GB2348581A

GB2348581A - Data communications method and data signal

Info

Publication number: GB2348581A
Application number: GB9907481A
Authority: GB
Inventors: John David Porter; Walter Charles Vester
Original assignee: Adaptive Broadband Ltd
Current assignee: Adaptive Broadband Ltd
Priority date: 1999-03-31
Filing date: 1999-03-31
Publication date: 2000-10-04
Anticipated expiration: 2019-03-31
Also published as: WO2000059260A1; AU3444200A; GB2348581B; 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, structure 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

2348581 Data Communications Method and Data Sigull The present invention

relates to a data communications method and a data signal for use in 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 in a point-to-multipoint data network in which bandwidth is a scarce resource being 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.

Data networks can be classified in many ways, but for the purpose of the present invention, it is useful to classify them by their means of accessing 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 being passed over the medium. Traffic is directed to individual network nodes via physical layer addresses that are attached to the data packets being sent over the medium. When multiple network nodes attempt to transmit data simultaneously, there is the possibility for con-teifion -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 terminals share the same medium, however it cannot be guaranteed that all terminals can hear each other. All that can be guaranteed is that all terminals can hear the central network node, referred to herein as the access point. For this reason, it is not enough for each terminal simply to monitor the channel III order to detect contentions. Feedback on success or failure of network contention must also be communicated back to the networ, terrninals by the access point.

2 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 definition 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 increases, the nondeterministic nature of random attempts at accessing the medium causes the throughput and efficiency of the network to fall off catastrophically. The uncertainty of the loading or the potential success of the medium contention results in non deterministic transit times for traffic being 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 loading. 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 maintained, and in particular under conditions of high network loading.

The problem of poor performance of networks using a broadcast medium at high network loading can be solved by 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 loading and with equal data loads f1rom each network terminal, at low network loading the efficiency of medium utilisation is very poor, 1 3 as each node must wait for its assigned time slot before transmitting.

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 gain access to the medium it must first be granted permission by the access point. The grant to a particular node of permission to access the medium is communicated by the access point to all network nodes, so that each node has substantially full priori knowledge of the structure of intended data traffic on the channel for a particular time period. By such a mechanism, medium access can be centrally controlled by the access point in a dynamically time - variable manner.

According to the present invention there is provided a data communications method for use in a point-to-multipoint network comprising a central control node and one or more remote subscriber nodes, said network having a bi-directional communications channel accessible to all network nodes, said method comprising the steps of..

a) transmitting a first data portion including control data onto the channel from the central control node exclusively within a first time period; - b) receiving said first data portion including control data at each of said remote subscriber nodes; c) transmitting further data portions onto the channel exclusively within a second time period, said further data portions being transmitted one at a time from particular of the remote subscriber nodes in response to the received control data; and d) receiving said further data portions at the central control node; wherein said control data indicates to each of said remote subscriber 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 4 control node.

The control data can indicate to each of the remote subscriber nodes the structure of the remainder of the first data portion, and also indicate to each of the remote subscriber nodes the expected content and structure of the finiher data portions to be transmitted from the remote nodes to the central control node at step c).

Furthermore, when a particular remote subscriber node has payload data to transmit to the central control node, the method includes 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 period defined by said control data, said contention period being a period within said second time period when the further data portions are not being transmitted; and receiving said channel reservation request at said central control node; acknowledging said channel reservation request in the next transmitted first data portion; granting permission to the remote subscriber 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 in the second time period immediately following the first data portion in which permission was granted.

According to another aspect of the present invention, there is provided a time-division duplex data signal for transmission onto a channel for use in a point-to-multipoint data network comprising a central control node and one or more remote subscriber nodes, said signal comprising:

a) a first data portion including a control data portion for transmission from the central control node over the channel to each of the remote subscriber nodes; and b) ficher data portions for transmission from particular of the remote subscriber nodes over the channel to the central control node in response to the control data portion; wherein said control data portion indicates to each of the remote subscriber nodes the particular of those nodes which are permitted to transmit one or more ftu-ther data portions, wherein access to the channel can be dynamically controlled by the central control node.

