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Basics, Network Entry Procedures, and Bandwidth Request/Grand Mechanism for IEEE Std. 802.16 Chen-Nien Tsai Institute of Computer Science and Information Engineering National Taipei University of Technology 2007.10.8 Outline ►A Brief Introduction to IEEE Std. 802.16. ► Overview of IEEE 802.16 ► MAC/PHY Basics ► Network Entry and Initialization ► Bandwidth Request/Grand Mechanism ► Summary 2 Introduction to IEEE Std. 802.16 ► The central aim of IEEE 802.16 technology is to support broadband access. Providing service at a rate of at least 1.544 Mbps. (ITU definition) ► Broadband Wireless Access (BWA) Broadband extension of the wireless access concept. ► Wireless Broadband Access A wireless implementation of broadband access concepts. 3 Introduction to IEEE Std. 802.16 ► IEEE Std. 802.16 is called the wirelessMAN® standard for wireless metropolitan area networks. Supports networks that are about the size of a city. Not limited to urban applications. Some of the most likely applications are in rural areas. ►Replace last-mile. 4 Wireless Technologies Bandwidth 1 Gbps IEEE 802.15 100 Mbps 10 Mbps 1 Mbps IEEE 802.11 IEEE 802.16 3GPP/3GPP2/ETSI 802.11n 802.15.3 High rate WPAN 802.15.1 Bluetooth <1m PAN Wi-Fi 802.11a/g WiMAX 802.16 3G Wi-Fi 802.11b 10m 4G 2.5G 100m LAN PAN: Personal area network LAN: Local area network Up to 50km MAN Up to 80km WAN MAN: Metropolitan area network WAN: Wide area network 5 IEEE 802.16 Project Timeline 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 IEEE Std 802.16-2001 IEEE Std 802.16a IEEE Std 802.16c IEEE Std 802.16-2004 IEEE Std 802.16-2004/Cor1 IEEE P802.16-2004/Cor2 IEEE P802.16Rev2 IEEE Std 802.16e (mobile) IEEE Std 802.16f (MIB) IEEE P802.16g (management) IEEE P802.16h (coexistence mechanism) IEEE P802.16i (management) IEEE P802.16j (multihop relay) IEEE Std 802.16k-2007 IEEE P802.16m IEEE Std 802.16/Conf01 IEEE Std 802.16/Conf02 IEEE Std 802.16/Conf03 IEEE Std 802.16/Conf04 IEEE Std 802.16.2-2001 6 IEEE Std 802.16.2-2004 Completed In progress Superseded Standards IEEE Standard Styles ► Amendment contains new material to be incorporated into an existing IEEE standard. Designated by a lowercase letter after the primary standard number. 802.16c, 802.16a. (amendment to 802.16-2001) ► Corrigendum allows corrections but prohibits new features. 802.16-2004/Cor1, 802.16-2004/Cor2. 7 IEEE Standard Styles ► Revision Base standard and its published amendments are editorially merged. 802.16-2004 (802.16-REVd), including 802.162001, 802.16c, 802.16a. 802.16Rev2 (under development) 8 Overview of IEEE 802.16 IEEE 802.16 ► Scope: Specifies the air interface of fixed BWA systems. ►Including the medium access control (MAC) layer and multiple physical (PHY) layer specifications. ► Purpose: Enables rapid worldwide deployment of costeffective BWA products. Facilitates competition in broadband access by providing alternatives to wireline broadband access. 10 Basic Network Architecture Wireless link Wired link Core network Users Base Station (BS) SS Subscribe Station (SS) SS 11 BS and SS ► Base station (BS) A generalized equipment set providing connectivity, management, and control of the SS. ► Subscribed station (SS) A generalized equipment set providing connectivity between subscriber equipment and a BS. 12 Typical Deployment Scenarios Mesh Node 13 Reference Model 14 Service-Specific Convergence Sublayer ► Functions Classification. Header suppression. ► Two CS specified ATM CS. Packet CS. 15 MAC Common Part Sublayer ► Functions System access. Bandwidth allocation. Call admission Connection management. ► Two operation modes Point-to-multipoint (PMP) Mesh 16 Security Sublayer ► Functions Authentication Secure key exchange Encryption 17 Physical Layer ► Four PHY specified WirelessMAN-SC PHY WirelessMAN-SCa PHY WirelessMAN-OFDM PHY WirelessMAN-OFDMA PHY 18 Physical Layer ► For 10-66 GHz (IEEE 802.16-2001) WirelessMAN-SC PHY ►Single-carrier ► For modulation. 