WO1999009690A2

WO1999009690A2 - System, device, and method for scheduling variable bit rate traffic in a communication network

Info

Publication number: WO1999009690A2
Application number: PCT/US1998/014894
Authority: WO
Inventors: Chester A. Ruszczyk; Whay Chiou Lee; Imrich Chlamtac
Original assignee: Motorola Inc.
Priority date: 1997-08-14
Filing date: 1998-07-17
Publication date: 1999-02-25
Also published as: CA2268794A1; AU8496798A; WO1999009690A3; EP0938789A1

Abstract

A system, device, and method for scheduling data transmission opportunities for variable bit rate traffic provides data transmission opportunities at a peak cell rate when the end user is actively transmitting data, and provides data transmission opportunities at a lower rate when the end user has no data to transmit. When the end user has no data to transmit, any excess data transmission opportunities (i.e., data transmission opportunities from the lower rate up to the peak cell rate) are available for statistically multiplexing traffic from other end users. Various embodiments utilize a lower rate anywhere between a zero rate and a sustainable cell rate, inclusive. Alternative embodiments allocate a burst of data transmission opportunities equivalent to a maximum burst size at the peak cell rate when the end user has data to transmit, after which the end user is once again provided data transmission opportunities at the lower rate.

Description

SYSTEM, DEVICE, AND METHOD FOR SCHEDULING

VARIABLE BIT RATE TRAFFIC IN A COMMUNICATION

NETWORK

Related Applications

This application claims priority from United States provisional patent application number 60/055,658 entitled System, Device, And

Method For Scheduling In A Communication Network filed on August 14, 1997, incorporated herein by reference in its entirety.

Background

1. Field of the Invention

- The invention relates generally to communication systems, and more particularly to scheduling transmission opportunities in a communication network.

2. Discussion of Related Art

In today's information age, there is an increasing need for high- speed communications that provide guaranteed quality of service (QoS) for an ever-increasing number of communications consumers. To that end, communications networks and technologies are evolving to meet current and future demands. Specifically, new networks are being deployed which reach a larger number of end users, and protocols are being developed to utilize the added bandwidth of these networks efficiently.

One technology that has been widely employed and will remain important in the foreseeable future is the shared-medium network. A shared medium network is one in which a single communications channel (the shared channel) is shared by a number of end users such that uncoordinated transmissions from different end users may interfere with one another. Since communications networks typically have a limited number of communications channels, the shared medium network allows many end users to gain access to the network over a single communications channel, thereby allowing the remaining communications channels to be used for other purposes. However, the shared medium network is only feasible when each end user only transmits data intermittently, allowing other end users to transmit during periods of silence.

One problem in a shared medium network involves scheduling end user transmissions to provide each end user with guaranteed bandwidth and QoS. In a cell-based network such as an Asynchronous Transfer Mode (ATM) network, one technique for scheduling end user transmissions is to provide each end user with data transmission opportunities at its peak cell rate, which is the maximum rate at which the end user can generate cells. Such a scheduling technique works well for continuous bit rate traffic which always generates cells at the peak cell rate. However, such a scheduling technique may be inefficient for variable bit rate traffic, which does not always generate cells at the peak cell rate. Furthermore, such a scheduling technique does not allow for statistical multiplexing (i.e., bandwidth sharing) of end user traffic. Therefore, an efficient technique for scheduling variable bit rate traffic that allows for statistical multiplexing is needed.

Brief Description of the Drawing

In the Drawing,

FIG. 1 is a block diagram showing an exemplary shared medium network in accordance with a preferred embodiment of the present invention;

FIG. 2 is a block diagram showing a headend unit and an access interface unit in accordance with a preferred embodiment of the present invention; FIG. 3 is a state diagram showing three possible states of a MAC

User and the transitions among them in accordance with various embodiments including prior art embodiments and a preferred embodiment of the present invention;

FIG. 4 is a logic diagram showing exemplary head end scheduler logic in accordance with a first embodiment of the present invention; FIG. 5 is a logic diagram showing exemplary head end scheduler logic in accordance with a second embodiment of the present invention;

FIG. 6 is a logic diagram showing exemplary MAC User logic in accordance with a third embodiment of the present invention;

FIG. 7 is a logic diagram showing exemplary head end scheduler logic in accordance with a third embodiment of the present invention;

FIG. 8 is a logic diagram showing exemplary head end scheduler logic in accordance with a fourth embodiment of the present invention;

FIG. 9 is a logic diagram showing exemplary head end scheduler logic in accordance with a fifth embodiment of the present invention;

Detailed Description An efficient technique for scheduling variable bit rate traffic that allows for statistical multiplexing is needed. The present invention provides such an efficient technique for scheduling variable bit rate traffic by providing data transmission opportunities at the peak cell rate when the end user is actively transmitting data, and providing data transmission opportunities at a lower rate when the end user has no data to transmit. When the end user has no data to transmit, any excess data transmission opportunities (i.e., data transmission opportunities from the lower rate up to the peak cell rate) are available for statistically multiplexing traffic from other end users. Various embodiments of the present invention utilize a lower rate anywhere between a zero rate and a sustainable cell rate, inclusive. Alternative embodiments of the present invention allocate a burst of data transmission opportunities equivalent to a maximum burst size at the peak cell rate when the end user has data to transmit, after which the end user is once again provided data transmission opportunities at the lower rate. FIG. 1 is a block diagram showing an exemplary shared medium network 100 in accordance with a preferred embodiment of the present invention. The shared medium network 100 allows a number of end users 110, through 110_N to access a remote external network 108 such as the Internet. The shared medium network 100 acts as a conduit for transporting information between the end users 1 10 and the external network 108.

