Introduction to Wireless Networking CS 490WN/ECE 401WN Winter 2007

Transcription

1 Introduction to Wireless Networking CS 490WN/ECE 401WN Winter 2007 Lecture 9: WiMax and IEEE Chapter 11 Cordless Systems and Wireless Local Loop I. Cordless Systems (Section 11.1) This section of the book discusses the following. Standards for digital cordless telephones. Methods for voice signal digitization and compression. Important topics, but we will not study them. II. Wireless Local Loop (Section 11.2) Traditional voice communications have been provided using a twisted pair cable. This cable provides an unbroken wire from a customer to a telephone local office. This is called a or a. Due to the pair of cables forming a loop to create the closed circuit. Twisted pair is still very commonly used. But coaxial cable and fiber are sometimes used for businesses. Access technologies for data networking. The mechanism for connecting a premise to a larger network is called an access network. LAN s are the access technologies for businesses and large facilities. Home access technologies are as follows. - Standard twisted pair telephone lines using modems. - Standard twisted pair telephone lines with special equipment on each end Digital Subscriber Line (DSL). - Coaxial cable Cable modems. Lecture 9, Page 1 of 11

2 Wireless technologies are increasingly seen as a viable access technology. They are given the terms or. - Can be narrowband or broadband. - Narrowband same bandwidth as a telephone line. - Broadband 2 Mbps and up. The Role of WLL Figure 11.8 illustrates a simple WLL configuration. WLL provider services one or more cells. Each cell has a base station, mounted on a building or tower. An individual subscriber has a fixed antenna mounted on a building or pole. - Has an unobstructed line of sight to the base station antenna. Lecture 9, Page 2 of 11

3 More complex configurations can be implemented, with levels of hierarchy. What are the advantages of using a WLL approach over other access technologies? 1. Less costly more costly equipment but much less cost by not installing (burying or hanging) cables. 2. Rapid installation. 3. Targeted installation to only those who are customers. Alternatives to WLL If existing wiring can be used, WLL may be more expensive. Cellular telephone systems can provide data connections just about anywhere. - But usually with much lower data rates. Frequency Allocations for WLL The U.S. FCC has allocated 15 frequency bands for WLL. From 2 to 40 GHz Higher frequencies above roughly 10 GHz are called. 10 GHz (c/10 GHz) = 30 mm. 300 GHz (c/300 GHz) = 1 mm. There are wide unused frequency bands in these ranges. Lecture 9, Page 3 of 11

4 Millimeter wave frequencies have difficulties. As we ve already seen, higher frequencies use more power. Rainfall and gaseous absorption are also problems. - Not a problem below 10 GHz. Multipath losses can be quite high millimeter waves reflect very easily. Figure shows how one might find a good set of frequencies to use in between the frequencies where water and oxygen have high absorption. What frequencies look to be good? 28 to 42 GHz. Lecture 9, Page 4 of 11

5 WLL Line of Sight These problems cause line-of-sight to be required above 10 GHz. Line of sight means the following. - No obstruction in the path. - AND no obstruction close to the path, at a radius called a Fresnel Zone. Can be a problem if trees make line of sight impossible. Might still work, but poorly. A newer technology called OFDM might be helpful here. OFDM = Orthogonal Frequency Division Multiplexing. MMDS and LMDS FCC allocations have created two types of services. MMDS Multichannel Multipoint Distribution Service Licensed frequencies in 2.15 GHz to 2.68 GHz. Can service an area with radius of 50 km. Equipment is inexpensive. Lower frequencies do not have problems with obstructions and rain absorption. But offers lower bandwidth. - Transfer rates of 27 Mbps. - Individual subscribers can have rates of 300 kbps to 3 Mbps. Mainly useful for residential and small business customers. LMDS Local Multipoint Distribution Service At frequencies around 30 GHz in the U.S. - Has trouble with obstructions, foliage. Has higher data rates than MMDS. - Makes it capable of providing video, telephone, data. Still relatively low cost compared to cabled alternatives. III. WiMax and IEEE IEEE Broadband Wireless Access (BBWA) Working Group Group started because they recognized a need for standards for LMDS. LMDS itself was just a name of the frequency bands allocated by the FCC, not a standard. Lecture 9, Page 5 of 11