The control data portion may fiu-ther comprise a downstream structure data portion indicating the structure of the remainder of the first data portion and an upstream structure portion indicating the expected contents and/or structure and/or timing of each-of the ftu-ther data portions.

The signal may further include a channel reservation request portion for transmission from particular of the remote subscriber nodes over the channel to central control node, said channel reservation request portion comprising one or more channel reservation requests each generated by a single remote subscriber node in response to a requirement to transmit data traffic to the central control node, wherein each of the channel reservation requests are transmitted by the respective remote subscriber nodes onto the channel during the channel reservation request portion, wherein each of the channel reservation requests is in contention with one another for channel capacity.

It is an advantage -o the present invention that the central control node has the ability to throttle any subscriber terminal's access to the medium.

It is another advantage of the present invention that it provides for dynamically variable 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 increases the nurnber of subscribers that can be provisioned service from a single access point.

It is a further advantage of the present invention that it provides high 6 efficiency of medium utilization dunng periods of high network loading and provides low latency medium access during periods of low network loading.

There is yet another advantage of the present invention in 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 terminals 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 error check each data cell of each received data portion independently, thus allowing implementation of a selective repeat automatic repeat query in a point-multipoint network.

It is a further feature of the present invention that position of data within a portion conveys information. 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 increases bandwidth efficiency.

Further features and advantages of the present invention win become apparent from the following detailed description of a particularly preferred embodiment thereof, presented by way of example only, and by reference to the accompanying drawings, in 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 single reservation request 1 7 acknowledgement cell of the present invention; Figure 4 shows the frame structure of a single 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 single subscriber reservation request cell of the data signal of the present invention; Figure 7 shows the frame structure of a single 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 single upstream payload cell without reservation request; Figure 10 demonstrates the sequence of messages involved in remote subscriber terminal registration in the present invention; Figure 11 shows the - sequence of messages involved in transmitting a single payload cell from upstream to the central access point from a remote subscriber terminal; and Figure 12 shows the sequence of messages involved in transmitting multiple payload cells upstream to the central access point from a remote subscriber terminal.

The method of data communication and data signal of the present invention are chiefly for use within a wireless access network, although it will be readily apparent to those skilled in the art that the said method and signal may equally be employed within a wired network. Within the particularly preferred embodiment to be described herein, the present invention is employed within a wireless network deployed in a cellular configuration. Each cell consists of a central access point and multiple subscriber units. Subscriber units communicate to the network only through the access points, making the network a point- 8 multipoint architecture. The access point 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.

Communication can occur on the wireless medium in both directions, and hence a means of duplexmg the wireless medium is required. Two common methods are frequency division duplexing (FDD) and time division duplexing JDD). In the case of FDD, the medium is broken into a downstream (data originating from the access point) frequency band and an upstream frequency band (data originating at the subscriber unit). TDD breaks a single frequency band into downstream time slots and upstream time slots. The data communications method and signal of the present invention uses TDD.

For the purpose of understanding the present invention, it is useful to view the network to which the invention is applied as consisting of multiple switches. On one end, corresponding to the access point, there is a switch with a single physical wired data port, and multiple wireless data ports. Disbursed throughout the cell are two port switches, each located at a subscriber terminal.

Each subscriber terminal has a single wireless port and a single physical wired port. The network model is complicated by the fact that a subscriber terminal can be powered on and off at will; thus there are times that the subscriber terminal switches are not accessible to the network. This gives rise to the concept of subscriber terminal registration, the procedure for which will be described in detail later. During the registration process, a subscriber terminal negotiates with an access point to be assigned a temporary port identifier, referred to as a subscriber unit access identification (SU_AID). Once a subscriber terminal has been granted an SU-AID, it is capable of proceeding with higher layer signalling to gain access to the network.

