Gsm Basics 2

Introduction Cellular telephones have revolutionized the communications arena, redefining how we perceive voice communications. Traditionally, cellular phones remained out of the hands of most consumers due to their high cost. As a result, cell phone carriers have invested time and resources into finding ways to give the systems higher capacity and thus lower cost. Cell systems are benefiting from this research and starting to develop into large-scale consumer products. How Cell Phones Work An Overview

It is common knowledge that Cellular Phones (referred to as "cell" phones from here on) are wireless phones; however, many are confused about how a cell phone actually works. Essentially, cell phones use highfrequency radio signals to communicate with "cell towers" located throughout the calling area. Cell phones communicate in the frequency range of 806-890 MHz and 1850-1990 MHz for the newly allocated "PCS" frequency range.

When the user wants to make a call, the cell phone sends a message to the tower, asking to be connected to a given telephone number. If the tower has sufficient resources to grant the request, a device called a "switch" patches the cell phone?s signal throughout to a channel on the "public switched telephone network" (otherwise known as the PSTN). This call now takes up a wireless channel as well as a PSTN channel that will be held open until the call is completed. The following figure illustrates this process.

Cells As the name implies, cell phone systems are made up of many small "cells." Each cell in a cell phone system represents the area served by one cell phone tower. The concept of cells is key behind the success of cell phones because by spacing many cells fairly close to each other, the cell phones may broadcast at very low power levels (typically 200mW ? 1W, depending on system). Since the cell phones may broadcast at low power levels, they use small transmitters and small batteries, and thus are able to fit in a shirt pocket, unlike amateur radios can occupy a tabletop.

Cells are typically spaced around 1-2 miles apart but can be spaced up to 20 miles apart in rural areas. In loaded areas or areas with many obstacles (such as tall buildings), the cell sites may be spaced closer together. Some technologies, like PCS, require closer cell spacing due to their higher frequency and lower power operation. Additionally, buildings interfere with cell signals coming from outside, so many buildings have their own "microcell." The Kingdome and New York subway are two examples of where microcells are used. Microcells may also be used to increase overall capacity within a heavily populated area such as a city?s core downtown area. In fact, homes may have "picocells" connected to the home?s PSTN connection to allow the cell phone to be used as a cordless phone. An example of typical microcell and picocell environments is pictured in the following figure.

Encoding and Multiplexing

Overview With thousands of cellular phone calls going on at any given time within a city, it certainly would not work for everyone to talk on the came channel at once (as in CB and short-wave radios). Therefore, several different techniques were developed by cell phone manufacturers to split up the available bandwidth into many channels each capable of supporting one conversation. The following sections will discuss each technology and how it works.

Analog vs. Digital While the distinction between analog and digital encoding is probably obvious to most readers, a short discussion is included for those who are not. Essentially, analog broadcasts audio as a series of continuously changing, voltage levels representing the amplitude of the voice conversation. When sent on the cell phone network using the standard frequency modulation (meaning voltage levels translate into frequency shifts) into channels separated by 30 kHz, we find that the amplitude can be effectively transmitted at 15 kHz due to Nyquist limitations. Instead of sending data as various voltage levels, a digital signal quantizes the voltage levels into a number of bins (typically 28 or 256 representing an 8-bit encoding). These bins are encoded as a binary number and sent as a series of ones and zeros. This allows for digital compression in the encoding stage enabling voice to be sent at as little as 8000 bits per second.

FDMA FDMA stands for "frequency division multiple access" and, though it could be used for digital systems, is exclusively used on all analog cellular systems. Essentially, FDMA splits the allocated spectrum into many channels. In current analog cell systems, each channel is 30 kHz. When a

FDMA cell phone establishes a call, it reserves the frequency channel for the entire duration of the call. The voice data is modulated into this channel?s frequency band (using frequency modulation) and sent over the airwaves. At the receiver, the information is recovered using a band-pass filter. The phone uses a common digital control channel to acquire channels. FDMA systems are the least efficient cellular system since each analog channel can only be used by one user at a time. Not only are these channels larger than necessary given modern digital voice compression, but they are also wasted whenever there is silence during the cell phone conversation. Analog signals are also especially susceptible to noise ? and there is no way to filter it out. Given the nature of the signal, analog cell phones must use higher power (between 1 and 3 watts) to get acceptable call quality. Given these shortcomings, it is easy to see why FDMA is being replaced by newer digital techniques.

TDMA TDMA stands for "time division multiple access." TDMA builds on FDMA by dividing conversations by frequency and time. Since digital compression allows voice to be sent at well under 10 kilobits per second (equivalent to 10 kHz), TDMA fits three digital conversations into a FDMA channel (which is 30 kHz). By sampling a person?s voice for, say 30 milliseconds, then transmitting it in 10 milliseconds; the system is able to offer 3 timeslots per channel in a round-robin fashion. This technique allows compatibility with FDMA while enabling digital services and easily boosting system capacity by three times. While TDMA is a good digital system, it is still somewhat inefficient since it has no flexibility for varying digital data rates (high quality voice, low quality voice, pager traffic) and has no accommodations for silence in a telephone conversation. In other words, once a call is initiated, the channel/timeslot pair belongs to the phone for the duration of the call. TDMA also requires strict signaling and timeslot synchronization. A digital control channel provides synchronization functionality as well as adding voice mail and message notification. Due to the digital signal, TDMA phones need only broadcast at 600 miliwatts.

