WO2000059260A1

WO2000059260A1 - Data communications method and data signal

Info

Publication number: WO2000059260A1
Application number: PCT/GB2000/001125
Authority: WO
Inventors: John David Porter; Walter Charles Vester
Original assignee: Adaptive Broadband Ltd.
Priority date: 1999-03-31
Filing date: 2000-03-24
Publication date: 2000-10-05
Also published as: AU3444200A; GB2348581B; GB2348581A; GB9907481D0

Abstract

A data communications method and data signal is presented wherein dynamic time-variable time-division duplexing can be achieved by virtue of control data in the form of a frame descriptor header, providing full a priori knowledge of every subscriber terminal in a cell of the expected contents, sturcture and/or timing of the remainder of the data to be transmitted onto a common channel in both the upstream and downstream direction in the remainder of the frame. The invention has the advantage that channel utilisation efficiency can remain high regardless of the symmetry of the upstream and downstream traffic.

Description

Data Communications Method and Data Signal

TECHNICAL FIELD

The present invention relates to a data communications method and a data signal for use m a data network where access to a common communications channel must be controlled

More particularly, the present invention provides a bandwidth efficient and highly integrated method of performing robust data transfer and bandwidth allocation m a pomt-to-multipomt data network m which bandwidth is a scarce resource bemg managed The advantages of the present invention over other similar implementations are that it provides the ability to dynamically assign bandwidth to multiple network terminals and multiple types of traffic BACKGROUND OF THE INVENTION

Data networks can be classified m many ways, but for the purpose of the present invention, it is useful to classify them by their means of accessmg the medium over which data is communicated The relevant classifications are broadcast and non-broadcast

An existing type of data network is Ethernet Ethernet uses broadcast medium access All network nodes sharing the network medium hear all traffic bemg passed over the medium Traffic is directed to mdividual network nodes via physical layer addresses that are attached to the data packets bemg sent over the medium When multiple network nodes attempt to transmit data simultaneously there is the possibility for contention among the nodes for access to the medium

A modification to the broadcast network is the broadcast network with hidden terminals In this network, all termmals share the same medium. however it cannot be guaranteed that all terminals can hear each other All that can be guaranteed is that all termmals can hear the central network node, referred to herein as the access pomt For this reason, it is not enough for each terminal simply to momtor the channel m order to detect contentions Feedback on success or failure of network contention must also be commumcated back to the network terminals by the access pomt

In contrast to the above, a non-broadcast network can be provided using connection-oriented protocols such as, for example, asynchronous transfer mode (ATM) ATM provides for the reservation of channel bandwidth by network nodes by the defmition of virtual channels within the medium More generally within a non-broadcast network, the medium that connects a network node to the rest of the network can only be accessed by two devices the network node itself, and the network switch to which it is attached The medium itself is full duplex. so there is no possibility for contention

Broadcast networks require some mechanism for distributing access to the medium This is because as network loading mcreases. the non-determmistic nature of random attempts at accessmg the medium causes the throughput and efficiency of the network to fall off catastrophically The uncertainty of the loadmg or the potential success of the medium contention results m non- determmistic transit times for traffic bemg passed across networks with broadcast medium access Furthermore, each node on the network is a peer, meaning it has equal priority when attempting access to the medium

By contrast non-broadcast networks gracefully approach maximum capacity until they reach very high medium loadmg Because there is no contention, traffic can be passed over the medium with known absolute limits to propagation delay time and bit error rate Network transfer efficiency can therefore be mamtamed. and m particular under conditions of high network loadmg

The problem of poor performance of networks usmg a broadcast medium at hiεh network loadmg can be solved bv emulating non-broadcast networks This is achieved by assigning fixed time slots to all nodes of the network, and restricting the transmissions of each node to its particular assigned time slot Such an arrangement is known as Time Division Multiple Access (TDMA) While such time-slot arrangements allow the network to operate well when at high levels of loadmg and with equal data loads from each network terrmnaL at low network loadmg the efficiency of medium utilisation is very poor, as each node must wait for its assigned time slot before transmitting SUMMARY OF THE INVENTION In contrast to the above, the present invention provides for dynamic time slot assignment via centralised control of medium access In order for a network node to gam access to the medium it must first be granted permission by the access pomt The grant to a particular node of permission to access the medium is commumcated by the access pomt to all network nodes, so that each node has substantially full a priori knowledge of the structure of mtended data traffic on the channel for a particular time peπod By such a mechanism, medium access can be centrally controlled by the access pomt m a dynamically time - variable manner

Accordmg to the present invention there is provided a data communications method for use m a pomt-to-multipomt network compπsmg a central control node and one or more remote subscπber nodes, said network having a bi-directional commumcations channel accessible to all network nodes, said method compπsmg the steps of a) transmitting a first data portion mcludmg control data onto the channel from the central control node exclusively within a first time penod, b) receiving said first data portion mcludmg control data at each of said remote subscπber nodes. c) transmitting further data portions onto the channel exclusively within a second time peπod, said further data portions bemg transmitted one at a time from particular of the remote subscπber nodes m response to the received control data, and d) receiving said further data portions at the central control node. wherem said control data indicates to each of said remote subscπber nodes those particular of the nodes which are permitted to transmit at step c) whereby access to the channel can be dynamically controlled by the central control node The control data can mdicate to each of the remote subscπber nodes the structure of the remamder of the first data portion, and also mdicate to each of the remote subscπber nodes the expected content and/or structure and/or timing of the further data portions to be transmitted from the remote nodes to the central control node at step c) Furthermore, when a particular remote subscnber node has payload data to transmit to the central control node, the method mcludes the further steps of generating a channel reservation request requesting permission to transmit onto the channel, transmitting the channel reservation request onto the channel within a contention peπod defined by said control data, said contention peπod bemg a peπod within said second time peπod when the further data portions are not bemg transmitted, and receiving said channel reservation request at said central control node, acknowledgmg said channel reservation request m the next transmitted first data portion, granting permission to the remote subscπber node to transmit the payload data onto the channel by indicating to the node in the control data of one of the subsequently transmitted first data portions that permission has been granted, and transmitting the payload data onto the channel as one of the further data portions m the second time penod immediately following the first data portion m which permission was granted

