2002-06-04 IEEE C802.16-02/05
Project IEEE 802.16 Broadband Wireless Access Working Group
This article was written by Carl Eklund, Roger B. Marks, Kenneth L. Stanwood, and Stanley Wang. It was published in IEEE Communications Magazine, June 2002, pp. 98-107.” For more details, see:
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2002-06-04 IEEE C802.16-02/05
Copyright Copyright ©2002 Institute of Electrical and Electronics Engineers, Inc. Reprinted, with Permission permission, from IEEE Communications Magazine, June 2002, pp. 98-107. This material is posted here with permission of the IEEE. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE (contact [email protected]).
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IEEE Standard 802.16: A Technical Overview of the WirelessMAN™ Air Interface for Broadband Wireless Access
Carl Eklund, Nokia Research Center Roger B. Marks, National Institute of Standards and Technology Kenneth L. Stanwood and Stanley Wang, Ensemble Communications Inc.
ABSTRACT lead to more ubiquitous broadband access. Such systems have been in use for several years, The broadband wireless access industry, which but the development of the new standard marks provides high-rate network connections to sta- the maturation of the industry and forms the tionary sites, has matured to the point at which basis of new industry success using second-gen- it now has a standard for second-generation eration equipment. wireless metropolitan area networks. IEEE Stan- In this scenario, with WirelessMAN technolo- dard 802.16, with its WirelessMAN™ air inter- gy bringing the network to a building, users inside face, sets the stage for widespread and effective the building will connect to it with conventional deployments worldwide. This article overviews in-building networks such as, for data, Ethernet the technical medium access control and physical (IEEE Standard 802.3) or wireless LANs (IEEE layer features of this new standard. Standard 802.11). However, the fundamental design of the standard may eventually allow for the efficient extension of the WirelessMAN net- INTRODUCTION AND working protocols directly to the individual user. ARKET PPORTUNITIES For instance, a central BS may someday exchange M O medium access control (MAC) protocol data with IEEE Standard 802.16-2001 [1], completed in an individual laptop computer in a home. The October 2001 and published on 8 April 2002, links from the BS to the home receiver and from defines the WirelessMAN™ air interface specifi- the home receiver to the laptop would likely use cation for wireless metropolitan area networks quite different physical layers, but design of the (MANs). The completion of this standard her- WirelessMAN MAC could accommodate such a alds the entry of broadband wireless access as a connection with full quality of service (QoS). major new tool in the effort to link homes and With the technology expanding in this direction, it businesses to core telecommunications networks is likely that the standard will evolve to support worldwide. nomadic and increasingly mobile users. For exam- As currently defined through IEEE Stan- ple, it could be suitable for a stationary or slow- dard 802.16, a wireless MAN provides network moving vehicle. access to buildings through exterior antennas IEEE Standard 802.16 was designed to communicating with central radio base stations evolve as a set of air interfaces based on a com- (BSs). The wireless MAN offers an alternative mon MAC protocol but with physical layer spec- to cabled access networks, such as fiber optic ifications dependent on the spectrum of use and links, coaxial systems using cable modems, and the associated regulations. The standard, as digital subscriber line (DSL) links. Because approved in 2001, addresses frequencies from wireless systems have the capacity to address 10 to 66 GHz, where extensive spectrum is cur- broad geographic areas without the costly infra- rently available worldwide but at which the Portions are U.S. Govern- structure development required in deploying short wavelengths introduce significant deploy- ment work, not subject to cable links to individual sites, the technology ment challenges. A new project, currently in the U.S. Copyright. may prove less expensive to deploy and may balloting stage, expects to complete an amend-
98 0163-6804/02/$17.00 © 2002 IEEE IEEE Communications Magazine • June 2002 ment denoted IEEE 802.16a [2] before the end chronous transfer mode (ATM) service cate- of 2002. This document will extend the air inter- gories as well as newer categories such as While extensive face support to lower frequencies in the 2–11 guaranteed frame rate (GFR). GHz band, including both licensed and license- The 802.16 MAC protocol must also support bandwidth exempt spectra. Compared to the higher fre- a variety of backhaul requirements, including allocation and quencies, such spectra offer the opportunity to both asynchronous transfer mode (ATM) and reach many more customers less expensively, packet-based protocols. Convergence sublayers QoS mechanisms although at generally lower data rates. This sug- are used to map the transport-layer-specific traf- gests that such services will be oriented toward fic to a MAC that is flexible enough to efficient- are provided, the individual homes or small to medium-sized ly carry any traffic type. Through such features details of enterprises. as payload header suppression, packing, and fragmentation, the convergence sublayers and scheduling and THE 802.16 WORKING GROUP MAC work together to carry traffic in a form reservation Development of IEEE Standard 802.16 and the that is often more efficient than the original included WirelessMAN™ air interface, along transport mechanism. management with associated standards and amendments, is Issues of transport efficiency are also the responsibility of IEEE Working Group addressed at the interface between the MAC are left 802.16 on Broadband Wireless Access (BWA) and the physical layer (PHY). For example, the unstandardized Standards (http://WirelessMAN.org). The Work- modulation and coding schemes are specified in ing Group’s initial interest was the 10–66 GHz a burst profile that may be adjusted adaptively and provide an