CDMA CDMA stands for "code division multiple access" and is both the most interesting and the hardest to implement multiplexing method. CDMA has been likened to a party: When everyone talks at once, no one can be understood, however, if everyone speaks a different language, then they can be understood. CDMA systems have no channels, but instead encodes each call as a coded sequence across the entire frequency spectrum. Each conversation is modulated, in the digital domain, with a unique code (called a pseudo-noise code) that makes it distinguishable from the other calls in the frequency spectrum. Using a correlation calculation and the code the call was encoded with, the digital audio signal can be extracted

from the other signals being broadcast by other phones on the network. From the perspective of one call, upon extracting the signal, everything else appears to be low-level noise. As long as there is sufficient separation between the codes (said to be mutually orthogonal), the noise level will be low enough to recover the digital signal. Each signal is not, in fact, spread across the whole spectrum (12.5 MHz for traditional cellular or 60 MHz in PCS cellular), but is spread across 1.25 MHz "pass-bands." CDMA systems are the latest technology on the market and are already eclipsing TDMA in terms of cost and call quality. Since CDMA offers far greater capacity and variable data rates depending on the audio activity, many more users can be fit into a given frequency spectrum and higher audio quality can be provide. The current CDMA systems boast at least three times the capacity of TDMA and GSM systems. The fact that CDMA shares frequencies with neighboring cell towers allows for easier installation of extra capacity, since extra capacity can be achieved by simply adding extra cell sites and shrinking power levels of nearby sites. CDMA technology also allows lower cell phone power levels (200 miliwatts) since the modulation techniques expect to deal with noise and are well suited to weaker signals. The downside to CDMA is the complexity of deciphering and extracting the received signals, especially if there are multiple signal paths (reflections) between the phone and the cell tower (called multipath interference). As a result, CDMA phones are twice as expensive as TDMA phones and CDMA cell site equipment is 3-4 times the price of TDMA equivalents.

An animated picture roughly demonstrating the differences between these strategies follows. This is another figure demonstrating the differences between FDMA, TDMA, and CDMA.

GSM GSM stands for "Global System for Mobile Communications." GSM is mostly a European system and is largely unused in the US. GSM is interesting in that it uses a modified and far more efficient version of TDMA. GSM keeps the idea of timeslots and frequency channels, but corrects several major shortcomings. Since the GSM timeslots are smaller than TDMA, they hold less data but allow for data rates starting at 300 bits per second. Thus, a call can use as many timeslots as necessary up to a limit of 13 kilobits per second. When a call is inactive (silence) or may be compressed more, fewer timeslots are used. To facilitate filling in gaps left by unused timeslots, calls do "frequency hopping" in GSM. This means that calls will jump between channels and timeslots to maximize the system?s usage. A control channel is used to communicate the frequency hopping and other information between the cell tower and the phone. To compare with the other systems, it should be noted that GSM requires 1 Watt of output power from the phone. Call Handoff It is apparent that cells must somehow overlap, and when a user travels between cells, one cell must hand the call off to the other cell. The cells must also not interfere with each other. This is accomplished by giving each cell a slightly different chunk of the frequency spectrum (note that CDMA does not do this) and by measuring power levels. When the power level of the user begins to fade, the cell tower determines which cell is the closest cell. Upon finding this information, the current cell tower sends an over-the-air message to the new cell tower and to the cell phone. At this point, the new cell tower picks up the call and the old one drops the call as the cell phone switches frequencies. This type of handoff is called a

"hard handoff" since the audio feed is lost for between 10 milliseconds and 100 milliseconds while the new tower picks up the signal. Often these "hard" handoffs fail when the new tower tries to pick the call up, leading to frequent dropped calls. In most systems, each cell tower typically receives a 1.8 MHz frequency spectrum. In normal cellular systems that have a 12.5 MHz spectrum (not the high-band PCS systems that have more bandwidth), this allows for 7 cells before cells have to reuse frequencies. Generally, there are 1-2 cells and 10-20 miles separating cells using the same frequency in order to minimize interference. A discussion of call handoff is not complete without CDMA technology. Since CDMA uses the entire spectrum available, there is no real distinction between cells in terms of frequency use. Since each call is scattered across a whole 1.25 MHz pass band in CDMA, every cell tower can access the whole 12.5 MHz spectrum (60 MHz in a PCS system). This means that there is no necessity to change frequencies during a handoff since everyone is using the same frequencies. Therefore, two cell towers intercept the signal where the cells overlap. This means no sudden switch, since this handoff (called a "soft handoff") is actually handled in the switch, changing from one weaker audio feed to a stronger audio feed. This technique removes the loud "pop" associated with normal "hard" handoffs and greatly reduces problems with dropped calls. The soft handoff concept is pictured below.