The present invention is directed towards controlling access of the subscriber terminals to the wireless medium through central control of the subscriber terminals by the access point. In order to achieve this the access point 9 is provided with a medium access controller (MAC) which administers the medium control. Similarly, each remote subscriber terminal is also provided with a compatible medium access controller for responding to the central MAC in a master-slave manner: the subscriber terminals request access to the medium and the access point 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 in the form of time slots when a subscriber terminal is granted the ability to access the wireless network medium, it is granted one or more time slots in which it can transmit. Within the granted time slot the entire medium capacity is available to the subscriber 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 includes reference to those control means required at both the central access point and at the subscriber terminals. In this respect, the medium access controller (MAC) therefore corresponds to those network means, whether hardware or software based, that would approximate in part to the Network-level and/or the Data-level of the ISO Open Systems Interconnection 7-layer Reference Model. The MAC may be implemented in a Field Programmable Gate Array (FPGA).

The MAC operates by controlling transmissions on the medium by the definition of a MAC frame, being the framework in which data transmissions take place. In order to fully understand the various features and advantages of the present invention, it is necessary to first describe in 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 single MAC frame. The MAC fi-ame consists of a downstream portion, generated by the access point and broadcast to all subscriber terminals, and an upstream portion, which consists of a contention interval and all data bursts being sent from subscriber terminals back to the access point.

The downstrearn 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 descriptor header (FDHDR) (4). The FWDR describes the complete contents of the remainder of the MAC frame. The size of the FDHDR may vary. The FWDR contains a map of all traffic, upstream and downstream, to occur within the MAC frame. After achieving bit synchronisation on the MAC frame via the preamble, subscriber terminals demodulate the FWDR and from that gain complete knowledge of the traffic that will occur within the remainder of the frame. Only one FDHDR may occur per MAC frame. The precise contents of the FWDR are shown in Figure 2 and described in detail in Table 1 below.

_Field Tag Description

SYNC Short 4 symbol sync burst.

BD_cnt Bursts Downstream Count. Number of subscriber units having payload data sent to them in this MAC frame BU-cnt Bursts Upstream Count. Number of subscriber units that will be sending payload data in this MAC frame.

Af_1D Access Point ID. Identifies the access point that originated the frame descriptor header.

RRA-crit Reservation Request Acknowledgment Count. Number of acknowledgments being sent in response to previous requests.

DA_crit Downstream Acknowledgment Count. Number of upstream cell acknowledgements being sent downstream M this MAC frame.

Downstream Identifies the subscriber unit being sent cells, the number of cells to Map be sent, and the traffic type being sent.

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

UA-ent Upstream Acknowledgment count. Total number of downstream cell acknowledgments being sent upstream M this MAC frame.

1 Field Tag Description

Upstream Identifies the subscriber units that have been granted reservations, Map the number of cells to be sent by each, and the traffic type allowed.

CRC Cyclic Redundancy Check. Allows each subscriber terminal to verify correct receipt of the frame descriptor.

SU-1D Subscriber Unit ID. Identifies the subscriber unit acting as the data source or sink in the burst.

Cell-Crit Cell Count. Total number of ATM cells to be sent in this particular burst.

Tr_type Traffic Type. Defines the type of traffic that the subscriber unit is allowed to send or will be receiving during the current frame.

Table 1: Erame Descriptor Header (FDHDR) Structure Following the FDHDR is the reservation request acknowledgement (RRA) portion 6. The RRA acknowledges a request by a subscriber for upstream time slots and can also communicate signal propagation delay. There is a single RRA for each reservation request that was made during the contention interval from the previous MAC frame, although in the case where no reservation requests were made in the previous MAC frame, then no acknowledgements will be sent.

The precise contents of the RRA are shown in Figure 3 and described in detail in Table 2 below.

Field Tag Description

Sync 8 bit framing synchronization sequence SU-1D ID of the subscriber unit that originated the reservation request, and to which the reservation request acknowledgment is directed.

RTRN Return Code. Communicates reservation status to SUs and SU-AID status to SUs performing registration.

