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`March 9, 1992
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`DOC: IEEE P802.11I92-39
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`Medium Access Control Protocol for Wireless LANs
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`( An Update )
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`K. S. Nataraian
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`C. C. Huang
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`0. F. Bantz
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`IBM Thomas J. Watson Research Center
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`P.O. Box 704. Yorktown Heights, NY 10598
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`Abstract
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`This contribution provides an updated and expanded description of a medium
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`access control protocol for wireless LANs that was proposed In [NATB1a]. The
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`proposal is described with respect to the 21 criteria that have been developed
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`for consideration of MAC proposals.
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`The medium access control protocol used is a hybrid of reservation and ran-
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`dom access based protocols. Channel time is structured as a sequence of
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`frames of equal duration. The protocol divides a frame into three intervals.
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`In the first two intervals, transmission is scheduled by the controller, and a
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`decentralized contention-based medium access control protocol is used in the
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`third interval. An adaptively movable boundary separates the contention-free
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`and contention-based portions in each frame. This provides for flexibility of
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`bandwidth allocation to meet a variety of asynchronous and isochronous ser-
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`vices that are anticipated in future wireless applications.
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`We assume Slow-Frequency-Hopping spread-spectrum radio transmissions for
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`isolating adjacent cells in a multicell network. However, the MAC protocol is
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`applicabie to a variety of other PHY layers that require different cell isolation
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`The communication architecture is flexible and permits several modes of op-
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`eration. in particular, wireless communication is supported:
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`0 When an infrastructure backbone network (i.e., a Distribution System) that
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`facilitates extended coverage and mobility is available, and
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`0 When there is no preexisting infrastructure to enable communication be-
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`tween mobile stations.
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`Page 1
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`ST. JUDE 1011
`ST. IUDE 1011
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`1
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`DOC: IEEE P802.11I92-39
`March 9, 1992
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`1 .lntroduction
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`The demand for wireless data communications is expected to grow in the
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`coming years as awide variety of user applications are developed and used -
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`in a number of operating environments. The following usage scenarios are
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`expected to become increasingly common in the future.
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`Infrastructure-based LANs: The network architecture will consist of a finite
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`number of Access Points that are attached to a Distribution System. The
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`Distribution System, typically an IEEE 802 network. would enable:
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`' Communication between mobile stations and fixed destinations ( ex. serv-
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`ers, applications, data etc) that are attached to the Distribution System.
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`Mobile stations communicate to an Access Point (a fixed station) that acts
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`as a “bridge" between the radio environment and the Distribution System.
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`The Access Point relays messages from/to stations that request its ser-
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`0 Communication between mobile stations
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`it communication is between two mobile stations that are not within
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`range of each other, this will occur utilizing the store-and—forward ca-
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`pability of one or more Access Points attached to a Distribution Sys-
`tern.
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`If communication is between two mobile stations that are within range
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`of each other. this can occur with direct or indirect support of an Ac~
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`cess Point that can serve both of them.
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`Adhoc LANs: A primary requirement for a segment of user applications would
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`be the capability to accomplish wireless communication without any depend-
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`ence on a preexisting infrastructure. An adhoc LAN consisting of a set of mo-
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`bile stations and shared resources like servers may be created. used for
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`wireless communication and “dismantled” when the needs have been satis-
`fied.
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`In this contribution we propose a communication architecture that is flexible
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`and encompasses the several modes of usage scenarios outlined above.
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`particular. wireless communication among participating stations is supported:
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`0 When an infrastructure backbone network (i.e.. a Distribution System) that
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`facilitates extended coverage and mobility is present and available for the
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`mobile station to use, and
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`0 When there is no preexisting infrastructure available to enable communi—
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`cation between mobile stations that wish to communicate.
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`in
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`Page 2
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`2
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`March 9, 1992
`DOC: iEEE P802.11192-39
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`2. Proposed Scheme