range. The 2–11 GHz amendment project that for each burst to each subscriber station. The important led to IEEE 802.16a was approved in March MAC can make use of bandwidth-efficient burst 2000. The 802.16a project primarily involves the profiles under favorable link conditions but shift mechanism for development of new physical layer specifica- to more reliable, although less efficient, alterna- tions, with supporting enhancements to the tives as required to support the planned 99.999 vendors to basic MAC. In addition, the Working Group percent link availability. differentiate their has completed IEEE Standard 802.16.2 [3] The request-grant mechanism is designed to (“Recommended Practice for Coexistence of be scalable, efficient, and self-correcting. The equipment. Fixed Broadband Wireless Access Systems”) to 802.16 access system does not lose efficiency address 10–66 GHz coexistence and, through when presented with multiple connections per the amendment project 802.16.2a, is expanding terminal, multiple QoS levels per terminal, and a its recommendations to include licensed bands large number of statistically multiplexed users. It from 2 to 11 GHz. takes advantage of a wide variety of request Historically, the 802.16 activities were initiated mechanisms, balancing the stability of con- at an August 1998 meeting called by the National tentionless access with the efficiency of con- Wireless Electronics Systems Testbed (N-WEST) tention-oriented access. of the U.S. National Institute of Standards and While extensive bandwidth allocation and Technology. The effort was welcomed in IEEE QoS mechanisms are provided, the details of 802, which opened a Study Group. The 802.16 scheduling and reservation management are left Working Group has held weeklong meetings at unstandardized and provide an important least bimonthly since July 1999. Over 700 individ- mechanism for vendors to differentiate their uals have attended a session. Membership, which equipment. is granted to individuals based on their atten- Along with the fundamental task of allocating dance and participation, currently stands at 130. bandwidth and transporting data, the MAC The work has been closely followed; for example, includes a privacy sublayer that provides authen- the IEEE 802.16 Web site received over 2.8 mil- tication of network access and connection estab- lion file requests in 2000. lishment to avoid theft of service, and it provides key exchange and encryption for data privacy. TECHNOLOGY DESIGN ISSUES To accommodate the more demanding physi- cal environment and different service require- MEDIUM ACCESS CONTROL ments of the frequencies between 2 and 11 GHz, The IEEE 802.16 MAC protocol was designed the 802.16a project is upgrading the MAC to for point-to-multipoint broadband wireless provide automatic repeat request (ARQ) and access applications. It addresses the need for support for mesh, rather than only point-to-mul- very high bit rates, both uplink (to the BS) tipoint, network architectures. and downlink (from the BS). Access and band- width allocation algorithms must accommo- THE PHYSICAL LAYER date hundreds of terminals per channel, with 10–66 GHz — In the design of the PHY speci- terminals that may be shared by multiple end fication for 10–66 GHz, line-of-sight propaga- users. The services required by these end users tion was deemed a practical necessity. With this are varied in their nature and include legacy condition assumed, single-carrier modulation time-division multiplex (TDM) voice and data, was easily selected; the air interface is designat- Internet Protocol (IP) connectivity, and packe- ed “WirelessMAN-SC.” Many fundamental tized voice over IP (VoIP). To support this design challenges remained, however. Because variety of services, the 802.16 MAC must of the point-to-multipoint architecture, the BS accommodate both continuous and bursty traf- basically transmits a TDM signal, with individu- fic. Additionally, these services expect to be al subscriber stations allocated time slots serial- assigned QoS in keeping with the traffic types. ly. Access in the uplink direction is by The 802.16 MAC provides a wide range of ser- time-division multiple access (TDMA). Follow- vice types analogous to the classic asyn- ing extensive discussions regarding duplexing, a
IEEE Communications Magazine • June 2002 99 The PHY TDM portion specification defined for Broadcast control TDM TDM TDM TDMA portion DIUC = 0 DIUC a DIUC b DIUC c
10–66 GHz uses Preamble burst single-carrier
modulation with TDMA TDMA TDMA TDMA DIUC d DIUC e DIUC f DIUC g
adaptive burst Preamble Preamble Preamble Preamble profiling in which transmission Burst start points parameters, including the DL-MAP UL-MAP modulation and Preamble codling schemes, my be adjusted Figure 1. The downlink subframe structure. individually to each subscriber burst design was selected that allows both time- • WirelessMAN-OFDMA: This uses orthogo- station on a division duplexing (TDD), in which the uplink nal frequency-division multiple access with and downlink share a channel but do not trans- a 2048-point transform. In this system, mul- frame-by-frame mit simultaneously, and frequency-division tiple access is provided by addressing a sub- duplexing (FDD), in which the uplink and down- set of the multiple carriers to individual basis. Both TDD link operate on separate channels, sometimes receivers. and burst FDD simultaneously. This burst design allows both Because of the propagation requirements, the TDD and FDD to be handled in a similar fash- use of advanced antenna systems is supported. variants are ion. Support for half-duplex FDD subscriber It is premature to speculate on further defined. stations, which may be less expensive since they specifics of the 802.16a amendment prior to its do not simultaneously transmit and receive, was completion. While the draft seems to have added at the expense of some slight complexity. reached a level of maturity, the contents could Both TDD and FDD alternatives support adap- change significantly in balloting. Modes could tive burst profiles in which modulation and cod- even be deleted or added. ing options may be dynamically assigned on a burst-by-burst basis. PHYSICAL LAYER DETAILS 2–11 GHz — The 