12 Field Tag Description DELAY Delay compensation bits. These bits are assigned during subscriber unit registration and cause a shift in subscriber unit timing. CRC Cyclic Redundancy Code. Used by the subscriber unit to verify 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 contains a downstream ack or nack of a single 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

Sync 4 symbol synchronization burst SU-ID 11) of the subscriber unit that originated the cells being acknowledged.

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

map CRC Cyclic Redundancy Code. Used to verify that the downstream acknowledgment has been received error free.

Table 3: Downstream Acknowledgement (DACK) cell structure Following the DACK portion comes the Downstream Burst (9). The MAC operate s on a principle of cell bursts for communicating payload data between the access point and the subscriber terminals by allowing multiple cells of data to be sent to or from a particular subscriber unit at a time. A burst must 1 - 13 always consist of at least one cell. In upstream bursts, this single cell must be an upstream cell with reservation request (UCELLR) (18). Additional cells in the upstream burst are in the -format of a UCELL - an upstream cell without reservation request (20). Upstream cells are discussed in more detail later.

Downstream bursts can also consist of multiple cells, but there is only one type - the downstream cell (DCELL) 10. There can be many DCELLs either several directed to a single subscriber terminal, or several directed to several subscriber terminals. 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 in a future implementation without departing from the scope of the present invention. The structure of a DCELL is shown in more detail in Figure 5, and described in Table 4 below.

Field Tag Description

Sync 4 symbol synchronization burst SU-ID ID of the subscriber 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 Channel Identifier ATM Header (VCI), Traffic Type, Cell Loss Priority.

Payload Payload data.

CRC Cyclic redundancy code. Used to verify correct 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 subscriber terminals. There then follows a slight delay due to subscriber tumaround time (STT) 12. The STT varies with distance to the farthest subscriber unit. A typical maximum distance to a subscriber unit could be, for example, 5km, although this obviously depends on 14 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 communicated 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 commurucate 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 point. Because the subscriber terminal has no active reservations, and because it is believed that at any given time the number of terminals making initial bandwidth requests will be small, it is reasonable to force the subscriber terminals to contend for reservations. This contention window is kept as small as possible while still allowing reasonable success probability by employing a novel implementation of aloha contention control schemes. Once the subscriber terminal's reservation request has been acknowledged by the access point, 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 in one MAC frame is communicated to the subscriber tem-Linals in the FDHDR. Multiple slots can be made available during times of heavy request traffic. Further-more, the start of the contention interval can be calculated by the 1 1 subscriber terminals by virtue of the FWDR 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

Preamble Physical layer synchronization sequence.

Sync 8 bit MAC framing synchronization burst SU-1D ID of the subscriber unit requesting a reservation.

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

Tr-Type Traffic type of the data for which time slots are being requested.

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

Table 5: Subscriber Reservation Request (SW structure Following the contention interval comes the upstream acknowledgement portion 16, containing upstream acknowledgement (UACK) cells of each downstream burstreceived during the downstream portion. Each UACK indicates upstream ack -or nack of a single downstream burst from a previous MAC frame. As many UACKs may be transmitted m each upstrewn acknowledgement portion as there were downstream bursts in the the downstream portion. The structure of each UACK cell is shown in Figure 7, and described in detail below in Table 6.

Field Tag Description

Sync 4 symbol synchronization burst SU-1D ID of the subscriber unit acknowledging receipt of downstream cells.

16 Field Tag Description UU Unused Ack/Nack One acknowledgment bit per cell. 1 = successful cell receipt. Map CRC Cyclic Redundancy Code. Used to verify that the downstream acknowledgment has been received error free.

Table 6: UpstreaM Acknowledgement (!JACK) structure Following the upstream acknowledgement portion comes the upstream burst portion 22, containing cell bursts from subscriber units which were granted permission in the FDHDR to transmit payload data to the access point.

