Wireless MAN Technologies

Wireless Metropolitan Area Network (WMAN), like WLAN and WPAN, is a generic term for networking confined to a geographical area and a set of specific networking technologies that provide wireless communications in metropolitan areas. WMAN is a new technology that will be a supplement to well-known wired technologies such as Resilient Packet Ring (RPR), Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH), SONET over IP, Gigabit Ethernet, and Wavelength Division Multiplexing (WDM). The area of coverage of WMAN falls between WLAN/WPAN, which are customer premises networks, and Wireless Wide Area Networks (WWAN), which are associated with cellular radio mobile networks. Methods of access to WMANs have some resemblance to those of broadband wired access technologies such as Digital Subscriber Line (DSL) and Data over Cable Service Interface Specifications (DOCSIS).

Conceptually, WMAN networks provide services to metropolitan or regional areas, either urban or rural, within a radius of 50 km. They can be used to connect WLANs/WPANs and provide access to data, voice, video, and multimedia services. Although WMANs provide city-wide coverage, the area may be as small as a university campus or even a group of buildings. WMANs belong to a network operator or a service provider, in many cases a wireless extension of services provided by wired or wireless carriers. WMANs can be implemented using a variety of wireless technologies: Local Multipoint Distributed Service (LMDS), Multi-Channel Multipoint Distributed Service (MMDS), Free Space Optics (FSO), Wireless Local Loop (WLL), and Wireless Interoperable Metropolitan Area Exchange (WiMAX).

Service Management Conceptual Model

In general terms, the notion of service is an abstraction. However, in an actual context such as communication service, it becomes more meaningful. If we add service qualifiers such as availability, speed, security, integrity, and responsiveness, we have something palpable to the user. The ultimate goal of any service is to satisfy the customer. That means services that assure the good functioning of a network, system, processes, and overall business in a cost-effective way. Service Management is an area of management that was designed specifically to address these issues. Service Level Management (SLM) is that part of management that encompasses networks and systems, binding together service providers with users of services. It does this by identifying, defining, tracking, and even proactively monitoring the services.

Communication services are provided on a contractual basis between customers (organizations, businesses, or individual subscribers) and one or multiple service providers. These services are to be delivered with the assurances that the exchange of information on data communication networks will be done under well-defined conditions and with guaranteed performance. A simplified communications service conceptual model is presented in Figure 3.1. Service providers with adequate applications and network infrastructure provide communication service to customer systems. Multiple service providers are usually involved in providing communications services because of the particular media used (wireless, fiber optic, cable, twisted pair), the type of network (access, connectivity, backbone), the type of information (voice, data, video, multimedia), and the communications technologies adopted.

The 802.11n amendment, more so than any previous 802.11 amendment, introduces many optional features geared toward specific market segments that will likely only be deployed in specific classes of devices. Many of the optional features are complex and, given time to market concerns and cost constraints, many implementations will only adopt a subset of the available features, perhaps phasing in features with time or in higher end products. Some features appear in multiple flavors. This is due to the many unknowns present at the time the features where being discussed during the standard development process. Very often there was no single clear direction to take that would clearly suit all situations and so variations were introduced.

The popularity of 802.11 also means that there are a large number of legacy 802.11 devices deployed making interoperability and coexistence with those devices essential.

This chapter discusses various features that help ensure interoperability and coexistence between 802.11n compliant devices as well as legacy 802.11 devices.

Station and BSS capabilities

With the large number of optional features in the 802.11n amendment a fair bit of signaling is required to establish device capabilities and ensure interoperability. Also, care must be taken to ensure that a feature used by one station does not adversely affect the operation of a neighboring station that is not directly involved in the data frame exchange.

Networks and Systems Management Concepts

To administer the enormous number of resources involved in communications, either wired or wireless, from customer premises to backbone networks and across all geographical and administrative boundaries, there is a need for specialized systems designed specifically for management. These systems comprise hardware and software components, applications and corresponding operations used together to monitor, control, operate, coordinate, provision, administer, diagnose and report faults, and account for the network and computing resources that allow communications to take place. As communications systems became an important part of any business, management systems evolved to support whole enterprises, i.e., to provide management of multivendor, multiprotocol and multitechnology network and systems environments. Management systems are more than just simple tools used to manage network and systems resources. They include standardized procedures and sophisticated communications protocols to collect and process the management information.

