobits per second.
Mobile Radio Networks
The infrastructure of cellular telephone technology is the backbone of wireless small computing mobile communications and enables these devices to connect to traditional communications systems. The forerunner of cellular telephone technology is a private radio technology. Service and trucking companies and government agencies use private radio technology to communicate with employees over frequencies isolated from other radio frequencies. For example, package carriers like Federal Express use private radio networks to track packages. Private radio transmitted analog information when first introduced but later expanded into digital communication as the need for paging and messaging services materialized.
Companies can operate their own private radio network by acquiring broadcast rights to a specified radio frequency from the Federal Communications Commission and purchasing the necessary broadcast equipment. Alternatively, companies can lease broadcast time from organizations that offer Specialized Mobile Radio (SMR) network services.
Cell Phones and Text Input
Traditional cellular telephones have a keypad that contains numbers and letters. (European traditional cellular telephones have only numbers.) Letters were designed to identify telephone exchanges-local switching stations that serviced a group of customers. Each switching station was referred to by a name that implied the location of the switching station. For example, there was a switching station called Murray Hill that covered the Murray Hill section of New York City.
The first two letters of the name of the switching station were used to replace the first two digits of a seven-digit phone number. Let's say a customer was assigned Murray Hill 5 1000 as a telephone number. A caller dials MU 5 1000. Today the person would call 685-1000 since the switching station naming convention was dropped decades ago.
Today customers expect to be able to enter textual information using the cellular telephone keypad. However, there are two problems with the keypad. First, the keypad doesn't contain the letters Q or Z. And each numeric key, except for the first key, contains three letters. A common solution to this problem is for software in the cellular telephone to count the number of times a key on the keypad is pressed to determine which letter of the alphabet was entered. For example, here's how the name Jim is entered using a cellular telephone keypad: Press the number 5 once, and then pause. Press the number 4 three times without pausing. And press the number 6 and pause.
Another solution is to use T9 technology. T9 technology uses special glasses that track eye movement, enabling a person to type by moving her eyes in one of eight directions.
Multiple letters are assigned to each direction. An algorithm was developed that predicted which one of the multiple letters a person wanted to type based on the previous letters that she selected. Let's say you entered 546. The number 5 could represent the letters JKL.
The number 4 could represent the letters GHI. And the number 6 could represent the letters MNO. Only one English word can be created by combining these letters, which is the word "JIM." However, all legitimate words that can be formed using the selected numbers are displayed on the cell phone screen. The person is then prompted to select the correct word.
Counting key presses and T9 technology are limited. Other technology such as voice recognition and new mobile communications devices have capitalized on these limitations and provide more efficient means to enter and send textual information over the cellular telephone network.
One of the first popular wireless mobile communications devices was a pager. A pager displays any series of numbers that were sent by a caller. Technically, the series of numbers represented the caller's telephone number and implied that the call be returned. Practically, the series of numbers could represent anything to which the caller and the receiver agreed on. For example, it was common for systems administrators to have their system send a 911 call to a pager whenever there was a systems problem. Some systems administrators even devised a code that was sent indicating which system transmitted the call and the nature of the problem. Drug dealers were also notorious for using pagers to send encoded messages of drug deliveries. A buyer would call the pager and send a series of numbers that told the dealer the type of drug, quantity, and delivery location for the buy.
Today's wireless mobile communications devices offer text messaging services that enable short textual messages to be sent to the device from any device that has access to the service. Cellular telephone companies offer three types of messaging services: Short Message Service (SMS), Cell Broadcast Service (CBS), and Unstructured Supplementary Services Data (USSD).
SMS type of messaging is capable of sending a maximum of 160 characters on the control channel of a cellular telephone network. A control channel is a communications channel used to manage cellular telephone calls. Messages sent using SMS are sent and received during a cellular telephone call because the telephone call and the message use different communications channels for transmission. However, there may be a slight delay between transmission of a message and when the message is received because SMS messaging uses store-forwarding technology, where the message is temporarily stored in a mailbox before being delivered to the receiver.
The CBS type of messaging broadcasts a maximum of 15 pages of 93 characters per page to every device on the network. Everyone on the network receives the same message, which is why CBS messaging has had limited success in the market.
The USSD type of messaging transmits a maximum of 182 characters along the control channel, similar to SMS messaging. However, USSD messaging does not use store forwarding technology. Instead, USSD messaging sends the message directly to the receiver, which enables the receiver to respond instantaneously.
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