2-11 GHz (IEEE 802.16a) WirelessMAN-SCa PHY ►Single-carrier modulation 19 Physical Layer ► For 2-11 GHz (IEEE 802.16a) WirelessMAN-OFDM PHY ►256-carrier OFDM (orthogonal-frequency division multiplexing) ►Multiple access is provided through TDMA (timedivision multiple access). WirelessMAN-OFDMA PHY ►2048-carrier OFDM. ►Multiple access is provided by assigning a subset of the carriers to an individual receiver. 20 MAC/PHY Basics MAC Support of PHY ► Several duplexing techniques are supported by the MAC. Time division duplexing (TDD) ►UL and DL transmission occur at different times and usually share the same frequency. Frequency division duplexing (FDD) ►UL and DL channels are located on separate frequencies. ►Full duplex ►Half duplex ► FDD or TDD? 22 Framing ► Each frame has a DL subframe and UL subframe. DL subframe begins with information necessary for frame synchronization and control. In the TDD case, the DL subframe comes first, followed by the UL subframe. In the FDD case, UL transmission occur concurrently with the DL frame. 23 TDD Frame Structure PS (Physical slot): a unit of time for allocating bandwidth. Rate: symbol rate for SC and SCa PHY, nominal sampling frequency for OFDM and OFDMA PHY. 24 FDD Bandwidth Allocation 25 TDD Downlink Subframe Burst profile for DIUC = 0 is well-known Downlink Interval Usage Code is a code identifying a particular burst profile synchronization BW allocation and other channel information for DL/UL 26 Downlink transmission ► Preamble Synchronization and equalization. ► Frame control DL-MAP ►How and when the DL data are transmitted. UL-MAP ►How and when the UL data are transmitted. DCD/UCD ►Channel description for UL/DL 27 TDD Uplink Subframe 28 Uplink Transmission ► Three classes of bursts may be transmitted in a UL subframe: Contention opportunities for initial ranging. Contention opportunities for BW requests. Contention-free periods assigned by BS to individual SSs. 29 Connections and Addressing ► Each SS has a unique 48-bit MAC address. It is used only during the initial ranging process or authentication process. Not carried in every MPDU. How to identify src. and dest.? CID ► 802.16 MAC is connection-oriented. Connection is a unidirectional mapping between BS and SS MAC peers. A connection identifier (CID) is a 16-bit value that identifies a connection. ►A maximum of 65535 connections are supported for 30 each DL and UL. Connection Types ► Basic connection Assigned to each SS after successful ranging. To transport delay-intolerant basic MAC messages. Identify the SS for managing per-SS functions. (BW grants in UL-MAP) ► Primary management connection Assigned to each SS after successful ranging. To transport delay-tolerant basic MAC messages. 31 Connection Types ► Secondary management connection Assigned to each “managed” SS during the registration process. To transport higher layer management messages (SNMP, TFTP, and DHCP). ► Transport connection Created and changed by Dynamic Service series messages (DSA, DSD, and DSC). To transport user data. 32 MAC Management Messages 33 Connection Identifiers ► Initial Ranging CID. ► Basic CID. ► Primary Management CID. ► Secondary Management CID. ► Transport CID. ► AAS Initial Ranging CID. ► Multicast Polling CID. ► Padding CID. ► Broadcast CID. 34 Well-known Addresses and Identifiers m is the maximum possible number of SSs that can be supported 35 MAC Headers ► Stand-alone MAC header 6 bytes. The smallest possible information unit that can be transported between two nodes. (with the exception of HARQ MAPs) None of the stand-alone headers can be used to encapsulate any payload. It is a misnomer to call them “headers.” BW request header and signaling header (defined in other std.) are stand-alone MAC headers. 