2–11 GHz bands, both The PHY specification defined for 10–66 GHz licensed and license-exempt, are addressed in uses burst single-carrier modulation with adap- IEEE Project 802.16a. The standard is in bal- tive burst profiling in which transmission param- lot but is not yet complete. The draft current- eters, including the modulation and coding ly specifies that compliant systems implement schemes, may be adjusted individually to each one of three air interface specifications, each subscriber station (SS) on a frame-by-frame of which provides for interoperability. Design basis. Both TDD and burst FDD variants are of the 2–11 GHz physical layer is driven by defined. Channel bandwidths of 20 or 25 MHz the need for non-line-of-sight (NLOS) opera- (typical U.S. allocation) or 28 MHz (typical tion. Because residential applications are European allocation) are specified, along with expected, rooftops may be too low for a clear Nyquist square-root raised-cosine pulse shaping sight line to a BS antenna, possibly due to with a rolloff factor of 0.25. Randomization is obstruction by trees. Therefore, significant performed for spectral shaping and to ensure bit multipath propagation must be expected. Fur- transitions for clock recovery. thermore, outdoor-mounted antennas are The forward error correction (FEC) used is expensive due to both hardware and installa- Reed-Solomon GF(256), with variable block size tion costs. and error correction capabilities. This is paired The three 2–11 GHz air interface specifica- with an inner block convolutional code to robust- tions in 802.16a Draft 3 are: ly transmit critical data, such as frame control • WirelessMAN-SC2: This uses a single-carri- and initial accesses. The FEC options are paired er modulation format. with quadrature phase shift keying (QPSK), 16- • WirelessMAN-OFDM: This uses orthogonal state quadrature amplitude modulation (16- frequency-division multiplexing with a 256- QAM), and 64-state QAM (64-QAM) to form point transform. Access is by TDMA. This burst profiles of varying robustness and efficien- air interface is mandatory for license- cy. If the last FEC block is not filled, that block exempt bands. may be shortened. Shortening in both the uplink
100 IEEE Communications Magazine • June 2002 SS transition Tx/Rx transition gap gap (TDD)
Initial Request SS 1 SS N maintenance contention scheduled scheduled opportunities opps data data (UIUC = 2) (UIUC = 1) (UIUC = i) (UIUC = j)
Access CollisionAccess Bandwidth Collision Bandwidth burst burst request request
Figure 2. The uplink subframe structure. and downlink is controlled by the BS and is implicitly communicated in the uplink map (UL- MAP) and downlink map (DL-MAP). The system uses a frame of 0.5, 1, or 2 ms. MAC PDU which has started First MAC PDU, Second MAC PDU, P This frame is divided into physical slots for the in previous TC PDU this TC PDU this TC PDU purpose of bandwidth allocation and identifica- tion of PHY transitions. A physical slot is Transmission convergence sublayer PDU defined to be 4 QAM symbols. In the TDD vari- ant of the PHY, the uplink subframe follows the Figure 3. TC PDU format. downlink subframe on the same carrier frequen- cy. In the FDD variant, the uplink and downlink subframes are coincident in time but are carried on separate frequencies. The downlink subframe headers rather than in the DL-MAP, SSs listen is shown in Fig. 1. to all portions of the downlink subframe they are The downlink subframe starts with a frame capable of receiving. For full-duplex SSs, this control section that contains the DL-MAP for means receiving all burst profiles of equal or the current downlink frame as well as the UL- greater robustness than they have negotiated MAP for a specified time in the future. The with the BS. downlink map specifies when physical layer tran- A typical uplink subframe for the 10–66 GHz sitions (modulation and FEC changes) occur PHY is shown in Fig. 2. Unlike the downlink, within the downlink subframe. The downlink the UL-MAP grants bandwidth to specific SSs. subframe typically contains a TDM portion The SSs transmit in their assigned allocation immediately following the frame control section. using the burst profile specified by the Uplink Downlink data are transmitted to each SS using Interval Usage Code (UIUC) in the UL-MAP a negotiated burst profile. The data are transmit- entry granting them bandwidth. The uplink sub- ted in order of decreasing robustness to allow frame may also contain contention-based alloca- SSs to receive their data before being presented tions for initial system access and broadcast or with a burst profile that could cause them to lose multicast bandwidth requests. The access oppor- synchronization with the downlink. tunities for initial system access are sized to In FDD systems, the TDM portion may be fol- allow extra guard time for SSs that have not lowed by a TDMA segment that includes an extra resolved the transmit time advance necessary to preamble at the start of each new burst profile. offset the round-trip delay to the BS. This feature allows better support of half-duplex Between the PHY and MAC is a transmis- SSs. In an efficiently scheduled FDD system with sion convergence (TC) sublayer. This layer per- many half-duplex SSs, some may need to transmit forms the transformation of variable length earlier in the frame than they receive. Due to MAC protocol data units (PDUs) into the fixed their half-duplex nature, these SSs lose synchro- length FEC blocks (plus possibly a shortened nization with the downlink. The TDMA preamble block at the end) of each burst. The TC layer allows them to regain synchronization. has a PDU sized to fit in the FEC block current- Due to the dynamics of bandwidth demand ly being filled. It starts with a pointer indicating for the variety of services that may be active, the where the next MAC PDU header starts within mixture and duration of burst profiles and the the FEC block. This is shown in Fig. 3. presence or absence of a TDMA portion vary The TC PDU format allows resynchroniza- dynamically from frame to frame. Since the tion to the next MAC PDU in the event that the recipient SS is implicitly indicated in the MAC previous FEC block had irrecoverable errors.
