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WiMAX for Broadband Wireless Access
By:Karim M. El DefrawyICSUCI-2005
Outline
What is WiMAX 802.16 Introduction 802.16 MAC Highlights 802.16 Reference Model MAC Convergence Sub-Layer (CS) MAC Common Part Sub-Layer (CPS) MAC Privacy Sub-Layer (PS) Questions
What is WiMAX?
Worldwide Interoperability for Microwave Access (WiMAX) is the common name associated to the IEEE 802.16a/REVd/e standards.
These standards are issued by the IEEE 802.16 subgroup that originally covered the Wireless Local Loop technologies with radio spectrum from 10 to 66 GHz.
IEEE 802.16 -- Introduction
IEEE 802.16 (2001) Air Interface for Fixed Broadband Wireless Access System MAC and
PHY Specifications for 10 – 66 GHZ (LoS) One PHY: Single Carrier Connection-oriented, TDM/TDMA MAC, QoS, Privacy
IEEE 802.16a (January 2003) Amendment to 802.16, MAC Modifications and Additional PHY
Specifications for 2 – 11 GHz (NLoS) Three PHYs: OFDM, OFDMA, Single Carrier Additional MAC functions: OFDM and OFDMA PHY support, Mesh
topology support, ARQ
IEEE 802.16d (July 2004) Combines both IEEE 802.16 and 802.16a Some modifications to the MAC and PHY
IEEE 802.16e (2005?) Amendment to 802.16-2004 MAC Modifications for limited mobility
IEEE 802.16 -- Introduction
Coverage range up to 50km and speeds up to 70Mbps(shared among users).
IEEE 802.16 -- Introduction
Source: WiMAX, making ubiquitous high-speed data services a reality, White Paper, Alcatel.
IEEE 802.16 MAC -- Highlights
WirelessMAN: Point-to-Multipoint and optional mesh topology
Connection-oriented Multiple Access: DL TDM & TDMA, UL TDMA;UL OFDMA
& TDMA, DL OFDMA & TDMA (Optional) PHY considerations that affect the MAC
Duplex: TDD, FDD, FDX FDD BS and SS, HDX FDD SS Adaptive burst profiles (Modulation and FEC) on both DL
and UL Protocol-independent core (ATM, IP, Ethernet) Flexible QoS offering (CBR, rt-VBR, nrt-VBR, BE) Strong security support
Reference Model
Adaptive PHY
Source: Understanding WiMAX and 3G for Portable/Mobile Broadband Wireless, Technical White Paper, Intel.
Adaptive Burst Profiles
Burst profile: Modulation and FEC On DL, multiple SSs can associate the
same DL burst On UL, SS transmits in an given time slot
with a specific burst Dynamically assigned according to link
conditions Burst by burst Trade-off capacity vs. robustness in real time
Duplex Scheme Support
The duplex scheme is Usually specified by regulatory bodies, e.g., FCC
Time-Division Duplex (TDD) Downlink & Uplink time share the same RF channel Dynamic asymmetry does not transmit & receive simultaneously (low
cost) Frequency-Division Duplex (FDD)
Downlink & Uplink on separate RF channels Full Duplexing (FDX): can Tx and Rx simultaneously; Half-duplexing (HDX) SSs supported (low cost)
IEEE 802.16 MAC – OFDM PHY TDD Frame Structure
DL Subframe
Frame n-1
pre.
Time
Adaptive
Frame n Frame n+1
UL subframe
FCHDL
burst 1DL
burst n
ULMAP
Broadcast Conrol msgs
...UL burst 1 UL burst m
DLMAP
DCDopt.
UCDopt.
...DLburst 2
UL TDMADL TDM
pre. pre.
IEEE 802.16 MAC – OFDM PHY FDD Frame Structure
DL Subframe
Frame n-1
pre.
Time
BroadcastControl Msgs
Frame n Frame n+1
UL subframe
FCHDL
burst 1DL
burst k...
DL TDMA
UL burst 1 UL burst m
DLburst 2
DLburst n
DLburst k+1
...
DL TDM
...
UL TDMA
DLMAP
ULMAP
DCDopt.
UCDopt.
pre.pre.
UL MAP for nextMAC frame UL
burstspre. pre.