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`The proposed MAC scheme is first described in the context of Infrastructure—
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`based LANs. The same scheme is used in Adhoc LANs as described in a later
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`section. A half-duplex wireless link is assumed. The link is shared between
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`inbound (mobile stations to Access Point) and outbound (Access Point to mo-
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`bile stations) traffic. Channel time is structured as a sequence of frames of
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`equal duration. The duration of a frame is subdivided into three intervals as
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`shown in Fig. 1. Centralized control is used in the first two intervals and de-
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`centralized control in the third interval.
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`|G|AH|
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`A
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`|BH|
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`B
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`|cu|
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`c
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`I-l--|lllllllll|--llll|l|lllllllulllllllllllll-l
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`[Broadcastl
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`| Contention I.
`I Contention]
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`I
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`[Free
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`M? to
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`lllobiles
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`| Hoblles to |
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`[AP
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`| Hobiies
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`[to AP
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`I HI“ TA -—-I
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`I---- TB ----I
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`i--- Tc ----1 l
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`AH - Header for Period A
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`8H - Header for Period B
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`CH - Header for Period C
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`AP - Access Point
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`Figure 1. Frame Structure of Medium Access Control Scheme
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`In the first interval (Period A), the link is used exclusively for outbound data
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`transfer from the Access Point to mobile stations. In the second interval (Pe-
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`riod B), bandwidth is allocated for contention-free inbound data transfer from
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`mobile stations to the Access Point. The allocation of bandwidth is performed
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`by a SCHEDULER resident in the Access Point wireless adapter. Bandwidth is
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`allocated In each frame for inbound and outbound transfers. The third interval
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`is used for transmission from mobile stations to Access Point in a random-
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`access mode of operation. Control as well as data packets will use this inter-
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`Page 3
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`3
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`March 9, 1992
`DOC: IEEE P802.11192-39
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`The control information for Periods A, B and C are AH, BH and CH respec-
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`tively. The medium access control protocol is now briefly described with re-
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`spect to Fig. 1.
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`2.1 Period A ( Outbound Interval )
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`Header AH (Broadcast from Access Point to mobile stations) is the interval
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`during which the Access Point broadcasts a special message to all the mobile
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`stations that identifies the beginning of Period A and contains additional con-
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`trol information shown in Fig. 2.
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`Period A (Broadcast from Access Point to mobile stations) is the interval dur-
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`ing which outbound traffic is transmitted. The Access Point broadcasts packets
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`and mobile stations receive packets addressed to them.
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`Header AH identifies the start of the information frame and contains the Net-
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`work ID, the Access Point ID, the frequency to be used in the next hop (as-
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`suming Slow Frequency Hopping is the underlying PHY), a list of receiving
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`stations and other system control information. The Network ID helps distin-
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`guish between several colocated autonomous LANs.
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`TA “ Length of Period A
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`Length of Period B
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`TC = Length of Period C
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`TAH = Length of Header AH
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`TBH
`Length of Header BH
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`TCH = Length of Header CH
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`TREMHOP = Remaining Length of Hop
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`BSID = Unique Id of the Access Point
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`NET_lD = Network Id
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`Next Frequency to be used in the Slow Frequency Hopping pattern
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`List of Receiving Stations
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`Broadcast Data indicator Flag
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`Figure 2. Control information In Header AH
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`In Period A the Access Point controls the transmissions outbound to the mo-
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`bile stations. The corresponding control information, Header AH, for this
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`Page 4
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`4
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`March 9, 1992
`DOC: IEEE P802.11192-39
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`interval is broadcasted by the Access Point. Each mobile station waits for the
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`header whose contents include those shown in Fig. 2. On correct reception
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`of Header AH, each mobile station sets a timer for TA so that it knows when to
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`receive Header BH and learn about the beginning of Period B. The parame-
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`ters TA, TB and TC lets the mobile stations know how much time is allocated
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`to the three intervals in the current frame. On correct reception of Header AH,