IEEE Communications Magazine • June 2002 101 referenced with 16-bit connection identifiers (CIDs) and may require continuously granted bandwidth or bandwidth on demand. As will be LEN Type (6) EKS described, both are accommodated. (2) msb (3) CI (1)
EC (1) Each SS has a standard 48-bit MAC address, Rsv (1) Rsv (1)
HT = 0 (1) but this serves mainly as an equipment identifi- er, since the primary addresses used during operation are the CIDs. Upon entering the network, the SS is assigned three management LEN lsb (8) CID msb (8) connections in each direction. These three con- nections reflect the three different QoS requirements used by different management levels. The first of these is the basic connec- tion, which is used for the transfer of short, CID Isb (8) HCS (8) time-critical MAC and radio link control (RLC) messages. The primary management connection is used to transfer longer, more delay-tolerant messages such as those used for authentication and connection setup. The sec- Figure 4. Format of generic header for MAC PDU. ondary management connection is used for the transfer of standards-based management mes- sages such as Dynamic Host Configuration Without the TC layer, a receiving SS or BS Protocol (DHCP), Trivial File Transfer Proto- would potentially lose the entire remainder of a col (TFTP), and Simple Network Management burst when an irrecoverable bit error occurred. Protocol (SNMP). In addition to these man- agement connections, SSs are allocated trans- EDIUM CCESS ONTROL ETAILS port connections for the contracted services. M A C D Transport connections are unidirectional to The MAC includes service-specific convergence facilitate different uplink and downlink QoS sublayers that interface to higher layers, above and traffic parameters; they are typically the core MAC common part sublayer that car- assigned to services in pairs. ries out the key MAC functions. Below the com- The MAC reserves additional connections for mon part sublayer is the privacy sublayer. other purposes. One connection is reserved for contention-based initial access. Another is SERVICE-SPECIFIC CONVERGENCE SUBLAYERS reserved for broadcast transmissions in the IEEE Standard 802.16 defines two general ser- downlink as well as for signaling broadcast con- vice-specific convergence sublayers for map- tention-based polling of SS bandwidth needs. ping services to and from 802.16 MAC Additional connections are reserved for multi- connections. The ATM convergence sublayer cast, rather than broadcast, contention-based is defined for ATM services, and the packet polling. SSs may be instructed to join multicast convergence sublayer is defined for mapping polling groups associated with these multicast packet services such as IPv4, IPv6, Ethernet, polling connections. and virtual local area network (VLAN). The primary task of the sublayer is to classify ser- MAC PDU Formats — The MAC PDU is the vice data units (SDUs) to the proper MAC data unit exchanged between the MAC layers of connection, preserve or enable QoS, and the BS and its SSs. A MAC PDU consists of a enable bandwidth allocation. The mapping fixed-length MAC header, a variable-length pay- takes various forms depending on the type of load, and an optional cyclic redundancy check service. In addition to these basic functions, (CRC). Two header formats, distinguished by the convergence sublayers can also perform the HT field, are defined: the generic header more sophisticated functions such as payload (Fig. 4) and the bandwidth request header. header suppression and reconstruction to Except for bandwidth request MAC PDUs, enhance airlink efficiency. which contain no payload, MAC PDUs contain either MAC management messages or conver- COMMON PART SUBLAYER gence sublayer data. Introduction and General Architecture — In Three types of MAC subheader may be pre- general, the 802.16 MAC is designed to support sent. The grant management subheader is used a point-to-multipoint architecture with a central by an SS to convey bandwidth management BS handling multiple independent sectors simul- needs to its BS. The fragmentation subheader taneously. On the downlink, data to SSs are mul- contains information that indicates the presence tiplexed in TDM fashion. The uplink is shared and orientation in the payload of any fragments between SSs in TDMA fashion. of SDUs. The packing subheader is used to indi- The 802.16 MAC is connection-oriented. All cate the packing of multiple SDUs into a single services, including inherently connectionless ser- PDU. The grant management and fragmentation vices, are mapped to a connection. This provides subheaders may be inserted in MAC PDUs a mechanism for requesting bandwidth, associat- immediately following the generic header if so ing QoS and traffic parameters, transporting and indicated by the Type field. The packing sub- routing data to the appropriate convergence sub- header may be inserted before each MAC SDU layer, and all other actions associated with the if so indicated by the Type field. More details contractual terms of the service. Connections are are provided below.
102 IEEE Communications Magazine • June 2002 Transmission of MAC PDUs — The IEEE 802.16 MAC supports various higher-layer pro- Frame n – 1 Frame n Frame n + 1 tocols such as ATM or IP. Incoming MAC SDUs from corresponding convergence sublayers are DL-MAP n – 1 DL-MAP n DL-MAP n + 1 formatted according to the MAC PDU format, UL-MAP n UL-MAP n + 1 UL-MAP n + 2 possibly with fragmentation and/or packing, Frame before being conveyed over one or more connec- control tions in accordance with the MAC protocol. Downlink After traversing the airlink, MAC PDUs are subframe reconstructed back into the original MAC SDUs so that the format modifications performed by Uplink the MAC layer protocol are transparent to the subframe receiving entity. IEEE 802.16 takes advantage of incorporat- ing the packing and fragmentation processes Round-trip delay + Tproc with the bandwidth allocation process to maxi- mize the flexibility, efficiency, and effectiveness Figure 5. Minimum FDD map relevance. of both. Fragmentation is the process in which a MAC SDU is divided into one or more MAC SDU fragments. Packing is the process in which multiple MAC SDUs are packed into a single Code (DIUC). Those for the uplink are each MAC PDU payload. Both processes may be ini- tagged with an Uplink Interval Usage Code tiated by either a BS for a downlink connection (UIUC). or an SS for an uplink connection. During initial access, the SS performs initial IEEE 802.16 allows simultaneous fragmenta- power leveling and ranging using ranging request tion and packing for efficient use of the band- (RNG-REQ) messages transmitted in initial width. maintenance windows. The adjustments to the SS’s transmit time advance, as well as power PHY Support and Frame Structure — The adjustments, are returned to the SS in ranging IEEE 802.16 MAC supports both TDD and response (RNG-RSP) messages. For ongoing FDD. In FDD, both continuous and burst down- ranging and power adjustments, the BS may links are supported. Continuous downlinks allow transmit unsolicited RNG-RSP messages com- for certain robustness enhancement techniques, manding the SS to adjust its power or timing. such as interleaving. Burst downlinks (either During initial ranging, the SS also requests to FDD or TDD) allow the use of more advanced be served in the downlink via a particular burst robustness and capacity enhancement tech- profile by transmitting its choice of DIUC to the niques, such as subscriber-level adaptive burst BS. The choice is based on received downlink profiling and advanced antenna systems. signal quality measurements performed by the The MAC builds the downlink subframe