Security One of the largest problems in wireless communication is security. There are two worries: Other people listening into phone calls and other people illegally billing time to a user?s account (called "phone cloning"). Unfortunately, analog phones transmit in plain FM, and provide no security. For instance, a few years ago, Newt Gingrich had a cell phone conversation taped by someone using a simple police scanner, which is designed to receive police activity on the CB frequencies. Since analog phones have such weak security, the architects of digital technology designed digital phones with much more robust security. Digital phones employ encryption to secure the phone and the conversation. Encryption is used in TDMA and CDMA to make sure that it is almost impossible to "latch" onto a conversation. The encryption works by picking a key that is used in an equation that compresses the audio. The encrypted key is sent to the cell tower so the cell tower knows how to decode the conversation. Therefore, even if the person with the scanner finds the channel and time slice you are using, they would need to find the encryption code to make sense of the signal. It is also important to mention that CDMA also uses its modulation code to provide increased security, resulting in over four billion possible encryption codes. Cell phones also must be protected from cloning. By encrypting the cell phone

number and related information when sending the information to the switch, cloning is prevented.

Switching Overview When a user places a call on a cell phone, the system must figure out how to route the call to the PSTN. Additionally, when someone calls the cell phone, the system must figure what cell the user is in. This section describes how this is done.

Finding the user Whenever you turn your cell phone on, the phone sends its identification to the cell phone tower. This includes the "MIN" (mobile identification number, usually the phone number) and the "ESN" (electronic serial number). The cell tower forwards this information to a centrally located switch via special leased phone lines that connect a switch to many cell sites (T-1 lines are often used). When the switch gets this information, it forwards it to any higher level switches.

Connecting the call Whenever a call comes in, it will come to the switch that serves the exchange (the exchange is the 555 in (206) 555-1212). This top-level switch will pass that call onto any lower level switches, if there is one, although there usually is not. When the call is passed to the lowest level switch, it checks to make sure the phone is still registered (it is turned on and in range). If it is registered, the phone is notified via the signaling channel and the phone begins ringing. When the user chooses to accept the call, the switch establishes the voice channel and the call begins.

Roaming Roaming was one of the most challenging issues the cell phone industry faced. The goal was simple: a phone could be used anywhere in the US or the world where compatible technology is used. The difficult part is getting various systems to communicate and pass routing and billing information to each other. When a user turns his or her cell phone on in a roaming area, the cell phone identifies itself to the switch. When the switch looks up the information and discovers it is not a local phone, it will attempt to find the "home" switch based on the exchange. When it locates the home switch, it will determine if roaming is possible. If roaming is possible, the switch (referred to here as the "roaming switch") sets up a "Visitor Location Register" (VLR) registering the phone in the locality. The home switch will also be notified about the change so that it can route calls to the switch in the roaming location. Outbound calls are handled through the roaming

switch as they would be handled if the user were at home. Incoming calls are routed from the home switch to the roaming switch after sending a message to the roaming switch requesting a "temporary local directory number" (TLDN). This TLDN will be used to make a connection from the home switch to the roaming switch across the PSTN. Finally, whenever the roaming phone is turned off, the phone is unregistered with the roaming switch and the home switch is notified. The process of registering the phone and notifying the home switch takes 2 seconds. A GSM network consists of the following components: •

Mobile station. The GSM mobile station (or mobile phone) communicates with other parts of the system through the base-station system.

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GSM Base station system (BSS).

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Base transceiver station (BTS). The base transceiver station (BTS) handles the radio interface to the mobile station. The base transceiver station is the radio equipment (transceivers and antennas)

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Base station controller (BSC). The BSC provides the control functions and physical links between the MSC and BTS. It provides functions such as handover, cell configuration data and control of RF power levels in base transceiver stations. A number of BSCs are served by a MSC.

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GSM Switching System

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Mobile services switching center (MSC). The MSC performs the telephony switching functions of the system. It also performs such functions as toll ticketing, network interfacing, common channel signalling, and others.

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Home location register (HLR). The HLR database is used for storage and management of subscriptions. The home location register stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status.

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Visitor location register (VLR). The VLR database contains temporary information about subscribers that is needed by the mobile services switching center (MSC) in order to service visiting subscribers. When a mobile station roams into a new mobile services switching center (MSC) area, the visitor location register (VLR) connected to that MSC will request data about the mobile station from the HLR, reducing the need for interrogation of the home location register (HLR).

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Authentication center (AUC). The AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The authentication center (AUC) also protects network operators from fraud.

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Equipment identity register (EIR). The EIR database contains information on the identity of mobile equipment to prevent calls from stolen,

unauthorized or defective mobile stations. •

Message center (MXE). The MXE is a node that provides integrated voice, fax, and data messaging.

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Mobile service node (MSN). The MSN is the node that handles the mobile intelligent network (IN) services.

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Gateway mobile services switching center (GMSC). A gateway mobile services switching center (GMSC) is a node used to interconnect two networks.

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GSM interworking unit (GIWU). The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GSM interworking unit (GIWU), users can alternate between speech and data during the same call.

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Operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of operation and support system is to offer support for centralized, regional, and local operational and maintenance activities that are required for a GSM network.