FDD MAPs Time Relevance
frame
Broadcast
Full Duplex Capable User
Half Duplex Terminal #1
Half Duplex Terminal #2
UPLINK
DOWNLINK
DL MAP
UL MAP
DL MAP
UL MAP
IEEE 802.16 MAC addressing and Identifiers
SS has 48-bit IEEE MAC address BS has 48-bit base station ID
Not a MAC address 24-bit operator indicator
16-bit connection ID (CID) 32-bit service flow ID (SFID) 16-bit security association ID (SAID)
IEEE 802.16 MAC – Convergence Sub-Layer (CS)
ATM Convergence Sub-Layer: Support for VP/VC switched connections Support for end-to-end signaling of dynamically
created connections ATM header suppression Full QoS support
Packet Convergence Sub-Layer: Initial support for Ethernet, VLAN, IPv4, and
IPv6 Payload header suppression Full QoS support
IEEE 802.16 MAC -- CS– Packet Convergence Sub-Layer
Functions: Classification: mapping the higher layer PDUs
(Protocol Data Units) into appropriate MAC connections
Payload header suppression (optional) MAC SDU (Service Data Unit), i.e, CS PDU,
formatting
Packet PDU(e.g., IP packet, Ethernet Packet)
PHSI
MAC SDU = CS PDU
Payload Header Suppression IndexOptional, Depending on upper layerprotocol
IEEE 802.16 MAC -- CPS– MAC PDU Format
CRC(optional)MAC PDU payload (optional)
Generic MACHeader
(6 bytes)
LENmsb(3)
HT
CID msb (8)LEN lsb (8)
Generic MAC Header Format(Header Type (HT) = 0)
BW Req. Header Format(Header Type (HT) =1)
msb lsb
EC
Type (6 bits)rsv
CI
EKS(2)
rsv
HCS (8)CID lsb (8)
BW Req.msb (8)
HT
CID msb (8)BWS Req. lsb (8)
EC
Type (6 bits)
HCS (8)CID lsb (8)
IEEE 802.16 MAC -- CPS-- Three Types of MAC PDUs
Data MAC PDUs HT = 0 Payloads are MAC SDUs/segments, i.e., data
from upper layer (CS PDUs) Transmitted on data connections
Management MAC PDUs HT =0 Payloads are MAC management messages or IP
packets encapsulated in MAC CS PDUs Transmitted on management connections
BW Req. MAC PDUs HT =1; and no payload, i.e., just a Header
IEEE 802.16 MAC -- CPS– Data Packet Encapsulations
PHSI
MAC PDU
Ethernet Packet
Ethernet Packet
Packet PDU(e.g., Ethernet)
CS PDU(i.e., MAC SDU)
HT
FEC block 1
CRCMAC PDU Payload
OFDMsymbol
1
PHY Burst(e.g., TDMA burst)
PreambleOFDMsymbol
2
OFDMsymbol
n
......
FECFEC Block 2 FEC block m
......FEC Block 3
IEEE 802.16 MAC – CPS-- MAC Management Connections
Each SS has 3 management connections in each direction: Basic Connection:
short and time-urgent MAC management messages MAC mgmt messages as MAC PDU payloads
Primary Management connection: longer and more delay tolerant MAC mgmt
messages MAC mgmt messages as MAC PDU payloads
Secondary Management Connection: Standard based mgmt messages, e.g., DHCP,
SNMP, …etc IP packets based CS PDU as MAC PDU payload
IEEE 802.16 MAC – CPS – MAC Management Messages
MAC mgmt message format:
MAC mgmt msg payloadmgmtmsgHD
8 bits
• MAC mgmt msg can be sent on: Basic connections; Primary mgmt connection; Broadcast connection; and initial ranging connections
• 41 MAC mgmt msgs specified in 802.16• The TLV (type/length/value) encoding scheme is used in MAC
mgmt msg, e.g., in UCD msg for UL burst profiles,
(type=1, length=1, value=1) QPSK modulation
(type=1, length=1, value=2) 16QAM modulation
(type=1, length=1, value=3) 64QAM modulation
IEEE 802.16 MAC – CPS – MAC PDU Transmission
MAC PDUs are transmitted in PHY Bursts The PHY burst can contain multiple FEC
blocks MAC PDUs may span FEC block
boundaries Concatenation Packing Segmentation Sub-headers
IEEE 802.16 MAC – CPS – MAC PDU Concatenation
MAC PDU 2
HT
FEC block 1
CRCMAC PDU Payload
OFDMsymbol
1
PHY Burst(e.g., TDMA burst)
PreambleOFDMsymbol
2
OFDMsymbol
n
......