start- SS before and during initial ranging. The BS ing with a frame control section containing the may confirm or reject the choice in the ranging DL-MAP and UL-MAP messages. These indicate response. Similarly, the BS monitors the quality PHY transitions on the downlink as well as band- of the uplink signal it receives from the SS. The width allocations and burst profiles on the uplink. BS commands the SS to use a particular uplink The DL-MAP is always applicable to the cur- burst profile simply by including the appropriate rent frame and is always at least two FEC blocks burst profile UIUC with the SS’s grants in UL- long. The first PHY transition is expressed in the MAP messages. first FEC block, to allow adequate processing After initial determination of uplink and time. In both TDD and FDD systems, the UL- downlink burst profiles between the BS and a MAP provides allocations starting no later than particular SS, RLC continues to monitor and the next downlink frame. The UL-MAP can, control the burst profiles. Harsher environmen- however, allocate starting in the current frame as tal conditions, such as rain fades, can force the long as processing times and round-trip delays SS to request a more robust burst profile. Alter- are observed. The minimum time between natively, exceptionally good weather may allow receipt and applicability of the UL-MAP for an an SS to temporarily operate with a more effi- FDD system is shown in Fig. 5. cient burst profile. The RLC continues to adapt the SS’s current UL and DL burst profiles, ever Radio Link Control — The advanced technolo- striving to achieve a balance between robustness gy of the 802.16 PHY requires equally advanced and efficiency. Because the BS is in control and radio link control (RLC), particularly the capa- directly monitors the uplink signal quality, the bility of the PHY to transition from one burst protocol for changing the uplink burst profile for profile to another. The RLC must control this an SS is simple: the BS merely specifies the pro- capability as well as the traditional RLC func- file’s associated UIUC whenever granting the SS tions of power control and ranging. bandwidth in a frame. This eliminates the need RLC begins with periodic BS broadcast of for an acknowledgment, since the SS will always the burst profiles that have been chosen for the receive either both the UIUC and the grant or uplink and downlink. The particular burst pro- neither. Hence, no chance of uplink burst profile files used on a channel are chosen based on a mismatch between the BS and SS exists. number of factors, such as rain region and equip- In the downlink, the SS is the entity that ment capabilities. Burst profiles for the downlink monitors the quality of the receive signal and are each tagged with a Downlink Interval Usage therefore knows when its downlink burst profile
IEEE Communications Magazine • June 2002 103 Uplink Scheduling Services — Each connec- tion in the uplink direction is mapped to a BS SS scheduling service. Each scheduling service is associated with a set of rules imposed on the BS DL data at DIUC n scheduler responsible for allocating the uplink capacity and the request-grant protocol between the SS and the BS. The detailed specification of C/(N + I) the rules and the scheduling service used for a too low particular uplink connection is negotiated at for DIUC n connection setup time. The scheduling services in IEEE 802.16 are based on those defined for cable modems in the RNG-REQ or DBPC-REQ Yes change to DIUC k DOCSIS standard [4]. Unsolicited grant service (UGS) is tailored Continue for carrying services that generate fixed units of monitoring data periodically. Here the BS schedules regular- Send DL data DL data ly, in a preemptive manner, grants of the size at DIUC k through negotiated at connection setup, without an DIUC n explicit request from the SS. This eliminates the overhead and latency of bandwidth requests in DL data at DIUC k No order to meet the delay and delay jitter require- RNG-RSP or DBPC-RSP ments of the underlying service. A practical limit on the delay jitter is set by the frame duration. If more stringent jitter requirements are to be met, Monitor DL output buffering is needed. Services that typical- data ly would be carried on a connection with UGS only through service include ATM constant bit rate (CBR) DIUC k and E1/T1 over ATM. When used with UGS, the grant management DL data at DIUC k subheader includes the poll-me bit (see “Band- width Requests and Grants”) as well as the slip indicator flag, which allows the SS to report that the transmission queue is backlogged due to fac- tors such as lost grants or clock skew between Figure 6. Transition to a more robust burst profile. the IEEE 802.16 system and the outside net- work. The BS, upon detecting the slip indicator flag, can allocate some additional capacity to the should change. The BS, however, is the entity in SS, allowing it to recover the normal queue control of the change. There are two methods state. Connections configured with UGS are not available to the SS to request a change in down- allowed to utilize random access opportunities link burst profile, depending on whether the SS for requests. operates in the grant per connection (GPC) or The real-time polling service is designed to grant per SS (GPSS) mode (see “Bandwidth meet the needs of services that are dynamic in Requests and Grants”). The first method would nature, but offers periodic dedicated request typically apply (based on the discretion of the opportunities to meet real-time requirements. BS scheduling algorithm) only to GPC SSs. In Because the SS issues explicit requests, the this case, the BS may periodically allocate a sta- protocol overhead and latency is increased, but tion maintenance interval to the SS. The SS can this capacity is granted only according to the use the RNG-REQ message to request a change real need of the connection. The real-time in downlink burst profile. The preferred method polling service is well suited for connections is for the SS to transmit a downlink burst profile carrying services such as VoIP or streaming change request (DBPC-REQ). In this case, video or audio. which is always an option for GPSS SSs and can The non-real-time polling service is almost be an option for GPC SSs, the BS responds with identical to the real-time polling service except a downlink burst profile change response that connections may utilize random access (DBPC-RSP) message confirming or denying transmit opportunities for sending bandwidth the change. requests. Typically, services carried on these Because messages may be lost due to irrecov- connections tolerate longer delays and are rather erable bit errors, the protocols for changing an insensitive to delay jitter. The non-real-time SS’s downlink burst profile must be carefully polling service is suitable for Internet access with structured. The order of the burst profile change a minimum guaranteed rate and for ATM GFR actions is different when transitioning to a more connections. robust burst profile than when transitioning to a A best effort service has also been defined. less robust one. The standard takes advantage Neither throughput nor delay guarantees are of the fact that an SS is always required to listen provided. The SS sends requests for band- to more robust portions of the downlink as well width in either random access slots or dedi- as the profile that was negotiated. Figure 6 cated transmission opportunities. The shows a transition to a more robust burst pro- occurrence of dedicated opportunities is sub- file. Figure 7 shows a transition to a less robust ject to network load, and the SS cannot rely burst profile. on their presence.