36 BW Request Header Format 000: incremental BR 001: aggregate BR Bandwidth request in bytes Encryption Control Header Check Sequence 37 MAC Headers ► Generic MAC header Followed by the optional variable-size payload. Payload may consist of: ►MAC subheaders ►Management messages ►Special payload ►Padding 38 Generic MAC Header Format Subheader type CRC Indictor Encryption Key Sequence Header Check Sequence 39 MAC Subheaders ► For generic MAC header only Packing and Fragmentation subheaders are mutually exclusive. Type 5 4 3 2 1 0 Mesh subheader ARQ Feedback payload Extended Type Indicate whether the Packing or Fragmentation Subheaders is extended. Fragmentation subheader Packing subheader Downlink: FAST-FEEDBACK allocation subheader Uplink: Grant Management subheader 40 Network Entry and Initialization Once the SS has powered up, it begins the network entry and initialization process. After completing the steps of the process, the SS has all the addresses and parameters it need to communicate with the rest of the network. Network Entry and Initialization ► The procedures for entering and registering a new SS or a new node to the network. ► The procedures described here apply only to PMP mode. Hey, I want to join the network. BS New SS 42 Phases ► Scanning and synchronization to the DL ► Obtain transmit parameters ► Initial ranging ► SS basic capability negotiation ► SS authorization and key exchange ► Registration ► Establish IP connectivity ► Establish time of day Optional ► Transfer operational parameters ► Establish provisioned connections 43 Scanning and synchronization to the DL ► Achieve PHY synchronization Scan the possible channels of the downlink frequency band of operation until it finds a valid downlink signal. ► Then try to acquire the channel control parameters for the DL and the UL. How? 44 Obtain Transmit Parameters (1/4) ► Achieve MAC synchronization. The SS achieves MAC synchronization once it has received at least one DL-MAP message. ► Obtain downlink parameters Retrieve parameters from the DCD messages. DCD messages contain: ►Frame duration, TTG size, RTG size, downlink center frequency, BS ID, and more. ►Downlink burst profiles. 45 Obtain Transmit Parameters (2/4) ► SS want to know when BS broadcasts channel parameters. SS can know it from DL-MAP. Note that DL-MAP is encoded with well-known parameters. ► SS want to know the downlink channel parameters. SS can know it from Downlink Channel Descriptor (DCD) Message. 46 Obtain Transmit Parameters (3/4) ► Obtain uplink parameters Retrieve parameters from the UCD messages. UCD messages contain: ►Uplink center frequency, bandwidth request opportunity size, ranging request opportunity size, and other PHY specific parameters. ►Uplink burst profiles. ► Receive the UL-MAP So that SS can perform initial ranging. Initial ranging opportunities. 47 Obtain Transmit Parameters (4/4) ► Now SS know downlink channel parameters, then it want to know uplink channel parameters. SS can know it from Uplink Channel Descriptor (UCD) Message. ► The next question is when SS can send requests to perform following procedures. SS can know it from UL-MAP 48 Message Flows BS Send DL/UL-MAP Send UCD/DCD Wireless channel DL/UL-MAP Power on sequence complete DL/UL-MAP Send DL/UL-MAP DL/UL-MAP Send DL/UL-MAP Send UCD SS power on UCD/DCD Send DL/UL-MAP Send DCD SS DCD DL/UL-MAP UCD Establish PHY synchronization Wait for UCD Obtain parameters for UL channel Send DL/UL-MAP DL/UL-MAP Extract slot info for uplink Send DL/UL-MAP DL/UL-MAP Wait for transmission opportunity to perform ranging Start ranging process 49 Initial Ranging (1/3) ► What is ranging? Ranging is the process of acquiring the correct timing offset and power adjustments. RNG-REQ/RNG-RSP messages. ► Two types of ranging Initial ranging: allow SS to join the network. Periodic ranging: allow SS to adjust transmission parameters and maintain the quality of RF communication link. 50 Initial Ranging (2/3) ► Initial Ranging accomplishes the following: The time advance of SS transmissions is adjusted to make the SS appear collocated with the BS. The transmission power of the SS is adjusted for optimal reception at the BS. The SS is allocated its Basic and Primary Management CIDs. 51 Initial Ranging (3/3) ► If two SS send there RNG-REQ in the same slot (opportunity) Collision. Call for Contention Resolution Algorithm. Binary exponential backoff is specified in the spec. 52 Basic Capability Negotiation ► SS informs BS of its basic capabilities by transmitting an SBC-REQ message with its capabilities set to “on”. ► BS responds with an SBC-RSP message with the intersection of SS’s and BS’s capabilities set to “on”. ► Capabilities includes: Bandwidth allocation support, max. transmit power, current transmit power, modulation type support, and more. 53 Authorization and Key Exchange ► Perform authorization and key exchange procedures. ► Details are skipped. 