The FDHDR from the downstream portion contains the instructions to the subscriber terminals on when to transmit a burst m the upstream burst portion, and what the burst is expected to contain. Each upstream burst contains one or more data cells with the same traffic type being sent from a particular subscriber terminal. Each upstream burst made in the upstream burst portion may be from a different subscriber unit, or alternatively may be from the same subscriber unit, depending upon the channel allocations granted to the subscriber units. In this way channel allocations can be dynamically arranged between the subscriber terminals from MAC frame to MAC frame, depending on the network traffic loading and the traffic priority. As mentioned earlier, each upstream burst must contain a single upstream cell with reservation request (UCELLR) 18, and zero or more upstream cells without reservation request (UCELL) 20. The condition that a burst must contain a UCELLR allows a subscriber terminal to maintain its channel reservation until all of its payload data has been sent, thus meaning that the subscriber erminal need not transmit again during the contention interval to request channel allocation to transmit the remainder of its data. This combination of the reservation maintenance request and the upstream cell into one message allows a single downstream acknowledgement to serve as both reservation request 17 acknowledgement and payload cell acknowledgement, thus improving bandwidth efficiency.

The structure of a UCELLR is shown in Figure 8, the contents of which are described below in Table 7.

_Field Tag Description

Preamble Physical layer synchronization sequence Sync 8 bit MAC framing synchronization sequence SU-1D ID of the subscriber unit from which the payload data is originated.

RSV_MAINT Reservation maintenance. Used by the subscriber terminal to continue requesting time slot reservations without contending for them.

Cells Number of time slots being requested by the subscriber unit for future MAC frames Tr-Type Traffic Type of the data to be sent by the subscriber unit in future MAC frames.

SEQ Sequende number. Used by the MAC to resequence cells that get out of order due to cell loss and retransmission Condensed Includes VPI, VCI, Traffic Type, Cell Loss Priority.

ATM Header Payload Payload data CRC Cyclic redundancy code. Used to verify correct receipt of the downstream cell.

Table 7: Upstream Cell with Reservaon Request (UCELLR) structure The structure of a UCELL is shown in Figure 9, the contents of which are described below in Table 8.

18 Field Tag Description

Preamble Physical layer synchronization sequence Sync 8 bit MAC framing synchronization sequence SU-113 ID of the subscriber unit from which the payload data is originated.

SEQ Sequence number. Used by the MAC to resequence cells that get out of order due to cell loss and retransmission. - - Condensed Includes VPI, VCI, Traffic Type, Cell Loss Priority.

Table 8: Upstream Cell with no Reservation Request (11CELL) Various features and advantages arising from the structure of the MAC frame of the present invention will now be described.

As mentioned earlier, channel access is mediated by the cential controller. Channel access information for a particular MAC frame is communicated to all network terminals by the access point by virtue of the FWDR. The MAC is fundamentally dependent on the subscriber terminals' knowledge of the timing of all network traffic from the FWDR. All traffic over the wireless network medium must therefore be contained within the MAC frame, whose contents, size, and time alignment are determined by the MAC at the access point. This requires that subscriber terminals have the ability to synchronise to the traffic in the network before they can transmit Within the contention interval defined by the access point. Subscriber terminals therefore achieve physical layer synchronisation by monitoring the channel for the 32 symbol correlation sequence referred to as the PREAMBLE in figure 1. They then acquire MAC frame synchronisation by searching for the 8 bit fi-ame synchronisation sequences located 1 - 19 at the start of each transmitted cell.

A particular advantage of the provision of the upstream and downstream acknowledgement portions will now be described.

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). Since 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 minimum of delay. The present invention therefore has the ability to perform quick tumaround automatic repeat query while still satisfying quality of service commitments of the wired network beyond the access point.

The retry mechanism relies on the upstream and downstream acknowledgements of the MAC (UACK and DACK), which contain bit maps corresponding to individual cells in an upstream or downstream burst. Each network node has the ability to detect bit errors using the CRC codes at the end of each cell. The recipient of a burst then conveys the success or failure of the arrival of a particular cell bysetting the corresponding bit of the burst map contained in the ACK. The original 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 in the bit map corresponds to the order in which a burst was received. In this way the -bit position conveys information in itself and therefore the acknowledgement does not require an additional identifier to be transmitted over the air.

