Class #9

1. Class #9 Chapter 9 - Wireless Transmission and Services

2. Wireless Transmission Objectives • Associate electromagnetic waves at different points on the wireless spectrum with their wireless services • Identify characteristics that distinguish wireless transmission from wire-bound transmission • Explain the architecture and access methods used in cellular net-works and services • Understand the differences between wireless and wireline local loops • Describe the most popular WLAN standards, including their advantages, disadvantages, and uses • Identify the major satellite positioning schemes and list several telecommunications services that rely on satellite transmission

3. The Electromagnetic Spectrum

4. The Wireless Spectrum

5. The Wireless Spectrum, cont.

6. Characteristics of Wireless Transmission Wireless is an unguided medium – that is, it uses the atmosphere as the transmission channel, and signals travel without physical guidance or path. Electrical signals travel from the transmitter to an antenna, when through the atmosphere as a series of electromagnetic waves.

7. Antennas • Radiation pattern - the relative strength over a three dimensional area of all the electromagnetic energy the antenna sends or receives. • Directional antenna - issues wireless signals along a single direction

8. Antennas, cont. • Omni-directional antenna - issues and receives wireless signals with equal strength and clarity in all directions.

9. Signal Propagation • Ideally, wireless uses Line of Sight propagation – nothing in between the transmitter and receiver antennas. • However, this is usually not the case. Different phenomena happen to the signal as obstacles are encountered by the signal. • Pass-through: the signal ignores the obstacle • Absorption: the signal is totally absorbed by an object. • Reflection - the wave encounters an obstacle and bounces back towards its source. • Diffraction - a wireless signal splits into secondary waves when it encounters an obstruction, particularly ones with sharp edges. • Scattering - the diffusion, or the reflection in multiple different directions of a signal.

10. Signal Propagation, cont.

11. Signal Propagation, cont. • Fading: a change in signal strength as result of some of the electromagnetic energy being scattered, reflected, or diffracted after being issued by the transmitter. • Multipath: the distortion, delay, weakening, or ghosting of a signal at the receiving end due to the signal arriving at the antenna at different times and causing phase distortion. • Antenna Diversity - the use of multiple antennas or multiple signal transmissions (sometimes in slightly different phases) to compensate for fading and delay.

12. Signal Propagation, cont. • Attenuation - after a signal has been transmitted, the farther it moves away from the transmission antenna, the more it weakens. • Interference - because wireless signals are a form of electromagnetic activity, they can be hampered by other electromagnetic energy (noise or other signals), resulting in interference.

13. Narrowband, Broadband, and Spread Spectrum Signals • Narrowband - a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies (ex: FM Radio). • Broadband - a type of signaling that uses a relatively wide band of the wireless spectrum (ex Television). • Spread spectrum - the use of multiple frequencies to transmit a signal (ex Military Communications, Cellular Phones). • Frequency Hopping (FHSS): the signal jumps between several different frequencies at a rate/sequence known to both the transmitter and receiver • Direct Sequence (DSSS): the signal’s bits are distributed over the entire spectrum all at once.

14. Cellular Communications • Mobile telephone service - a system for providing telephone services to multiple, mobile receivers using two-way radio communication over a limited number of frequencies. • Mobile wireless evolution: • First generation – 1984. Analog signals and an inefficient use of frequencies • Second generation – 1990. Multiple digital signals over a limited band of frequencies • Third generation – Next generation. QOS, prioritizing of signals, faster transmission, more efficient spectrum usage.

15. Principles of Cellular Technology A Cell is a geographical area where the base station transmission can use the full spectrum of frequencies available to the cell provider without causing interference in other nearby cells.

16. Principles of Cellular Technology,cont. Handoff is when a cellular user moves from one cell to another and the transmission channel is changes. When handoff is completed (seamlessly, it is hoped), the frequency used by the phone in the 1st cell is freed up to be used by another caller. To ensure that signals do not interfere with each other, frequencies are not reused in adjacent cells. A “guardband” type of situation is used, called a cluster, where a group of 7 cells each use different frequencies.

