In a two way cable system that includes a controller and a plurality of user terminals, a method of granting rights for upstream data transmission from the user terminals includes processing contention requests to generate contention grants and maintaining a list of connections, each connection having specified therefor a predetermined bit rate. The controller sends a downstream data stream to the user terminals and receives an upstream data stream from the user terminals. The downstream data stream includes a plurality of grant fields, and the upstream data stream includes upstream data slots and upstream control slots. The method further includes scheduling preemptive grants for upstream data slots using selected fields of the plurality of grant fields so as to provide preemptive grants for upstream data transport at a bit rate specified for each connection, sending to the user terminals a data transmission grant in a grant field of the plurality of grant fields for which a preemptive grant is scheduled, and sending to the user terminals a data transmission grant in a grant field of the plurality of grant fields in which a contention grant is pending and no preemptive grant is scheduled.
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105. A method employed in a two way communications system comprising a controller and a plurality of user terminals, the two way communications system capable of carrying information flows and bursts of data of the plurality of user terminals, the controller transmitting downstream information to the plurality of user terminals on at least one downstream channel, the plurality of user terminals transmitting upstream information to the controller on at least one shared upstream channel of a shared communications medium, the bandwidth of the at least one shared upstream channel being divided into upstream time slots, the upstream time slots capable of carrying reservation requests and data, at least one media access control mechanism being used to resolve contention for transmission on the at least one shared upstream channel, an upstream information transmission of at least one first user terminal of the plurality of user terminals capable of colliding on the at least one shared upstream channel with an upstream information transmission of at least one second user terminal of the plurality of user terminals, the method comprising the steps of:
maintaining a first information flow, the first information flow being associated with at least one service parameter, the first information flow having a quality of service indicated by the at least one service parameter;
receiving notice of a first grant from the controller responsive to the controller scheduling of the first grant, the first grant message indicating an allocation of at least one first upstream time slot to carry data that is at least part of the first information flow; and
disregarding notice of a second grant from the controller, the notice of the second grant indicating the allocation of a second grant of at least one second upstream time slot to the at least one second user terminal, the second grant generated by the controller responsive to the at least one second user terminal sending a reservation request that solicits the second grant to carry a requested burst of data, the requested burst of data having a requested burst size of finite amount.
93. A method employed in a two way communications system comprising a controller and a plurality of user terminals, the two way communications system capable of carrying information flows and bursts of data of the plurality of user terminals, the controller transmitting downstream information to the plurality of user terminals on at least one downstream channel, the plurality of user terminals transmitting upstream information to the controller on at least one shared upstream channel of a shared communications medium, the bandwidth of the at least one shared upstream channel being divided into upstream time slots, the upstream time slots capable of carrying reservation requests and data, at least one media access control mechanism being used to resolve contention for transmission on the at least one shared upstream channel, an upstream information transmission of at least one first user terminal of the plurality of user terminals capable of colliding on the at least one shared upstream channel with an upstream information transmission of at least one second user terminal of the plurality of user terminals, the method comprising the steps of:
sending a reservation request that solicits a first grant of at least one first upstream time slot to carry a requested burst of data, the requested burst of data having a requested burst size of finite amount;
maintaining a first information flow, the first information flow being associated with at least one service parameter, the first information flow having a quality of service indicated by the at least one service parameter;
receiving notice of the first grant from the controller responsive to the reservation request that solicits the first grant, the notice of the first grant indicating an allocation of the at least one first upstream time slot to carry a granted burst of data, the granted burst of data having a granted burst size and comprising a finite amount of data; and
receiving notice of a second grant from the controller responsive to the controller scheduling of a second grant, the notice of the second grant indicating an allocation of the at least one second upstream time slot to carry data that is at least part of the first information flow.