Accordmg to another aspect of the present invention, there is provided a time-division duplex data signal for transmission onto a channel for use m a pomt-to-multipomt data network compπsmg a central control node and one or more remote subscπber nodes, said signal compπsmg a) a first data portion mcludmg a control data portion for transmission from the central control node over the channel to each of the remote subscπber nodes, and b) further data portions for transmission from particular of the remote subscπber nodes over the channel to the central control node in response to the control data portion. wherein said control data portion mdicates to each of the remote subscnber nodes the particular of those nodes which are permitted to transmit one or more further data portions, wherem access to the channel can be dynamically controlled by the central control node

The control data portion may further compnse a downstream structure data portion indicating the structure of the remamder of the first data portion and an upstream structure portion mdicatmg the expected contents and/or structure and/or timing of each of the further data portions

The signal may further mclude a channel reservation request portion for transmission from particular of the remote subscπber nodes over the channel to central control node, said channel reservation request portion compπsmg one or more channel reservation requests each generated by a smgle remote subscπber node in response to a requirement to transmit data traffic to the central control node, wherem each of the channel reservation requests are transmitted by the respective remote subscπber nodes onto the channel duπng the channel reservation request portion, wherem each of the channel reservation requests is m contention with one another for channel capacity

It is an advantage of the present invention that the central control node has the ability to throttle any subscπber terminal^'s access to the medium

It is another advantage of the present invention that it provides for dynamically vaπable time division duplexing thus allowing medium utilisation efficiency to stay very high regardless of the symmetry of the upstream and downstream traffic

Furthermore, the present invention allows persistent bandwidth allocations for support of constant bit rate traffic, and by virtue of dynamic bandwidth allocation also enables statistical multiplexing, which mcreases the number of subscπbers that can be provisioned service from a smgle access pomt It is a further advantage of the present invention that it provides high efficiency of medium utilization during peπods of high network loadmg and provides low latency medium access duπng peπods of low network loadmg

There is yet another advantage of the present invention m that it provides dynamic bandwidth allocation while minimizing contention and minimizing reservation grant latency. Furthermore, the use of reservation grants decouples contention success from the quantity of queued-up traffic load. It is a yet further advantage of the present invention that communication of the complete contents and timing of the expected contents and structure of the further data portions to all network termmals takes place via the control data portion. This allows the control data to serve the additional purpose of granting reservations to subscriber terminals. It is a feature of the present invention that each network node has the ability to eπor check each data cell of each received data portion mdependently, thus allowing implementation of a selective repeat automatic repeat query m a point-multipoint network.

It is a further feature of the present invention that position of data within a portion conveys mformation. More specifically, acknowledgements of the receipt of data bursts are ordered the same as the bursts themselves, therefore the acknowledgement does not require an additional identifier to be transmitted over the air. This mcreases bandwidth efficiency BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the present invention will become apparent from the following detailed descπption of a particularly preferred embodiment thereof, presented by way of example only, and by reference to the accompanying drawings, m which:-

Figure 1 shows the overall frame structure of the time division duplex data signal of the present invention;

Figure 2 shows the frame structure of the control data portion used in the present invention;

Figure 3 shows the frame structure of a smgle reservation request acknowledgement cell of the present invention;

Figure 4 shows the frame structure of a smgle downstream acknowledgement cell of the present invention;

Figure 5 illustrates the structure of a single downstream payload cell of the the present invention; Figure 6 shows the structure of a smgle subscπber reservation request cell of the data signal of the present invention;

Figure 7 shows the frame structure of a smgle upstream acknowledgement cell of the present invention;

Figure 8 illustrates the frame structure of an upstream payload cell with reservation request of the present invention;

Figure 9 illustrates the frame structure of a smgle upstream payload cell without reservation request;

Figure 10 demonstrates the sequence of messages mvolved in remote subscπber terminal registration m the present invention; Figure 11 shows the sequence of messages mvolved in transmitting a smgle payload cell from upstream to the central access pomt from a remote subscriber terminal;

Figure 12 shows the sequence of messages mvolved m transmitting multiple payload cells upstream to the central access pomt from a remote subscriber terminal; and

Figure 13 shows a schematic representation of the wireless access network within which the present invention is employed. DESCRIPTION OF THE PREFERRED EMBODIMENT

The method of data communication and data signal of the present mvention are chiefly for use within a wireless access network, although it will be readily apparent to those skilled m the art that the said method and signal may equally be employed within a wired network Within the particularly preferred embodiment to be descπbed herem. the present mvention is employed within a wireless network deployed in a cellular configuration, as shown m Figure 13 Each cell 102 consists of a central access pomt 104 and multiple subscnber units 106 Subscπber units communicate to the network only through the access pomts, makmg the network a pomt-multipomt architecture Each access pomt is the center of all wireless network communication for the particular cell, and thus is the locus of control of access to the wireless medium for the cell Each access pomt is connected to a wide area network 120 via a respective backhaul link 108 Higher layer network functions are provided by means of a network server 110

Although it will be appreciated that Figure 13 shows multiple cells and access pomts foπning the network, the following descπption concentrates on the operation of the present mvention m a smgle cell only

Communication can occur on the wireless medium in both directions, and hence a means of duplexing the wireless medium is required Two common methods are frequency division duplexing (FDD) and time division duplexing (TDD) In the case of FDD, the medium is broken mto a downstream (data oπgmating from the access pomt) frequency band and an upstream frequency band (data oπgmatmg at the subscπber unit) TDD breaks a smgle frequency band mto downstream time slots and upstream time slots The data communications method system and signal of the present mvention uses TDD