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`each receiving mobile station can determine whether or not it will receive
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`packets from the Access Point (either broadcast or explicitly directed toward
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`If there is no outbound traffic in a frame. then TA will be set to zero In
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`Header AH of the frame.
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`2. Period 8 ( Inbound Slotted Interval )
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`Header BH (Broadcast from Access Point to mobile stations) is the interval
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`during which the Access Point broadcasts a special message to all the mobile
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`stations signifying the end of the Period A and the beginning of Period B. It
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`also contains additional control information shown in Fig. 3.
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`Period B (Contention Free Transfer from Mobile stations to Access Point) is the
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`interval during which mobile stations take turn to transmit according to the slot
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`allocation specified in BH. At the end of Period A, each mobile station waits for
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`Header BH. When received, each mobile station sets a timer for TB so that it
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`knows when to receive Header CH and learn about the beginning of Period c,
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`the contention interval. A mobile station that requested slot allocation in an
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`earlier frame will check to see if it has been allocated any slots.
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`length of Period B
`TB
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`TC — length of Period 0
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`NTR 1—" Number of mobile stations that have slots allocated in the
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`. 8(1)) = User I can transmit 8(1) packets.
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`There will be one such pair for each of the NTR mobile stations that hav
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`slots allocated in the current frame.
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`Figure 3. Control Information in Header BH.
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`Header BH specifies a list of ordered pairs of the term (i, 8(I) > that indi-
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`cates mobile station l is allocated 50') slots in the current frame. Since the list
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`is ordered, the order in which they are allowed to transmit to the Access Point
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`5
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`'
`March 9, 1992
`DOC: IEEE P802.11/92-39
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`_________________________________
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`is known to the mobile stations. Since each mobile station knows the list of
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`stations that precede it and their allocations, It can determine when it should
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`begin its transmission. At its designated time, the mobile station transmits for
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`a fixed period of time whose duration depends on the number of slots allo-
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`cated to it. Thus. contention-tree transfer from mobile stations to Access Point
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`occurs utilizing the slot allocation information specified in BH. If a mobile sta-
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`tion fails to receive BH correctly, then it will not make use of any slots that may
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`have been allocated to it in the current frame. At the beginning of a frame, it
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`there is no pending request from any mobile station for inbound data transfer.
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`then TB (the length of Period 8) ln Header BH of the frame is set to zero.
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`Adjustment of TA and TB in each frame enables the bandwidth to be allocated
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`on a demand-driven basis. If both TA and T8 are set to zero, then the MAC'
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`reduces to exclusively random-access mode of operation. For a large class
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`of applications. outbound traffic (from Access Point to mobile stations) tends
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`to be significantly large compared to inbound traffic (mobile stations to Access
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`Point). In such cases, TA is expected to be the dominant component in a frame.
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`2.3 Period C ( inbound Contention Interval )
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`Header CH (Broadcast from Access Point to mobile stations) is the interval
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`during which the Access Point broadcasts a special message to all the mobile
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`stations, identifying the end of the Period B and the beginning of the Period
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`C. The message also conveys the information In Fig. 4.
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`0
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`TC = length of Period C
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`K = Current estimate of users actively attempting transmission
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`in Random Access Section ( expected to be much smaller than the
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`number of users registered with the Access Point).
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`Control information in Header CH.
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`Figure 4.
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`Period C (Random access from mobile stations to Access Point) is the interval
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`during which any station may contend for the channel and transmit a message
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`without any explicit allocation from the Access Point. A Slotted-ALOHA pro—
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`tocol with suitable modifications for retransmission scheduling is used in this
`interval.‘
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`1 The proposed choice of Slotted-ALOHA in Period C allows eliicient support for simple and
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`inexpensive radio PHY layers (e.g. radios with long transmit-to—receive turnaround times).
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`However. we note that Period C admits the use of random access protocols that. under a
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`different set of PHY layer assumptions. are theoretically more efficient than Slotted-ALOHA.
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`in partiCUlar, a Carrier Sense Multiple Access-based protocol can be used in Period C. Use
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`of such CSMA-based alternative choices. are considered to be within the scope ofthis MAC
`proposal.
`
`6
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`