0. 117. A method for scheduling data transmissions, wherein the method is employed in a two-way communications system comprising a controller and a plurality of stations, the two-way communications system capable of carrying information flows and bursts of data of the plurality of stations, the controller transmitting downstream information to the plurality of stations on at least one downstream channel, the plurality of stations transmitting upstream information to the controller on at least one shared upstream channel of the communications medium, the bandwidth of the at least one shared upstream channel being divided into upstream time slots, the upstream time slots capable of carrying reservation requests and data, the method comprising:
processing a request for recurring time slots that includes soliciting a first grant of at least one first upstream time slot the at least one shared upstream channel of the communications medium, the at least one shared upstream channel capable of carrying an upstream transmission from a first station, the transmission capable of colliding with an upstream transmission from at least one second station, the time slots for carrying bursts of data having a burst size of finite amount;
scheduling a plurality of unsolicited upstream transmission grants comprising a second grant of at least one second upstream timeslot, the scheduling being based at least upon a first entry in a list of at least one information flow and at least one service parameter, the second grant being scheduled to provide a first information flow with a quality of service indicated by the at least one service parameter, the first information flow being associated with the first entry in the list of at least one information flow;
sending notice of the first grant to the plurality of stations responsive to the request for recurring time slots, the notice of the first grant indicating an allocation of the at least one first upstream time slot to carry a granted burst of data, the granted burst of data having a granted burst size and comprising a finite amount of data; and
sending notice of the second grant to the plurality of stations responsive to the scheduling of the second grant, the notice of the second grant indicating an allocation of the at least one second upstream time slot to carry data that is at least part of the first information flow.
0. 132. A controller in a two-way communications system comprising a controller and a plurality of stations, the two-way communications system capable of carrying information flows and bursts of data of the plurality of stations, the controller transmitting downstream information to the plurality of stations on at least one downstream channel, the plurality of stations transmitting upstream information to the controller on at least one shared upstream channel of the communications medium, the bandwidth of the at least one shared upstream channel being divided into upstream time slots, the upstream time slots capable of carrying reservation requests and data, the controller comprising:
logic configured to process a request for recurring time slots that includes soliciting a first grant of at least one first upstream time slot in the at least one shared upstream channel of the communications medium, the at least one shared upstream channel capable of carrying an upstream transmission from a first station, the transmission capable of colliding with an upstream transmission from at least one second station, the time slots for carrying bursts of data having a burst size of finite amount;
logic configured to schedule a plurality of unsolicited upstream transmission grants comprising a second grant of at least one second upstream timeslot, the scheduling being based at least upon a first entry in a list of at least one information flow and at least one service parameter, the second grant being scheduled to provide a first information flow with a quality of service indicated by the at least one service parameter, the first information flow being associated with the first entry in the list of at least one information flow;
logic configured to send notice of the first grant to the plurality of stations responsive to the request for recurring time slots, the notice of the first grant indicating an allocation of the at least one first upstream time slot to carry a granted burst of data, the granted burst of data having a granted burst size and comprising a finite amount of data; and
logic configured to send notice of the second grant to the plurality of stations responsive to the scheduling of a second grant, the notice of the second grant indicating an allocation of the at least one second upstream time slot to carry data that is at least part of the first information flow.
1. A method employed in a two way communications system comprising a controller and a plurality of user terminals, the two way communications system capable of carrying information flows and bursts of data of the plurality of user terminals, the controller transmitting downstream information to the plurality of user terminals on at least one downstream channel, the plurality of user terminals transmitting upstream information to the controller on at least one shared upstream channel of a shared communications medium, the bandwidth of the at least one shared upstream channel being divided into upstream time slots, the upstream time slots capable of carrying reservation requests and data, at least one media access control mechanism being used to resolve contention for transmission on the at least one shared upstream channel, an upstream information transmission of at least one first user terminal of the plurality of user terminals capable of colliding on the at least one shared upstream channel with an upstream information transmission of at least one second user terminal of the plurality of user terminals, the method comprising the steps of:
processing a reservation request that solicits a first grant of at least one first upstream time slot to carry a requested burst of data, the requested burst of data having a requested burst size of finite amount;
maintaining a list of at least one information flow, the list of at least one information flow being associated with at least one service parameter;
scheduling a second grant of at least one second upstream time slot, the scheduling being based at least upon a first entry in the list of the at least one information flow and the at least one service parameter, the second grant being scheduled to provide a first information flow with a quality of service indicated by the at least one service parameter, the first information flow being associated with the first entry in the list of at least one information flow;
sending notice of the first grant to the plurality of user terminals responsive to the reservation request that solicits the first grant, the notice of the first grant indicating an allocation of the at least one first upstream time slot to carry a granted burst of data, the granted burst of data having a granted burst size and comprising a finite amount of data; and
sending notice of the second grant to the plurality of user terminals responsive to the scheduling of the second grant, the notice of the second grant indicating an allocation of the at least one second upstream time slot to carry data that is at least part of the first information flow.
