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Advances in Wireless Networks:
IEEE 802.16(WiMAX)
Vinh Do
Comp 529
California State University of Northridge
Outline

Background
 IEEE 802.15: PAN
 IEEE 802.11: Wireless LANs
 802.11 last-mile network
 Mesh network
 IEEE 802.16
-Standards
-Physical layer
-MAC layer
 IEEE 802.20(proposed)
Elements of a wireless network
network
infrastructure
wireless hosts
 laptop, PDA, IP phone
 run applications
 may be stationary (nonmobile) or mobile
– wireless does not
always mean mobility
Elements of a wireless network
network
infrastructure
base station
 typically connected to
wired network
 relay - responsible for
sending packets between
wired network and
wireless host(s) in its
“area”
– e.g., cell towers
802.11, 802.16
access points
Elements of a wireless network
network
infrastructure
wireless link
 typically used to connect
mobile(s) to base station
 also used as backbone link
 multiple access protocol
coordinates link access
 various data rates,
transmission distance
Characteristics of selected wireless link
standards
54 Mbps
5-11 Mbps
802.11{a,g}
802.11b
.11 p-to-p link
1 Mbps
802.15
3G
UMTS/WCDMA, CDMA2000
384 Kbps
2G
IS-95 CDMA, GSM
56 Kbps
Indoor
Outdoor
Mid range
outdoor
Long range
outdoor
10 – 30m
50 – 200m
200m – 4Km
5Km – 20Km
Elements of a wireless network
network
infrastructure
infrastructure mode
 base station connects
mobiles into wired or
mesh network
 handoff: mobile changes
base station providing
connection into wired
network
Elements of a wireless network
Ad hoc mode
 no base stations
 nodes can only transmit to
other nodes within link
coverage
 nodes organize
themselves into a network:
route among themselves
Wireless Link Characteristics
Differences from wired link ….
– decreased signal strength: radio signal attenuates as it
propagates through matter (path loss)
– interference from other sources: standardized wireless
network frequencies (e.g., 2.4 GHz) shared by other
devices (e.g., phone); devices (motors) interfere as well
– multipath propagation: radio signal reflects off objects
ground, arriving ad destination at slightly different times
…. make communication across (even a point to point)
wireless link much more “difficult”
Wireless network characteristics
Multiple wireless senders and receivers create additional
problems (beyond multiple access):
C
A
B
A
B
Hidden terminal problem
 B, A hear each other
 B, C hear each other
 A, C can not hear each other
means A, C unaware of their
interference at B
C
C’s signal
strength
A’s signal
strength
space
Signal fading:
 B, A hear each other
 B, C hear each other
 A, C can not hear each other
interferring at B
802.15: personal area network(PAN)





replacement for cables (mouse,
keyboard, headphones)
ad hoc: no infrastructure
master/slaves:
– slaves request permission to send
(to master)
less than 10 m diameter
– master grants requests
802.15: evolved from Bluetooth
specification
– 2.4-2.5 GHz radio band
– up to 721 kbps
P
S
P
radius of
coverage
M
S
P
S
P
M Master device
S Slave device
P Parked device (inactive)
IEEE 802.11 Wireless LAN

802.11b
– 2.4-5 GHz unlicensed radio
spectrum
– up to 11 Mbps
– direct sequence spread
spectrum (DSSS) in
physical layer
• all hosts use the same
chipping code
– widely deployed, using base
stations

802.11a
– 5-6 GHz range
– up to 54 Mbps
– Orthogonal frequency
division multiplexing(OFDM)
 802.11g
– 2.4-5 GHz range
– up to 54 Mbps
– Orthogonal frequency
division multiplexing(OFDM)
 All use CSMA/CA for multiple
access
 All have base-station and adhoc network versions
Wi-Fi with directional antennas
Wi-Fi with directional antennas

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Increase range of 802.11
Fixed access/ Last mile usage-802.11 with high speed antennas
802.11g is often selected
-speed
-ability to handle interference(OFDM)
-interoperability with 802.11b-based devices
Limitation
-efficiency of the network decreases as the number of users on 802.11
increases due to the overhead of managing additional subscriber
-CSMA/CA contributed to network traffic
802.11 Mesh network as MANs


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

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Interconnect 802.11x based nodes by wireless 802.11 links
802.11a standard commonly used in AP to AP links (performance and nonchanel overlapped with 802.11b/g)
Properties
– 2.4GHz or 5 GHz unlicensed spectrum
– up to 54 Mbps
– Portable access
Automatic learn and maintain dynamic path configuration
Small nodes act as a simple router
Connection is shared across nodes
Based on propriety solutions
– May provide VoIP and QoS
– Coverage range can be over 10km
– Performance up to 100Mbps
Better suited to blanket large areas with 802.11 access
Mesh network topology
802.11 Mesh network(Cont.)