The shared medium network 100 includes a Headend Unit (HU) 102 that is coupled to the external network 108. The HU 102 is in communication with a plurality of Access Interface Units 104, through 104_N (collectively referred to as "AIUs 104" and individually as an "AIU 104") by means of a shared physical medium 106. Each end user 1 10 interfaces to the shared medium network 100 by means of an AIU 104. A single AIU 104 may support one or a number of the end users 110. The shared physical medium 106 includes a plurality of channels over which information can be transferred between the HU 102 and the AIUs 104. In the preferred embodiment, each channel is unidirectional; that is, a particular channel either carries information from the HU 102 to the AIUs 104 or from the AIUs 104 to the HU 102. Those channels that carry information from the HU 102 to the AIUs 104 are typically referred to as "downstream channels." Those channels that carry information from the AIUs 104 to the HU 102 are typically referred to as "upstream channels." In alternative embodiments, these various upstream and downstream channels may, of course, be the same physical channel, for example, through time-division multiplexing/duplexing, or separate physical channels, for example, through frequency-division multiplexing/duplexing.

In the preferred embodiment, the shared medium network 100 is a data-over-cable (DOC) communication system wherein the shared physical medium 106 is a two-way hybrid fiber-optic and coaxial cable (HFC) network. The HU 102 is a headend device typically referred to as a "cable router." The AIUs 104 are cable modems. In other embodiments, the shared physical medium 106 may be coaxial cable, fiber-optic cable, twisted pair wires, and so on, and may also include air, atmosphere, or space for wireless and satellite communication.

In the shared medium network 100 of the preferred embodiment, the downstream channels are typically situated in a frequency band above approximately 50 MHz, although the particular frequency band may vary from system to system, and is often country-dependent. The downstream channels are classified as broadcast channels, since any information transmitted by the HU 102 over a particular downstream channel reaches all of the AIUs 104. Any of the AIUs 104 that are tuned to receive on the particular downstream channel can receive the information.

In the shared medium network 100 of the preferred embodiment, the upstream channels are typically situated in a frequency band between approximately 5 through 42 MHz, although the particular frequency band may vary from system to system, and is often country- dependent. Each upstream channel is divided into successive time slots, and is therefore often referred to as a "slotted channel." A slot may be used to carry a protocol data unit including user or control information, or may be further divided into mini-slots used to carry smaller units of information. The upstream channels are classified as shared channels, since only one AIU 104 can successfully transmit in a slot or mini-slot at any given time, and therefore the upstream channels must be shared among the plurality of AIUs 104. If more than one of the AIUs 104 simultaneously transmit in a slot or mini-slot, there is a collision that corrupts the information from all of the simultaneously transmitting AIUs 104.

In order to allow multiple AIUs 104 to share a single upstream channel, the HU 102 and the AIUs 104 participate in a medium access control (MAC) protocol. The MAC protocol provides a set of rules and procedures for coordinating access by the AIUs 104 to the shared channel. Each AIU 104 participates in the MAC protocol on behalf of its end users. For convenience, each participant in the MAC protocol is referred to as a "MAC User."

A MAC User typically represents a connection supporting a particular end user application. Although it is not elaborated upon, a MAC User could also represent an aggregate of connections that share similar traffic characteristics. In many modern communication networks, each MAC User has specific traffic parameters and QoS requirements, which are agreed upon when the MAC User establishes a connection in the network and which are guaranteed by the network. For example, a MAC User may require a guaranteed minimum amount of bandwidth, a guaranteed maximum transfer delay for the data it transmits, or a minimum time variation between transmission opportunities provided by the HU 102. In a cell-based network such as an Asynchronous Transfer Mode (ATM) network, a number of parameters are used to characterize the bandwidth requirements of each MAC User. Specifically, a set of ATM Traffic Descriptors is used for characterizing the type of traffic generated by the MAC User, and a set of QoS Parameters is used for specifying the network services required by the MAC User.

The ATM Traffic Descriptors include Peak Cell Rate (PCR), Sustainable Cell Rate (SCR), Minimum Cell Rate (MCR), and Maximum Burst Size (MBS). The PCR is an indication of the maximum data rate (cells per second) that will be generated by the MAC User. The SCR represents the long-term average data rate generated by the MAC User. The MCR is the minimum data rate (cells per second) required by the MAC User. The MBS is the maximum size (number of cells) of any burst generated by the MAC User.