In order that the full operation of the present invention can be understood, various operational scenarios will now be described, with reference to Figures 10, 11, and 12.

With reference to Figure 10, the procedure for subscriber tem-iinal registration using the method and signal of the present invention is first described below.

When a subscriber unit (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 point (AP); access network authentication is completed by ot her controllers outside the MAC. However, the initiation of the access network autherification process is what also initiates access network registration. The signalling that is passed for authentication purposes is mentioned in this description because of its close coupling with the registration process.

Upon power up, the subscriber unit (SU) generates an authorisation initiation cell which must be sent to the access point. It is placed in 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 descriptor header (32). From the contents of the frame descriptor header, the SU knows exactly the framing and timing of the remainder of the MAC frame, and thus the exact location of the contention interval for its reservation request (SRR in figure 4). During the subscriber reservation contention interval, the SU transmits a reservation request for a one-cell reservation (34). Since the subscriber unit has not yet been granted an SU_AID, in this reservation request the SU-AID will be set to zero. Because only a single cell is in the traffic queue, the number of cells requested is set to one.

If the access point 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-AID assigned by the access point to the subscriber unit. The subscriber terminal will now use this SU-AID in all future 21 upstream bursts. As a result of the reservation acknowledgement, the subscriber terminal now has knowledge that its reservation request has been received by the access point. It waits an indeterminate 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 I'DHDR (40) will be sent out with an upstream burst map (Upstream Map in figure 2) that includes the SUs SU_All). 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 arrival at the access point causes a downstream cell acknowledgement to be generated for transmission during the downstream portion of the next MAC frame (44). The authentication initiation cell is routed (46) to the access network control server, which generates a challenge message and routes it back to the subscriber terminal. This cell enters one of the access point's traflic queues. When a time slot is available for sending traffic, it is sent downstream to the SU by the AP (48).

Successful arrival 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 point is aware that it sent a burst of downstrearn traffic to the SU, it expects a UACK in 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 in one of the subscriber 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 arrival by the AP, and passage of additional access network signalling between the SU and the AP in the manner described above, completes the registration and authentication process (52).

22 With reference to Figure I I a single cell transfer from an SU to the AP will now be described.

This scenario describes 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 this description of the MAC layer. Therefore, this explanation proceeds assuming that the establishment of the virtual circuit through the wireless network was successful. This description 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 monitors the downstream bursts (60) from the AP in order to demodulate the FDHDR and thus have full knowledge of the remainder of the MAC frame. When a single payload cell (62) arrives at the MAC of the SU, it enters the queue corresponding 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 correctly, the AP generates a reservation request acknowledgement (66) and transmits it in the downstream portion of the next MAC frame. The SU receives the acknowledgement and continues to monitor the downstream portion and demodulate the FDHDRs.

Some non-deterministic time later, the AP grants a time slot to the SU (68). The grant is communicated via the FDHDR, which includes burst maps for each burst to be sent in the remainder ofthe MAC frame. The SU detects its SU_AID in one of the burst maps of the FDHDP, 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; in this example only one cell was requested so only one cell will be granted.

The SU generates an upstream cell (UCELLR) which, in addition to the payload data, includes a reservation request that indicates to the AP whether 1 23 the reservation is to be maintained or released. In this case, since the traffic to be sent consists of only one cell, the reservation request will request 0 cells (the Cells field of the UCELLR in 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 in which the SUs burst map appeared in the FDHDR is the same time slot in which the UCELLR is to appear in the upstream portion of the MAC frame.