69. A controller in a two way communications system comprising the controller and a plurality of user terminals, the two way communications system capable of carrying information flows and bursts of data of the plurality of user terminals, the controller transmitting downstream information to the plurality of user terminals on at least one downstream channel, the plurality of user terminals transmitting upstream information to the controller on at least one shared upstream channel of a shared communications medium, the bandwidth of the at least one shared upstream channel being divided into upstream time slots, the upstream time slots capable of carrying reservation requests and data, at least one media access control mechanism being used to resolve contention for transmission on the at least one shared upstream channel, an upstream information transmission of at least one first user terminal of the plurality of user terminals capable of colliding on the at least one shared upstream channel with an upstream information transmission of at least one second user terminal of the plurality of user terminals, the controller comprising:
logic configured to process a reservation request that solicits a first grant of at least one first upstream time slot to carry a requested burst of data, the requested burst of data having a requested burst size of finite amount;
logic configured to maintain a list of at least one information flow, the list of at least one information flow being associated with at least one service parameter;
logic configured to schedule a second grant of at least one second upstream time slot, the scheduling being based at least upon a first entry in the list of the at least one information flow and the at least one service parameter, the second grant being scheduled to provide a first information flow with a quality of service indicated by the at least one service parameter, the first information flow being associated with the first entry in the list of at least one information flow;
logic configured to sending send notice of the first grant to the plurality of user terminals responsive to the reservation request that solicits the first grant, the notice of the first grant indicating an allocation of the at least one first upstream time slot to carry a granted burst of data, the granted burst of data having a granted burst size and comprising a finite amount of data; and
logic configured to sending send notice of the second grant to the plurality of user terminals responsive to the scheduling of the second grant, the notice of the second grant indicating an allocation of the at least one second upstream time slot to carry data that is at least part of the first information flow.
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creating the first entry in the list of at least one information flow responsive to establishment of the first information flow, the establishment signaled by the at least one message that carries information flow signaling; and
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monitoring at least one message that carries information flow signaling; and
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98. The method of clam 93, wherein the upstream time slots comprise a first type of upstream time slots and a second type of upstream time slots, the first type of upstream time slots limited to carrying reservation requests, and the second type of upstream time slots capable of carrying data that is not limited to reservation requests.
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0. 118. The method of claim 117, further comprising: sending a response to the request for the recurring time slots.
0. 119. The method of claim 117, further comprising: receiving the request for recurring time slots in the upstream channel.
0. 120. The method of claim 117, further comprising: maintaining a collection of information flows, the collection of information flows being associated with at least one service parameter.
0. 121. The method of claim 117, further comprising: sending the plurality of unsolicited upstream transmission grants, the grants meeting the requested quality of service of the first information flow defined by the at least one service parameter.
0. 122. The method of claim 121, further comprising: receiving the bursts of data on the upstream channel responsive to at least one of the plurality of unsolicited grants.
0. 123. The method of claim 117, wherein upon determining that additional information in excess of the burst size is required to be sent: processing a supplemental upstream data request.
0. 124. The method of claim 123, wherein the supplemental upstream data request is transmitted within a previously scheduled data burst.
0. 125. The method of claim 123, further comprising: scheduling at least one supplemental upstream transmission grant responsive to the supplemental upstream data request.
0. 126. The method of claim 125, further including: transmitting the at least one supplemental grant responsive to the supplemental upstream data request.
0. 127. The method of claim 126, further comprising: receiving at least one supplemental burst of data on the upstream channel responsive to the at least one supplemental grant.
0. 128. The method of claim 117, wherein the communications medium is at least part of a cable television networking environment.
