Chapter 10: Wireless Networking School of information science and Engineering, SDU
10.1 Introduction to Wireless Networks Requirement to the networks: Provides ubiquitous personal communications; Accommodate mobile voice and data users who move throughout buildings, cities, or countries. Market ---- A coverage throughout a particular territory, that the cellular telephone system can provide services. Also called coverage region. Network ---- The interconnection of many markets, that provides service to mobile users throughout a country or continent.
Typical cellular system
Base stations forms an integrated network, and provide sufficient radio coverage to all mobile users. Each base station may handle on the order of 50 simultaneous calls. They are connected to a central hub called the mobile switching center (MSC). MSC provides connectivity between the public switched telephone network (PSTN) and the numerous base stations. Ultimately between all of the wireless subscribers. A typical MSC is responsible for connecting as many as 100 base stations to the PSTN (as many as 5,000 calls at one time) PSTN forms the global telecommunications grid which connects conventional (landline) telephone switching centers (called central offices) with MSCs throughout the world. Common air interface (CAl) is a carefully defined communication protocol to connect mobile subscribers to the base stations. Specifies exactly how mobile subscribers and base stations communicate over radio frequencies, and also defines the control channel signaling methods. Must provide a great deal of channel reliability to ensure that data is properly sent and received.
10.2 Differences Between Wireless and Fixed Telephone Networks PSTN: Using landline trunked lines (called trunks), capacity is large. Comprised of fiber optic cables, copper cables, microwave links, and satellite links. Networks configurations are virtually static. Wireless networks: Network configuration changed dynamically (within a small intervals of time). Capacity is limited.
10.2.1 The Public Switched Telephone Network (PSTN) Included in a undergraduate course mobile communications. Skipped.
10.2.2 Limitations in Wireless Networking Wireless communications system is extremely complex. Requires an air interface. to provide telephone grade communications; Propagation conditions are quite different; User locations are arbitrary. Number of BS is large to assure adequate area coverage Sometimes the number is hundreds. Each of BS must be connected to the MSC. MSC must provide connection for each of the mobile users to the PSTN. Requires simultaneous connections to the LEC, one or more IXCs, and to other MSCs via a separate cellular signaling network.
Problems to wireless networks: The radio channel is the extremely hostile and random. The users may request service from any physical location, while traveling over a wide range of velocities; The MSC is forced to switch calls imperceptibly between base stations throughout the system. Wireless systems are constrained to operate in a fixed bandwidth to support an increasing number of users over time. Need some vital techniques: Spectrally efficient modulation; Frequency reuse; Geographically distributed radio access points. As wireless systems grow, the number of BS and the switching burden of the MSC increase. The geographical location of a user changes constantly, results in extra overhead at all aspects. Particularly at the MSC (ensure seamless communications)
10.3 Development of Wireless Networks 10.3.1 First Generation Wireless Networks Based on analog technology. All cellular systems use FM modulation. All cordless telephones use a single BS to communicate with a single portable terminal. Typical example: Advanced Mobile Phone Services (AMPS). Transport architecture:
Transport architecture: Provide analog speech and inefficient, low-rate, data transmission. Speech signals are usually digitized using a standard, time division multiplex format for transmission between BS and MSC, and are always digitized for distribution from MSC to PSTN.
Functions of MSCs: Maintains all mobile related information. Controls each mobile handoff. performs all of the network management functions: Call handling and processing; Billing; Fraud detection within the market. Connection of MSCs: MSC is interconnected with the PSTN via landline trunked lines (trunks) and a tandem switch. MSCs also are connected with other MSCs via dedicated signaling channels for exchange of location, validation, and call signaling information.
PSTN is a separate network from the SS7 signaling network. Long distance voice traffic is carried on the PSTN. Signaling information used to provide call set-up and to inform MSCs about a particular user is carried on the SS7 network.
10.3.2 Second Generation Wireless Networks Features: Employ digital modulation Employ advanced call processing capabilities. Introduce new network architectures that have reduced the computational burden of the MSC. for example, GSM has introduced the concept of a base station controller (BSC) which is inserted between several base stations and the MSC. Use digital voice coding and digital modulation. Employ dedicated control channels (common channel signaling) within the air interface for simultaneously exchanging voice and control information between the subscriber, the base station, and the MSC. Provide dedicated voice and signaling trunks between MSCs, and between each MSC and the PSTN.