The high level paradigm of management consists of two entities: the Managing Entity and the Managed Entity. The relationship between managing entities and managed entities can be modeled as manager-agent, client-server, mainframe-terminal, master-slave, or peer-to-peer. In the manager-agent model, the management entity, also called the manager, represents the managing process while the managed entity, also called agent, represents the managed process. These models are depicted in Figure 2.1. Both manager and agent processes are software applications. The manager application provides management functions and services while the agent application provides access to the management information related to managed resources or managed objects.

Fixed-Mobile Convergent Network Management

In Chapters 2 and 3 we provided an extensive introduction to the concepts of network and service management, with no specific reference there to wireless networks or fixed-mobile convergent networks. At this point, we use the layered architecture known as Telecommunications Management Network (TMN) to analyze these concepts. The layered TMN architecture is presented in Figure 18.1.

TMN consists of five logical/functional layers implemented as interconnected management applications. The first three layers, Network Element Layer (NEL), Element Management Layer (EML), and Network Management Layer (NML) deal with network management as a whole and network management sub-divisions such as EML and NEL. The main characteristics of each layer are shown in Figure 18.1.

The next layer, the Service Management Layer (SML), provides management of services, monitors the connectivity between multiple service providers, and deals with specific aspects of service management such as Quality of Service and Service Level Agreements.

Management of convergent networks means management of all those networks that are part of an integrated mobile and fixed wireless network. Network management includes monitoring and controlling the capabilities of all vital network components, including diagnosing faults, modifying configurations, measuring performance, providing billing/charging information, and securing network operations. A high-level depiction of convergent wireless networks management is shown in Figure 18.2.

Convergent network management can be achieved through dedicated network management systems that provide separation of management aspects from the switching and transport aspects.

The capability to perform adaptive transmit beamforming is provided in the 802.11n standard. With transmit beamforming (TxBF), we apply weights to the transmitted signal to improve reception. The weights are adapted from knowledge of the propagation environment or channel state information (CSI). Since by definition transmit beamforming weights are derived from channel information, spatial expansion as defined in Section 6.2 is not considered transmit beamforming.

The key advantage with transmit beamforming is the ability to significantly improve link performance to a low cost, low complexity device. This advantage is illustrated in Figure 12.1, which depicts a beamforming device with four antennas. Such a device could be an AP or a home media gateway. The device at the other end of the link has only two antennas, typical of a small client device. Such a system would benefit from 4 × 2 transmit beamforming gain from device A to device B. However, when transmitting from device B to device A, the system gain would be matched with 2 × 4 SDM with MRC as described in Section 6.1. Therefore link performance would be balanced in both directions.

In Figure 12.2, the generic MIMO system is modified to illustrate the application of beamformer weights to the transmitted signal. To simplify notation, the system description is given in terms of the frequency domain for a single subcarrier. Transmit beamforming as described is applied to each subcarrier in the frequency band.

What is the IP-based Multimedia Subsystem?

The IP-based Multimedia Subsystem (IMS) is the Next Generation Network (NGN) architecture, set of components, and interface specifications that allow convergence of wired and wireless networks. Convergence/integration of fixed and mobile wireless networks, the subject of this book, is part of the broader convergence just defined. The IMS convergent network will emerge from an Internet Protocol (IP)-based network infrastructure and a common service platform, allowing the development of a large array of telecommunications and multimedia applications. IMS is a user/operator-centric architectural framework that shifts much of its intelligence to the network periphery. Users of IMS services will benefit because of full mobility and service transparency across all networks.