54 Registration ► The process by which SS is allowed entry into the network. ► SS sends a REG-REQ message to BS. ► BS responds with a REG-RSP message. ► The SS is allocated its Secondary management CID if the SS is managed. ► Also negotiate the version of IP and the QoS parameters for the secondary management connection. 55 Establish IP connectivity ► SS and BS shall negotiate IP version during REG-REQ/RSP exchange if the SS is managed. ► After registration, SS shall invoke DHCP mechanisms in order to obtain an IP address and any other parameters needed to establish IP connectivity. 56 Establish Time of Day ► That the SS and BS have the current date and time is required for time-stamping logged events by the management system. ► The protocol by which the time of day shall be retrieved is defined in IETF RFC 868 (Time protocol). 57 Transfer Operation Parameters ► If the SS has a configuration file, the name is indicated in DHCP response. ► SS shall download the configuration file using TFTP (Trivial File Transfer Protocol). ► SS notify the BS by transmitting a TFTPCPLT message when the file download has completed successfully. ► BS responds a TFTP-RSP message. 58 Establish Provisioned Connections ► In the case of a managed SS: The reception of the TFTP-CPLT message triggers the BS to start connection setup. ► In the case of a unmanaged SS: The successful completion of registration serves as the trigger. ► Both are BS-initiated. ► After at least one service flow has been activated, the SS is capable of sending and receiving user data. 59 Dynamic Service Establishment ► BS-initiated Wireless channel DSA-RSQ SS DSA-RSP BS DSA-ACK 60 Dynamic Service Establishment ► SS-initialed DSA The standard does not go into details on what actually triggers the DSA. Triggering is just assumed to happen, stimulated by the upper layers when needed. Wireless channel DSA-RSP This allows BS to take it time determining whether to admin the service flow DSA-ACK 61 DSA-RSQ SS DSX-RVD BS Bandwidth Request/Grand Mechanism The BW request/grand mechanism for the IEEE 802.16 standard was chosen to be efficient, low-latency, and flexible. Requests ► The mechanism that SS use to indicate to the BS that they need uplink bandwidth allocation. ► Requests are made on a per-connection basis. ► Grants are made to the SS (Basic CID), not to the connection. ► No explicit acknowledgments of requests. 63 Requests (1/2) ► Contention-based bandwidth requests. Transmit during contention period. Broadcast polling. Multicast group polling. Focused contention transmission. (OFDM PHY) CDMA-based bandwidth requests. (OFDMA PHY) ► Contention-free bandwidth requests. Unicast polling. PM bit. 64 Requests (2/2) ► Bandwidth stealing SS uses a portion of allocated BW for a connection to send another BW requests rather than sending data. ► Piggyback Request The bandwidth request is piggybacked onto a MAC PDU on an existing connection with allocated BW. 65 Grants ► GPC mode Grand Per Connection mode. Only optionally allowed in IEEE Std 802.16-2001. No longer specified in IEEE Std 802.16-2004. ► GPSS mode Grand per Subscriber Station mode. Improves efficiency and latency. (smaller MAP) An addition scheduler is required to allocate the granted bandwidth in each SS. 66 The Problems ► The reality at the SS and the perception at the BS can get out of sync.: BS does not hear a BW request. SS does not hear the allocation in the MAP. BS scheduler decides it does not have BW right now for the particular service. SS used BW for a purpose different from that originally requested. (e.g., bandwidth stealing) 67 Aggregate Requests ► BW request/grant mechanism is designed to be self-correcting. ► After a period, if the SS still needs BW for a service, it simply asks again. SS issues an aggregate request. To avoid BS’s perception becoming further askew from reality by duplicate requests. An aggregate request tells BS that the current state of SS’s queue for that service, allowing BS to reset its perception of that service’s needs. 