Chapter 4 Wireless Metropolitan Area Networks
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