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Mobile Telephone
Mobile Systems

by: Lawrence Harte

Mobile systems are composed of include base stations (BS), switching systems and various data processing functions. The radio system portion of mobile networks is called a radio access network (RAN). 


Mobile networks inter-connect wireless devices with nearby radio towers that route calls through switching systems to other wireless telephones to other telephones or data networks. Creating and managing a wireless network involves equipment selection and installation, implementation methods, inter-connection to the public switched telephone network (PTSN) and other networks such as the Internet.


The mobile system uses either a mobile switching center (MSC) for voice and medium-speed data or a packet data service node (PDSN) for packet data services. The MSC coordinates the overall allocation and routing of calls throughout the wireless system. The PDSN ensures packets from the mobile devices can reach their destination.


Figure 1.13 shows a simplified functional diagram of a mobile network. This diagram shows that the mobile system is composed of 3 key parts; the user equipment (UE), radio access network (RAN) and a core interconnecting network (CN). The UE is divided into 2 parts, the mobile equipment (ME) and the subscriber identity module (SIM) card. The RAN is composed of base stations and base station controllers 

This article is Part 3 of a 9 Part Series
Mobile Telephone List

Month
Mobile Technologies Oct 06
Mobile Devices Nov 06
Mobile Systems Dec 06
Mobile Systems Operation Jan 07
Analog Systems Feb 07
Digital Cellular Systems Mar 07
Packet Digital Cellular Systems Apr 07
Wideband Digital Cellular May 07
Mobile Services Jun 07

(BSCs). This example shows that the BSCs connect voice calls to mobile switching centers (MSCs) and connects data sessions to packet data service nodes (PDSNs). The core network is basically divided into circuit switched (primarily voice) and packet switched (primarily data) parts. The core network circuit switch parts contain the serving MSC (SMSC) and a gateway MSC (GMSC). The serving SMSC connects to the RAN system and the gateway GMSC connects to the public telephone network. The core network packet switched parts contain the serving general packet radio service (GPRS) support node (SGSN) and a gateway GPRS service node (GGSN). The SGSN connects to the RAN system and the GGSN connects to data networks such as the Internet. 

Figure 1.13., Mobile Network

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Base Stations


Base stations may be stand alone transmission systems or part of a cell site and is composed of an antenna system (typically a radio tower), building, and base station radio equipment. Base station radio equipment consists of RF equipment (transceivers and antenna interface equipment), controllers, and power supplies. Base station transceivers have many of the same basic parts as a mobile device. However, base station radios are coordinated by the mobile system's BSC and have many additional functions than a mobile device.


The radio transceiver section is divided into transmitter and receiver assemblies. The transmitter section converts a voice or data signal to RF for transmission to wireless devices and the receiver section converts RF from the wireless device to voice or data signals routed to the MSC or packet switching network. The controller section commands insertion and extraction of signaling information.


Unlike end user wireless devices (such as a mobile telephone or laptop computer), the transmitter, receiver, and control sections of an access point may be grouped into equipment racks. For example, a single equipment rack may contain all of the RF amplifiers or voice channel cards. Unlike analog or early-version digital cellular systems that dedicated one transceiver in each base station for a control channel, the mobile system combines control channels and voice channels are mixed on a single physical radio channel.


Radio Antenna Towers


Wireless base station antenna heights can vary from a few feet to more than three hundred feet. Radio towers raise the height of antennas to provide greater area coverage. There may be several different antenna systems mounted on the same radio tower. These other antennas may be used for paging systems, a point to point microwave communication link, or land mobile radio (LMR) dispatch systems. Shared use of towers by different types of radio systems in this way are very common due to the economies realized by sharing the cost of the tower and shelter. However, great care must be taken in the installation and testing to avoid mutual radio interference between the various systems.


Communication Links


Communication links carry both data and digital voice information between the base station and the mobile network. The physical connection options for communication links include copper wire, microwave radio, or fiber optic links. Duplicate and/or alternate communication links are sometimes used to help ensure communication 

may continue in the event of the failure of a communication line (such as when a cable seeking backhoe cuts a line). 


Mobile Switching Center (MSC)


The mobile switching center (MSC) processes requests for service from mobile devices and land line callers, and routes calls between the base stations and the public switched telephone network (PSTN). The MSC receives the dialed digits, creates and interprets call-processing tones, and routes the call paths. 


Authentication, Authorization, and Accounting (AAA)


Authentication, Authorization, and Accounting are the processes used in validating the claimed identity of an end user or a device, such as a host, server, switch, or router in a communication network. Authentication is a process of exchanging information between a communications device (typically a user device such as a mobile phone or computing device) and a communications network that allows the carrier or network operator to confirm the true identity of the user (or device). Authorization is the act of granting access rights to a user, groups of users, system, or a process. Accounting is the method to establish who, or what, performed a certain action, such as tracking user connection and logging system users.

Interworking Function (IWF)


Interworking functions are systems and/or processes that attach to a communications network that is used to process and adapt information between dissimilar types of network systems. IWFs in the mobile system may include data gateways that convert circuit switched data from the MSC to the Internet.

Message Center (MC)


The message center is a node or network function within a communications network that accommodates messages sent and received via short messaging service (SMS).