Advantages over single hop and directional last-mile alternatives
– Robustness and resiliency
– The shorter transmission range limit interference allowing simultaneously,
spatially separated data flows

Benefits
– Lower costs to the operator due to product availability
– Balanced traffic
– Flexibility over wired installations can be achieved

Limitations
– A large subscriber base is needed to cover large areas
– Using omni-directional antennas produces noise into network
– Shared bandwidth: more users translate into less banwidth
– Latency: latency increases with every hop
– Lack of standardization leads to unavailability of QoS.
IEEE 802.16 standards

802.16.1
– 10-66GHz unlicensed band
– LOS
– Up to 134Mbps

802.16.2: minimizing interference between coexisting WMANs

802.16-2004 (replace 802.16a/REVd)
– 2.5GHz, 3.5GHz licensed bands
– 5.8GHz licensed exempt band
– NLOS
– up to 75 Mbps
– Fixed end point
– 3 to 5 miles; Maximum range 30 miles based on tower height, antenna
gain and transmit power.
IEEE 802.16 standards(Cont.)

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802.16e
– 2-6 GHz license band
– NLOS
– up to 15 Mbps
– Mobility, regional roaming
– Support mobile user traveling at speeds up to 95 miles/hr
– 1 to 3 miles
Interoperability
Built in QoS
High performance
Smart antennas
Intelligent APs to monitor traffic
Point to Multipoint Wireless MAN

Base Station(BS) connected to public networks
 BS serves Subscriber Stations(SSs)
– SS typically serves a building(business or residence)
– Provide SS with first-mile access to public networks

Multiple services with different QoS
 Compare to a wireless LAN
–
–
–
–
Many more users
Multimedia QoS
Longer distance
Higher data rate
WIMAX network topology(fixed endpoints)
WIMAX backhaul for a Wi-Fi mesh topology
WIMAX as an intra mesh backhaul option
WIMAX as a client connection option
IEEE 802.16 standards
Physical layer characteristics


Line of sight(LOS)- because of 10-66GHz
Broadband chanels
– Wide channels(20,25 or 28 MHz)
– High capacity(down and up links)

Multiple Access
– TDM/TDMA
– High rate burst modems

Adaptive burst profile on both uplink and downlink
 Multiple duplex schemes
– Time division Duplex (TDD)
– Frequency division duplex (FDD)-including burst FDD
• Support for half duplex terminals

Adaptive modulation
– QPSK, QAM16, QAM64
Adaptive Modulation

Allow a wireless system to choose the higher modulation depending on the
channel conditions
– Lower modulation(QPSK) for higher range
– Higher modulation(QAM) for lower range(increase throughput)
Baud Rate and Channel Size(10-66 GHz)

Flexible plan--allowing manufacturers to choose according to
spectrum requirements
Channel
Width
(MHz)
20
25
28
QPSK
Bit Rate
16-QAM
Bit Rate
64-QAM
Bit Rate
(Mbits/s)
32
40
44.8
(Mbits/s)
64
80
89.6
(Mbits/s)
96
120
134.4
Adaptive Burst profile

Burst profile
– Modulation
– Reed Solomon FEC(forward error correction)
• to recover error frame lost due to frequency selective fading or
burst error
• Automatic repeat request (ARQ) is used to correct errors that can
not be corrected by FEC

Dynamically assigned according to link conditions
– Burst by burst, per subscriber station
– Trade-off capacity vs robustness in real time

Roughly double capacity for the same cell area
 Burst profile for downlink channel is well known and robust
– Up to 12 burst profiles can be defined
– The parameters of each are communicated to the SSs via MAC
messages during the frame control section of the downlink frame
Duplex scheme
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The downlink channel is time division multiplex(TDM)
– Information for each SS multiplexed onto a single stream of data and
received by all SSs within the same sector
The uplink is time division multiple access(TDMA)
– Channel is divided into a number of time slots which are assigned various
uses(registration, user traffic)
Frequency division duplex(FDD)
– DL and UL on the separate RF channel
– Support half-duplex SSs (SS does not transmit/receive simultaneously)
Time division duplex(TDD)
– DL and UL time-shared the same RF channel
– SS does not transmit/receive simultaneously
TDD Frame(10-66GHz)
Frame duration: .5ms, 1ms, 2ms
Physical slot(PS) = 4 QAM symbols(1QAM symbol = 4bits)
TDD downlink subframe

