INTRODUCTION
Local Multipoint Distribution Service (LMDS) is a relatively new Wireless Local Loop (WLL) or fixed wireless access service to deliver TV signals and two-way broadband communications, operating at millimeter frequencies. In US the allocated frequencies are near 30 GHz, while in Europe the allocated frequencies are near 40 GHz. LMDS has the following advantages:
- relatively high data rates, in the Mbps range;
- capable of providing video, telephony, and data;
- relatively low cost in comparison with cable alternatives.
With the growing interest in LMDS-WLL services, a need was recognized to develop standards for this service. The IEEE 802 committee set up the 802.16 working group to develop broadband wireless standards that:
- use wireless links with microwave or millimeter wave radios;
- use licensed spectrum;
- are metropolitan in scale;
- provide public network service to fee-paying customers;
- use point-to-multipoint architecture with stationary rooftop or tower-mounted antennas;
- provide efficient transport of heterogeneous traffic supporting quality of service (QoS);
- are capable of broadband transmissions (>2 Mbps).
In essence, IEEE 802.16 standardizes the air interface and related functions associated with LMDS.
Three working groups have been set up:
- IEEE 802.16.1: Air Interface for 10 to 66 GHz;
- IEEE 802.16.2: Coexistence of Broadband Wireless Access Systems;
- IEEE 802.16.3: Air Interface for Licensed Frequencies 2 to 11 GHz;
The following three tables define the scope and purposes of these three groups as provided by their project authorizations:
IEEE 802.16.1: Air Interface for 10-66 GHz
IEEE 802.16.2: Coexistence of Broadband Wireless Access Systems
IEEE 802.16.3: Air Interface for Licensed Frequencies, 2-11 GHz
The 802.16.1 and 802.16.3 fit into a larger context of wireless services as it is presented in the following figure:
A Multitier Perspective of Wireless Transmission and Distribution Systems
IEEE 802.16 ARCHITECTURE
System Reference Architecture.
The 802. 16 standards are designed with the respect to the abstract system reference model shown bellow:
IEEE 802.16 System Reference Points
An 802.16 wireless service provides a communication path between a subscriber site (single subsc device or a network on subscriber’s premises as a LAN, PBX, IP- based network) and a core network ( p telephone network, Internet). The IEEE 802.16 standards are concerned with the air interface between the subscriber’s transceiver station and the base transceiver station.
Protocol Architecture
Protocols defined specifically for wireless transmission address issues relating to the transmission of t blocks of data over the network. In OSI terms, high-layer protocols (layer 3 or 4 above) are independe network architecture and are applicable to a variety of networks and communications interface. A discussion of the four protocol layers defined in the 802.16 protocols suite is concerned with the lowes layers of the OSI model, as is depicted in the next figure:
IEEE 802.16 Protocol Architecture
Physical and Transmission layers functions:
- encoding/decoding of signals;
- preamble generation/removal (for synchronization);
- bit transmission/reception.
In addition, the physical layer of the 802 model includes a specification of the transmission medium an frequency band.
Medium Access Control (MAC) layer functions:
- on transmission: assemble data into a frame with address and error detection fields;
- on reception: disassemble frame, and perform address recognition and error detection;
- govern access to the wireless transmission medium.
The MAC protocol defines how and when a base station or a subscriber station may initiate transmissi the channel. The complexity of MAC protocols depends on the direction of transmission: upstream (m complex)/downstream (less complex).
Convergence layer functions:
- encapsulate PDU framing of upper layers into native 802.16 MAC/PHY frames;
- map upper layer’s addresses into 802.16 addresses;
- translate upper layer QoS parameters into native 802.16 MAC format;
- adapt time dependencies of upper layer traffic into equivalent MAC service.
In some cases, such as digital audio and video, no convergence layer is needed.
An example of the protocol structure supported by the convergence layer is the handling of TCP/IP b traffic, as in the next figure:
IEEE 802.16 Protocols in Context
Services
Requirements for the IEEE 802.16 standards are defined in terms of bearer services that the 802.16 systems support. A bearer service refers to the type of traffic generated by a subscriber network or c network. Separate bearer services have been defined for 802.16.1 and 802.16.3.
IEEE 802.16.1 Services
- Digital audio/video multicast: usually one-way streams to subscribers;
- Digital telephony: multiplexed streams;
- ATM: must support various QoS services defined for ATM;
- Internet protocol: supports the transfer of IP datagrams
- Bridged LAN: similar to the IP-based support
- Back-haul: for cellular or digital wireless telephone networks, as trunks for base stations;
- Frame relay: uses variable-length frames in contrast to the fixed-length cells of ATM.
Another way of viewing the service requirements for 802.16.1 is shown in the next table, where the bearer services are grouped in three categories:
- Circuit based;
- Variable packet;
- Fixed-length Cell/Packet.
Each category is characterized by the requirements concerning: the data rate that must be supported, error performance and the maximum one-way delay (transit delay). The standard 802.16 defines thre types of delays: medium access delay, transit delay and end-to-end delay, as it was shown in the IEE 802.16 System Reference Points previous figure.
IEEE 802.16.1 Services and QoS Requirements
IEEE 802.16.3 Services
The services supported by IEEE 802.16.3 are the following:
- voice transport: packet based, equivalent to that of the PSTN;
- data transport: IP based, including Q0S requirements;
- bridged LAN: similar to the IP-based support; it enables transfer of data between two LANs with
switching at the MAC layer.
IEEE 802.16.1 MAC LAYER
Data transmitted over the 802.16.1 air interface from or to a given subscriber are structured as a sequence of MAC frames : protocol data unit, PDU, that includes MAC protocol control
information and higher-level data.