0. 129. The method of claim 117, wherein the bursts of data are carried in anyone of ATM cells or packets.
0. 130. The method of claim 117, wherein the bursts of data comprise anyone of digitized voice or digitized video.
0. 131. The method of claim 117, wherein the information flow is from a local area network (LAN).
0. 133. The controller of claim 132, further comprising: logic configured to send a response to the request for the recurring time slots.
0. 134. The controller of claim 132, further comprising: logic configured to receive the request for recurring time slots in the upstream channel.
0. 135. The controller of claim 132, further comprising: logic configured to maintain a collection of information flows, the collection of information flows being associated with at least one service parameter.
0. 136. The controller of claim 132, further comprising: logic configured to send the plurality of unsolicited upstream transmission grants, the grants meeting the requested quality of service of the first information flow defined by the at least one service parameter.
0. 137. The controller of claim 136, further comprising: logic configured to receive the bursts of data on the upstream channel responsive to at least one of the plurality of unsolicited grants.
0. 138. The controller of claim 132, further comprising: logic configured to process a supplemental upstream data request upon determining that additional information in excess of the burst size is required to be sent.
0. 139. The controller of claim 138, wherein the supplemental upstream data request is transmitted within a previously scheduled data burst.
0. 140. The controller of claim 138, further comprising: logic configured to schedule at least one supplemental upstream transmission grant responsive to the supplemental upstream data request.
0. 141. The controller of claim 140, further including: logic configured to transmit the at least one supplemental grant responsive to the supplemental upstream data request.
0. 142. The controller of claim 141, further including: logic configured to receive at least one supplemental burst of data on the upstream channel responsive to the at least one supplemental grant.
0. 143. The controller of claim 132, wherein the communications medium is at least part of a cable television networking environment.
0. 144. The controller of claim 132, wherein the bursts of data are carried in ATM cells or packets.
0. 145. The controller of claim 132, wherein the bursts of data comprise anyone of digitized voice or digitized video.
0. 146. The controller of claim 132, wherein the information flow is from a local area network (LAN).
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This application is a continuation of U.S. Application No. 08/732,668, filed October 16, 1996, now U.S. Patent No. 5,966,163, which claims the benefit of the priority of U.S. Provisional Application No. 60/005,747, filed October 20, 1995.
1. Field of the Invention
The present invention relates to a mechanism to provide constant bit rate upstream data transport in a two way communication system (e.g., a cable TV system) that has a known contention based upstream data transport mechanism. In particular, the invention relates to a mechanism to provide the constant bit rate upstream data transport, such as for a telephone voice channel, in a way that is compatible with known contention based available bit rate upstream data transport mechanisms so that jitter requirements of the constant bit rate transmission are fulfilled while not adversely effecting the latency of the available bit rate upstream data transport.
2. Description of Related Art
The upstream channel of a cable system is expected to carry a variety of services ranging from CBR (Constant Bit Rate) to ABR (Available Bit Rate) as defined by the ATM Forum (Asynchronous Transfer Mode Forum). These two services have unique sets of quality of service requirements such as jitter. For the CBR services, bounded jitter is required but not for the ABR services. ABR performance is measured on the system response time where the access delay plays a key roll CBR and ABR data are also different on the prospects of the traffic patterns. CBR sources produce data in a constant rate fashion while the ABR sources are usually in the burst mode. In the shared upstream channel of the cable television (CATV) environment, the services can not be optimized by a single Medium Access Control (MAC) protocol due to the above differences. The contention based Medium Access Control algorithm has been shown as an appropriate protocol for the local area network (LAN) traffic to provide instant access but it can not provide a guaranteed access environment for CBR sources to achieve bounded jitter.
One family of the contention algorithms, Distributed Queue Random Access Protocol (DQRAP), which uses a separate field (other than the data field) for resolving collisions while the user data transmission is taking place at the same time has been proposed to optimize the system throughput for ABR traffic. DQRAP has shown 85% of utilization with reasonable average access delay. It falls short of providing guaranteed access in order to support CBR services.
What is needed is a priority preempt mechanism to support guaranteed access in a contention based media access protocol in order to optimize these two services in the same system.