The QoS parameters include Maximum Cell Transfer Delay (MaxCTD), Cell Delay Variation (CDV), and Cell Loss Ratio (CLR). The MaxCTD specifies the maximum delay (including the access delay and propagation delay of the underlying communications network) that will be tolerated by the MAC User. The CDV specifies the maximum time variation between data transmission opportunities that will be tolerated by the MAC User. The CLR specifies the MAC User's allowance of cells that may be dropped by the network, in terms of a ratio of lost cells to all the cells transmitted by the MAC User.

MAC Users having similar traffic characteristics are categorized into service categories. ATM defines five service categories, namely continuous bit rate (CBR), real-time variable bit rate (RT-VBR), non-realtime variable bit rate (NRT-VBR), available bit rate (ABR), and-unspecified bit rate (UBR). Each MAC User is categorized into one of the service categories. CBR connections are characterized by traffic having a fixed bit rate and requiring real-time delivery of cells. The ATM Traffic Descriptor for the CBR service category is PCR. The QoS Parameters for the CBR service category are MaxCTD, CDV, and CLR.

RT-VBR connections are characterized by traffic having a variable bit rate and requiring real-time delivery of cells. In a typical RT-VBR connection, the traffic over the connection will have periods where cells are generated in bursts at the PCR and periods of silence where no cells are generated (voice and real-time video are good examples of RT-VBR traffic). The ATM Traffic Descriptors for the RT-VBR service category are PCR, SCR, and MBS. The QoS Parameters for the RT-VBR service category are MaxCTD, CDV, and CLR.

NRT-VBR connections are characterized by traffic having a variable bit rate (similar to RT-VBR connections), but not requiring realtime delivery of cells. NRT-VBR connections typically require a low CLR. ABR connections are characterized by traffic requiring a minimum cell rate, but willing to accept additional bandwidth if and when such additional bandwidth becomes available. The ATM Traffic Descriptors for the ABR service category are the MCR and PCR. The MCR is the minimum guaranteed cell rate required by the connection. The PCR is the maximum cell rate at which the connection can transmit if allowed by the network. Consequently, the transmission rate of the ABR connection lies somewhere between the MCR and the PCR. No QoS Parameters are defined for the ABR service category. ABR connections are typically subject to a flow control mechanism with feedback that requires the source of the connection to adapt its rate in response to changing ATM layer transfer characteristics. UBR connections are not guaranteed any bandwidth and have no

QoS requirements. The ATM Traffic Descriptor for the UBR service category is the PCR. The PCR is the maximum cell rate at which the connection can transmit if allowed by the network.

FIG. 2 is a block diagram 200 showing the HU 102 and the AIU 104 in accordance with a preferred embodiment of the present invention. The block diagram 200 shows the HU 102 in communication with one of the AIUs 104 by means of a downstream channel 230 and an upstream channel 240. In the preferred embodiment, the downstream channel 230 and the upstream channel 240 are separate channels carried over the shared physical medium 106. The AIU 104 supports at least one End User 110. The HU 102 transmits data and control messages to the AIU 104 by means of downstream channel 230, and receives reservation requests and data from the AIU 104 by means of upstream channel 240. The HU 102 includes a Connection Manager (CM) 215. The CM 215 is responsible for connection admission control for the network. More specifically, the CM 215 is responsible for establishing and terminating connections in the network.

Before an End User such as the End User 110 can communicate over the network, a connection must be established. Therefore, when the End User 110 requests admission to the network, a connection request message is forwarded to the CM 215. The connection request message specifies the bandwidth and QoS requirements for the connection in the form of an ATM Service Category and associated ATM Traffic Descriptors and QoS Parameters. The CM 215 decides whether or not to establish the connection, and thereby admit the End User 110 to the network, based on the ability (or inability) of the network to meet the bandwidth and QoS requirements of the End User 110. The HU 102 also includes a Headend Scheduler (HES) 214 that is coupled to the CM 215 and to an Adaptive Reservation Manager (ARM) 211. The HES 214 is responsible for scheduling data transmission opportunities for those End Users that have been admitted to the network by the CM 215. When the CM 215 establishes a connection, the CM 215 sends a connection setup message to the HES 214 including a connection identifier and the ATM Service Category and associated ATM Traffic Descriptors and QoS Parameters for the established connection. Similarly, when the CM 215 terminates a connection, the CM 215 sends a connection release message to the HES 214 including a connection identifier for the terminated connection. The HES 214 uses the connection information received from the CM 215 together with feedback information received from the ARM 211 to control the timing of control messages transmitted by the ARM 211. The ARM 211 is responsible for implementing the MAC protocol in the HU 102. The ARM 211 includes a Reservation Manager (RM) 212 and a Feedback Controller (FC) 213. The RM 212 monitors the contention mini-slots on the upstream channel 240 to determine the result of contention for each contention mini-slot. The RM 212 sends the contention results to the FC 213 and to the HES 214. The FC 213 maintains the state information for each priority class (if multiple priority classes are implemented to support differentiated quality of service), determines the assignment of contention mini-slots for each contention cycle, and formats the control messages to be transmitted on the downstream channel 230. The FC 213 bases the timing of control message transmissions on, among other things, timing information received from the HES 214.