FECFEC Block 2 FEC block m
......FEC Block 3
MAC PDU 1
HT CRCMAC PDU Payload ......
MAC PDU k
HT CRCMAC PDUPayload
Multiple MAC PDUs are concatenated into the same PHY burst
IEEE 802.16 MAC – CPS – MAC PDU Fragmentation
FEC block1
OFDMsymbol
1
PHY Burst
Pre.
MAC SDU
OFDMsymbol
n1
......
FEC FEC Blockm1
......
MAC SDUseg-1
HT CRCMAC PDU PayloadHT CRC
MAC PDUPayload
A MAC SDU can be fragmented into multiple segments, eachsegment is encapsulated into one MAC PDU
FEC block1
OFDMsymbol
1
PHY Burst
Pre.OFDMsymbol
n2
......
FEC Blockm2
......
HT CRCMAC PDUPayload
MAC SDUseg-2
MAC SDUseg-3
FSH
FSH
FragmentationSub-Header
(8 bits)
FSH
IEEE 802.16 MAC – CPS – MAC PDU Packing
MACSDU 1
Fixed size MSDUs, e.g., ATMCells, on the same connection
HT CRCMAC PDU Payload
HT CRC
Packing with fixed size MAC SDUs (no packing sub-header is needed)
......
PSH
MACSDU 2
MACSDU k
Packing with variable size MAC SDUs (Packing Sub-Heade is neeeded)
PSH ...... PSH
MAC SDU orseg. 1 MAC SDU or seg 2
MAC SDU orseg n
Variable sizeMSDUs or MSDUsegments, e.g.,IP packets, on
the sameconnection
PackingSub-Heder
(16 bits)
IEEE 802.16 MAC – CPS QoS
Three components of 802.16 QoS Service flow QoS scheduling Dynamic service establishment Two-phase activation model (admit first, then activate)
Service Flow A unidirectional MAC-layer transport service characterized
by a set of QoS parameters, e.g., latency, jitter, and throughput assurances
Identified by a 32-bit SFID (Service Flow ID) Three types of service flows
Provisioned: controlled by network management system Admitted: the required resources reserved by BS, but not
active Active: the required resources committed by the BS
IEEE 802.16 MAC – CPS – Uplink Service Classes
UGS: Unsolicited Grant Services rtPS: Real-time Polling Services nrtPS: Non-real-time Polling
Services BE: Best Effort
IEEE 802.16 MAC – CPS – Uplink Services: UGS
UGS: Unsolicited Grant ServicesFor CBR or CBR-like services, e.g.,
T1/E1.The BS scheduler offers fixed size
UL BW grants on a real-time periodic basis.
The SS does not need to send any explicit UL BW req.
IEEE 802.16 MAC – CPS – Uplink Services: rtPS
rtPS: Real-time Polling Services For rt-VBR-like services, e.g., MPEG
video. The BS scheduler offers real-time,
periodic, UL BW request opportunities. The SS uses the offered UL BW req.
opportunity to specify the desired UL BW grant.
The SS cannot use contention-based BW req.
IEEE 802.16 MAC – CPS – Uplink Services: nrtPS
nrtPS: non-real-time polling services For nrt-VBR-like services, such as,
bandwidth-intensive file transfer. The BS scheduler shall provide timely
(on a order of a second or less) UL BW request opportunities.
The SS can use contention-based BW req. opportunities to send BW req.
IEEE 802.16 MAC – CPS – Uplink Services: BE
BE: Best EffortFor best-effort traffic, e.g., HTTP,
SMTP.The SS uses the contention-based
BW request opportunities.