For the purpose of understanding the present mvention. it is useful to view the network m each cell to which the mvention is applied as consisting of multiple switches On one end, coπesponding to an access pomt 104. there is a switch with a smgle physical wired data port, and multiple wireless data ports Disbursed throughout the cell are two port switches, each located at a subscπber terminal 106 Each subscnber terminal has a smgle wireless port and a smgle physical wired port The network model is complicated by the fact that a subscπber termmal can be powered on and off at will, thus there are tunes that the subscπber termmal switches are not accessible to the network This gives πse to the concept of subscnber termmal registration, the procedure for which will be descπbed m detail later Duπng the registration process, a subscπber termmal negotiates with an access pomt to be assigned a temporary port identifier, referred to as a subscπber unit access identification (SU_ALD) Once a subscπber termmal has been granted an SU_AID. it is capable of proceedmg with higher layer signalling to gam access to the network The present mvention is directed towards controlling access of the subscnber termmals to the wireless medium through central control of the subscπber termmals by the access pomt In order to achieve this the access pomt is provided with a medium access controller (MAC) which administers the medium control Similarly, each remote subscnber termmal is also provided with a compatible medium access controller for respondmg to the central MAC m a master-slave manner the subscπber terminals request access to the medium and the access pomt has the ability to grant access or fail to grant access based on the current level of network utilisation Access to the network is granted m the form of time slots - when a subscπber termmal is granted the ability to access the wireless network medium, it is granted one or more time slots m which it can transmit Within the granted time slot the entire medium capacity is available to the subscnber node to transmit its payload data By referring to a medium access controller, it is to be understood that either a hardware or software based control means is envisaged and that reference to to a controller as such implicitly mcludes reference to those control means required at both the central access pomt and at the subscπber termmals In this respect, the medium access controller (MAC) therefore corresponds to those network means, whether hardware or software based, that would approximate m part to the Network-level and/or the Data-level of the ISO Open Systems Interconnection 7-layer Reference Model In its best mode, the MAC should be implemented m a Field Programmable Gate Array (FPGA) provided with memory

The MAC operates by controlling transmissions on the medium by the definition of a MAC frame, bemg the framework m which data transmissions take place In order to fully understand the vaπous features and advantages of the present mvention. it is necessary to first descnbe m detail the constituent parts of a MAC frame This will be performed by reference to Figures 1 through 9

Figure 1 shows the overall structure of a smgle MAC frame The MAC frame consists of a downstream portion, generated by the access pomt and broadcast to all subscπber termmals. and an upstream portion, which consists of a contention interval and all data bursts bemg sent from subscπber termmals back to the access pomt

The downstream portion consists first of a downstream preamble (2) The preamble is a Physical layer synchronization sequence of fixed length, used for frame acquisition and channel estimation. Only one downstream preamble may occur per MAC frame Immediately following the preamble is the frame descπptor header (FDHDR) (4) The FDHDR descπbes the complete contents of the remainder of the MAC frame The size of the FDHDR may varv although only one FDHDR may occur per frame The FDHDR contains a map of all traffic upstream and downstream, to occur within the MAC frame m both the remamder of the downstream and the complete upstream directions .After achieving bit synchronisation on the MAC frame via the preamble, subscπber termmals demodulate the FDHDR and from that gam complete knowledge of the traffic that will occur within the remamder of the frame Only one FDHDR may occur per MAC frame The precise contents of the FDHDR are shown m Figure 2 and descnbed m detail m Table 1 below Field Tag Description No. of Bits

SYNC Short 4 symbol sync burst 8

BD_cnt Bursts Downstream Count Number of 4 subscnber units having payload data sent to them m this MAC frame

BU_cnt Bursts Upstream Count. Number of subscnber 4 units that will be sendmg payload data m this

MAC frame

AP ID Access Pomt ID Identifies the access pomt that ongmated the frame descπptor header

RRA cnt Reservation Request Acknowledgment Count

Number of acknowledgments bemg sent m response to previous requests

DA cnt Downstream Acknowledgment Count Number of upstream cell acknowledgements bemg sent downstream m this MAC frame

Downstream Identifies the subscnber units bemg sent cells. 16 Map the number of cells to be sent, and the traffic type bemg sent

RR cnt Reservation Request count Total number of reservation request slots that will be made available m this MAC frame

UA cnt Upstream Acknowledgment count Total number of downstream cell acknowledgments bemg sent upstream m this MAC frame

Upstream Identifies the subscnber units that have been 16 Map granted reservations, the number of cells to be sent by each, and the traffic type allowed Field Tag Description No. of Bits

CRC Cyclic Redundancy Check. .Allows each 8 subscriber teiminal to verify conect receipt of the frame descriptor.

Table 1 : Frame Descriptor Header (FDHDR) Structure

When there is data traffic to be sent from the AP to one or more subscriber terminals per MAC frame the downstream map in the FDHDR consists of respective one or more data sequences to identify each SU to which data traffic is to be sent and to inform the SU of the number of cells bemg sent and the traffic type. The data sequence is repeated for each SU to which data is bemg sent m the present MAC frame, with the appropriate SU_AID at the start of each sequence and traffic information in the form of a Cell_cnt value and a TRjype flag. The downstream burst maps therefore communicate to all of the sus which of the SUs are to be sent downstream bursts in the present MAC frame, and it should be understood that multiple downstream bursts may be sent to multiple different SUs in the same MAC frame.

