Wireless_ Guide to Wireless Communications

							Chapter 9 Summary

Overview
By now, you understand the tremendous impact that wireless communications has and will
continue to have on the world around us. Wireless networks allow users to be connected as they
move about, freeing then from cables and phone lines.

However the WPANs and WLANs you have learned about thus far have restricted both
connections and mobility, letting users roam only a few hundred feet from the source of the RF
signal. Limits on the strength RF signals – to prevent interference in unlicensed bands- have also
restricted user mobility in Wi-Fi networks.

Chapter 9 describes wireless metropolitan area networks (WMANs). Students will learn why
WMANs are needed. They will also see the components and modes of operation of a WMAN.
Chapter 9 also lists the range of WMAN technologies, including FSO, LMDS, MMDS, and 802.16
(WiMAX). In addition, students will learn how WMANs function and the security features of
WMANs.


Objectives
      Explain why wireless metropolitan area networks (WMANs) are needed
      Describe the components and modes of operation of a WMAN
      List the range of WMAN technologies, including FSO, LMDS, MMDS, and 802.16
       (WiMAX)
      Explain how WMANs function
      Outline the security features of WMANs



What is a WMAN?
   1. Define a wireless metropolitan area network (WMAN) as a group of technologies that
      provide wireless connectivity across a substantial geographical area such as a large city.

   2. WMANs have two primary goals.

   3. Describe the primary goals of WMANs, including:
      a. Extend the reach of wired networks beyond a single location without the expense of
         high-speed cable-based connections
      b. Extend user mobility throughout a metropolitan area
      c. Provide high-speed connections to areas not serviced by any other method of
         connectivity
Last Mile Wired Connections

   1. Define a last mile connection as the link between a customer and ISP. Even today most last
      mile connections are based on copper wiring. Installing fiber is slow, inconvenient, and
      expensive.

   2. Most last mile connections are based on copper wiring. Copper-based digital
      communications lines require the signal to be regenerated every 6,000 feet.

      Describe the various connection options for homes and offices that require a last mile
      connection. (users take advantage of DSL and cable TV connection at 5mbps)

   3. Last mile delivery of telephone and data lines has long been a problem for the carrier, who
      must be able to justify the cost of installing wired connections to remote areas.

Last Mile Wireless Connection

   1. Microwave links were introduced in the early 1950s by AT&T, bringing about a new era in
      communications.

   2. Microwaves are higher frequency RF waves in the 3 to 30 GHz range of the
      electromagnetic spectrum, known as super high frequency (SHF) band. Microwave towers
      are installed roughly 35 miles (56 kilometers) apart from each other.

   3. Using wireless as the last mile connection for buildings is called fixed wireless because
      buildings are fixed in one location.

   4. Mention that as an alternative to last mile wired connections, many users have turned to
      wireless options. Using wireless as the last mile connection for buildings is known as fixed
      wireless.

Baseband vs. Broadband

   1. Explain the differences between baseband and broadband transmissions, as described in this
      section. Use Figure 9-2 to illustrate your explanation.

   2. A broadband transmission sends multiple signals at different frequencies. This allows
      many different signals to be sent simultaneously.

   3. A baseband transmission treats the entire transmission medium as if it were only one
      channel. The signals are transmitted at one set frequency, allowing only one signal to be
      sent at a time.


Land-Based Fixed Broadband Wireless

   1. This section describes the following land-based fixed broadband wireless solutions:
         a. Free Space Optics
         b. Local multipoint distribution service
         c. Multichannel multipoint distribution service
Free Space Optics

   1. Free space optics (FSO) as an optical, wireless, point-to-point, line-of-sight broadband
      technology. FSO is an excellent alternative to high-speed fiber-optic cable since it can
      transmit up to 1.25 Gbps at a distance of 4 miles (6.4 kilometers) in full-duplex mode.

   2. FSO uses infrared (IR) transmission instead of RF. These transmissions are sent by low-
      powered invisible infrared beams through the open air. Use Figure 9-3 to show an FSO
      transceiver.


   3. The advantages of FSO, including:
         a. Cost – FSO installation cost significantly less then installing new fiber optic cables
             or even leasing lines from a local carrier.
         b. Speed of installation – it can be days for FSO compared for months or even years
             for fiber optic cables.
         c. Transmission rate- can be scaled to meet users needs anywhere from 10Mbps to
             1.25Gbps
         d. Security – is a key advantage as IR transmission cannot be easily intercepted and
             decoded as some RF transmission.

   4. The disadvantages of FSO, including:
         a. The primary disadvantage of FSO is that atmospheric conditions can have impact
             FSO transmissions
         b. To deal with fog, FSO can increase the transmit power of the signal
         c. Signal interference
         d. Tall buildings or towers can sway due to wind or seismic activity, affecting the aim
             of the beam

   5. Scintillation as the temporal and spatial variations in light intensity caused by atmospheric
      turbulence. FSO overcomes scintillation by sending the data in parallel streams from
      several separate laser transmitters. This solution is called spatial diversity. Use Figure 9-4
      to better describe spatial diversity.