6 Started in 1999 Objectives - Licensed spectrum - Metropolitan in scale - Use stationary rooftop or tower-mounted antennas. - Supports specialized treatment of packet streams for Quality of Service (QoS). - Capable of data rates over 2 Mbps. Standardizes air interface (modulation, coding) and related functions (MAC, management). WiMax is an industry forum to promote and interoperability among vendors. Same idea we saw with and. WiMax = Worldwide Interoperability for Microwave Access Standards are as follows. We will first study the textbook material on and a. But which area do you think is causing great interest? Mobile users no need for rooftop antennas! Lecture 9, Page 6 of 11

7 IEEE Architecture The objective is to provide a communication path between a subscriber site and a core network. Subscriber sites can be a single subscriber or a network on a customer s premises (for example, a LAN). Three interfaces are defined in this model. Air interface How the devices communicate through the air. What have we learned are functions that are included? Modulation Error Control Coding SNI and BNI Interfaces between transceiver stations and the networks behind them. - Details are out of the scope of Repeater system model includes optional use of repeaters to supplement the air interface. Lecture 9, Page 7 of 11

8 Protocol Architecture is concerned with the two lowest layers of the OSI model Data Link Layer - Includes Convergence and Medium Access Control - Assembles data into frames with address and error correction fields. - Disassembles frames upon reception, recognizes addresses, and performs error checking. - Governs access to the wireless medium (MAC layer) provides the ability to meet the demands of the protocols it services. - Some, like ATM (Asynchronous Transfer Mode, a type of data transfer protocol) require specialized quality of service (QoS) levels to meet their requirements. - When multiple stations are competing for the wireless medium, this results in a complex MAC protocol. - Note that protocols like do not provide QoS at all, unless one uses e which provides a little QoS. Lecture 9, Page 8 of 11

9 - Convergence layer provides functions specific to the service being provided. - Encapsulating data units from upper layers into frames. - Converting addresses from upper layers to addresses. - Translating upper layer QoS parameters to format. - Adapting time dependencies of upper layer traffic into the MAC service. - Some types of applications (like audio and video) may not require convergence functionality. Physical Layer - Includes Transmission and Physical layers - They deal with encoding of signals, synchronization, and bit transmission/reception (modulation, coding). - Also considers choice of the frequency band over which to transmit Services defines a set of that support different types of traffic from users is designed to support the following bearer services. Digital video/audio multicast - One-way digital audio/video streams to all subscribers (broadcast) or a subset of subscribers (multicast). - Examples are broadcast radio and TV. - This type of traffic requires steady delivery of packets. Digital telephony - Classic WLL service to replace wired local loops. ATM - ATM defines different types of QoS. - Constant Bit Rate - Variable Bit Rate real-time or non-real-time. - Unspecified Bit Rate supports these Internet Protocol - The Internet now also has types of QoS that supports. - Includes Differentiated Services. Bridged LAN - Enables transfer of data between two LAN s. Lecture 9, Page 9 of 11

10 Back-haul - For cellular or digital wireless telephone networks to connect together base stations. Frame relay - Older technology similar to ATM. Bearer Services grouped intro three broad categories Circuit-based - Allows virtual circuit switching to be used. Variable packet - Internet protocol and many others that use variable sized packets. Fixed-length cell/packet - ATM uses this. Table summarizes these three categories and gives specifications for bearer services in those categories. Lecture 9, Page 10 of 11

11 Types of requirements as seen in Figure a few pages back in these notes. - Medium access delay - How much time a transmitting station must wait before it can send a packet. - Transit delay - Delay from SNI to BNI or BNI to SNI. - Includes medium access delay. - Plus processing required at the MAC layer for transmission or reception. - Maximum Delay (one-way) in Table above refers to transit delay. - End-to-end delay - Total delay between the terminal in the subscriber network and the ultimate service beyond the core network. Next lecture: Details on the MAC Layer. Lecture 9, Page 11 of 11