Chapter 4

Wireless Metropolitan Area Networks

Learning objectives

• To know about wireless metropolitan area

networks (WMANs)

• To know the architecture of WMANs

• To know IEEE 802.16 standards

• To know protocols of WMANS

• To know the wireless broadband networks

• To know the applications of WMANs

Wireless metropolitan area networks

(WMANs)

• Wireless Metropolitan area networks (WMANs) are

large computer networks connected by wireless

technology, usually spanning a city (up to 50 km)

• It is a promising Broadband Wireless Access (BWA)

technology providing high-speed, high bandwidth

efficiency and high-capacity multimedia services

• Residential and enterprise applications

• IEEE 802.16 standard, commonly known as WiMAX

(Worldwide interoperability for Microwave Access)

Illustration

WiMAX base station

WiMAX

• WiMAX Forum (formed in June 2001)

• Description by forum

A standards based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and digital subscriber line (DSL), especially when there is no wire broadband access.

IEEE 802.16

• Air interface between a subscriber's transceiver (SS) station and a base transceiver station (BS)

• Goal - high-speed Internet access to home and business subscribers, without wires.

• The standards (Stallings) – IEEE 802.16 – one MAC for WMAN. Physical layer: 10 to 66

GHz.

– IEEE 802.16a - MAC modification to 802.16.1. Physical layer: 2 to 11 GHz.

– IEEE 802.16c - 10-66 GHz detailed

– IEEE 802.16e – MAC for fixed and mobile operations.

– IEEE 802.16.2 - 10 to 66 GHz, minimizing interference between coexisting fixed broadband wireless access systems

IEEE 802.16 properties

• Broad bandwidth: Supports up to 134 Mbits in 28

MHz channel (in 10-66 GHz)

• Efficiently transports IPv4, IPv6, asynchronous transfer mode (ATM), Ethernet, etc. (supports multiple services simultaneously)

• MAC designed for efficient use of spectrum

• Point-to-multipoint topology (P2MP), with mesh extensions

• Supports for adaptive antennas (that increase spectral efficiency) and space-time coding.

Illustration: P2MP

WiMAX base station

Indoor customer

premise equipment

outdoor customer

premise equipment

Illustration: Indoor CPE and Outdoor CPE

WiMAX base station

Illustration: Mesh extensions

WiMAX base station

IEEE 802.16 standards (WiMAX):

some details

Illustration: QAM (Quadrature Amplitude Modulation)

8 different phases and 2 different amplitudes

16 different

possible values

log224 = 4

C = 2B

Recall Nyquist formulation: C = 2B log2 M

Advantages of IEEE 802.16

• Last Mile connectivity

• Roaming between networks

• Flexibility

• Scalability

Illustration: last mile

IEEE 802.16 Vs. IEEE 802.11

WMAN network architecture

Components

• WiMAX Base Station

– provides coverage to a very large area up to a radius of 6

miles.

– Any wireless device within the coverage area would be

able to access the Internet.

• WiMAX Receiver

– also referred as Customer Premise Equipment (CPE).

• Backhaul

– connection from the access point to the base station and

base station to the core network.

WiMAX features

WiMAX mobility support

• IEEE 802.16e-2005 standard supports mobility

management

• Four mobility-related usage scenarios

1. Nomadic: fixed subscriber station with

different points of attachment

2. Portable: portable device

3. Simple mobility: subscriber moving at speeds

up to 60 km/h

4. Full mobility: Up to 120 km/h mobility and

seamless handoff

802.16 protocol stack

Service Specific Convergence Sublayer

(CS)

MAC Common Part Sublayer (MAC CPS)

Security Sublayer

Physical Layer (PHY)

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

LLC

MAC Sublayer

Physical Layer

ISO

IEEE

IEEE 802.16 specifies the MAC and PHY layers

The MAC layer consists of 3 sublayers

802.16

802.16 protocol stack (Contd..) • MAC CS (Convergence Sublayer) receives higher level data and provides

mapping to MAC SDU. Interfacing with various protocols (IP and ATM).

• MAC CPS (Common Part Sublayer) is the core part of the MAC layer. Receives MAC CS PDU and provides mapping to MAC CPS SDU. Defines medium access method, provides functions related to duplexing and channelization (Cf. a slide No 32) and provides bandwidth allocation and connection control.

• MAC SS (Security Sublayer) provides authentication, encryption and data transferring to PHY layer.

Higher level data

MAC CS SDU

MAC CPS SDU

MAC CS PDU

MAC CPS PDU

MAC SS SDU MAC SS PDU

Router, Host,…(Cf next slide).

IP packet,

ATM cell,

block of

digital

speech

…..

MAC CS

MAC CPS

MAC SS

Illustration: higher level data sources

Fixed

subscriber

Mobile

subscriber

IEEE 802.16 Physical Layer

• 802.16 defines multiple PHY layer options:

– Single Carrier (SC),

– OFDM (Orthogonal Frequency Division Multiplexing: all the subchannels are dedicated to a single source),

– OFDMA (Multiple Access, multi-user version of OFDM),

– Scalable OFDMA (SOFDMA), …

IEEE 802.16 Physical Layer (Contd..)