IMS development was initiated in 1999 by a group of leading mobile service providers in conjunction with the promotion of future generations of mobile networks. This work was taken over by the 3G Partnership Project (3GPP) and presented for the first time in 3GPP Release 5 (3GPP R5) specifications. Release 5 was augmented by the addition, in Release 6 (3GPP R6), of the Internet Engineering Task Force (IETF) Session Initiation Protocol (SIP). In 3GPP Release 7, IMS incorporated the NGN concepts promoted by the European Telecommunications Standard Institute's (ETSI) Telecommunications and Internet Converged Services and Protocols for Advanced Networking (TISPAN) division. Also, the mobile networks support for IMS was extended from GSM/UMTS to CDMA2000 through 3GPP2 working specifications.

This chapter introduces some of the advanced channel access techniques in the 802.11 standard. In addition to, and built upon, the distributed channel access techniques described in Chapter 7, the 802.11 standard includes two centrally coordinated channel access techniques. The PCF was introduced in the original 802.11 specification and HCCA was introduced in the 802.11e amendment to support parameterized QoS and to fix some of the deficiencies in the PCF. The chapter then introduces new channel access techniques in the 802.11n amendment.

A very simple technique called the reverse direction protocol was introduced with one bit of additional signaling and some simple changes to the rules for operating a TXOP. This technique is particularly effective under EDCA for improving throughput for certain traffic patterns.

During the development of the 802.11n amendment, a strong interest emerged among many participants for improving the power efficiency of the MAC protocol. While outside the scope of the 802.11n PAR this resulted in the power-save multi-poll (PSMP).

PCF

Infrastructure network configurations may optionally include the point coordination function (PCF). With PCF, the point coordinator (PC), which resides in the AP, establishes a periodic contention free period (CFP) during which contention free access to the wireless medium is coordinated by the PC. During the CFP the NAV of all nearby stations is set to the maximum expected duration of the CFP. In addition, all frame transfers during the CFP use an inter-frame spacing that is less than that used to access the medium under DCF, preventing stations from gaining access to the medium using contention-based mechanisms.

Communications Networks

Communications by voice and physical signaling are common means of interaction between human beings. In the simplest forms, there is an emitting entity of information and a receiving entity of information. As the sources move apart the need for telecommunications appears self-evident. This simple model of communications becomes more complex when the information transmitted is not just sound, speech, or music, but full motion video images or various forms of data such as text, shared files, facsimile, graphics, still images, computer animation or instrumentation measurements. This information can be transmitted using electrical or optical signals, the native analog information undergoing numerous conversions and switching to accommodate various communications technologies. Telecommunications can take place over various media be that twisted copper pairs, coaxial cable, fiber optic, or wireless radio, microwave, satellite, and infrared links. Communication can be limited to a small group of people or extended to departments, compounds, or campuses and covering whole metropolitan areas, regions, countries, or continents. Hence, a shared infrastructure is needed, i.e., a communications network.

Communications networks can be classified in many ways, as there are distinct technologies and network equipment needed for voice communications, data/computer communications, and video communications. Among these types of communications, data communications, by the virtue of digitization of any type of information, has become the convergent system.

This section provides details on the MAC frame formats. The information provided here is sufficiently detailed to act as a reference for the topics discussed in this book, but it does not provide an exhaustive list of all field elements, particularly in the management frames. For a detailed treatment of the frame formats refer to the actual specification (IEEE 2007a, 2007b).

General frame format

Each MAC frame consists of the following:

• a MAC header

• a variable length frame body that contains information specific to the frame type or subtype

• a frame check sequence or FCS that contains a 32-bit CRC.

This frame format consists of a set of fields that occur in a fixed order as illustrated in Figure 11.1. Not all fields are present in all frame types.

Frame Control field

The Frame Control field is shown in Figure 11.2 and is composed of a number of subfields described below.

Protocol Version field

This field is 2 bits in length and is set to 0. The protocol version will only be changed when a fundamental incompatibility exists between a new revision and the prior edition of the standard, which to date has not happened.

Type and Subtype fields

The Type and Subtype fields together identify the function of the frame. There are three frame types defined: control, data, and management. Each frame type has several subtypes defined and the combinations are listed in Table 11.1.