The AIU 104 includes a User Interface (Ul) 225 for interfacing with the End User 110. Data transmitted by the End User 1 10 is received by the Ul 225 and stored in a Memory 224. The Ul 225 also stores in the Memory 224 a time stamp indicating the arrival time of the data. The AIU 104 also includes a Control Message Processor (CMP) 222 that is coupled to the Memory 224. The CMP 222 is responsible for processing data and control messages received from the HU 102 by means of Receiver 221. The CMP 222 participates as a MAC User in the MAC protocol on behalf of the End User 110.

A number of different MAC protocols have been developed for use in the shared medium network 100. These protocols can generally be categorized as contention-free protocols and contention-based protocols. Contention-free protocols, such as time-division multiple-access (TDMA) and round-robin polling, avoid collisions on the shared channel by authorizing only one MAC User to transmit at a time. Contention-based protocols, such as certain reservation-based protocols, do not avoid collisions but instead resolve any collisions that do occur on the shared channel. In a preferred embodiment, the MAC protocol uses a combination of polling and contention-based reservation for scheduling upstream transmissions. Contention-based reservation requires a MAC User to make a reservation before the HU 102 will allocate bandwidth to the MAC User. The HU 102 provides reservation opportunities to the MAC Users by transmitting special control messages (described in detail below) to the MAC Users by means of the downstream channel 230. A MAC User makes a reservation by transmitting a reservation request message in response to a reservation opportunity provided by the HU 102. Each reservation opportunity typically authorizes multiple MAC Users to respond, and therefore the MAC Users must contend for a reservation. Not all MAC Users are required to make a reservation. Certain MAC Users are allocated bandwidth regularly without having to make reservations.

Each MAC User that is required to make a reservation maintains a MAC User state machine as known in the prior art and described with reference to FIG. 3. The MAC User starts in the INACTIVE state 302, and remains there so long as it has no data to transmit. When the MAC User receives data to be transmitted, the MAC User transitions into the CONTENTION state 304. In the CONTENTION state 304, the MAC User contends for access to the shared channel until it is able to make a successful reservation for itself. Upon making a successful reservation in state 304, the MAC User transitions into the ACTIVE state 306. Here, the MAC User receives opportunities from the HU 102 to transmit data, and remains in the ACTIVE state 306 so long as it has data to transmit. Upon transmitting all of its data, the MAC User is considered to be "fulfilled," and the MAC User transitions back into the INACTIVE state 302. Each MAC User that is not required to make a reservation remains in the ACTIVE state 306 so long as its connection is active.

In a preferred embodiment, the MAC protocol divides each upstream channel into discrete time slots. A MAC User transmits reservation request messages and data in slots designated by the HU 102. In the preferred embodiment, the upstream channel supports two types of slots, namely contention slots and data slots. Contention slots are typically of shorter duration than data slots, and are therefore often referred to as "mini-slots" or "contention mini-slots." Contention mini-slots are used for transmitting reservation request messages, while data slots are used for transmitting data messages. Each data slot is typically capable of carrying the equivalent of a single protocol data unit, which, in a cell-based network such as Asynchronous Transfer Mode (ATM), is a cell.

Although not required, a typical embodiment divides the upstream channel 240 into consecutive contention cycles, where each contention cycle is typically composed of a predetermined number of contention mini-slots and a predetermined number of data slots set at fixed locations within the contention cycle. It is typical for the contention mini-slots (and therefore the data slots also) to be contiguous within the contention cycle. For the sake of simplicity, the contention mini-slots are typically scheduled in clusters, each of which consists of the maximum number of contention mini-slots that can fit within a data slot. For each contention cycle, the HU 102 transmits an entry poll message to the MAC Users by means of the downstream channel 230. The entry poll message includes an assignment of contention mini-slots for the contention cycle, indicating which MAC User(s) are permitted to contend in each contention mini-slot, and an assignment of data slots for the contention cycle, indicating which MAC User is permitted to transmit data in each data slot. Each entry poll message also includes feedback information (described in detail below) for each contention mini-slot in the preceding contention cycle. The feedback information allows each MAC User that contended in the preceding contention cycle to determine the outcome of its contention (i.e., success or failure).

Typically, only MAC Users that are in the CONTENTION state 304 that have data to transmit may contend for a reservation by transmitting a reservation request message in a designated contention mini-slot. The HU 102 monitors each contention mini-slot to determine the result of contention for each contention mini-slot. Specifically, the HU 102 receives either (1 ) no transmission, indicating that no MAC User transmitted in the contention mini-slot; (2) a reservation request message, indicating that a single MAC User transmitted in the contention mini-slot and identifying that MAC User; or (3) a collision, indicating that more than one MAC User transmitted in the contention mini-slot. For convenience, the three states are referred to as IDLE, SUCCESS, and COLLISION, respectively.