Serving General Packet Radio Service Support Node (SGSN)


A serving general packet radio service support node is a switching node that coordinates the operation of packet radios that are operating 

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within its service coverage range. The SGSN operates in a similar process of a MSC and a VLR, except the SGSN performs packet switching instead of circuit switching. The SGSN registers and maintains a list of active packet data radios in its network and coordinates the packet transfer between the mobile radios. 


Gateway GPRS Support Node (GGSN)


A gateway GPRS support node is a packet switching system that is used to connect a GPRS packet data communication network to other packet networks such as the Internet.


Base Station Controller (BSC)


A base station controller is an automatic coordinator (controller) in a mobile system that allows one or more base transceiver stations (BTS) to communicate with a mobile switching center and/or a packet data communication system. A BSC is called a Radio Network Controller (RNC) in the WCDMA system. 

Voice Message System (VMS)


The voice mail system is a telecommunications system that allows a subscriber to receive and play back messages from a remote location (such as a PBX telephone or mobile phone). The VMS consists primarily of memory storage (for messages), telephone interfaces (to connect to the communication system), and message recording, playback, and control features (typically via DTMF tones).


Public Switched Telephone Network (PSTN)


Public switched telephone networks are communication systems that are available to the public to allow users to interconnect communication devices. Public telephone networks within countries and regions are standard integrated systems of transmission and switching facilities, signaling processors, and associated operations support systems that allow communication devices to communicate with each other when they operate.


Public Packet Data Network (PPDN)


A packet data network is a network that is generally available for commercial users (the public). An example of a PPDN is the Internet.

Network Databases


Network databases are information storage and retrieval systems that are accessible by a network. There are many network databases in the mobile network. Some of the key network databases include a master subscriber database (home location register), temporary active user subscriber database (visitor location register), an unauthorized or suspect user database (equipment identity register), billing database, and authorization and validation center (authentication).


Home Location Register (HLR)


The home location register (HLR) is a subscriber database containing each customer's international mobile subscriber identity (IMSI) and international mobile equipment identifier (IMEI) to uniquely identify each customer. There is usually only one HLR for each carrier even though each carrier may have many MSCs.
The HLR holds each customer's user profile which includes the selected long distance carrier, calling restrictions, service fee charge rates, and other selected network options. The subscriber can change and store the changes for some feature options in the HLR (such as call forwarding.) The MSC system controller uses this information to authorize system access and process individual call billing. 


The HLR is a magnetic storage device for a computer (commonly called a hard disk). Subscriber databases are critical, so they are usually regularly backed up, typically on tape or CDROM, to restore the information if the HLR system fails.


Visitor Location Register (VLR)


The visitor location register (VLR) contains a subset of a subscriber's HLR information for use while a mobile telephone is active on a particular MSC. The VLR holds both visiting and home customer's information. The VLR eliminates the need for the MSC to continually check with the mobile telephone's HLR each time access is attempted. The user's required HLR information is temporarily stored in the VLR memory and then erased either when the wireless telephone registers with another MSC , registers in another system or after a specified period of inactivity.


Equipment Identity Register (EIR)


The equipment identity register is a database that contains the identity of telecommunications devices (such as wireless telephones) and the status of these devices in the network (such as authorized or not-authorized). The EIR is primarily used to identify wireless telephones that 

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may have been stolen or have questionable usage patterns that may indicate fraudulent use. The EIR has three types of lists; white, black and gray. The white list holds known good mobile devices. The black list holds invalid (barred) mobile device. The gray list holds mobile devices that may be suspect for fraud or are being tested for validation.


Billing Center (BC)


A separate database (called the billing center) keeps records on billing. The billing center receives individual call records from MSCs and other network equipment. The switching records (connection and data transfer records) are converted into call detail records (CDRs) that hold the time, type of service, connection points, and other details about the network usage that is associated with a specific user identification code. These billing records are then transferred via tape or data link to a separate computer typically by electronic data interchange (EDI) to a billing system or company that can settle bills between different service providers (a clearinghouse company).


Authentication Centre (AuC)


The Authentication Centre stores and processes information that is required to validate (authenticate) the identity of a wireless device before service is provided. During the authentication procedure, the AuC provides information to the system to allow it to validate the mobile device. 


Number Portability Database (NPDB)


Number portability is the ability for a telephone number to be transferred between different service providers. This allows customers to change service providers without having to change telephone numbers. Number portability involves three key elements: local number 

portability, service portability and geographic portability. To enable number portability, the mobile system maintains a number portability database (NPDB). This database helps to route calls to their destination which may have an assigned telephone number that is different (number has been ported) than the destination phone number.


IP Backbone Network


A backbone network is the core infrastructure of a network that connects several major network components together. A backbone system is usually a high-speed communications network such as ATM or FDDI. The mobile system uses a backbone network that can provide end-to-end IP transmission capability. 


Backbone network is a communications network that connects the primary switches or nodes within the network. The backbone network is usually composed of high-speed switches and communication lines.


The focus on using IP communication allows carriers to use off-the-shelf IP network equipment. This typically lowers the equipment cost (due to a large selection of vendors and equipment options), reduces operation and maintenance costs due to one type of system to maintain (less training and processes), and allows for the use of standard software (traffic monitoring and management).
Series Source:
Introduction to Mobile Telephone 2nd Edition
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