104 IEEE Communications Magazine • June 2002 Bandwidth Requests and Grants — The IEEE 802.16 MAC accommodates two classes of SS, differentiated by their ability to accept bandwidth BS SS grants simply for a connection or for the SS as a whole. Both classes of SS request bandwidth per DL data at DIUC n connection to allow the BS uplink scheduling algorithm to properly consider QoS when allocat- ing bandwidth. With the grant per connection (GPC) class of SS, bandwidth is granted explicitly C/(N+I) high to a connection, and the SS uses the grant only enough for for that connection. RLC and other management DIUC m protocols use bandwidth explicitly allocated to the RNG-REQ or DBPC-REQ management connections. Yes With the grant per SS (GPSS) class, SSs are change to DIUC m granted bandwidth aggregated into a single grant to the SS itself. The GPSS SS needs to be more intelligent in its handling of QoS. It will typically Start monitoring DL data through use the bandwidth for the connection that DIUC m requested it, but need not. For instance, if the QoS situation at the SS has changed since the last request, the SS has the option of sending the No higher QoS data along with a request to replace RNG-RSP or DBPC-RSP this bandwidth stolen from a lower QoS connec- tion. The SS could also use some of the band- width to react more quickly to changing Send DL data at DIUC m environmental conditions by sending, for instance, a DBPC-REQ message. The two classes of SS allow a trade-off DL data at DIUC m between simplicity and efficiency. The need to explicitly grant extra bandwidth for RLC and requests, coupled with the likelihood of more than one entry per SS, makes GPC less efficient and scalable than GPSS. Additionally, the ability Figure 7. Transition to a less robust burst profile. of the GPSS SS to react more quickly to the needs of the PHY and those of connections enhances system performance. GPSS is the only The SS has a plethora of ways to request class of SS allowed with the 10–66 GHz PHY. bandwidth, combining the determinism of uni- With both classes of grants, the IEEE 802.16 cast polling with the responsiveness of con- MAC uses a self-correcting protocol rather than tention-based requests and the efficiency of an acknowledged protocol. This method uses unsolicited bandwidth. For continuous band- less bandwidth. Furthermore, acknowledged pro- width demand, such as with CBR T1/E1 data, tocols can take additional time, potentially the SS need not request bandwidth; the BS adding delay. There are a number of reasons the grants it unsolicited. bandwidth requested by an SS for a connection To short-circuit the normal polling cycle, any may not be available: SS with a connection running UGS can use the • The BS did not see the request due to poll-me bit in the grant management subheader irrecoverable PHY errors or collision of a to let the BS know it needs to be polled for contention-based reservation. bandwidth needs on another connection. The BS • The SS did not see the grant due to irrecov- may choose to save bandwidth by polling SSs erable PHY errors. that have unsolicited grant services only when • The BS did not have sufficient bandwidth they have set the poll-me bit. available. A more conventional way to request band- • The GPSS SS used the bandwidth for anoth- width is to send a bandwidth request MAC er purpose. PDU that consists of simply the bandwidth In the self-correcting protocol, all of these request header and no payload. GPSS SSs can anomalies are treated the same. After a timeout send this in any bandwidth allocation they appropriate for the QoS of the connection (or receive. GPC terminals can send it in either a immediately, if the bandwidth was stolen by the request interval or a data grant interval allocat- SS for another purpose), the SS simply requests ed to their basic connection. A closely related again. For efficiency, most bandwidth requests method of requesting data is to use a grant are incremental; that is, the SS asks for more management subheader to piggyback a request bandwidth for a connection. However, for the for additional bandwidth for the same connec- self-correcting bandwidth request/grant mecha- tion within a MAC PDU. nism to work correctly, the bandwidth requests In addition to polling individual SSs, the BS must occasionally be aggregate; that is, the SS may issue a broadcast poll by allocating a request informs the BS of its total current bandwidth interval to the broadcast CID. Similarly, the needs for a connection. This allows the BS to standard provides a protocol for forming multi- reset its perception of the SS’s needs without a cast groups to give finer control to contention- complicated protocol acknowledging the use of based polling. Due to the nondeterministic delay granted bandwidth. that can be caused by collisions and retries, con-