Specifically designed to provide paging, and other data services such as facsimile and high-data rate network access. Network controlling structure is more distributed. Handoff process is mobile assisted handoff (MAHO) Mobile units perform more functions than that in first generation subscriber units: Received power reporting; Adjacent BS scanning; Data encoding; Encryption. In general: Second generation systems have been designed to reduce the computational and switching burden at the base station or MSC, while providing more flexibility in the channel allocation scheme so that systems may be deployed rapidly and in a less coordinated manner.
Examples of 2G networks: Global System for Mobile (GSM) TDMA and CDMA U.S. digital standards (the Telecommunications Industry Association IS-54 and 15-95 standards) Second Generation Cordless Telephone (CT2), British standard for cordless telephony, Personal Access Communications System (PACS) local loop standard Digital European Cordless Telephone (DECT)
10.3.3 Third Generation Wireless Networks CDMA 2000; WCDMA; TD-SCDMA; All are deployed in china.
10.4 Fixed Network Transmission Hierarchy American hierarchy and European herarchy. 10.5 Traffic Routing in Wireless Networks connection-oriented services (virtual circuit routing), and connectionless services (datagram services). 10.6 Wireless Data Services Cellular digital packet data (CDPD) and GPRS. 10.8 Inlegrated Services Digital Network (ISDN) (included in the undergraduate course communication networks analysis and design.) 10.7 Common Channel Signaling (CCS) 10.9 Signaling System No. 7(SS7) 10.10 An example of SS7 Global Cellular Network Interoperability (included in the undergraduate course modern switch techniques and networks)
10.11 Personal Communication Services/Networks (PCS/PCN) Objective of PCS/PCN: Provide ubiquitous wireless communications coverage, enabling users to access the telephone network for different types of communication needs, without regard for the location of the user or the location of the information being accessed. Base of PCS/PCN: Advanced intelligent network (AIN). The mobile and fixed networks will be integrated to provide universal access to the network and its databases. AIN will also allow its users to have a single telephone number to be used for both wireless and wireline services.
Three levels of AIN: Intelligent level Contains databases for the storage of information about the network users. Transport level Handles the transmission of information. Access level Provides ubiquitous access to every user in the network and contains databases that update the location of each user in the network. Features of PCS: High user densities Large amount of signaling Common channel signaling efficient signaling protocols (SS7)
Approximate data requirements that PCS/PCN networks will be expected to carry
10.12 Protocols for Network Access Packet radio contention technique may be used to transmit on a common channel. The advantage of packet contention techniques is the ability to serve a large number of terminals with extremely low overhead. PRMA (Packet Reservation Multiple Access) can be used for packed voice terminals in a cellular system. PRMA has an advantage in that it can utilize the discontinuous nature of speech with the help of a voice activity detector (VAD) to increase capacity of the radio channel. Digital packet data and speech data are simultaneously supported with PRMA. The problem of contention is taken care of by designing the system using a probabilistic model based on trunking theory to predict the availability of time slots.
The availability of time slots depends on the usage of the network, and if there are too many users, call set-up will be prolonged. If encounter congestion, data packets are dropped, and speech packets are given priority. Speech requires the packets be delivered in order. ARQ is used, the packets are retransmitted if a mobile receives a negative acknowledgment.
Summary of this chapter: Modern cellular networks are based on digital radio and digital network technologies to maximize capacity and quality of service. Digital radio techniques shown below provide superior airinterface performance and spectral efficiency: Equalization Channel coding Interleaving Speech coding Wide range of signals (including speech and data) are stored and organize into packets for transmission over the air-interface. Digital air-interface is often designed to work well with the network architecture of the wireless system, and as wireless systems continue to emerge, the distinction between the digital air-interface and the backbone network architecture of personal communication systems will blur. SS7 has become an important part of the wireless background throughout the world. The first step towards a universal packet-based network. Fiber optic infrastructure support enormous bandwidth. Packet-based mobile services will thrive.