IEEE 802.16 MAC – CPS – Bandwidth Grant
BW grants are per Subscriber Station: Allows real-time reaction to QoS need, i.e., SS may re-
distribute bandwidth among its connections, maintaining QoS and service-level agreements
Lower overhead, i.e., less UL-MAP entries compare to grant per connection
Off- loading base station’s work Requires intelligent subscriber station to redistribute the
allocated BW among connections
IEEE 802.16 MAC – CPS – BW Request/Grant Mechanisms
Implicit requests (UGS): No actual requests BW request messages, i.e., BW req. header
Sends in either a contention-based BW req. slot or a regular UL allocation for the SS;he special B
Requests up to 32 KB with a single message Request Incremental or aggregate, as indicated by MAC header–
Piggybacked request (for non-UGS services only) Presented in Grant Management (GM) sub-header in a data
MAC PDU of the same UL connection is always incremental Up to 32 KB per request for the CID
Poll-Me bit Presented in the GM sub-header on a UGS connection request a bandwidth req. opportunity for non-UGS services
IEEE 802.16 MAC – CPS -- Contention UL Access
Two types of Contention based UL slots Initial Ranging
Used for new SS to join the system Requires a long preamble
BW Request Used for sending BW req Short preamble
Collision Detection and Resolution Detection: SS does not get the expected response in
a given time Resolution: a truncated binary exponential backoff
window
IEEE 802.16 MAC – CPS UL Sub-Frame Structure
Source: http://www.cygnuscom.com/pdf/WP_PN_Article.pdf
IEEE 802.16 MAC – CPS – Ranging
Ranging is a process of acquiring the correct timing offset, and PHY parameters, such as, Tx power level, frequency offset, etc. so that the SS can communicate with the BS correctly.
BS performs measurements and feedback.
SS performs necessary adjustments. Two types of Ranging:
Initial ranging: for a new SS to join the system Periodic ranging (also called maintenance
ranging): dynamically maintain a good RF link.
IEEE 802.16 MAC – CPS – Automatic Repeat reQuest (ARQ)
A Layer-2 sliding-window based flow control mechanism.
Per connection basis. Only effective to non-real-time applications. Uses a 11-bit sequence number field. Uses CRC-32 checksum of MAC PDU to check
data errors. Maintain the same fragmentation structure for
Retransmission. Optional.
IEEE 802.16 MAC – Privacy Sub-layer (PS)
Two Major Functions: Secures over-the-air transmissions Protects from theft of service
Two component protocols: Data encryption protocol A client/server model based Key
management protocol (Privacy Key Management, or PKM)
IEEE 802.16 MAC – PS -- Security Associations
A set of privacy information, e.g., encryption keys, used encryption algorithm
Three types of Security Associations (SAs) Primary SA: established during initial registration Static SA: provisioned within the BS Dynamic SA: dynamically created on the fly
Identified by a 16-bit SAID Connections are mapped to SAs
IEEE 802.16 MAC – PS -- Multi-level Keys and Their Usage
Public Key Contained in X.509 digital certificate Issued by SS manufacturers Used to encrypt AK
Authorization Key (AK) Provided by BS to SS at authorization Used to derive KEK
Key Encryption Key (KEK) Derived from AK Used to encrypt TEK
Traffic Encryption Key (TEK) Provided by BS to SS at key exchange Used to encrypt traffic data payload
IEEE 802.16 MAC – PS -- Data Encryption
Use DES (Data Encryption Standard) in CBC (Cipher Block Chaining) mode with IV (Initialization Vector).
CBC IV is calculated from IV parameter in TEK keying info; and PHY synchronization field in DL-MAP.
Only MAC PDU payload (including sub-headers) is encrypted.
MAC PDU headers are unencrypted. Management messages are
unencrypted.
IEEE 802.16 MAC – one big item is out of scope
Scheduler
Questions ??
References
IEEE802.16-2004 Alcatel White Paper: WiMAX, making
ubiquitous high-speed data services a reality
Intel White Paper: Understanding WiMAX and 3G for Portable/Mobile Broadband Wireless
WiMAX Forum: www.wimaxforum.com http://en.wikipedia.org/wiki/WiMax
IEEE 802.16 MAC – commonly used terms
BS – Base Station SS – Subscriber Station, (i.e., CPE) DL – Downlink, i.e. from BS to SS UL – Uplink, i.e. from SS to BS FDD – Frequency Division Duplex TDD – Time Division Duplex TDMA – Time Division Multiple Access TDM – Time Division Multiplexing OFDM – Orthogonal Frequency Division
Multiplexing OFDMA - Orthogonal Frequency Division Multiple
Access QoS – Quality of Service
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