IEEE Communications Magazine • June 2002 105 tention-based requests are allowed only for cer- SS’s public key and used to secure further trans- In general, service tain lower QoS classes of service. actions. Upon successful authorization, the SS will flows in IEEE Channel Acquisition — The MAC protocol register with the network. This will establish the 802.16 are includes an initialization procedure designed to secondary management connection of the SS eliminate the need for manual configuration. and determine capabilities related to connection preprovisioned, Upon installation, an SS begins scanning its fre- setup and MAC operation. The version of IP quency list to find an operating channel. It may used on the secondary management connection and setup of the be programmed to register with a specified BS, is also determined during registration. service flows is referring to a programmable BS ID broadcast by each. This feature is useful in dense deployments IP Connectivity — After registration, the SS initiated by the where the SS might hear a secondary BS due to attains an IP address via DHCP and establishes BS during SS selective fading or when the SS picks up a side- the time of day via the Internet Time Protocol. lobe of a nearby BS antenna. The DHCP server also provides the address of initialization. After deciding on which channel or chan- the TFTP server from which the SS can request nel pair to attempt communication, the SS a configuration file. This file provides a standard However, service tries to synchronize to the downlink transmis- interface for providing vendor-specific configura- flows can also be sion by detecting the periodic frame pream- tion information. bles. Once the physical layer is synchronized, dynamically the SS will look for the periodically broadcast Connection Setup — IEEE 802.16 uses the established by DCD and UCD messages that enable the SS concept of service flows to define unidirectional to learn the modulation and FEC schemes transport of packets on either downlink or uplink. either the BS or used on the carrier. Service flows are characterized by a set of QoS parameters such as latency and jitter. To most the SS. Initial Ranging and Negotiation of SS Capa- efficiently utilize network resources such as band- bilities — Upon learning what parameters to width and memory, 802.16 adopts a two-phase use for its initial ranging transmissions, the SS activation model in which resources assigned to a will look for initial ranging opportunities by particular admitted service flow may not be actu- scanning the UL-MAP messages present in every ally committed until the service flow is activated. frame. The SS uses a truncated exponential Each admitted or active service flow is mapped to backoff algorithm to determine which initial a MAC connection with a unique CID. ranging slot it will use to send a ranging request In general, service flows in IEEE 802.16 are message. The SS will send the burst using the preprovisioned, and setup of the service flows is minimum power setting and will try again with initiated by the BS during SS initialization. How- increasingly higher transmission power if it does ever, service flows can also be dynamically estab- not receive a ranging response. lished by either the BS or the SS. The SS typically Based on the arrival time of the initial rang- initiates service flows only if there is a dynamical- ing request and the measured power of the sig- ly signaled connection, such as a switched virtual nal, the BS commands a timing advance and a connection (SVC) from an ATM network. The power adjustment to the SS in the ranging establishment of service flows is performed via a response. The response also provides the SS with three-way handshaking protocol in which the the basic and primary management CIDs. Once request for service flow establishment is respond- the timing advance of the SS transmissions has ed to and the response acknowledged. been correctly determined, the ranging proce- In addition to dynamic service establishment, dure for fine-tuning the power can be performed IEEE 802.16 also supports dynamic service using invited transmissions. changes in which service flow parameters are re- All transmissions up to this point are made negotiated. Like dynamic service flow establish- using the most robust, and thus least efficient, ment, service flow changes also follow a similar burst profile. To avoid wasting capacity, the SS three-way handshaking protocol. next reports its PHY capabilities, including the modulation and coding schemes it supports and Privacy Sublayer — IEEE 802.16’s privacy pro- whether, in an FDD system, it is half-duplex or tocol is based on the Privacy Key Management full-duplex. The BS, in its response, can deny the (PKM) protocol of the DOCSIS BPI+ specifica- use of any capability reported by the SS. tion [5] but has been enhanced to fit seamlessly into the IEEE 802.16 MAC protocol and to bet- SS Authentication and Registration — ter accommodate stronger cryptographic meth- Each SS contains both a manufacturer-issued ods, such as the recently approved Advanced factory-installed X.509 digital certificate and Encryption Standard. the certificate of the manufacturer. These cer- tificates, which establish a link between the 48- Security Associations — PKM is built around bit MAC address of the SS and its public RSA the concept of security associations (SAs). The key, are sent to the BS by the SS in the Autho- SA is a set of cryptographic methods and the rization Request and Authentication Informa- associated keying material; that is, it contains the tion messages. The network is able to verify the information about which algorithms to apply, identity of the SS by checking the certificates which key to use, and so on. Every SS establishes and can subsequently check the level of autho- at least one SA during initialization. Each con- rization of the SS. If the SS is authorized to nection, with the exception of the basic and pri- join the network, the BS will respond to its mary management connections, is mapped to an request with an Authorization Reply containing SA either at connection setup time or dynami- an Authorization Key (AK) encrypted with the cally during operation.