`

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`DOC: IEEE P802.11192-39
`March 9, 1992
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`Period C is used for the following types of information:
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`0 Request messages for registering with an Access Point
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`0 Bandwidth reservation requests
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`0 Data packets
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`2.3.1 Registration with Access Point
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`The process by which a mobile station introduces itself and requests the ser-
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`vices ot an Access Point is called Registration. The set of registered users at
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`an Access Point will change dynamically with time. An Access Point does not
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`assume a priori knowledge about the number or the identity of mobile stations
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`desiring its services.
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`After a mobile station has monitored the radio environment and chosen an
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`Access Point to register with. it sends a Request Registration Control Packet.
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`The packet contains the Access Point ID as well as the Network ID of the LAN
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`that the mobiie station wishes to join. Other information included are the mo-
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`bile station MAC address. and other information that may be required for ac-
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`cess control purposes. -
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`On receipt of a Request Registration control packet, the designated Access
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`Point processes the request. It responds to the mobile station with a message
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`called Registration Response packet.
`if the registration request is accepted,
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`then the Access Point becomes the Owner of the mobile station. An Access
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`Point provides the MAC functions for all mobile stations for which it is the
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`owner. Ownership of a mobile station can change if it roams within an Ex-
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`tended Service Area and a handoff sequence is initiated.
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`2.3.2 Reservation Requests for Bandwidth
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`Mobile stations request bandwidth for transmission in Contention Free mode
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`(Period B). Such requests can be supported by transmitting the appropriate
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`bandwidth reservation request to the Access Point. Both isochronous or non-
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`isochronous services are supported by transmitting the appropriate bandwidth
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`reservation request to the Access Point. Allocations based on reservation re»
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`quests are specified in the BH header of the subsequent frames.
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`Bandwidth reservation requests are transmitted by mobile stations to the Ac-
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`cess Point using random access Slotted-ALOHA protocol in the third interval
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`(Period C). The Access Point receives reservation requests and processes
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`them according to the SCHEDULER allocation algorithm.
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`Page 7
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`7
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`

`

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`March 9, 1992
`DOC: IEEE P802.11I92-39
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`The actual slot allocation information for inbound transmission is conveyed in
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`Header 81-! of one or more subsequent frames.
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`Reservation requests have the form shown In Fig. 5. Such a request means
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`mobile station with MAC Address MSID requests Access Point APiD to allo-
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`cate REQ_SIZE slots for inbound transmission (i.e.. number of slots needed for
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`transmission in Period 3). Successful (Le. collision-free and error-free) recep-
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`tion of reservation requests are acknowledged by the Access Point as part of
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`the contention protocol
`in Period C.
`The field SERVICE_TYPE indicates
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`whether an asynchronous or isochronous service is required.
`if asynchonous
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`service is required. then no periodicity of request is assumed. If isochronous
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`service is indicated, then REQfiSIZE slots will be allocated in every frame until
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`a cancellation of the request is indicated in a subsequent control packet from
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`the mobile station.
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`INon—Iso) Figure 5. Reservation Request Control Packet
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`Reservation
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`Request packet
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`MAC Address
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`Network ID
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`Access Point
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`ID
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`Reservation
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`Request Size
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`Type of Service
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`(lso.
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`APID
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`REQ SIZE
`-
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`SERVICE_TYPE
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`The SCHEDULER maintains a queue of pending requests for inbound data
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`transfer, each request containing information shown in Fig. 6.
`If there is not
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`sufficient bandwidth to satisfy all reservation requests in a frame. then the
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`unfulfilled requests will be taken up in the following frame. Note that the Ac-
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`cess Point processes the reservation requests within Period A of the next
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`frame and communicates it as part of Header BH.
`‘w—MWW“
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`Page 8
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`8
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`