DIUC: Downlink interval
usage code
 Tr/Rx: gap between the
downlink burst and
subsequent uplink bust
–
Allows time for the BS to switch
from transmit to receive mode
and SSs to switch from receive
to transmit mode
FDD framing
Example of FDD bandwidth allocation
FDD downlink subframe
TDMA portion: transmit data to some half-duplex SSs(the ones
scheduled to transmit earlier in the frame than they receive)
-Need preamble to re-sync(carrier phase)
Uplink subframe
Uplink subframe descriptions

Initial maintenance opportunities
– Ranging
– To determine network delay or to request power or profile
change
– Collisions may occur in this interval
 Request contention opps
– SSs request bandwith in response to polling from BS.
– Collisions may occur in this interval
 Schedule data
– SSs transmit data bursts in the intervals granted by the BS
– Transition gaps between data intervals for synchronization
purposes.
MAC Layer


Designed for Point-to-multipoint broadband wireless access
apps
Support difficult user environments
– High bandwidth, hundreds of user per channel
– Continuous and burst traffic
– Very efficient use of spectrum

Protocol independent core
– ATM, IP, Ethernet,…

Flexible QoS offerings
– Best Effort(BF), rt-VBR,nrt-VBR, ATM CBR
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Security
Support PHY alternatives
– Adaptive mod, TDD/FDD, single-carrier, OFDM/OFDMA
Service-specific convergence sublayers
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ATM convergent sublayer defined for ATM services
Packet convergent sublayer
– Defined for mapping services such as IPv4, IPv6, Ethernet
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Preserve or enable QoS
Enable bandwidth allocation
Classify service data units(SDUs) to the proper MAC
connection
MAC addressing

SS has 48bits IEEE MAC address
– Use mainly as equipment id

16-bit Connection ID(CID)
– Used in MAC PDUs
MAC PDU format
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The Generic MAC header has fixed format
One or more MAC sub-headers may be part of the payload
The presence of sub-headers is indicated by a Type field in the
Generic MAC header
Generic MAC header
LEN: PDU length in bytes(2048 max)
CID: Connection ID
EKS: Encryption Key Sequence
HT: header Type
EC: Encryption Control
CI: CRC indicator
Type: subheader, …
HCS: Header Check
Sequence
MAC PDU Transmission
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MAC PDUs are transmitted in PHY burst
A single burst can contain multiple Concatenated MAC PDUs
The PHY burst can contain multiple FEC blocks
MAC PDUs may span FEC block boundaries
The TC(Transmission convergence) layer between the MAC and
PHY allows for capturing the start of the next MAC PDU in case
of erroneous FEC blocks
Downlink Transmissions
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Two kinds of bursts: TDM and TDMA
TDMA bursts have resync preamble
Each terminal listens to all bursts at its operational IUC or a
more robust one
Each burst may contain data for several terminals
SS must recognize the PDUs with known CIDs
DL-MAP message signals downlink usage
Burst profiles
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Each burst profile has mandatory exit threshold and minimum
entry threshold
SS allowed to request a less robust DIUC once above the
minimum entry level
SS must request fall back to more robust DIUC once at
mandatory exit threshold
Requests to change DIUC done with Downlink burst profile
change REQ(DBPC-REQ) or RNG-REG messages
Transition to more robust burst profile
Transition to less robust burst profile
Uplink Transmissions

Transmissions in contention slots
– Bandwidth requests
– Contention resolved using truncated exponential backoff

Transmissions in initial ranging slots
– Ranging requests(RNG-REQ)
– Contention resolved using truncated exponential backoff
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Bursts defined by UIUCs
Transmissions allocated by the UL-MAP message
All transmissions have synchronization preamble
Uplink Services

Unsolicited Grant Services (UGS)
– Used for constant-bit-rate (CBR) service flows (SFs)

Best Effort (BE)
– For best-effort traffic

Real time Polling Services (rtPS)
– For rt-VBR SFs such as MEPEG video
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None Real time Polling Services (nrtPS)
– For nrt SFs with better than BE service such as bandwidth-intensive file
transfer
Request/Grant scheme

Bandwidth Requests are always per Connection
 Self Correcting
– No acknowledgement

Grants are either per Connection (GPC) or per SS (GPSS)
– Grants (given as durations) are carried in the UL-MAP messages
– SS needs to convert the time(durations) to amount of data using
information about the UIUC