106 IEEE Communications Magazine • June 2002 Cryptographic Methods — Currently, the REFERENCES PKM protocol uses X.509 digital certificates with [1] IEEE 802.16-2001, “IEEE Standard for Local and Metropoli- Through the RSA public key encryption for SS authentication tan Area Networks — Part 16: Air Interface for Fixed and authorization key exchange. For traffic Broadband Wireless Access Systems,” Apr. 8, 2002. dedicated service [2] IEEE P802.16a/D3-2001: “Draft Amendment to IEEE Stan- encryption,the Data Encryption Standard (DES) of many running in the cipher block chaining (CBC) mode dard for Local and Metropolitan Area Networks — Part 16: Air Interface for Fixed Wireless Access Systems — Medium with 56-bit keys is currently mandated. The CBC Access Control Modifications and Additional Physical Layers volunteers, the initialization vector is dependent on the frame Specifications for 2–11 GHz,” Mar. 25, 2002. counter and differs from frame to frame. To [3] IEEE 802.16.2-2001, “IEEE Recommended Practice for Local IEEE 802.16 reduce the number of computationally intensive and Metropolitan Area Networks — Coexistence of Fixed Broadband Wireless Access Systems,” Sept. 10, 2001. Working Group public key operations during normal operation, [4] SCTE DSS 00-05, Data-Over-Cable Service Interface the transmission encryption keys are exchanged Specification (DOCSIS) SP-RFIv1.1-I05-000714, “Radio succeeded in Frequency Interface 1.1 Specification,” July 2000. using 3DES with a key exchange key derived from quickly designing the authorization key. [5] SCTE DSS 00-09, DOCSIS SP-BPI+-I06-001215, “Baseline Privacy Plus Interface Specification,” Dec. 2000. The PKM protocol messages themselves are [6] H. Krawczyk, M. Bellare, and R. Canetti, “HMAC: Keyed- and forging a authenticated using the Hashed Message Hashing for Message Authentication,” IETF RFC 2104, Authentication Code (HMAC) protocol [6] with Feb. 1997. standard based SHA-1 [7]. In addition, message authentication [7] Federal Information Processing Standards Publication 180–1, “Secure Hash Standard,” Apr. 1995. on forward-look- in vital MAC functions, such as the connection setup, is provided by the PKM protocol. ing technology. BIOGRAPHIES
CARL EKLUND ([email protected]) is a senior research IEEE Standard SUMMARY AND CONCLUSION engineer with Nokia Research Center, Helsinki, Finland. He chaired the MAC Task Group that developed the IEEE 802.16 is the The WirelessMAN™ air interface specified in 802.16 medium access control protocol and served as a IEEE Standard 802.16 provides a platform for lead MAC editor of IEEE Standard 802.16-2001. He received foundation of the development and deployment of standards- his M.Sc. in engineering physics from Helsinki University of the wireless based metropolitan area networks providing Technology in 1996. He is currently a guest researcher at the National Institute of Standards and Technology (NIST), broadband wireless access in many regulatory Boulder, Colorado. metropolitan environments. The standard is intended to area networks of allow for multiple vendors to produce interop- ROGER B. MARKS [F] ([email protected]) is with NIST, Boulder, erable equipment. However, it also allows for Colorado. In 1998 he initiated the effort that led to the IEEE 802.16 Working Group on Broadband Wireless Access, the next few extensive vendor differentiation. For instance, chairing it since inception. He served as technical editor of the standard provides the base station with a IEEE Standards 802.16-2001 and 802.16.2-2001. He decades. set of tools to implement efficient scheduling. received his A.B. in physics in 1980 from Princeton Univer- However, the scheduling algorithms that deter- sity and his Ph.D. in applied physics in 1988 from Yale Uni- versity. Author of over 80 publications, his awards include mine the overall efficiency will differ from ven- the 1995 IEEE Morris E. Leeds Award (an IEEE Technical dor to vendor and may be optimized for Field Award) and the Broadband Wireless Hall of Fame. He specific traffic patterns. Likewise, the adaptive developed the IEEE Radio and Wireless Conference and burst profile feature allows great control to chaired it from 1996 through 1999. optimize the efficiency of the PHY transport. KENNETH L. STANWOOD ([email protected]) is currently Innovative vendors will introduce clever principal member of technical staff and manager of sys- schemes to maximize this opportunity while tems engineering at Ensemble Communications, San Diego, maintaining interoperability with compliant California, where he was the primary designer of the MAC and transmission convergence layers of Ensemble’s propri- subscriber stations. etary Adaptix™ broadband wireless access system. He grad- The publication of IEEE Standard 802.16 is a uated with a B.S. degree in mathematical sciences from defining moment in which broadband wireless Oregon State University in 1983 and an M.S. in computer access moves to its second generation and begins science from Stanford University in 1986. He has been heavily involved in the IEEE 802.16 10–66 GHz project its establishment as a mainstream alternative for (serving as a lead MAC editor) as well as its European broadband access. Through the dedicated service counterpart, ETSI BRAN HIPERACCESS. He is technical work- of many volunteers, the IEEE 802.16 Working ing group chair for the Worldwide Interoperability for Group succeeded in quickly designing and forg- Microwave Access (WiMAX) Forum, which is dedicated to producing test specifications and system option profiles to ing a standard based on forward-looking tech- ensure interoperability of systems built to IEEE Standard nology. IEEE Standard 802.16 is the foundation 802.16. He chairs 802.16’s Task Group c, which is creating of the wireless metropolitan area networks of profiles for 10–66 GHz 802.16 systems. the next few decades. STANLEY WANG ([email protected]) is currently a director of RedDot Wireless Inc., San Jose, California, ACKNOWLEDGMENTS where he is leading the development and implementation As lead PHY editors of IEEE Standard 802.16- of MAC protocols. Before joining RedDot Wireless, he was 2001, Jay Klein and Lars Lindh played key with Ensemble Communications Inc., where he worked on IEEE 802.16 standards, serving as a lead MAC editor of roles in the completion of the 10–66 GHz IEEE Standard 802.16-2001. He received his Ph.D. in com- physical layer discussed here. Mr. Klein also puter engineering in 1994 from the University of Southern chaired the PHY Task Group that led its California. He was with the faculty of the Computer Sci- development. Mr. Lindh and Ken Peirce each ence Department at California State University-San Marcos from 1994 through 2000, where he received the Harry E. provided a helpful technical review of this Brakebill Outstanding Professor Awards in 1996 and his manuscript. tenure in 1999.
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