Similarly, the upstream map m the FDHDR takes the same form, that is one or more respective data sequences to identify those SUs which have been allocated a transmission slot (i.e. allocated an upstream burst) and which are therefore expected to transmit in the upstream portion of the present MAC frame, the number of cells they are to transmit and the traffic type. The data sequence is repeated for each SU which is expected to transmit with the appropπate change in SU_ALD at the start of the sequence, Cell_cnt value, and TR_type flag. As will be apparent, the upstream burst maps therefore communicate to all of the SUs which of the SUs have been allocated an upstream burst slot m the upstream portion of the present MAC frame, and it should be understood that multiple upstream bursts may be received from multiple different SUs m the same MAC frame. The format of the upstream and downstream maps in the FDHDR is also shown m Figure 2, and given m detail m Table 1 1 below -

Field Tag Description No. of Bits

SU_AID SU_AID of SU to which data is bemg sent 8 to/expected to be received from

Cell_cnt No of payload cells to be sent by AP/expected 4 to be received by AP (allowed values 1 to 6)

TRjype Traffic pnonty type (allowed values 0 to 3) 4

Table 1 1 Downstream and Upstream Map Structure m the FDHDR

Following the FDHDR is the reservation request acknowledgement (RRA) portion 6 The RRA acknowledges a request by a subscπber for upstream time slots and can also commumcate signal propagation delay There is a smgle RRA for each reservation request that was made during the contention interval from the previous MAC frame, although m the case where no reservation requests were made m the previous MAC frame, then no acknowledgements will be sent The precise contents of the RRA are shown m Figure 3 and descπbed m detail m Table 2 below

Field Tag Description No. of Bits

Sync 8 bit framing synchronization sequence 8

SU_ID ID of the subscπber unit that ongmated the 8 reservation request, and to which the reservation request acknowledgment is directed

RTRN Return Code Communicates reservation status 4 to SUs and SU_AID status to SUs performing registration Field Tag Description No. of Bits

DELAY Delay compensation bits These bits are 12 assigned during subscnber unit registration and cause a shift m subscnber unit timing.

CRC Cyclic Redundancy Code Used by the 8 subscπber unit to venfy that the frame has been received error free

Table 2 Reservation Request Acknowledgement (RRA) Structure

Following the RRA comes the Downstream Acknowledgement (DACK) portion 8 containing DACK cells. Each DACK cell contams a downstream ack or nack of a smgle upstream burst from a previous MAC frame There is a single DACK cell for each upstream burst from the previous MAC frame, although in the event that there were no previous upstream bursts then no DACKs will be sent. The precise contents of a DACK cell are shown in Figure 4 and described in detail in Table 3 below.

Field Tag Description No. of Bits

Sync 4 symbol synchronization burst SU_ID ID of the subscriber unit that originated the cells bemg acknowledged.

UU Unused

Ack/Nack One acknowledgment bit per cell. 1 = map successful cell receipt.

CRC Cyclic Redundancy Code. Used to verify that the downstream acknowledgment has been received eπor free. Table 3 Downstream Acknowledgement fDACK cell structure

Following the DACK portion comes the Downstream Burst (9) The MAC operates on a principle of cell bursts for communicating payload data between the access pomt and the subscnber termmals by allowing multiple cells of data to be sent to or from a particular subscnber unit at a time A burst must always consist of at least one cell. In upstream bursts, this smgle cell must be an upstream cell with reservation request (UCELLR) ( 18) Additional cells m the upstream burst are m the format of a UCELL - an upstream cell without reservation request (20) Upstream cells are discussed m more detail later Downstream bursts can also consist of multiple cells, but there is oniy one type - the downstream cell (DCELL) 10. There can be many DCELLs per frame - either several directed to a smgle subscnber termmal, or several directed to several subscπber termmals. Each DCELL contains one ATM cell of payload data. Currently the MAC allows bursts to have a maximum size of six cells, although more or less cells may be designated per burst if required m a future implementation without departing from the scope of the present mvention. The structure of a DCELL is shown m more detail m Figure 5, and descπbed m Table 4 below

Field Tag Description No. of Bits

Sync 4 symbol synchronization burst SU ID LD of the subscπber unit to which the payload data is directed.

SEQ Sequence number. Used by the MAC to resequence cells that get out of sequence due to cell loss and subsequent cell repetition

Condensed Includes Virtual Path Identifier (VPI), Virtual 24 ATM Header Channel Identifier (VCI), Traffic Type, Cell

Loss Prioπty Field Tag Description No. of Bits

Payload Payload data. 48*8

CRC Cyclic redundancy code. Used to verify 16 coπect receipt of the downstream cell.

Table 4 : Downstream Cell (DCELL) Structure

The downstream burst concludes the downstream portion transmitted by the access point and received at all subscπber termmals. There then follows a slight delay due to subscriber turnaround time (STT) 12. The STT vanes with distance to the farthest subscriber umt. A typical maximum distance to a subscriber unit could be , for example, 5km, although this obviously depends on the network configuration and the size of each network cell.

Following the STT comes the Upstream Portion of the MAC frame, being data transmitted from the subscriber units to the access point. The entire expected structure of the upstream portion has already been commumcated to each and every subscriber terminal in the FDHDR transmitted in the downstream portion. Therefore, each subscriber terminal knows whether or not it is permitted to transmit in the upstream portion, what data it is to transmit and when it is to transmit this data. In this way absolute control of the contents of the upstream portion can be controlled by the access point. With such a mechanism, however, it becomes necessary to define a period in which subscriber terminals can first communicate a request for transmission permission to the access point without which no subsequent permission would ever be granted. This period forms the first part of the upstream portion, being the subscriber reservation request (SRR) portion 14.

The SRR is a contention based reservation request interval. If a subscriber terminal has been sitting idle with empty data queues, the arrival of a burst of data on its physical port will force it to request a time slot reservation from the access pomt. Because the subscriber terminal has no active reservations, and because it is believed that at any given time the number of termmals makmg initial bandwidth requests will be small, it is reasonable to force the subscπber terminals to contend for reservations. This contention window is kept as small as possible while still allowing reasonable success probability by employmg a novel implementation of aloha contention control schemes. Once the subscπber terminal^'s reservation request has been acknowledged by the access pomt, the subscriber terminal ceases requesting bandwidth in the contention slots, allowing other terminals access to the contention interval. The number of SRR's that may occur m one MAC frame is commumcated to the subscπber termmals m the FDHDR. Multiple slots can be made available during times of heavy request traffic. Furthermore, the start of the contention interval can be calculated by the subscriber termmals by virtue of the FDHDR indicating to each terminal the number of RRAs, DACKs and the structure of the downstream burst in the subsequent downstream portion of the MAC frame. The contention interval then begins immediately after the end of the downstream burst, allowing for the STT. The structure of a single SRR to be transmitted during the contention interval is shown in Figure 6, and described in detail in Table 5 below

Field Tags Description No. of Bits

Preamble Physical layer synchronization sequence 32

Sync 8 bit MAC framing synchronization burst 8

SU D ID of the subscriber unit requesting a 8 reservation.