`

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`-
`DOC: lEEE P802.11192-39
`March 9, 1992
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`At the end of Period B, each mobile station waits for Header CH corresponding
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`to Period C. The header contains TC. the length of the third interval. When re-
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`ceived, each mobile station sets a timer for TC so that it knows when to expect
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`the header for the next frame.
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`HAG Address Request
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`Size
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`isochronous or
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`Non-lsochronous Figure 6. An Entry in the Queue of Outstanding Reservation Requests
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`Phase C is devoted to the transmission of messages using a Slotted-ALOHA
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`protocol. Note that the duration of the Slotted-ALOHA subframe can change
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`from one frame to the next in a manner determined by the Access Point and
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`conveyed in the control headers AH, BH and CH.
`in the Slotted-ALOHA sub-
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`lrame, each mobile station that has a message to transmit will do so oniy at the
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`beginning of a time slot. At the end of each transmission, the users must know
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`if their packets were received correctly (I.e., without collisions) or not. If a col-
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`lision is detected (by lack of a positive ACK). the mobile station schedules a
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`retransmission of the collided packet according to a retransmission scheduling
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`algorithm. Since bursty data, reservation and registration requests must have
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`short response time, parameter TC. the length of the random access portion '
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`of a frame. is always lower bounded by a value. TC__MIN. TC_MIN Is the min-
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`imum number of slots per frame that are always available for use in contention
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`mode (eg, TC_MIN = 20% of the frame length).
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`At the end of each frame, the Access Point may have outstanding packets to
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`be transmitted to mobile stations registered with it. These are first scheduled
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`for transmission in Period A. Recall that TB is the length of Period B. The
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`SCHEDULER allocates bandwidth subject to the constraint (TA + T8) is never
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`allowed to exceed 80% of irame length.
`If there is no outbound traffic and
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`there are no outstanding reservation requests, TC is increased to 100%.
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`When two stations are within a Basic Service Area and registered with the
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`same Access Point, then there are two choices for communication between
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`them. Fig. 7 shows an Access Point and two mobile stations that are registered
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`with it.
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`Page 9
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`9
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`

`

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`March 9, 1992
`DOC: IEEE P802.11/92-39
`
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`-
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`The data transfer can occur in:
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`Store-and-Forward (84’) repeat mode: The Access Point receives from the
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`source station. and retransmits to the destination. This requires two trans-
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`missions but has potential to provide coverage over a larger area.
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`0 Direct Data Transfer mode: Stations communicate directly with a single
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`transmission from source to destination station (no active intervention by
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`Access Point during data transfer).
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`S-F repeat
`mode
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`ACCESS
`POINT
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`S-F repeat
`mode
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`direct transfer mode
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`--«————————-——+
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`A mobile station may (or may not) be able to communicate directly to another
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`mobile station registered with the same Access Point.
`in the Store—and-
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`Forward repeat mode. a mobile station communicates through the Access
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`Point to another mobile station in the network.
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`The following are two modes of system operation for direct data transfer be-
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`tween mobile stations.
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`0 Mode 1: The first mode of operation is Random Access Direct Data Trans-
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`fer. In this mode. a mobile station communicates directly to another mobile
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`station in Period C. The Access Point will not receive the packet for
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`store-and-forward transmission.
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`mode I
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`source
`address
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`destination
`address
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`Data
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`
`Page 10
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`10
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`10
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`

`

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`March 9, 1992
`DOC: IEEE P802.11I92—39
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`Mode 2: The second mode of operation is Reserved Access Direct Data
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`Transfer.
`In this mode, a mobile station communicates directly to another
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`mobile station in Period B of the frame. Bandwidth allocation ls requested
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`from the Access Point in Period C. The Access Point then includes the fol-
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`lowing in Header BH of Period B in a subsequent frame. The message has the
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`following form:
`‘
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`mode 2
`
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`source
`address
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`destination
`address
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`allocated
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`time slots
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`The message allows both the sender and the receiver know when data trans-
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`mission will happen, and that the second mode operation is active in the as-
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`signment slots.
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`SUCCESS
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`RANDOM ACCESS
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`
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`DIRECT DATA
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`TRANSFER HODE
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`ACCESS POINT
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`S—F REPEAT MODE
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`RESERVED ACCESS
`DIRECT DATA
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`TRANSFER MODE
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`Mode 2
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`SUCCESS
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`Figure 8. Different Modes of Data Transfer In a Basic Service Area
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`The above two modes of operation permit a mobile station to use Direct Data
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`Transfer mode whenever it is in range of the destination station. The source
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`station first tries the Direct Data Transfer mode.
`If not successful (lack of suc-
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`cess in a number of retries), it enters the S-F repeat tnode. Using the Access
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`Point. the packets are delivered to the destination. Fig. 8 summarizes the dif-
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`ferent modes of operation.
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`Page 11
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`11
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`11
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`