Connections and Service Flow.
The IEEE 802.16.1 MAC protocol is connection oriented: a logical connection is set up between peer entities (MAC users) prior to exchange of data between those entities. Each MAC frame includes a connection ID, which is used by the MAC protocol to deliver the incoming data to the correct MAC user.
Service flows provide a mechanism for upstream/downstream QoS management in terms of:
latency (maximum acceptable delay), jitter (maximum acceptable delay variation) and throughput (minimum acceptable bit rate)
IEEE 802.16.1 Frame Format
The frame format consists of three sections as in the following figure:
IEEE 802.16.1 Frame Format
- Header: protocol control information needed by the MAC protocol;
- Payload: either higher-level data (an ATM cell, an IP packet, a block of digital
speech) or a MAC control message;
- CRC: error-detecting code field.
Three header formats are defined:
- Downlink header: used by the base station;
- Uplink header: used by the subscriber to convey bandwidth management needs to base
station;
- Bandwidth request header: used by subscriber to request additional bandwidth.
The structures of the IEEE 802.16 MAC Header Formats are illustrated in the following figures:
IEEE 802.16 MAC Header Formats
The downlink header, shown in (a) has the following fields:
- Encryption control (1 bit): encrypted/non-encrypted payload;
- Encryption key sequence (4 bits): an index into a vector encryption key information;
- Length (11 bits): length in bytes of the entire MAC frame;
- Connection identifier (16 bits): a unidirectional MAC layer address that identifies a
connection to equivalent peers in the subscriber and base station MAC;
- Header type (1 bit): a generic/bandwidth request header;
- ARQ indicator (1bit): indicates if the frame belongs to an ARQ enabled connection;
- Fragment control (2 bits): used in fragmentation and reassembly;
- Fragment sequence number (4 bits): sequence number of the current fragment;
- Header check sequence (8 bits): an 8 bit CRC used to detect errors in the header.
The uplink header format, shown in figure (b), contains also an 8-bit grant management field, GM, which is used by the subscriber to convey bandwidth management needs to the base station. The subfields within GM include the following:
- Slip indicator: (1 bit): a slip of uplink grants relative to the uplink queue depth;
- Poll-me (1 bit): requests a poll by the base station;
- Grants per interval (7 bits): the number of grants required by a concatenation;
- Piggyback request (8 bits): the number of bytes of uplink capacity required by the
subscriber for this connection.
The bandwidth request header is used by the subscriber to request additional bandwidth.
MAC Management Messages.
IEEE 802.16.1 defines a number of messages that are used to exchange operating parameters and status and encryption-related information, for capacity management, between base station and the subscriber. The names of these messages are presented bellow:
- Uplink and downlink channel descriptor;
- Uplink and downlink access definition;
- Ranging request and response;
- Registration request, response and acknowledge;
- Privacy key management request and response;
- Dynamic service addition request, response and acknowledge;
- Dynamic service change request, response, and acknowledge;
- Dynamic service deletion request and response;
- Multicast polling assignment request and response;
- Downlink data grant type request;
- ARQ acknowledgment.
IEEE 802.16.1 PHYSICAL LAYER
The IEEE 802.16.1 physical layer supports a different structure for the point-to-multipoint downstream channels and the multipoint-to-point upstream channels (next figure). Most systems require greater downstream capacity to individual subscribers to support asymmetric data
connections such as Web applications over the Internet. The capacity requirements aspects for the upstream and downstream transmissions are reflected in the physical layer specification.
IEEE 801.16.1 Physical Layer Modes
Upstream Transmission.
This transmission uses:
- a DAMA-TDMA (demand assignment multiple access- time division multiple access)
technique;
- an error correction uses Reed-Solomon code;
- a modulation scheme based on QPSK.
Downstream Transmission.
In downstream transmission direction, the standard specifies two modes of operation: one targeted to support a continuous transmission stream (mode A), and one targeted to support a burst transmission stream (mode B).
Continuous transmission stream (audio, video) uses a simple TDM scheme for channel access.
Duplexing technique- frequency division duplex (FDD) - is used for allocating capacity between upstream and downstream traffic. FDD implies that all subscribers can transmit and receive simultaneously, each on their own assigned frequencies.
Burst transmission downstream mode (IP-based traffic) uses the DAMA -TDMA scheme for channel access. Three alternatives techniques are available for duplexing traffic between upstream and downstream:
- FDD with adaptive modulation;
- frequency shift division duplexing (FSDD);
- time division duplexing (TDD).
LITERATURE
BOLC01 Bolocskei, H., et al. Fixed Broadband Wireless Access: State of the Art, Challanges, and Future Directions.
IEEE Communications Magazine, January 2001
PHIL98 Phillips, J., and Namee, G. Personal Wireless Communications with DECT and PWT.
Boston: Artech House. 1998
STAL02 Stallings, W. Wireless Communications and Networks.
Upper Saddle River, NJ: Prentice Hall 2002
WEBB00 Webb, W. Introduction to Wireless Local Loop: Broadband and Narrowband Systems Boston: Artech House, 2000
The IEEE 802.16 Working Group on Broadband Wireless Access Standards (Web site)
REVIEW QUESTIONS
1. What a the key differences among IEEE 802.16.1, IEEE 802.16.2, and IEEE 802.16.3?
2. Present a multitier perspective of Wireless Transmission and Distribution Systems [IEEE 802.16].
3. Relate the four protocol layers defined in the 802.16 protocol architecture to the OSI model.
4. What are the three categories of delay defined in the 802.16 standards?
5. Within IEEE 802.16.1 Physical Layer structure comment the upstream and downstream transmissions.