      Research – Beam divergence and Active tracking

   6. FSO applications, including:
        a. Last mile connection FSO can be used in high speed links that connect end users
            with internet service providers or other network
        b. LAN connections – Connect building separated by street or other obstacles
        c. Fiber-optic backup – in case of a break in the cable
        d. Backhaul – FSO can be used to carry cellular telephone traffic from antenna towers
            back to facilities wired into the public switched telephone network.

Local Multipoint Distribution Service (LMDS)

   1. Local multipoint distribution service (LMDS) as a fixed broadband technology that can
      provide a wide variety of wireless services, including:
         a. High-speed Internet access
         b. Real-time multimedia file transfer
       c. Remote access to local area networks
       d. Interactive video, video-on-demand, video conferencing
       e. Telephone service

2. LMDS can transmit from 51 to 155 Mbps downstream and 1.54 Mbps upstream over a
   distance of up to about 5 miles (8 kilometers). Use Figure 9-5 to describe LMDS
   transmissions.


3. LMDS, as explained in this chapter.
     1. (Local) L - Refers to the area of coverage of LMDS systems.
     2. (Multipoint) M – Indicates that signals are transmitted out to the the remote stations
        in a point-multipoint fashion.
     3. (Distribution) D – Refers to the distribution of the various types of information that
        can be distributed.
     4. (Services) S – means that there are a variety of services available.

4. Explain the architecture of LMDS. An LMDS network is composed of cells much like a
   cellular telephone system. The main difference is that LMDS is a fixed wireless technology
   for buildings.

5. Use Figure 9-6 through Figure 9-8 to describe the factors that determine the cell size,
   including:
       a. Line-of-sight - a direct line of sight between the transmitter and receiver is essential.
       b. Antenna height – another factor to consider is the height of transmission and
          reception antennas.
       c. Overlapping cells - because LMDS is line of sight technology a signal sent from on
          tower may not reach all of the recipient so overlapping cells will improve
          transmission.
       d. Rainfall - cell sizes are determined by the amount of local rainfall. Since LMDS
          signals are microwave these signals react with water and loose strength.

6. LMDS signals are broadcast from radio hubs that are deployed throughout the carrier’s
   market, the area in which the LMDS provider has a license to use a certain frequency. The
   Hub connects to the service provider’s central office, which can connect to other networks,
   such as the Internet. Use Figure 9-9 to show the LDMS infrastructure.

7. Use Figure 9-19 to describe the equipment used at the receiving site, such as:
      a. 12- to 15-inch diameter directional antenna
      b. Digital radio modem
      c. Network interface unit

8. LMDS systems can use either time division multiple access (TDMA) or frequency division
   multiple access (FDMA). Modulation techniques include quadrature phase shift keying
   (QPSK) and quadrature amplitude modulation (QAM).


9. Describe the main advantages and disadvantages of LMDS, as explained in this chapter.
Multichannel Multipoint Distribution Service (MMDS)

   1. MMDS as a fixed broadband wireless technology similar to LMDS, which can transmit
      video, voice, or data signals at 1.5 to 2 Mbps downstream and 320 Kbps upstream at
      distances of up to 35 miles (56 kilometers). MMDS is sometimes called wireless cable.

      Layout
   2. An MMDS hub (transmitter) is typically located on a high point such as a mountain, tower
      or building and uses a point-to-multipoint architecture that multiplexes communications to
      multiple users. The tower has a backhaul connection to the carrier’s network. The carrier
      network connects with the Internet.

      Pizza Box Antenna
      Is a directional antenna approximately 13 by 13 inches (33 cm) is aimed at the hub to
      receive the MMDS signal. It is installed on the roof of a building so that it has a direct line
      of sight to the radio transmitter.


   3. Since MMDS signals can travel longer distances, they can provide service to an entire area
      with only a few radio transmitters. An MMDS cell size can have a radius of up to 35 miles
      (56 kilometers).


   4. Describe the main advantages and disadvantages of MMDS, as explained in this chapter.



IEEE 802.16 (WiMAX)
   1. IEEE 802.16 is a standard for wireless broadband metropolitan area networks. 802.16
      supports enhancements and extensions to the MAC protocols so a base station (BS) can
      communicate with another BS and also directly with subscriber stations (SS).

   2. WiMAX Forum promotes the implementation of 802.16 by testing and certifying
      equipment. WiMAX stands for worldwide interoperability for microwave access.