Cells Number of cell time slots being requested by 4 the subscriber unit.

Tr Type Traffic type of the data for which time slots 4 are being requested. Field Tags Description No. of Bits

CRC Cyclic redundancy code Used to venfy correct receipt of this upstream burst

Table 5 Subscnber Reservation Request (SRR) structure

Following the contention interval comes the upstream acknowledgement portion 16 . containing upstream acknowledgement (UACK) cells of each downstream burst received duπng the downstream portion Each UACK indicates upstream ack or nack of a smgle downstream burst from a previous MAC frame As many UACKs may be transmitted m each upstream acknowledgement portion as there were downstream bursts m the the downstream portion The structure of each UACK cell is shown m Figure 7, and descnbed m detail below m Table 6

Field Tag Description No. of Bits

Sync 4 symbol synchronization burst 8

SUJD ID of the subscnber unit acknowledgmg receipt 8 of downstream cells

UU Unused 8

Ack/Nack One acknowledgment bit per cell 1 = 8

Map successful cell receipt

CRC Cyclic Redundancy Code Used to venfy that 8 the downstream acknowledgment has been received enor free

Table 6 Upstream Acknowledgement (UACK structure

Following the upstream acknowledgement portion comes the upstream burst portion 22. containing cell bursts from subscπber units which were granted permission m the FDHDR to transmit payload data to the access pomt The FDHDR from the downstream portion contams the instructions to the subscπber terminals on when to transmit a burst m the upstream burst portion, and what the burst is expected to contam Each upstream burst contams one or more data cells with the same traffic type bemg sent from a particular subscnber terminal Each upstream burst made m the upstream burst portion may be from a different subscnber unit, or alternatively may be from the same subscnber unit depending upon the channel allocations granted to the subscnber units In this way channel allocations can be dynamically arranged between the subscnber terminals from MAC frame to MAC frame, depending on the network traffic loadmg and the traffic pnoπty As mentioned earlier, each upstream burst must contam a smgle upstream cell with reservation request (UCELLR) 18. and zero or more upstream cells without reservation request (UCELL) 20 The condition that a burst must contam a UCELLR allows a subscnber termmal to maintain its channel reservation until all of its payload data has been sent, thus meaning that the subscriber termmal need not transmit again during the contention interval to request channel allocation to transmit the remamder of its data This combination of the reservation maintenance request and the upstream cell mto one message allows a smgle downstream acknowledgement to serve as both reservation request acknowledgement and payload cell acknowledgement, thus improving bandwidth efficiency

The structure of a UCELLR is shown m Figure 8, the contents of which are descnbed below m Table 7

Field Tag Description No of Bits

Preamble Physical layer synchronization sequence 32

Sync 8 bit MAC framing synchronization sequence 8

SU_ID ID of the subscnber unit from which the 8 payload data is ongmated Field Tag Description No of Bits

RSV_MAINT Reservation maintenance Used by the subscπber termmal to continue requesting time slot reservations without contendmg for them

Cells Number of time slots bemg requested by the 4 subscnber unit for future MAC frames

Tr-Type Traffic Type of the data to be sent by the 4 subscnber unit m future MAC frames

SEQ Sequence number Used by the MAC to 8 resequence cells that get out of order due to cell loss and retransmission

Condensed Includes VPL VCI, Traffic Type, Cell Loss 24 ATM Header Pnonty Payload Payload data 48*8 CRC Cyclic redundancy code Used to venfy 16 coπect receipt of the downstream cell

Table Upstream Cell with Reservation Request (UCELLR) structure

The structure of a UCELL is shown m Figure 9. the contents of which are descnbed below m Table 8

Field Tag Description No. of Bits

Preamble Physical layer synchronization sequence 32

Sync 8 bit MAC framing synchronization sequence 8

SU_ID ID of the subscnber unit from which the 8 payload data is ongmated Field Tag Description No. of Bits

SEQ Sequence number. Used by the MAC to 8 resequence cells that get out of order due to cell loss and retransmission.

Condensed Includes VPI, VCI. Traffic Type, Cell Loss 24

ATM Header Prionty

Payload Payload data 48*8

CRC Cyclic redundancy code. Used to venfy 16 coπect receipt of the downstream cell.

Table 8 Upstream Cell with no Reservation Request (UCELL)

Various features and advantages ansing from the structure of the MAC frame of the present invention will now be descnbed.

As mentioned earlier, channel access is mediated by the central controller Channel access information for a particular MAC frame is communicated to all network termmals by the access pomt by virtue of the FDHDR The MAC is fundamentally dependent on the subscπber termmals^' knowledge of the timing of all network traffic from the FDHDR. All traffic over the wireless network medium must therefore be contamed within the MAC frame, whose contents, size, and time ahgnment are determmed by the MAC at the access pomt. This requires that subscπber termmals have the ability to synchromse to the traffic m the network before they can transmit within the contention interval defined by the access pomt. Subscnber termmals therefore achieve physical layer synchronisation by monitoring the channel for the 32 symbol correlation sequence refened to as the PREAMBLE m figure 1 They then acquire MAC frame synchronisation by searching for the 8 bit frame synchronisation sequences located at the start of each transmitted cell. A particular advantage of the provision of the upstream and downstream acknowledgement portions will now be descπbed

The wireless medium over which the data is sent is by nature unreliable The wireless network operates at a speed that prohibits the use of forward error correction (FEC) to improve the raw channel bit error rate In the absence of FEC, the network performs selective repeat automatic repeat query (ARQ) Smce the network is supporting the transfer of data requiring fixed and guaranteed latencies, the acknowledgement and retry process is implemented at the MAC layer, where it can be handled with a muirmum of delay The present mvention therefore has the ability to perform quick turnaround automatic repeat query while still satisfying quality of service commitments of the wired network beyond the access pomt

