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WiMAX for Broadband Wireless Access. By: Karim M. El Defrawy ICS UCI-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?.

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wimax for broadband wireless access

WiMAX for Broadband Wireless Access


Karim M. El Defrawy



  • 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
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 -- 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 introduction5
IEEE 802.16 -- Introduction

Coverage range up to 50km and speeds up to 70Mbps(shared among users).

ieee 802 16 introduction6
IEEE 802.16 -- Introduction

Source: WiMAX, making ubiquitous high-speed data services a reality, White Paper, Alcatel.

ieee 802 16 mac highlights
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
adaptive phy
Adaptive PHY

Source: Understanding WiMAX and 3G for Portable/Mobile Broadband Wireless, Technical White Paper, Intel.

adaptive burst profiles
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
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)
fdd maps time relevance
FDD MAPs Time Relevance









Half Duplex T

erminal #1

Full Duplex Capable User

Half Duplex T

erminal #2

ieee 802 16 mac addressing and identifiers
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
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
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
ieee 802 16 mac cps three types of mac pdus
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 mac management connections
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
IEEE 802.16 MAC – CPS – MAC Management Messages
  • MAC mgmt message format:
  • 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
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 qos
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
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
IEEE 802.16 MAC – CPS – Uplink Services: UGS
  • UGS: Unsolicited Grant Services
    • For 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
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
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
IEEE 802.16 MAC – CPS – Uplink Services: BE
  • BE: Best Effort
    • For best-effort traffic, e.g., HTTP, SMTP.
    • The SS uses the contention-based BW request opportunities.
ieee 802 16 mac cps bandwidth grant
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
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
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
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
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
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
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
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
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
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.
  • 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
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