WiMAX Applications

   1. The WiMAX applications, including:
      a. Suitable for backhaul applications for business
      b. Last mile delivery applications
      c. Supports simultaneous voice, video, and data transmission
      d. Suitable for voice-over-IP (VoIP) connections
      e. Enables vendors to create customer premises equipment (CPE)
      f. Can also be deployed as a point-to-point network to provide broadband access to rural
         and remote areas
      g. WiMAX CPE devices will support TV (video), telephone (voice), and data on the same
         network
   2. WiMAX MAC layer makes it easy for carriers to deploy the network.

   3. Mention that the range of a WiMAX network is measured in miles.

Standards Family Overview

   1. 802.16-2001 and 802.16-2004 standards define the interface specification for fixed, point-
      to-multipoint broadband WMANs. 802.16a adds support for systems operating in the 2
      GHz to 11 GHz band.

   2. 802.16c provided clarifications related to performance evaluation and testing. 802.16e
      defines specifications for a mobile version of WiMAX.

WiMAX Protocol Stack

   1. PHY layer supports multiple frequency bands and several modulation techniques. WiMAX
      MAC layer is connection oriented and includes service-specific convergence sublayers that
      interface to the upper OSI layers.

   2. WiMAX offers multiple simultaneous services through the same link, such as asynchronous
      transfer mode (ATM), IPv4, IPv6, Ethernet, and VLAN.

   3. There are five variations of the PHY layer in 802.16. The first two are based on the
      modulation of a single carrier signal. When transmitting at a single frequency, transmission
      is half-duplex. Each frame is subdivided into one uplink subframe and one downlink
      subframe. Subframes are further divided into a series of time slots. Burst is a data
      transmission to or from a single device. These two methods use time division duplexing
      (TDD).

   4. WiMAX also allows the use of two different frequency channels, one for downlink and
      another for uplink. This mechanism is called frequency division duplexing.

   5. The two variations of the PHY layer specified in the standard:
         a. WirelessMAN-SC (single carrier)
         b. WirelessMAN-SCa (single-carrier access)
  6. The two additional Physical layer transmission mechanisms used by 802.16 to support
     NLOS applications:
        a. WirelessMAN-OFDM
        b. WirelessMAN-OFDMA


  7. 802.16 uses forward error correction and automatic repeat requests (ARQ) for error
     correction. In addition, 802.16 dynamically changes the modulation technique used when
     transmitting. Use Table 9-3 and Figure 9-16 to illustrate your explanation.

  8. 802.16 defines several transmission profiles, which are sets of predefined connection
     parameters. Define system profiles as the combination of the basic profile and one of the
     transmission profiles.

  9. The frequency range and throughput supported by 802.16. Use Table 9-4 to illustrate your
     explanation.

  10. 802.16 MAC layer. Most wireless MAN implementations function in a point-to-multipoint
      basis with one BS and potentially hundreds of SSs. 802.16 MAC dynamically allocates
      bandwidth to individual SSs for the uplink.

  11. An advanced antenna system (AAS) transmits multiple simultaneous signals in different
      directions to stations that fall within the range. WiMAX can also take advantage of multiple
      in multiple out (MIMO) antenna systems.

  12. To address a burst to a particular SS, an 802.16 BS uses a 16-bit connection identifier
      (CID). Stations can request additional dedicated bandwidth (for QoS).


WiMAX Coexistence

  1. One of the main concerns with wireless communications is that as the number of
     transmitters grows, so does interference. WiMAX is not limited to the 2.4 GHz or the 5
     GHz bands. The U-NII band offers 12 channels and about 300 MHz of bandwidth, and
     since WiMAX signals are limited to between 30 and 35 miles, interference may not be a
     serious problem.

  2. Adaptive modulations, variable data rates, signal power levels, and FEC also help with
     interference.
WMAN Security
  1. The security advantages of using FSO systems. In addition, describe the security problems
     regarding LMDS and MMDS system.

WiMAX Security

  1. WiMAX standard was initially designed to include very powerful security measures. The
     MAC layer includes a privacy sublayer. The privacy sublayer provides a client/server
     authentication and key management protocol using digital certificates.

  2. The components in the privacy sublayer:
        a. An encapsulation protocol for encrypting packet data
        b. A privacy key management protocol that provides secure key distribution

  3. The traffic encryption key (TEK) as a security key used to encrypt the data. SSs must renew
     the keys periodically with the BS. The default TEK lifetime is 12 hours.

  4. The encryption algorithms supported by 802.16, including:
        a. 3-DES
        b. RSA with 1024-bit key
        c. AES with 128-bit key



Class Discussion Topics
  1. What is free space optics (FSO)?

  2. What are the differences between broadband and baseband transmissions?


Additional Projects
  1. Write a report comparing free space optics (FSO), local multipoint distribution service
     (LMDS), and multichannel multipoint distribution service (MMDS). They should compare
     characteristics such as frequency range, distance, data rate, multiplexing techniques, and
     modulation techniques, among others.

  2. Use the Internet to research the security problems faced by the 802.16 (WiMAX) standards
     and write a one-page report summarizing the most important points.