The retry mechanism relies on the upstream and downstream acknowledgements of the MAC (UACK and DACK), which contam bit maps coπesponding to mdividual cells m an upstream or downstream burst Each network node has the ability to detect bit errors usmg the CRC codes at the end of each cell The recipient of a burst then conveys the success or failure of the aπival of a particular cell by setting the corresponding bit of the burst map contained m the ACK The onginal sender of the burst reads the bitmap m the ACK and then re-sends the cells that were lost The bit map is arranged so that the position of a bit m the bit map corresponds to the order m which a burst was received In this way the bit position conveys information m itself and therefore the acknowledgement does not require an additional identifier to be transmitted

In order that the full operation of the present mvention can be understood, vanous operational scenanos will now be descnbed. with reference to Figures 10, 1 1, and 12

With reference to Figure 10. the procedure for subscπber termmal registration usmg the method and signal of the present mvention is first descπbed below

When a subscπber umt (SU) is first powered on. it is not immediately able to access the network medium It first has to go through the process of access network registration and access network authentication Access network registration is handled purely within the medium access controller of the access pomt (AP), access network authentication is completed by other controllers outside the MAC However, the initiation of the access network authentication process is what also initiates access network registration The signalling that is passed for authentication purposes is mentioned m this descnption because of its close coupling with the registration process

Upon power up, the subscπber umt (SU) generates an authoπsation initiation cell which must be sent to the access pomt It is placed m a traffic queue, which causes the MAC to proceed through the process of requesting a reservation When sitting idle, the SU searches for the preamble sequence (30) of the downstream portion of the MAC frame (see figure 1) Once it has detected the preamble, the SU demodulates the frame descπptor header (32) From the contents of the frame descπptor header, the SU knows exactly the framing and timing of the remamder of the MAC frame, and thus the exact location of the contention interval for its reservation request (SRR m figure 4) During the subscπber reservation contention interval, the SU transmits a reservation request for a one-cell reservation (34) Smce the subscπber umt has not yet been granted an SU_AID, m this reservation request the SU_AID will be set to zero Because only a smgle cell is m the traffic queue, the number of cells requested is set to one

If the access pomt receives the reservation request error free, it will send a reservation request acknowledgement (RRA) m the next MAC frame (36) Contained within the RRA will be the SU_ALD assigned by the access pomt to the subscnber umt The subscπber termmal will now use this SU AID m all future upstream bursts As a result of the reservation acknowledgement, the subscnber termmal now has knowledge that its reservation request has been received by the access pomt It waits an mdetermmate amount of time (38) to be granted a timeslot in which to transmit the authentication initiation cell The SU continues to demodulate FDHDRs that are sent out by the AP Eventually, an FDHDR (40) will be sent out with an upstream burst map (Upstream Map m figure 2) that mcludes the SU's SU_AID This indicates to the SU that it must transmit its cell within the upstream portion of the current MAC frame (42)

At the MAC layer, successful cell arnval at the access pomt causes a downstream cell acknowledgement to be generated for transmission duπng the downstream portion of the next MAC frame (44) The authentication initiation cell is routed (46) to the access network control server (110), which generates a challenge message and routes it back to the subscπber termmal This cell enters one of the access pomt's traffic queues When a time slot is available for sendmg traffic, it is sent downstream to the SU by the AP (48)

Successful arnval of this cell at the SU causes an upstream cell acknowledgement (UACK) to be generated for transmission during the upstream portion of the next MAC frame (50) Because the access pomt is aware that it sent a burst of downstream traffic to the SU. it expects a UACK m the following MAC frame The position of the UACK relative to other possible UACKs is identical to the position of the downstream burst relative to the other downstream bursts within the previous MAC frame

A radio control application that executes on the SU receives the challenge cell and generates the authentication response This cell is placed m one of the subscπber terminal' s traffic queues, which causes the MAC to generate another SRR -After the AP has granted the reservation, the SU sends the cell upstream Acknowledgement of cell arnval by the AP, and passage of additional access network signalling between the SU and the AP m the manner descπbed above, completes the registration and authentication process (52)

With reference to Figure 11 a smgle cell transfer from an SU to the AP will now be descπbed

This scenaπo descπbes the transmission of a short burst of data from the SU to the AP For the purpose of discussion, the burst of data is smaller than one ATM cell The complete transfer of the data requires that an ATM virtual circuit be open between the SU and the AP. the precise details of which are not relevant to the present mvention but will be apparent to the man skilled the art Therefore, this explanation proceeds assuming that the establishment of the virtual circuit through the wireless network was successful This descnption follows the ATM cell from its ingress to the SU MAC through to its egress from the AP MAC

While sitting idle, the SU continuously momtors the downstream bursts (60) from the AP m order to demodulate the FDHDR and thus have full knowledge of the remamder of the MAC frame When a smgle payload cell (62) arπves at the MAC of the SU, it enters the queue coπespondmg to its traffic type The SU MAC then generates an SRR(64) and transmits it during the reservation contention interval of the next MAC frame If the SRR is received conectly. the AP generates a reservation request acknowledgement (66) and transmits it m the downstream portion of the next MAC frame The SU receives the acknowledgement and continues to momtor the downstream portion and demodulate the FDHDRs

Some non-deterrmmstic time later, the AP grants a time slot to the SU (68) The grant is commumcated via the FDHDR. which mcludes burst maps for each burst to be sent m the remamder of the MAC frame The SU detects its SU_ALO m one of the burst maps of the FDHDR. and takes that as an indication that it is to send some of its traffic upstream The number of cells granted is also part of the burst map, m this example only one cell was requested so only one cell will be granted The SU generates an upstream cell (UCELLR) which, m addition to the payload data, mcludes a reservation request that mdicates to the AP whether the reservation is to be maintained or released In this case, smce the traffic to be sent consists of only one cell, the reservation request will request 0 cells (the Cells field of the UCELLR m figure 8 is set to 0) The UCELLR (70) is then sent upstream during the time slot assigned to it by the AP The same time slot m which the SU's burst map appeared m the FDHDR is the same time slot m which the UCELLR is to appear m the upstream portion of the MAC frame