As discussed above, the HU 102 includes feedback information in each entry poll message. Specifically, in each entry poll message, the HU 102 indicates a feedback state (i.e., IDLE, SUCCESS, or COLLISION) for each contention mini-slot in the preceding contention cycle. The feedback information allows each MAC User that transmitted a reservation request message in the preceding contention cycle to determine the result of its contention attempt. Specifically, each MAC User examines the feedback state corresponding to the contention mini- slot in which the MAC User transmitted its reservation request message to determine if the result is SUCCESS or COLLISION. If the feedback state indicates SUCCESS, then the MAC User transitions from the CONTENTION state 304 to the ACTIVE state 306 and awaits data transmission opportunities from the HU 102; otherwise, the MAC User remains in the CONTENTION state 304 and awaits further contention opportunities from the HU 102.

The HU 102 provides data transmission opportunities to the active MAC Users (i.e., those MAC Users that have made a successful reservation or require no reservation). Specifically, the HES 214 schedules data transmission opportunities to the active MAC Users by allocating data slots on the upstream channel 240 such that each MAC User receives a sufficient amount of bandwidth within its QoS constraints.

_ CBR connections require bandwidth at a constant rate within specified delay and jitter constraints. Therefore, CBR MAC Users are allocated data transmission opportunities at regular intervals at the PCR. CBR MAC Users are not required to make reservations, and therefore remain in the ACTIVE state 306 as long as the CBR connection is active.

RT-VBR connections are allocated a sufficient amount of bandwidth to meet the bandwidth and real-time delivery requirements of the connection. One approach is to treat the RT-VBR connection like a CBR connection by allocating bandwidth at regular intervals at the PCR. This guarantees that the bandwidth and real-time delivery requirements of the connection will be met. However, it may also be an inefficient use of bandwidth, since the RT-VBR connection does not generate data constantly at the PCR.

Treating a RT-VBR connection like a CBR connection may be acceptable for certain RT-VBR connections, specifically those having an SCR close to the PCR, but is less desirable if there is a significant difference between the SCR and the PCR. Therefore, it is convenient to categorize each RT-VBR connection according to the ratio of SCR to PCR. If the ratio of SCR to PCR exceeds a predetermined value, for example 0.5, then the connection is considered to be a CBR-like connection that can be treated like a CBR connection with little loss of bandwidth efficiency. However, if the ratio of SCR to PCR is less than or equal to the predetermined value, then the connection is considered to be a "bursty" connection, and is preferably not treated like a CBR connection.

For a "bursty" RT-VBR connection, bandwidth allocation at the PCR is excessive and is therefore inefficient and undesirable. Likewise, it is also undesirable to allocate bandwidth at the SCR, since the network would then be unable to guarantee real-time delivery of cells for the connection. Therefore, various embodiments of the present invention allocate data transmission opportunities for "bursty" RT-VBR traffic at the PCR when the MAC User has data to transmit and at a lower rate when the MAC User has no data to transmit. For convenience, the MAC User is said to be "active" when it is determined that the MAC User requires data transmission opportunities at the PCR and "inactive" when it is determined that the MAC User does not require data transmission opportunities at the PCR.

In a first embodiment, the HES 214 allocates data transmission opportunities at the PCR while the MAC User is "active" and at a zero rate while the MAC User is "inactive". In this first embodiment, the MAC User begins in the INACTIVE state 302, and transitions into the CONTENTION state 304 when the MAC User receives data to be transmitted. In the CONTENTION state 304, the MAC User contends as described above. Once a successful reservation is made, the MAC User transitions into the ACTIVE state 306, and the HES 214 allocates data slots to the MAC User at the PCR. The HES 214 monitors the data slots allocated to the MAC User to determine whether the MAC User continues to transmit data, in which case the MAC User is active, or the MAC User is no longer transmitting data, in which case the MAC User is inactive. After a predetermined number of data slots in which no data is transmitted by the MAC User, the MAC User transitions back to the INACTIVE state 302, and the HES 214 stops allocating data slots to the MAC User. Exemplary HES 214 logic for scheduling data transmission opportunities in accordance with the first embodiment of the present invention is shown in FIG. 4. The HES 214 allocates data transmission opportunities for the MAC User at the zero rate when the MAC User is "inactive" (402). When the MAC User makes a reservation (404), the HES 214 allocates data transmission opportunities for the MAC User at the PCR (406). The HES 214 continues allocating data transmission opportunities at the PCR as long as the MAC User continues transmitting data. When the MAC User has not transmitted data for a predetermined number of consecutive data transmission opportunities (408), the HES 214 returns to allocating data transmission opportunities for the MAC User at the zero rate (402).

This first embodiment for handling "bursty" RT-VBR traffic is bandwidth efficient, at least to the degree that the MAC User is not allocated bandwidth in excess of its required bandwidth (other than the overhead of waiting until the MAC User has not transmitted data for a predetermined number of consecutive data transmission opportunities before the MAC User is considered to be "inactive"). However, because the MAC User is required to contend for access following a period of inactivity, the MAC User is required to buffer data during the period of contention and therefore is delayed in transmitting the data. The contention access delay may vary each time the MAC User contends, since there is no a priori knowledge as to how many contention cycles it will take before a successful reservation is made. Therefore, the contention access delay and its variability makes it difficult for the HES 214 to meet the guaranteed CDV for the MAC User.