Historically, guaranteed access is provided via a TDMA (time division multiple access) arrangement. However, the TDMA approach can only provide services of an integer multiplier of a base rate. For example, these services are possible, 16 Kbps, 32 Kbps, 48 Kbps, etc., if the base rate is 16 kbps. Any request between two layers results in bandwidth waste.
TDMA is based on a cyclic framing structure. Access to the media is usually restricted until the start of the next cycle. It can be a long time if the frame is large or the transmission rate is low.
Another scheme often employed for guaranteed performance is when a station is polled periodically and the station can indicate if there is information to send. Performance of the source data can be guaranteed but at a significant waste of bandwidth because of the need to send the poll to a station and the bandwidth wasted when the station has nothing to send.
A media access control mechanism such as DQRAP uses a separated fields called Control Mini-Slots (CMS) to resolve collided transmissions while actual data transmission occurs at the data slot. Several CMSs associated with a data slot form the basic transmission unit. A station with packets ready for transmission choose one of the CMSs randomly to gain the right to transmit. Those stations that have already obtained the transmission rights are scheduled in a virtual queue called TQ (each station knows the total number of waiting members and its position among them). The requesting stations are informed of the success of the access requests by feedback sent by the Central Controller on the downstream channel with what is called contention grants. DQRAP has shown good performance in terms of Utilization vs. Access Delays. It is difficult and expensive to implement a priority scheme during the contention phase without priority protection. DQRAP can not guarantee bounded jitter which is required in order to support CBR services.
XDQRAP (Extended Distributed Queue Random Access Protocol) is a Medium Access Control (MAC) protocol designed to satisfy a wide range of performances required by the services that the IEEE 802 14 network is expected to provide. It is a contention-based protocol that provides LAN type of traffic an instant access environment. With a central feedback mechanism that is a natural result of the tree-and-branch network topology, it seamlessly marries a reservation scheme to offer deterministic access for the Constant Bit Rate (CBR) services. The MAC was presented in the paper. A proposal to Use XDQRAP for the IEEE 802 14 (IEEE 802 14/95-068) incorporated herein by reference. Simulation results of the basic XDQRAP were presented in the paper Simulation of the Performance of XDQRAP under a Range of Conditions (IEEE 802 14/95-049) incorporated herein by reference.
In the XDQRAP model, the channel time is partitioned into a sequence of fixed-sized time slots. Each unit consists of a data field where the actual data transmissions takes place and a control field used for requesting data slots. The request field consists of two areas called Control Mini Slots (CMS) Stations with data to transmit follow a set of rules to put the transmission requests in one of the control mini slots. The contention resolution algorithm is a tree based approach since there are potentially more than one request in the same control mini slot. By taking advantage of the central feedback, XDQRAP keeps the newly arrived packets out of contention to achieve fast-coverage resolution cycle.
The status of the control mini-slots and the data slots are constantly monitored at the headend. Based on the information, the global queuing information is fed back to the network via the common downstream channel. An individual station, therefore, adjusts its local state-machine accordingly. The contention winning stations position themselves in the global transmission sequence. Stations that lose in the contention retry at the next scheduled time.
Voice and some video conferencing data streams require that data arrives at the receiver within a narrow window of time (i.e., minimum jitter). Having contention for data slots is thus not a good mechanism to use for Constant Bit Rate (CBR) data streams. What is needed is a mechanism whereby a station can be guaranteed to send data at fixed intervals without having to use the control mini-slots to request data slots.
It is an object to the present invention to provide a priority preempt mechanism to support guaranteed access in a contention based media access protocol. It is a further object of the present invention to provide a mechanism for constant bit rate upstream data transport, such as for a telephone voice channel, in a way that is compatible with known contention based available bit rate upstream data transport mechanisms so that jitter requirements of the constant bit rate transmission are fulfilled while not adversely effecting the latency of the available bit rate upstream data transport.