Since the AP MAC granted a reservation to the SU in the downstream portion of the current frame, it is expecting to receive the cell in 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 arrives, the AP MAC has assembled most of the bits comprising the DACK, minus the bit map containing the acknowledgements of the individual cells. When the AP receives the single upstream cell correctly (verified using its CRC), it sets the corresponding bit in the DACK (72). When the SU receives the DACK in the downstream portion of the subsequent MAC frame, it will know that both the upstream data cell and the attached reservation request were received correctly by the AP.

The request for 0 additional cell reservations by the SU is received by the AP MAC and processed accordingly. At this point the SU has completed the process of cell transfer. It sits idle, continuing to monitor the downstream bursts and awaiting the arrival of data from its external port.

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

This scenario- differs from the previous scenario in that it demonstrates the MAC's combination of payload data ARQ and reservation request acknowledgement into a single 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 24 maintain short cell latencies even during retries.

When the payload data (80) arrives at the SUs MAC layer, it is put into the appropriate traffic queue for transmission. The arrival of the data causes the SU to initiate the reservation request process. As described 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.

Currently the SU can request a maximum of six slots. The SU generates an SRR (84) and transmits it during the contention interval. Since the number of cells to be transmitted in 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 during the contention window. When the AP receives the SRP, it acknowledges the request by placing an RRA (86) in the downstream portion of the next MAC frame.

Some time later, the AP grants time slots to the SU (88). The grant is communicated via the FDHDR, which includes burst maps for each burst to be sent in the remainder of the MAC frame. The SU detects its SU AID in 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; the access point could grant anywhere from one to six cells. In this example six cells are granted to the subscriber terminal.

The SU generates an upstream cell (UCELLR) which, in addition to the payload data, includes a reservation request that indicates to the AP that it is requesting time slots for four additional cells. The SU also generates cells to fill the remaining five slots that it has been granted for the current burst. These remaining cells need not contain 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 in the upstream portion of the MAC frame.

Since the AP MAC granted reservations to the SU in the downstream portion of the current frame, it is expecting to receive the same number of cells in the upstream portion of the same frame, and is expecting to send a single DACK for the entire upstream burst in the subsequent MAC frame. By the time the upstream cell arrives, the access point NIAC has assembled the DACK, minus the bit map containing the acknowledgements of the individual cells. When the AP MAC verifies that the upstream cells have been received correctly, it sets the corresponding bits in the DACK.

The AP MAC expects the first cell of the burst to contain a reservation request. In this case, it finds that the SU has requested four additional time slots. As mentioned m the previous scenario, the DACK also acts as an acknowledgement of the reservation request. The AP therefore grants four time slots in 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 in UCELLR. The upstream burst is sent as before (94).

The AP has now updated the number of reservations being is 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 in order to re-send the single cell that was in error. Rather, any time the AP receives a cell in error, it increments 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 increments the number of cells 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 in the DACK map If the AP veri fies correct reception of the four cells, it sets the corresponding bits in the DACK. Since the UCELLR contained 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 description of the particularly preferred embodiment

26 of the present invention presented above has referred to various of the data cells, and in particular various of the data payload cells as being ATM cells. It is to be understood that the data cells need not be ATM cells exclusively, but may instead 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.

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Claims

CLAIMS:

1. A data communications method for use in a point-multipoint network comprising a central control node and one or more remote subscriber nodes, said network having a bi-directional communications channel accessible to all network nodes, said method comprising the steps of.

a) transmitting a first data portion including control data onto the channel from the central control node exclusively within a first time period; b) receiving said first data portion including control data at each of said remote subscriber nodes; c) transmitting further data portions onto the channel exclusively within a second time period, said further data portions being transmitted one at a time from pailicular of the remote subscriber nodes in response to the received control data; and d) receiving said further data portions at the central control node; wherein said control data indicates to each of said remote subscriber 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 according to claim 1, wherein the transmitting step a) includes the step of transmitting a known data sequence as part of said first data portion prior to the transmission of the remainder of said first data portion, and the receiving step b) includes the steps of receiving the known data sequence prior to receiving the remainder of said first data portion, and using said known data sequence at each of the remote subscriber nodes to synchronise the 28 nodes with the transmitted first data portion.