Bandwidth Grant per Subscriber Station (GPSS)
–
–
–
–
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BS grants bandwidth to the SS
SS may re-distribute bandwidth among its connections
Suitable for many connections per terminal
Low overhead but requires intelligent SS
Bandwidth Grant per Connection (GPC)
– BS grants bandwidth to a connection
– Mostly suitable for few users per SS
– High overhead, but allows simpler SS
Bandwidth Requests

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Come from the Connection
Implicit requests (UGS)
– No actual messages, negotiated at connection setup

BW request messages
– Uses special BW request header
– Requests up to 32 KB with a single message
Maintaining QoS in GPSS
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BS sees the requests for each connection; based
on this, grants bandwidth to the SSs (maintaining
QoS and fairness)
SS scheduler maintains QoS among its
connections and is responsible to share the BW
among the connections (maintaining QoS and
fairness)
Algorithm in BS and SS can be very different
SS Initialization

Channel Acquisition
– Scan frequency list to find an operation channel
– Establish synchronization with the BS
– Obtains the modulation and FEC schemes used on the carrier
via Uplink Channel Description (UDC)

Perform ranging and Capabilities Negotiation
– SS send a RNG_REQ in the ranging window
– BS measures arrival time and signal power; calculates timing
advance and power adjustment
– BS send adjustment in RNG-RSP
– SS adjusts timing advance and power; sends new RNG-REQ
– Continue until power and timing is ok

Authorize SS and perform key exchange
SS Initialization(Cont.)

Perform registration
– SS send a list of capabilities and parts of the configuration file to the BS
in the REG-REG message
– BS replies with the REG-RSP message(indicates with capabilities are
supported/allowed
– SS acknowledges the REG-RSP with REG-ACK message
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Establish ID connectivity (via DHCP)
Set up connections
– BS passes Service Flow Encodings to the SS in multiple Dynamic
Service Addition Request (DSA-REQ) messages
– SS replies with DSA-RSP messages
– Service Flow Encodings contain either
• Full definition of service attributes
• Service class name (ASCII string which is known at the BS and
which indirectly specifies a set of QoS parameters such as jitter and
latency)
SS Authentication and Registration
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Trust relation assumed between equipment manufacturer and
network operator
Each SS contains both the manufacturer’s X.509 certificate and the
the manufacturer’s certificate.
SS sent both certificates to the BS in the Authorization Request and
Authentication Information messages
BS verifies the identity of the SS by checking the certificates and
level of authentication of the SS
BS response with an Authorization Reply containing the
Authorization key (AK) encrypted with the SS’s public key if the SS
is authorized to join the network
The SS registers with the network upon successful authorization
Privacy and Encryption
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Secures over-the-air transmissions
Protocol based on Privacy Key Management (PKM) from
DOCSIS(Data over Cable Service Interface Specification)
Designed to allow new/multiple encryption algorithms
Data encryption
– Currently 56-DES (Data Encryption Standards) in CBC (cipher block
chaining) mode
– Initialization Vector (IV) based on frame number

Authentication
– X.509 certificates with RSA public key encryption
– Strong authentication of SSs (prevents theft of service)
– Prevents cloning

Message authentication
– Most important MAC management messages authenticated with one-way
hashing using Hashed Message Authentication Code(HMAC) with SHA-1
Security Associations

A set of privacy information
– Shared by a BS and one or more of its client SSs in order to support
secured communications
– Includes Traffic Encryption Keys (TEKs) and CBC IVs

Security Association Establishment
– Primary SA established during initial registration
– Other SAs may be provisioned or dynamically created within the BS
IEEE 802.20 Wireless WANs( proposed)

Similar to 802.16e, 3G
– Mobility, regional roaming

Differences
– < 3.5 GHz
– Cell ranges up to 8 miles
– Support mobile user
traveling at speeds up to
155miles/hr
References
1.
IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface
for Broadband Wireless Access
2.
IEEE 802.16-2001, “IEEE Standard for local and Metropolitan Area Networks—
Part 16: Air Interface for Fixed Broadband Wireless Access Systems”
3.
WiMAX: The Critical Wireless Standard, Carolyn Gabriel
4.
Understanding Wi-Fi and WiMAX as Metro-Access Solutions,
5.
802.16: A Look Under the Hood by Beth Cohen and Debbie Deutsch (www.wifiplanet.com)
6.
WiMAX Anticlimax by Andy Dornan (www.networkmagazine.com)