Smce the AP MAC granted a reservation to the SU m the downstream portion of the current frame, it is expecting to receive the cell m the upstream portion of the same frame, and is expecting to send a DACK in the subsequent MAC frame. By the time the upstream cell arπves, the AP MAC has assembled most of the bits compnsmg the DACK, minus the bit map containing the acknowledgements of the mdividual cells When the AP receives the smgle upstream cell correctly (verified usmg its CRC), it sets the coπesponding bit m the DACK (72) When the SU receives the DACK m the downstream portion of the subsequent MAC frame, it will know that both the upstream data cell and the attached reservation request were received conectly by the AP

The request for 0 additional cell reservations by the SU is received by the AP MAC and processed accordmgly At this pomt the SU has completed the process of cell transfer It sits idle, continumg to momtor the downstream bursts and awaiting the arnval of data from its external port

The transfer of multiple payload cells from a SU to the AP will now be descnbed with reference to Figure 12

This scenaπo differs from the previous scenaπo m that it demonstrates the MAC^'s combination of payload data ARQ and reservation request acknowledgement mto a smgle message By combining the reservation request with a cell of payload data, the MAC is able to use the payload cell acknowledgement for both the reservation request acknowledgement and the payload cell acknowledgement Because cell acknowledgement and retry is handled at the MAC layer, it is possible for the wireless access network to maintain short cell latencies even during retries

5 When the payload data (80) arπves at the SU's MAC layer, it is put mto the appropπate traffic queue for transmission The arnval of the data causes the SU to initiate the reservation request process As descnbed before, the SU waits for the next downstream burst (82), then demodulates the FDHDR to determine the location of the contention interval within the current MAC frame 0 Currently the SU can request a maximum of six slots The SU generates an SRR (84) and transmits it during the contention interval Smce the number of cells to be transmitted m this example is greater than the maximum number that can be requested, the SU requests six slots The SU generates the request and transmits it duπng the contention window When the AP receives the SRR, it acknowledges 5 the request by placmg an RRA (86) m the downstream portion of the next MAC frame

Some time later, the .AP grants time slots to the SU (88) The grant is commumcated via the FDHDR. which mcludes burst maps for each burst to be sent m the remamder of the MAC frame The SU detects its SU_AT-D m one of the 0 burst maps of the FDHDR, and takes that as an mdication that it is to send some of its traffic upstream The number of cells granted is also part of the burst map. the access pomt could grant anywhere from one to six cells In this example six cells are granted to the subscπber terminal

The SU generates an upstream cell (UCELLR) which, m addition ι <; to the payload data, mcludes a reservation request that mdicates to the .AP that it is requesting time slots for four additional cells The SU also generates cells to fill the remainmg five slots that it has been granted for the current burst These remainmg cells need not contam reservation requests - they follow the format of the UCELL (see figure 9) The burst of six cells is then sent upstream (90) during the time slots allocated to the SU m the upstream portion of the MAC frame

Smce the AP MAC granted reservations to the SU m the downstream portion of the current frame, it is expecting to receive the same number of cells m the upstream portion of the same frame, and is expecting to send a smgle DACK for the entire upstream burst in the subsequent MAC frame By the time the upstream cell arπves. the access pomt MAC has assembled the DACK. minus the bit map containing the acknowledgements of the mdividual cells When the AP MAC venfies that the upstream cells have been received conectly. it sets the corresponding bits m the DACK

The AP MAC expects the first cell of the burst to contam a reservation request In this case, it finds that the SU has requested four additional time slots As mentioned m the previous scenano. the DACK also acts as an acknowledgement of the reservation request The AP therefore grants four time slots m the upstream burst portion of the present MAC frame (92) The SU generates its UCELLR plus three UCELLs. and places a reservation request of 0 slots m UCELLR The upstream burst is sent as before (94)

The .AP has now updated the number of reservations bemg maintained for the SU However, if any of the four cells that the .AP expected is received m error, the SU will need to retransmit it It is not practical to require the SU to request a reservation and await the reservation grant m order to re-send the single cell that was m error Rather, any time the AP receives a cell m error, it mcrements the number of slots reserved for the particular SU In this example, the AP had received a UCELLR with a reservation request of 0 cells (94) However. it also received one cell m error, so it mcrements the number of ceils reserved for the SU by one When the SU receives the FDHDR and the DACK of the next MAC frame, it will know that it has one slot reserved for it and it will know which cell to re-transmit from the position of the nack m the DACK map If the .AP verifies correct reception of the four cells, it sets the conesponding bits in the DACK. Smce the UCELLR contamed a reservation request for 0 slots, no further reservations are needed and the transfer of cells from SU to AP is complete upon downstream receipt of the DACK (96) by the SU.

The detailed descnption of the particularly preferred embodiment of the present mvention presented above has referred to vaπous of the data cells, and in particular various of the data payload cells as bemg ATM cells It is to be understood that the data cells need not be ATM cells exclusively, but may mstead be data cells of a different structure which still satisfy and support ATM quality of service requirements. In this case, such data cells of a different structure are ATM compatible data cells.

In summary, the present mvention presents a data communications method and system and a data signal wherein dynamic time vaπable time-division duplexing can be achieved by virtue of control data in the form of a frame descπptor header providing full a pnoπ knowledge to every subscnber termmal in a cell of the expected contents, structure and/or timing of the remamder of the data to be transmitted onto a common channel m both the downstream and subsequent upstream direction m the remamder of the frame. The provision of this is a pπori knowledge to each subscnber termmal via the frame descnptor header facilitates the many advantages of the present mvention as descπbed earlier.