The contention access delay also affects the CLR. If the contention access delay is too large, some cells become "obsolete" and are consequently dropped by the MAC User. Although typical RT-VBR connections can tolerate a certain amount of cell loss, it is desirable to minimize such degradation if possible. Moreover, the smaller the CLR, the more closely the SCR requirement is matched. Thus, it is preferable that a contention-based reservation access scheme that supports multiple priority classes be used so that the RT-VBR users can be given high priority for reservation access. One contention-based reservation access scheme that supports multiple priority classes is described in U.S. Patent Application No. 08/865,924 entitled System, Device, and Method for Sharing Contention Mini-Slots Among Multiple Priority Classes, filed on May 30, 1997 in the name of Chester A. Ruszczyk, Whay Chiou Lee, and Imrich Chlamtac (Attorney Docket No. CX097020) with a preferred collision resolution technique described in U.S. Patent Application No. 08/866,865 entitled System, Device, and Method for Contention-Based Reservation in a Shared Medium Network, filed on May 30, 1997 in the name of Chester A. Ruszczyk, Whay Chiou Lee, and Imrich Chlamtac (Attorney Docket No. CX096053). Even with the use of an efficient contention-based reservation access scheme that supports multiple priority classes, it is important that the CM 215 only admit an additional RT-VBR user if the system can support the user without excessive contention access delay.

In a second embodiment, the HES 214 allocates data transmission opportunities at the PCR rate while the MAC User is "active" and at a lower non-zero rate, preferably at the SCR, while the MAC User is "inactive". In this second embodiment, HES 214 allocates data transmission opportunities for the MAC User regularly (although the allocation rate varies), so the MAC User remains permanently in the ACTIVE state 306 and is not required to make a reservation. The HES 214 typically begins by allocating bandwidth at the lower non-zero rate and monitoring for data transmissions by the MAC User. As soon as the MAC User begins transmitting data in the designated data slots, the HES 214 increases the bandwidth allocation to the PCR and monitors for the MAC User to stop transmitting data. The HES 214 continues allocating bandwidth at the PCR so long as the MAC User is actively transmitting data. When the MAC User stops transmitting data, the HES 214 throttles the bandwidth allocation back to the lower non-zero rate. This throttling can occur immediately upon a determination that the MAC User is "inactive" (e.g., following a predetermined number of data transmission opportunities in which no data was transmitted) or gradually by decreasing the bandwidth allocation in steps. By providing a minimum bandwidth equal to the lower non-zero rate and providing PCR on demand, the HES 214 is more easily able to meet the delay and jitter requirements of the MAC User because the slots allocated at the lower non-zero rate are sent at known intervals, and the HES 214 is able to react quickly when the MAC User requires bandwidth at the PCR._ Exemplary HES 214 logic for scheduling data transmission opportunities in accordance with the second embodiment of the present invention is shown in FIG. 5. The HES 214 allocates data transmission opportunities for the MAC User at the lower non-zero rate when the MAC User is "inactive" (502). When the MAC User transmits data in response to an allocated data transmission opportunity (504), the HES 214 allocates data transmission opportunities for the MAC User at the PCR (506). The HES 214 continues allocating data transmission opportunities at the PCR as long as the MAC User continues transmitting data. When the MAC User has not transmitted data for a predetermined number of consecutive data transmission opportunities (508), the HES 214 returns to allocating data transmission opportunities for the MAC User at the lower non-zero rate (502).

One advantage of this second embodiment is that the MAC User "knows" that it is being provided regular data transmission opportunities. Therefore, any data received between scheduled data transmission opportunities can be buffered until the next scheduled data transmission opportunity, and the MAC User is not required to contend in order to get transmission opportunities. However, if the MAC User is frequently "inactive" for a comparatively long time with respect to the time it is "active," much bandwidth is wasted. Moreover, the ability for the system to statistically multiplex RT-VBR connections is diminished. In this second embodiment, it is also possible that under certain circumstances, the reduced allocation rate will be insufficient to guarantee that data received by the MAC User between polls can be transmitted within the QoS constraints simply by waiting for the next data transmission opportunity. In this situation, it is preferable for the MAC User to contend for a reservation, prompting the HES 214 to provide a data transmission opportunity to the MAC User sooner than the next scheduled data transmission opportunity.

In a third embodiment, the MAC User decides whether or not to contend when it receives data between scheduled data transmission opportunities. When the MAC User receives data to be transmitted, the MAC User first determines the amount of time before the next scheduled data transmission opportunity, and then determines whether the QoS constraints will be violated if the MAC User waits until the next scheduled data transmission opportunity. If waiting until the next scheduled data transmission opportunity will violate the QoS constraints, then the MAC User contends for a reservation; otherwise, the MAC User waits for the next scheduled data transmission opportunity.