68 Incremental Requests ► There is a chance that a repeated aggregate request crosses the grant for that same bandwidth in the same frame. It can cause wasted allocations to the SS. It can be easily avoided by adding the concept of incremental requests. BS just add this BW requests to it current perception of the BW needs for that service. 69 Incremental or Aggregate? ► In general, the airlink should be reliable. ► Therefore: Most BW requests typically would be incremental. Only periodic aggregate requests to ensure BS does not deviate too far from reality. 70 Other BW Request Options. ► SI (Slip Indicator) bit. SS can set this bit, requesting BS to slightly increase the rate at which it automatically allocates BW to SS. (up to 1% additional BW) ► PM (Poll Me) bit. SS can set this bit, indicating it has a BW need on another connection. When BS sees the PM bit set, it knows the SS needs to make a BW request and may poll it immediately. 71 Usage Rules Service Type Polling Contention Requests UGS PM bit Not allowed rtPS Unicast Not allowed PiggyBack Requests Bandwidth Stealing Not allowed Not allowed Allowed Allowed nrtPS All Allowed Allowed Allowed BE All Allowed Allowed Allowed 72 Summary ► MAC/PHY Basics Frame structure Connections Types Header formats ► Network Entry Procedures After completing the procedures, SS can communicate with the network. ► Bandwidth Request/Grand Mechanism Contention-based bandwidth requests Contention-free bandwidth requests 73 Summary ► More about IEEE 802.16 QoS Scheduling Mesh mode PHY details 74 References [1] IEE Std 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access. [2] Carl Eklund et al., WirelessMAN: Inside the IEEE 802.16 Standard for Wireless Metropolitan Area Networks, IEEE Press, 2006. [3] http://www.ieee802.org/16/. 75 The End Backup Materials Duplexing ► Duplexing defines how bidirectional communication is achieved between two devices or between a BS and a set of client devices in a PMP system. Frequency Division Duplexing (FDD) Time Division Duplexing (TDD) ►Half-duplex: transmit or receive but not both simultaneously. ►Full-duplex: transmit and receive simultaneously. 78 Multiplexing ► Refers to a mechanism in which a single device transmits to multiple devices on a single channel. Frequency Division Multiplexing (FDM) ►The transmitting device divides the time domain into multiple slots to communicate with multiple devices. Time Division Multiplexing (TDM) ►The transmitting device uses different frequencies to communicate with multiple devices. Orthogonal FDM (OFDM)? 79 Multiple Access ► Refers to the way that multiple devices access the medium, regardless of whether the communication is many-to-one or manyto-many. Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) Orthogonal FDMA (OFDMA) Code Division Multiple Access (CDMA) 80 Message Formats 上窮碧落下黃泉 Downlink Channel Descriptor Message ► Define the characteristics of a DL physical channel. 82 DCD Channel Encoding (partial) 83 SC Downlink_Burst_Profile 84 DCD Burst Profile Encodings – SC (partial) 85 DIUC Allocation – SC 86 Uplink Channel Descriptor Message ► Define the characteristics of a UL physical channel. 