Claims

1 A data communications method for use m a pomt-multipomt network compnsmg a central control node and one or more remote subscπber nodes, said network having a bi-directional communications channel accessible to all network nodes, said method compnsmg the steps of

a) transmitting a first data portion mcludmg control data onto the channel from the central control node exclusively within a first time peπod.

b) receiving said first data portion mcludmg control data at each of said remote subscnber nodes,

c) transmitting further data portions onto the channel exclusively within a second time penod, said further data portions bemg transmitted one at a time from particular of the remote subscnber nodes m response to the received control data, and

d) receiving said further data portions at the central control node,

wherem said control data mdicates to each of said remote subscnber nodes those particular of the nodes which are permitted to transmit at step c), whereby access to the channel can be dynamically controlled by the central control node

2 A data communications method accordmg to claim 1. wherem the transmitting step a) mcludes the step of transmitting a known data sequence as part of said first data portion pnor to the transmission of the remamder of said first data portion, and the receivmg step b) mcludes the steps of receivmg the known data sequence pnor to receivmg the remamder of said first data portion, and usmg said known data sequence at each of the remote subscnber nodes to synchromse the nodes with the transmitted first data portion

3 A data communications method accordmg to claim 2, wherem the control data is transmitted as part of said first data portion immediately after the known data sequence has been transmitted, wherem said control data is recogmsed as such by said remote subscnber nodes

4 A data communications method according to any of the precedmg claims, wherem the control data mdicates to each of the remote subscπber nodes the structure of the remamder of the first data portion, and mdicates to each of the subscriber nodes the content and structure of the further data portions to be transmitted at step c)

5 A data commumcations method accordmg to any of the precedmg claims, wherem the first data portion further mcludes at least one acknowledgement portion mdicatmg acknowledgement of receipt of earlier further data portions sent from said remote subscnber nodes

6 A data commumcations method accordmg to any of the precedmg claims, wherem the first data portion further mcludes one or more payload data portions

A data commumcations method according to any of the precedmg claims, wherem when a particular remote subscπber node has payload data to transmit to the central control node, said method further compnses the steps of

a) generating a channel reservation request requesting permission to transmit onto the channel.

b) transmitting the channel reservation request onto the channel within a contention penod defined by said control data, said contention penod bemg a penod within said second time period when the further data portions are not bemg transmitted, and

c) receivmg said channel reservation request at said central control node,

d) acknowledgmg said channel reservation request m the next transmitted first data portion,

e) granting permission to the remote subscπber node to transmit the payload data onto the channel by mdicatmg to the node m the control data of one of the subsequently transmitted first data portions that permission has been granted, and

f) transmitting the payload data onto the channel as one of the further data portions m the second time peπod immediately following the first data portion m which permission was granted

8 A method accordmg to claim 7, wherem said contention peπod is a period within said second time peπod before the further data portions are transmitted J j

9 A data commumcations method accordmg to claims 7 or 8. wherem each remote subscnber node with payload data to transmit generates a channel reservation request and transmits the channel reservation request m the contention penod of the second time penod, wherem all transmitted channel reservation requests are transmitted m contention with one another

10 A data commumcations method accordmg to any of the precedmg claims, wherem the time division-duplex commumcations channel is a wireless commumcations channel

11 A data commumcations method accordmg to any of the precedmg claims, wherem said data payloads are ATM compatible cells

12 A data commumcations method accordmg to any of the precedmg claims, wherem when a particular remote subscnber node is permitted to transmit onto the channel, the entire channel capacity is made available to the particular remote subscnber node

13 A time-division duplex data signal for transmission onto a channel for use m a pomt-multipomt data network compnsmg a central control node and one or more remote subscnber nodes, said signal compπsmg

a) a first data portion mcludmg a control data portion for transmission from the central control node over the channel to each of the remote subscnber nodes, and

b) further data portions for transmission from particular of the remote subscπber nodes over the channel to the central control node in response to the control data portion;

wherein said control data portion mdicates to each of the remote subscnber nodes the particular of those nodes which are permitted to transmit one or more further data portions, wherem access to the channel can be dynamically controlled by the central control node.

14. A signal according to claim 13. wherem said first data portion includes a known data sequence at the start of said portion, the known data sequence allowing each of the remote subscnber nodes to synchromse with the first data portion.

15. A signal according to claim 14, wherein said known data sequence immediately precedes the control data portion.

16. A signal according to any of claims 13 to 15 wherem the control data portion compnses a downstream structure portion mdicatmg the structure of the remamder of the first data portion and an upstream structure portion mdicatmg the structure of the further data portions.

17. A signal according to any of claims 13 to 16 wherem the first data portion further mcludes at least one payload data portion containing data traffic for at least one of the remote subscriber terminals.

18 A signal accordmg to any of claims 13 to 17 wherem each of the further data portions contam at least one payload data portion containing data traffic.

19 A signal accordmg to claims 17 or 18 wherem each further data portion contams an acknowledgement portion acknowledgmg the successful receipt or otherwise of a payload data portion sent to the respective remote subscnber node m the first data portion

20 A signal accordmg to claims 18 or 19. wherem the first data portion further mcludes an acknowledgement portion acknowledgmg the successful receipt or otherwise of payload data portions sent by the respective remote subscπber nodes m the further data portions

21 A signal according to any of claims 13 to 19 and further compnsmg a channel reservation request portion for transmission from particular of the remote subscnber nodes over the channel to the central control node, said channel reservation request portion compnsmg one or more channel reservation requests each generated by a smgle remote subscnber node m response to a requirement to transmit data traffic to the central control node, wherem each of the channel reservation requests are transmitted by the respective remote subscnber nodes onto the channel during the channel reservation request portion, wherem each of the channel reservation requests is m contention with one another for channel capacity

22 A signal accordmg to claim 21, wherem the channel reservation request portion is transmitted before said further data portions are transmitted

23 A signal according to claim 21 or 22 wherem said first data portion further mcludes at least one reservation request acknowledgement portion acknowledgmg successful receipt or otherwise of channel reservation requests sent in the channel reservation request portion.

24. A signal according to any of claims 13 to 23, wherein said data traffic are ATM compatible cells.

25. A signal according to any of claims 13 to 24 wherem said signal is a wireless signal.