These and other objects are achieved in a method of granting rights for upstream data transmission from user terminals that includes processing contention requests to generate contention grants and maintaining a list of connections, each connection having specified a predetermined bit rate. The method is practiced in a two way cable system that includes a controller and a plurality of the user terminals. The controller sends a downstream data stream to the user terminals and receives an upstream data stream from the user terminals. The downstream data stream includes a plurality of grant fields, and the upstream data stream includes upstream data slots and upstream control slots. The method further includes scheduling preemptive grants for upstream data slots using selected fields of the plurality of,grant fields so as to provide preemptive grants for upstream data transport at a bit rate specified for each connection, sending to the user terminals a data transmission grant in a grant field of the plurality of grant fields for which a preemptive grant is scheduled, and sending to the user terminals a data transmission grant in a grant field of the plurality of grant fields in which a contention grant is pending and no preemptive grant is scheduled.
The invention will be described in detail in the following description of preferred embodiments with reference to the following figures wherein:
The preemptive grant mechanism can be used on top of DQRAP so that CBR sources can use access mechanisms such as DQRAP to obtain guaranteed access. The preemptive grant is issued by the Central Controller without a request from the station and bypasses the contention at the control mini-slot.
The preemptive grant mechanism is briefly described by the following:
The central controller operation is briefly described as follows:
For example, a feedback data field in the downstream channel from the controller to a user terminal will include a station or terminal ID and a transmission right type. The transmission right type may be encoded as:
00:
No Request
01:
Contention Grant
10:
Collision
11:
Preemptive Grant
Since the contention grants and preemptive grant share the control fields on the downstream channel, the Central Controller will try to minimize the impacts of the regular contention operation by choosing the feedback data field based on the following order: no request, then collision, then contention grant.
When a preemptive grant is generated by the scheduler, the Central Controller scans through the incoming control mini-slot field and chooses a feedback data field based on the above criteria. It also overwrites the ID field by the CBR connection ID and the feedback field by ‘11’.
The user terminal (station) operation is briefly described as follows. The basic idea is to treat a preemptive grant as if it were a collision indication.
The station having newly arrived messages.
The stations treat it as TQ>0 cases once they see FB=11.
The stations holding TQ positions.
The station holding the first position will be preempted and not transmit on the current slot. The stations holding the positions other than the first will not have an effect since the TQ value broadcasted by the Central Controller is not changed.
The station in the contention process.
The stations putting their request in the control mini-slot overwritten by the Central Controller treat it as a collision; it is equivalent to FB=‘10’. Those stations using a different mini-slot are not impacted.
The CBR station
Transmit the cell on the current slot. Essentially, the station which requests a CBR service needs to maintain two queues: the TQ and a CBR queue. XDQRAP does an excellent job of handling bursty computer data by providing immediate access for data transmission during light loads and high utilization through fast contention resolution during heavy loads. To summarize XDQRAP protocol:
1. A station randomly chooses one of the Control Mini-Slots (CMS) to put on the connection ID and it indicates the number of cells it wishes to transmit (reservation mode). With modifications to XDQRAP, the number of control mini-slots may be dynamically varied.
Many stations vie for usage for the data slots and it is possible for delays to occur as stations resolve the contention for these data slots. As seen by the receiver of this data, there could be variation of arrival times even if the information was sent on a periodic basis. For data streams that require minimal buffering, trying to accommodate this variation in arrival time is unacceptable. Embodiment of present invention to handle Continuous Bit Rate (CBR) data streams are described as follows.
For available bit rate traffic (ABR traffic), the headend broadcast upstream control slot (e.g., control mini-slot) assignment data to the user terminals. The user terminals store this assignment data and use it to locate where requests may be placed in the upstream data stream.
When a user terminal has ABR data to send to the head-end, the user terminal sends a request in an assigned control slot to request an allocation of upstream data slots to carry the ABR data. Then the head-end acknowledges receipt of the request for allocation.
For constant bit rate (CBR) and variable but rate (VBR) traffic, the user terminals communicate with the head-end to establish a connection. The connection is good until cancelled. It does not have to be renewed with each quantum of data to be transferred. The communication channel may be over the cable network or outside of the cable network (e.g., via telephone line).