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`March 9, 1992
`DOC: IEEE P802.11192-39
`——-—-—————-————-————-———-———-—————~———...__...__
`
`Response to 21 Criteria
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`1) Unauthorized network access impact on throughput
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`The proposed MAC protocol is a hybrid of Reservation TDMA and random
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`access schemes. Slow Frequency Happing Spread Spectrum communication
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`is used for intercell isolation in a multicell radio network. Users in a BSA can
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`utilize the channel only after successfully registering with an Access Point.
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`The registration procedure will exclude an unauthorized user trom accessing
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`a network. Users in adjacent cells of the same network as well as cells of
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`colocated networks will
`typically be hopping in non-interfering channels
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`(please see response to item 8). Their access of their networks should have
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`marginal impact of system throughput.
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`in an Adhoc-LAN. only the users that belong to the network will impact each
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`other. Users belonging to other networks will again be isolated in frequency
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`domain and will have marginal impact on system throughput.
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`2) Establish peer-to—peer connectivity without prior knowledge
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`Peer-to-peer connectivity can be established by creation of Adhoc LAN net-
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`works. Such networks are created to facilitate communication between mobile
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`stations without relying on the existence of a distinguished entity a la Access
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`Point. The networks are created on-the~fly on a temporary basis consisting of
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`mobile stations that wish to participate in such a network.
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`This section describes-a simple approach to create Adhoc networks. With this
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`approach, mobile stations would require implementation of Access Point
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`functions in software in mobile station adapter cards. A mobile station is ex-
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`plicitly designated to provide the basic control functions of an Access Point.
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`The designated mobile station, called an Adhoc-type Access Point, performs
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`only the core functions of an Infrastructure-type Access Point. These include
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`the following:
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`1. Bandwidth allocation based on reservation requests
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`2. Registration of stations that wish to ioin the Adhoc network
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`3. Timing and framing information for coordinating the stations in the Adhoc
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`network. With a Frequency Hopping PHY layer, the control lunctlons in-
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`cludes specifying the frequency hopping pattern to use, time instants when
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`the stations should hop and how long they should stay in a hop. The above
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`Page 12
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`12
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`12
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`DOC: IEEE P802.11I92-39
`March 9, 1992
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`control information facilitates synchronized frequency-hopping by all mobile
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`stations that belong to the Adhoc LAN. Data transfer will occur using the same
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`MAC protocol as in infrastructure-based LAN. The primary mode of operation
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`is Direct Data Transfer between stations that belong to the Adhoc LAN. The
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`Adhoc Access Point need not function as a Store—and-Forward repeater for
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`data communication between peer stations that are in direct range of each
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`other. However, Store-and-Forward repeat is a backup option available at the
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`Adhoc Access Point and can be used for extending the coverage range of the
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`Adhoc LAN.
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`A promising direction for automatic creation of Adhoc LANs is to create sys-
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`tems in which a station is elected as an #Adhoc-type Access Point, performs the
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`core functions of an Access Point for some duration and the responsibility for
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`performing Adhoc-type Access Point functions is rotated among the partic-
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`ipating stations in an equitable manner.
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`3) Throughput
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`is a hybrid of contention-free (Periods A and B) and
`The MAC protocol
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`random—access protocols (Period C). Suppose the fraction of bandwidth used
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`in Periods A and B is f. The fraction used in Period C is (1 — f). The maximum
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`throughput achievable is [100(f+ kmmfi -- 0)]%, where Aswan = 0.36. If we
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`set I = 0.8, the maximum throughput is about 07.2%}' The average throughput
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`is a function that depends very much on the traffic model assumptions.
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`The following simulations show that high link utilization in an environment of
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`heavy traffic is achievable and the system is stable even under high load con-
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`ditions. Periods A and B are based on reservation and are unconditionally
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`stable. Stability in Period C is achieved using adaptive retransmission strate-
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`The system modele