Exemplary MAC User logic in accordance with the third embodiment of the present invention is shown in FIG. 6. The logic begins in step 602, and upon receiving data to be transmitted in step 604, proceeds to determine the amount of time before the next scheduled data transmission opportunity, in step 606. The logic then determines whether the QoS constraints for the MAC User will be violated if the MAC User waits until the next scheduled data transmission opportunity, in step 608. If the QoS constraints will not be violated if the MAC User waits until the next scheduled data transmission opportunity (NO in step 610), then the logic buffers the data until the next scheduled data transmission opportunity, in step 612. However, if the QoS constraints will be violated if the MAC User waits until the next scheduled data transmission opportunity (YES in step 610), then the logic contends for a reservation, in step 614. The logic terminates in step 699. Exemplary HES 214 logic for scheduling data transmission opportunities in accordance with the third embodiment of the present invention is shown in FIG. 7. The HES 214 allocates data transmission opportunities for the MAC User at the lower non-zero rate when the MAC User is "inactive" (702). When the MAC User either transmits data in response to a data transmission opportunity or makes a reservation (704), the HES 214 allocates data transmission opportunities for the MAC User at the PCR (706). The HES 214 continues allocating data transmission opportunities at the PCR as long as the MAC User continues transmitting data. When the MAC User has not transmitted data for a predetermined number of consecutive data transmission opportunities (708), the HES 214 returns to allocating data transmission opportunities for the MAC User at the lower non-zero rate (702).

In the embodiments described above, the HES 214 utilizes only the PCR and SCR traffic descriptors for scheduling data transmission opportunities. However, RT-VBR traffic is also characterized by the MBS, which specifies the maximum amount of data that the MAC User can transmit at the PCR during any one burst. Alternative embodiments utilize the MBS to simplify scheduling by automatically allocating a burst of data transmission opportunities (referred to as a "burst allocation") equivalent to the MBS at the PCR when the MAC User becomes "active." During the burst allocation, the HES 214 does not have to monitor and react to the status of the MAC User (i.e., "active" or "inactive"), and therefore the HES 214 logic is simplified. Upon completion of the burst allocation, the HES 214 automatically treats the MAC User as being

"inactive" and allocates data transmission opportunities at the lower rate. By automatically providing a burst allocation when the MAC User becomes "active," the HES 214 provides for efficient bandwidth utilization if the MAC User transmits a maximum size burst each time it becomes "active." However, utilizing the burst allocation is inefficient if the actual burst size is variable or is consistently less than the MBS. When the MBS is considerably larger than a typical burst size, the system's ability to statistically multiplex RT-VBR connections is diminished.

Exemplary HES 214 logic for scheduling data transmission opportunities in accordance with a fourth embodiment of the present invention is shown in FIG. 8. The HES 214 allocates data transmission opportunities for the MAC User at the zero rate when the MAC User is "inactive" (802). When the MAC User makes a reservation (804), the HES 214 makes a burst allocation for the MAC User at the PCR (806). When the burst allocation is complete (808), the HES 214 returns to allocating data transmission opportunities for the MAC User at the zero rate (802).

Exemplary HES 214 logic for scheduling data transmission opportunities in accordance with a fifth embodiment of the present invention is shown in FIG. 9. The HES 214 allocates data transmission opportunities for the MAC User at the lower non-zero rate when the MAC User is "inactive" (902). When the MAC User either transmits data in response to a data transmission opportunity or makes a reservation (904), the HES 214 makes a burst allocation for the MAC User at the PCR (906). When the burst allocation is complete (908), the HES 214 returns to allocating data transmission opportunities for the MAC User at the lower non-zero rate (902).

Although the above embodiments are described with reference to a particular reservation-based MAC protocol that utilizes a slotted upstream channel, the present invention is not limited to either the reservation-based MAC protocol or the slotted upstream channel. The described scheduling techniques are applicable to other MAC protocols and other upstream channel configurations. In particular, the described scheduling techniques are applicable to a MAC protocol commonly referred to as Multimedia Cable Network System (MCNS). It will be apparent to a skilled artisan how the described scheduling techniques can be applied to the MCNS protocol and to other MAC protocols and upstream channel configurations. All logic described herein can be embodied using discrete components, integrated circuitry, programmable logic used in conjunction with a programmable logic device such as a Field Programmable Gate Array (FPGA) or microprocessor, or any other means including any combination thereof. Programmable logic can be fixed temporarily or permanently in a tangible medium such as a read-only memory chip, a computer memory, a disk, or other storage medium. Programmable logic can also be fixed in a computer data signal embodied in a carrier wave, allowing the programmable logic to be transmitted over an interface such as a computer bus or communication network. All such embodiments are intended to fall within the scope of the present invention.

The present invention may be embodied in other specific forms without departing from the essence or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.

Claims

We claim:

1. A method for scheduling data transmission opportunities for an end user generating variable bit rate data, the method comprising the steps of: determining that the end user has data to transmit; allocating data transmission opportunities at a peak cell rate upon determining that the end user has data to transmit; and allocating data transmission opportunities at a lower rate when the end user has no data to transmit, wherein the lower rate is equal to one of: a zero rate, such that the end user is required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate; and a non-zero rate, such that the end user is not required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate.