Licensed

OFDMA

OFDM

OFDMA

Mechanisms of the Physical Layer

• Robust Error Control Mechanism

• Adaptive Modulation and Coding

• Space -Time Block Codes (STBC)

• Adaptive Antenna System (AAS)

Robust Error Control Mechanism

• Two phases:

– The outer Reed-Solomon code corrects burst errors at the level of bytes.

– The inner convolutional code corrects independent bit errors.

Adaptive Modulation and Coding

• WiMAX supports a variety of modulation (BPSK, QPSA, 16 QAM, …) and coding schemes and allows for the scheme to change on a burst-by-burst basis per link, depending on channel conditions.

• Using the channel feedback indicator, the subscriber station (SS) can provide the BS with feedback on the downlink channel quality.

• For the uplink, the BS can estimate the channel quality, based on the received signal quality.

Downlink

Uplink

Space-Time Bloc Codes (STBC)

• Space-time codes combine spatial and temporal signal copies.

• The signal copy is not transmitted only from another antenna but also at another time.

• The delayed transmission is called delayed diversity.

• Improve performance.

Adaptive Antenna System (AAS)

• Used with OFDM

• Each of the antennas transmits one group of

subcarriers

• Receivers provide transmitters with feedback (Cf.

next slide)

• A BS can serve multiple SSs

WiMAX signal transmission scenario

using AAS

• AAS works by adjusting the width and the angle of the antenna radiation

pattern.

• It can focus its transmission energy to the direction of the receiver.

• SSs deliver quality channel feedback.

MAC Layer

• IEEE 802.16 MAC

– designed for P2MP broadband wireless access applications

– to provide an interface between the higher layers and the

physical layer

– describes a number of Convergence Sub-Layers (i.e., how

wireline technologies such as Ethernet, IP, ATM,…are

encapsulated on the air interface, etc.)

– describes how secure communications are delivered

– include power saving mechanisms (using sleep mode and

idle mode) and handover

Channelization (recall MAC Common Part

Sublayer - MAC CPS) • Differentiate between MAC frame and WiMAX (TDMA) frame

• TDMA frame consists of a sequence of time slots, each dedicated to a given subscriber.

• A TDMA time slot may contain exactly one MAC frame, a fraction of a MAC frame, or multiple MAC frames.

• The sequence of time slots across multiple TDMA frames that is dedicated to one subscriber forms a logical channel, and MAC frames are transmitted over that logical channel.

SS1 SS2 SS3 … … … … …. … SSn time

SS1 SS2 … SSn SS1 SS2 … SSn time

SS1: logical channel 1 SS2: logical channel 2 SSn: logical channel n

WiMAX frame

• WiMAX Frame consists of two sub-frames

– uplink (UL) sub-frame

– downlink (DL) sub-frame

• Initial maintenance: first access by stations to detect round-trip time to BS as well as necessary transmission power (random choice of a time slot in that field by backoff mechanism). Collisions are possible.

• Request contention: demands reservations in coming UL maps (backoff mechanism). Collisions are possible.

MAC Quality of Service (QoS)

• A key feature of 802.16 is that it is a connection-oriented technology. The SS cannot transmit data until it has been allocated a channel by the BS.

• 802.16e provides strong support for QoS.

• QoS is supported by allocating each connection between the SS and the BS to a specific QoS class.

MAC Quality of Service (QoS)

(Contd..)

IEEE 802.16 MAC Layer defines five service

classes

• Unsolicited Grant Service (UGS)

• Real-time Polling Service (rtPS)

• Extended Real-time Polling Service (ertPS)

• Non-real-time Polling Service (nrtPS)

• Best Effort (BE) service

Unsolicited Grant Service (UGS)

• The BS provides fixed size bursts in the UL at periodic

intervals for the service flow (which eliminates the overhead and latency of SS request ).

• Typical application: VoIP (without silence suppression)

Real-time Polling Service (rtPS)

• Supports variable size grants for optimum data transport efficiency.

• Allows the SS to specify the size of the desired UL .

• Application: streaming video.

Extended Real-time Polling Service

(ertPS)

• Supports real-time applications where the applications require guaranteed data rate and delay.

• Application: VoIP (with silence suppression)

Non-real-time Polling Service

(nrtPS)

• Supports non-real time service flows that require variable size bursts in the UL on a regular (but not strictly periodic) basis.

• SSs contend for bandwidth (for UL) transmission during contention request opportunities.

Best Effort (BE) service

• It is intended to be used for best effort traffic

where no throughput or delay guarantees are provided.

• SSs contend for bandwidth (for UL) transmission during contention request opportunities.

Scheduling

• Two types of scheduling

– Centralized - base station assigns capacity to the other

stations

– Decentralized - stations exchange scheduling information

with their neighbors