17. Cellular Call Completion • In traditional analog cellular phone networks, there are 2 groups of frequencies in the 800MHz range used: • 824-849 MHz are the forward path frequencies (phone to base) • 869-894 MHz are the reverse path frequencies (base to phone) • There is a 20 MHz guardband between paths. • Each call uses about 30 Khz

18. Cellular Call Completion, cont. • Components of a signal: • Mobile Identification Number (MIN) - an encoded representation of the mobile telephone’s 10-digit telephone number. • Electronic Serial Number (ESN) - a fixed number assigned to the telephone by the manufacturer. Sort of like a MAC address. • System Identification Number (SID) - a number assigned to the particular wireless carrier to which the telephone’s user has subscribed. Each carrier has a different SID.

19. Cellular Call Completion, cont. After a mobile user’s ID numbers are issue to the base station, the base transmits this info to a mobile telephone switching office (a CO with SLICs designed for cell bases and LGMs designed to handle handoff). The MTSO is then trunked to a regular CO.

20. Advanced Mobile Phone Service (AMPS) • A first generation cellular technology that encodes and transmits speech as analog signals. • AMPS uses Frequency Division Multiple Access (FDMA) to divide the cell spectrum into 832 separate channels with guardbands. • FDMA tends to be inefficient, as it uses 2 channels to complete a call and uses 30 MHz per channel. • The analog signals used are also subject to noise.

21. Time Division Multiple Access (TDMA) TDMA is a 2nd generation technology that breaks down digital signals so that multiple calls can use a single frequency through 3-slot time-slicing. An analogy would be allowing 3 conversations is a room, but only allowing 1 speaker to speak at a time.

22. Code Division Multiple Access (CDMA) Each voice signal is digitized and assigned a unique code, and then small components of the signal are issued over multiple frequencies using the spread spectrum technique. An analogy would be that of several groups of people talking in several languages at the same time. A listener would first choose to listen only to conversations in their own language, then choose to only listen to their own conversation.

23. Global System for Mobile Communications (GSM) • It is a European version of TDMA technology, because it divides frequency bands into channels and assigns signals time slots within each channel. • Makes more efficient use of limited bandwidth than the IS-136 TDMA standard common in the United States. • Makes use of silences in a phone call to increase its signal compression, leaving more open time slots in the channel (like statistical multiplexing).

24. Emerging Third Generation (3G) Technologies • The promise of these technologies is that a user can access all telecommunication services from one mobile phone (video, voice, data). However, the FCC has yet to allocate frequencies for 3G cell phones. • GSM in America: American deployment of European technology • CDMA2000 - a packet switched version of CDMA. • Wideband CDMA (W-CDMA) - based on technology developed by Ericson, is also packet-based and its maximum throughput is also 2.4 Mbps. • PCS: a category of digital wireless services (including telephony and paging) in the 1900 MHz band. Different PCS services use a variety of available technologies (GSM, TDMA, CDMA, etc).

25. Wireless Local Loops (WLLs) • WLL is a generic term that describes a wireless link used in the PSTN to connect LEC central offices with subscribers without the use of a physical media. • Acts the same as a copper local loop, but is used in situations where laying of a physical loop is physically or financially prohibitive. • CLECs use this technology to build their own PSTN infrastructure, instead of renting from ILECs. • Used to transmit both voice and data signals. • WLL access technology is often similar to that used on cellular networks (TDMA and CDMA), but using fixed receivers instead of mobile ones, the 1.8-3.7 GHz spectrum. Some technologies also take advantage of broadband wireless techniques.

26. Wireless Local Loops (WLLs), cont. There are 2 types of WLL access technologies: • Local Multipoint Distribution Service (LMDS) is a point-to-multipoint, fixed wireless technology that was conceived to supply wireless local loop service in densely populated urban areas and later on a trial basis to issue television signals. • Uses the high microwave frequency bands (28, 30, and 31 GHz). The use of these short bandwidths means that LMDS can carry lots of data. • A disadvantage is that its use of very high frequencies limits its signal’s transmission distance to no more than 4km between antennas. LDMS is also highly dependant on a clear line-of-site.