The head-end schedules upstream data slots to carry data for all CBR and VBR connections and then schedules upstream data slots to carry data for the pending ABR requests. The head-end then broadcasts grants to the user terminals that give data transmission rights to specifically identified station IDs to carry data upstream in specifically identified upstream data slots based on the above described schedule. In this way CBR and VBR jitter requirements can be fulfilled in systems with ABR contention based mechanisms.
The user terminal responds to a grant by sending the specified data (e.g., defined by station ID) upstream in the specified data slot. Receipt of the upstream data transmission is then acknowledged by the head-end.
The downstream data frame (e.g.,
In the cable environment there needs to be some centralized control (e.g., at the headend) to allow for authorizing stations to use the network, providing security, and statistics gathering for billing. Generically, this function is called network control and there is at least one element responsible for network resources. The entity responsible for the network resources is called the network control element.
Prior to transmitting data, a station must request a connection be setup by the network. As part of the connection setup process, it informs the network about the characteristics of the path. One of the ways to define the characteristics is called Q.2931, an ITU standard incorporated herein by reference. The process of sending and receiving information about the connection is called signaling. In our environment the signaling information is sent to a network control element located at the head-end.
Q.2931 signaling information consists of the bandwidth, delay and jitter requirements for this connection. CBR data streams would request a specific data rate, maximum end-to-end delay and jitter tolerance. ABR traffic would have the CBR parameters plus minimum bandwidth and burst size information. Several network entities may be involved in the connection such as a switch, intermediate controllers and the destination. Each of these entities can accept, modify or reject the various parameters of the signaling information. This ensures that the network resources exist to support the type of connection required for a particular data stream.
The CMS and data slot can be viewed as a single entity and part of an infinitely long time domain multiplex (TDM) frame starting at number 1 and going on indefinitely (see
When a station initiates a CBR connection, the head-end begins to allocate a ‘TDM slot’ periodically at a rate equal to the requested data rate. It can do this because it knows what information rate has been requested and the amount of jitter the connection requires. From the first frame, the head-end is able to calculate the slot number for every data transmission for that connection. Because its centralized control can have other stations transmit data on data slots not previously reserved by CBR connections. This mechanism for providing a station a data slot (TQ) number without the station having to request it via a Control Mini-Slot is called a preemptive grant.
As an example, assume a station wants to request a connection having a bandwidth of 64 Kb/s and minimal jitter. The network control element calculates that the connection requires 167 ATM cells (64 Kb/sec)/(8 bits /byte)/(48 bytes/ATM cell)) to be available in the upstream direction every second. For an upstream QPSK signal of 1 MHz, a 1.544 Mb/s data rate is possible. Our ‘TDM slot’ consists of 82 bytes and this translates into 2,353 ((1.5 Mb/sec)/(8 bits/byte)/(82 bytes/frame)) available frames in a second. The network controller will allocate every 14th (167/2,353) ‘TDM slot’ to the station requesting the 64 Kb/sec CBR connection.
As an example,
The controller received Station B's request and will issue a starting frame of 548 in response to Station B's request. Because the controller knows that Station A's frame will occur in the middle of the 5 frames, the controller will only allow Station B to have 3 ATM cells transmitted Station B can issue an immediate request for another 2 ATM cells and have that resolved before the completion of the CBR Frame and transmit the remaining 2 ATM cells starting at frame 552.
In order to make the CBR connections work and to maintain the quality of service for the stations, the network controller must receive the desired network service level from the originating station. As mentioned before, an existing standard from the International Telecommunications Union (ITU) known as Q.2931 may be a useful mechanism to convey the connection characteristics to the network. A subset of this standard (Q.931, 0.932, Q.933) is used in ISDN, Frame Relay and ATM signaling.
Multiple connections from a modem can be multiplexed over the same physical link. Each connection can have a different class of service such as CBR, VBR, ABR and UBR. The network also guarantees the amount of maximum delay the packet will incur through the network and the amount of variation in arrival time (packet arrival jitter).
The quality of the connection between the source and destination modems is determined by the source modem specifying the characteristics of the connection. The quality of service parameters are negotiated during the connection setup phase. If the modem does not specify a value for a particular parameter, then the default value is assumed. Default values may be network specific or may be established at connection set-up time.