87 UCD Channel Encoding (partial) 88 SC Uplink_Burst_Profile 89 UCD Burst Profile Encodings – SC (partial) 90 UIUC Allocation – SC 91 DL-MAP Message 92 SC DL-MAP IE 93 UL-MAP Message 94 SC UL-MAP IE 95 RNG-REQ and RNG-RSP 96 SBC/REQ and SBC-RSP 97 REG-REQ and REG-RSP 98 DSA-REQ and DSA-RSP 99 DSA-ACK 100 Service Flow Encodings 101 Grant Management Subheader 102 FDD or TDD? Advantages of FDD Systems ► Continuous UL and DL Transmissions. Reduce delay for MAC, ARQ, and channel information feedback. ► Higher Immunity to System Interference. Due to a large guard band. BS-to-BS and SS-to-SS (or MS-to-MS) interference are generally negligible. Note that there are still interferences between BS and MS. 104 Issues and Challenges of FDD Systems ► Feedback Required for CSIT Acquisition. CSIT: Channel State Information at the Transmitter. UL and DL channels are generally uncorrelated, so the quality of CSIT will degrade. ► Inflexible Traffic Allocation. Data traffic and Internet service have more variation in traffic symmetry. It would be desirable if the system could allocate bandwidth dynamically with regard to traffic demand. 105 Issues and Challenges of FDD Systems ► Restrictive Band Allocation. FDD systems require a pair of frequency channels, it makes the FDD systems harder to fit into the scarce resource of spectrum. ► Guard Band. It represents a waste of resource. ► Higher Hardware Cost. Requires a separate oscillator of different frequency, an expensive duplexer, and a sharp RF filter. 106 Advantages of TDD Systems ► Channel Reciprocity. Channel state information at the receiver provides CSIT. Better CSIT quality. ► Dynamic Traffic Allocation/Traffic Asymmetry. Can distribute the bandwidth between UL and DL easily by altering their subframe durations. 107 Advantages of TDD Systems ► Higher Frequency Diversity. Diversity is a well-known technique to enhance the system reliability in fading channels. DL and UL signals have wider bandwidth, which corresponds to an increase in frequency diversity. ► Unpaired Band Allocation. Only one single contiguous channel is needed. ► Lower Hardware Cost. The sharing of a single oscillator and the absence of a duplexer. 108 Issues and Challenges of TDD Systems ► Guard Time between DL/UL Transitions. Reduces the efficiency of the system. ► Duplexing Delay in MAC and ARQ. The traffic in both directions is discontinuous, and there is a delay between consecutive UL/DL subframes, called the duplexing delay. ► Outdated CSIT. The estimated CSIT may be outdated due to duplexing delay. 109 Issues and Challenges of TDD Systems ► Cross-Slot Interference. This interference arises when neighboring TDD BSs either have different traffic symmetries or do not synchronize their frames. A major challenge in TDD systems. ► Interoperator Interference. Different operators neither coordinate in network planning nor synchronize their frames and traffic asymmetry. Would cause strong adjacent channel interference. 110 FDD or TDD? ► TDD has received significant attention because: Traffic asymmetry of high-bit-rate multimedia application. The flexibility of unpaired spectrum. ► To alleviate cross-slot interference, the employment of sectored antennas and time slot grouping are very effective. 111 FDD or TDD ► More detailed discussion can be found in: Petwer W. C. Chan et al., “The Evolution Path of 4G Networks: FDD or TDD,” IEEE Communications Magazine, vol. 44, issue 12, Dec. 2006, pp. 42-50. 112 Review of the OFDM System ► OFDM stands for Orthogonal Frequency Division Multiplexing. ► It was proposed in mid-1960s and used in several high-frequency military system. ► It is a multicarrier transmission technique. Divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream. 113 Single carrier and Multicarrier Transmission ► Single carrier transmission Each user transmits and receives data stream with only one carrier at any time. ► Multicarrier transmission A user can employ a number of carriers to transmit data simultaneously. 114 Single carrier and Multicarrier Transmission Single carrier transmission Multicarrier transmission cos(2 f1t ) s (t ) bi cos(2 f 2t ) s (t ) bi cos(2 fct ) ∑ S/P cos(2 f N t ) N oscillators are required 115 116 Service Classes ► UGS (Unsolicited Grant Service) ► rtPS (Real-Time Polling Service) ► nrtPS (Non-Real-Time Polling Service) ► BE (Best Effort) 117