27. Wireless Local Loops (WLLs), cont. • Multipoint Multichannel Distribution System (MMDS) uses lower microwaves with frequencies in the 2.1 to 2.7 GHz range to transmit convergent signals. • One advantage is that because of its lower frequency range, MMDS is less susceptible to interference, and has a range of up to 30 miles. • MMDS does not require a line-of-sight path between the transmitter and receiver.

28. WLAN Architecture A Wireless LAN is a data network (or portion thereof) that uses wireless signaling between nodes. Pictured here are several wireless NICs. Devices like these make clients more mobile, and reduce construction/cabling costs. However, they tend to be slower than their physical media counterparts, cost more, and require line-of-site.

29. WLAN Architecture, cont. In addition to WNICs, a WLAN mist also have at least 1 fixed access point – a node on the network which acts as a wireless antenna/transceiver.

30. WLAN Architecture, cont. It is common for a WLAN to include several access points. Each access point services between 10-100 wireless nodes in a limited region (300 foot line-of-site radius). WLANS can also be used to connect 2 different parts of a LAN without incurring the hassles of installing horizontal wiring or backbones. Since such connectivity usually means fixed connection points, the line of site radius could be as far as 1000 feet.

31. Wireless Networking Standards • 802.11 - IEEE’s Radio Frequency half-duplex wireless networking standard committee. It has several sub-committees that have developed the following sub-standards • 802.11b/WiFi - uses direct sequence spread spectrum (DSSS) signaling. Also used the congested 2.4 - 2.4835 GHz frequency range and separates it into 14 overlapping 22-MHz channels. Maximum bandwidth is around 11 Mbps. • 802.11g - designed to be just as affordable as 802.11b while increasing its maximum capacity to 54 Mbps through different encoding techniques. • 802.11a - uses multiple frequency bands in the less congested 5 GHZ range. Like 802.11g, 802.11a provides a maximum throughput of 54 Mbps. • Bluetooth - A Ericsson mobile wireless networking standard that uses direct sequence spread spectrum (DSS) signaling in the 2.4 GHz band to achieve a maximum throughput of less than 1 Mbps. • Designed to be used on small inexpensive networks composed of personal communications devices, also known as personal area networks.

32. Personal Area Networks

33. Satellite Positioning • The original method for positioning satellites above the earth was in geosynchronous orbit. • Geosynchronous satellites are positioned approximately 35,800 km (22,300 miles) above the earth’s equator. • An alternative to GEO satellites are low earth orbiting (LEO, 700-1400 km) and medium earth orbiting (MEO, 10350 km) satellites.

34. Satellite Positioning

35. Satellite Services • Digital broadcasting - To deliver content to subscribers, networks (or other multimedia providers) uplink their audio and video signals to a satellite, which then downlinks the signals, in a broadcast fashion, to earth. • Analog broadcasting - Traditional analog television and radio signals can be issued from a terrestrial transmitter to a satellite and then downlinked to another terrestrial location within seconds. • Mobile Wireless - Services such as cellular telephone, paging, and other PCS applications are well suited to LEO or MEO satellite transmission.

36. Satellite Services • Tracking and monitoring - Two-way satellite communications can be used to monitor the whereabouts and condition of wildlife, mobile weather sensors, marine vessels, and so on anywhere in the world. • Global positioning service (GPS) - A service that expands on remote monitoring functions, GPS allows a mobile station on earth to exchange signals with a satellite to determine its precise location. • Wide area networks - Private companies use satellite transmission to connect multiple locations on their WANs.

37. Summary • The wireless spectrum, the range of frequencies within the electromagnetic spectrum that are used for telecommunications services, starts at 9 KHz and ends at 300 GHz. • Cellular telephone service is distinguished from other mobile two-way radio services by its use of cells to reuse limited frequencies within a certain geographical area. • Wireless LANs (WLANs) use the same protocols and a similar architecture as wire-bound LANs.

38. Assignments • Review Questions, (chap 9)1-25

39. End of Class Quiz • If a wireless signal collides with an object whose dimensions are larger than the signal’s wavelength, what happens to the signal? • Which antenna type issues signals with equal strength and clarity in all directions? • At what altitude does a geosynchronous satellite orbit the Earth? • What IEEE 802.xx subcomittee creates standards for wireless LANs? • How many generations of cell phone technology are there?