3. A data communications method according to claim 2, wherein the control data is transmitted as part of said first data portion immediately after the known data sequence has been transmitted, wherein said control data is recognised as such by said remote subscriber nodes.

4. A data communications method according to any of the preceding claims, wherein the control data indicates to each of the remote subscriber nodes the structure of the remainder of the first data portion, and indicates to each of the subscriber nodes the content and structure of the further data portions to be transmitted at step c).

5. A data communications method according to any of the preceding claims, wherein the first data portion further includes at least one acknowledgement portion indicating acknowledgement of receipt of earlier further data portions sent from said remote subscriber nodes.

6. A data communications method according to any of the preceding claims, wherein the first data portion further includes one or more payload data portions.

7. A data communications method according to any of the preceding claims, wherein when a particular remote subscriber node has payload data to transmit to the central control node, said method further comprises 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 period defined by said control data, said 1 29 contention period being a period within said second time period when the further data portions are not being transmitted; and c) receiving said channel reservation request at said central control node; d) acknowledging said channel reservation request in the next transmitted first data portion; e) granting permission to the remote subscriber 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 f) transmitting the payload data onto the channel as one of the further data portions in the second time period immediately following the first data portion in which permission was granted.

8. A method according to claim 7, wherein said contention period is a period within said second time period before the further data portions are transmitted.

9. A data communications method according to claims 7 or 8, wherein each remote subscriber node with payload data to transmit generates a channel reservation request and transmits the channel reservation request m the contention period of the second time- Period, wherein all transmitted channel reservation requests are transmitted in contention with one another.

10. A data communications method according to any of the preceding claims, wherein the time division-duplex communications channel is a wireless communications channel.

11. A data communications method according to any of the preceding claims, wherein said data payloads are ATM compatible cells.

12. A data communications method according to any of the preceding claims, wherein when a particular remote subscriber node is permitted to transmit onto the channel, the entire channel capacity is made available to the particular remote subscriber node.

13. A time-division duplex data signal for transmission onto a channel for use in a point-multipoint data network comprising a central control node and one or more remote subscriber nodes, said signal comprising:

a) a first data portion including a control data portion for transmission from the central control node over the channel to each of the remote subscriber nodes; and b) finiher data portions for transmission from particular of the remote subscriber nodes over the channel to the central control node in response to the control data portion; wherein said control data portion indicates to each of the remote subscriber nodes the particular of those nodes which are permitted to transmit one or more further data portions, wherein access to the channel can be dynamically controlled by the central control node.

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

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

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16. A signal according to any of claims 13 to 15 wherein the control data portion comprises a downstream structure portion indicating the structure of the remainder of the first data portion and an upstream structure portion indicating the structure of the further data portions.

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

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

19. A signal according to claims 17 or 18 wherein each further data portion contains an acknowledgement portion acknowledging the successful receipt or otherwise of a payload data portion sent to the respective remote subscriber node in the first data portion.

20. A signal according to claims 18 or 19, wherein the first data portion finiher includes an acknowledgement portion acknowledging the successful receipt or otherwise of payload data portions sent by the respective remote subscriber nodes in the further data portions.

21. A signal according to any of claims 13 to 19 and further comprising a channel reservation request portion for transmission from particular of the remote subscriber nodes over the channel to the central control node, said channel reservation request portion comprising one or more channel reservation requests each generated by a single remote subscriber node in response to a requirement to transmit data traffic to the central control node, wherein each of the channel 32 reservation requests are transmitted by the respective remote subscriber nodes onto the channel during the channel reservation request portion, wherein each of the channel reservation requests is in contention with one another for channel capacity.

22. A signal according to claim 2 1, wherein the channel reser-vation request portion is transmitted before said further data portions are transmitted.

23. A signal according to claim 21 or 22 wherein said first data portion further includes at least one reservation request acknowledgement portion acknowledging 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 wherein said signal is a wireless signal.