2. The method of claim 1 wherein the step of allocating data transmission opportunities at the peak cell rate upon determining that the end user has data to transmit comprises allocating a burst of data transmission opportunities equivalent to a predetermined maximum burst size at the peak cell rate upon determining that the end user has data to transmit, and wherein the step of allocating data transmission opportunities at the lower rate when the end user has no data to transmit comprises allocating data transmission opportunities at the lower rate upon completion of the burst of data transmission opportunities.

3. A device for scheduling data transmission opportunities for an end user generating variable bit rate data, the device comprising: determining logic operably coupled to determine that the end user has data to transmit; first allocating logic responsive to the determining logic to allocate data transmission opportunities at a peak cell rate upon determining that the end user has data to transmit; and second allocating logic responsive to the first allocating logic to allocate data transmission opportunities at a lower rate when the end user has no data to transmit, wherein the lower rate is equal to one of: a zero rate, such that the end user is required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate; and a non-zero rate, such that the end user is not required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate.

4. An apparatus comprising a computer usable medium having embodied therein a computer readable program for scheduling data transmission opportunities for an end user generating variable bit rate data, the computer readable program comprising computer readable program instructions enabling a computer to perform the steps of: determining that the end user has data to transmit; allocating data transmission opportunities at a peak cell rate upon determining that the end user has data to transmit; and allocating data transmission opportunities at a lower rate when the end user has no data to transmit, wherein the lower rate is equal to one of: a zero rate, such that the end user is required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate; and a non-zero rate, such that the end user is not required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate.

5. A data signal embodied in a carrier wave, wherein embodied in the data signal is a computer readable program for scheduling data transmission opportunities for an end user generating variable bit rate data, the computer readable program comprising computer readable program instructions enabling a computer to perform the steps of: determining that the end user has data to transmit; allocating data transmission opportunities at a peak cell rate upon determining that the end user has data to transmit; and allocating data transmission opportunities at a lower rate when the end user has no data to transmit, wherein the lower rate is equal to one of: a zero rate, such that the end user is required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate; and a non-zero rate, such that the end user is not required to contend for data transmission opportunities when data transmission opportunities are allocated at the lower rate.

6. A method for transmitting end user data by an access interface unit in a shared medium communication network, the end user data having associated quality of service constraints, the method comprising the steps of: receiving end user data to be transmitted; determining an amount of time before a next scheduled data transmission opportunity; determining whether the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity based on the determined amount of time before the next scheduled data transmission opportunity; buffering the data until the next scheduled data transmission opportunity, if the quality of service constraints will not be violated by waiting until the next scheduled data transmission opportunity; and contending for a data transmission opportunity, if the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity.

7. A device for transmitting end user data in a shared medium communication network, the end user data having associated quality of service constraints, the device comprising: receiving logic operably coupled to receive end user data; determining logic responsive to the receiving logic to determine an amount of time before a next scheduled data transmission opportunity and to determine whether the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity based on the determined amount of time before the next scheduled data transmission opportunity; buffering logic responsive to the determining logic, the buffering logic buffering the data until the next scheduled data transmission opportunity, if the quality of service constraints will not be violated by waiting until the next scheduled data transmission opportunity; and contending logic responsive to the determining logic, the contending logic contending for a data transmission opportunity, if the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity.

8. An apparatus comprising a computer usable medium having embodied therein a computer readable program for transmitting end user data by an access interface unit in a shared medium communication network, the end user data having associated quality of service constraints, the computer readable program comprising computer readable program instructions enabling a computer to perform the steps of: receiving end user data to be transmitted; determining an amount of time before a next scheduled data transmission opportunity; determining whether the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity based on the determined amount of time before the next scheduled data transmission opportunity; buffering the data until the next scheduled data transmission opportunity, if the quality of service constraints will not be violated by waiting until the next scheduled data transmission opportunity; and contending for a data transmission opportunity, if the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity.

9. A data signal embodied in a carrier wave, wherein embodied in the data signal is a computer readable program for transmitting end user data by an access interface unit in a shared medium communication network, the end user data having associated quality of service constraints, the computer readable program comprising computer readable program instructions enabling a computer to perform the steps of: receiving end user data to be transmitted; determining an amount of time before a next scheduled data transmission opportunity; determining whether the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity based on the determined amount of time before the next scheduled data transmission opportunity; buffering the data until the next scheduled data transmission opportunity, if the quality of service constraints will not be violated by waiting until the next scheduled data transmission opportunity; and contending for a data transmission opportunity, if the quality of service constraints will be violated by waiting until the next scheduled data transmission opportunity.

10. A system comprising a headend unit in communication with an access interface unit over a shared medium communication network, wherein the headend unit includes: a headend scheduler operably coupled to schedule data transmission opportunities for an end user generating variable bit rate data, the headend scheduler determining that the end user has data to transmit, allocating data transmission opportunities at a peak cell rate upon determining that the end user has data to transmit, and allocating data transmission opportunities at a lower rate when the end user has no data to transmit.