There are three phases of a connection: connection establishment, data transfer, and termination. Connection established is the process of establishing a path through the network that has the characteristics specified. After establishing the connection, the information flows may commence data transfer. After the information has been transferred, the source is required to notify the network to terminate the connection. This termination notification is in the form of a disconnect sequence, and it is required to allow the network to reclaim network resources dedicated to this connection.
Messages are packets that flow between the source modem to the network in order to perform a desired function. The network may return the information requested or request information from devices outside the network.
As discussed above, there is a need to establish parameters that the headend controller will use when the scheduling algorithm. As a useful mechanism for relaying the connection requirements, ITU standard Q.2931 may be employed.
Each packet in a message contains one or more fields called ‘Information Elements’. Information elements contain the logical parameters associated with that element. For example, the information element for the ‘End-to-End Transit Delay’ contains the initiator's requested transit delay and the maximum transit delay that would be accepted. The main Information Elements are listed in Table 1. Some of the connection control messages are described in Table 2.
TABLE 1
INFORMATION ELEMENT IDENTIFIER CODING
IDENTIFIER # FOR IE
INFORMATION ELEMENT
8765
4321
NAME
0000
0100
Bearer Capability
0100
0010
End-to-End Transit Delay
0000
1000
Cause
0001
1001
Network Connection Identifier
0100
1000
Bandwidth Control Parameters
0110
1100
Source Address
0110
1101
Source Sub-address
0111
0000
Destination Address
0111
0001
Destination Sub-address
0111
1110
User-User
1010
0000
More Data
TABLE 2
CONNECTION CONTROL MESSAGES
Setup - This message is sent by the initiating modem to the network and
by the network to the destination to initiate connection establishment.
Connection Proceeding - This message is sent by the destination to the
network or by network to the source. It indicates that the requested
connection established has been initiated and no more connection
establishment information will be accepted.
Connect - This message is sent by the destination to the network and by
the network to the source to indicate connection acceptance by the
destitnation.
Alerting - This message is sent by the destination to the network and by
the network to the source to indicate that destination alerting has been
initiation.
Release - This message is sent by the initiator or the network to indicate
that the equipment sending the message has disconnected the connection
and intends to release the network connection identifier (if any) and the
connection reference. The receiving equipment should release the network
connection identifier and prepare to release the connection reference
after sending RELEASE COMPLETE.
Release Complete - This message is sent by the initiator or the network to
indicated that the equipment sending the message has released the
connection and, if appropriate, the channel. The channel, if released, is
available for reuse. The receiving equipment shall release the connection
Disconnect - This message may be sent by either to request the network to
clear the connection or it may be sent by the network to indicate that the
connection is cleared.
Status Enquiry - This message may be sent by the user or the network at
any time to solicit a STATUS message from the peer layer 3 entity.
Sending a STATUS message in response to a STATUS ENQUIRY
message is mandatory. This message is sent by the initiator or the network
at any time during a connection to report certain error conditions as listed
in section 5.8 of Recommendation Q.931
User Information - This message is sent by the user to the network to
transfer information to the remote user. This message is also sent by the
network to the user to deliver information from the other user. This
message is used if the user-to-user transfer is part of an allowed
information transfer. This message is sent by the user or the network at
any time to solicit a STATUS message from the peer layer 3 entity.
Sending a STATUS message in response to a STATUS ENQUIRY
message is mandatory.
Generically, these messages are called connection control messages. A typical connection control packet would include a Protocol Discriminator, a Connection Reference, a Message Type and one or more Information Elements.
A particular message may contain more information than a particular network needs or can understand. All equipment should be able to ignore any extra information present in a message that is not required for the proper operation of that equipment. For example, the source may ignore the source address during the CONNECT message since it should be its own address.
The purpose of the Protocol Discriminator is to distinguish messages for network connection control from other messages within Q.931. It also distinguishes messages of Q.931 from OSI network layer protocol units that are coded to other ITU Recommendations and other standards.
The value for this field should be x‘08’ (hexadecimal 08) which is the value for the Q.931/(I 451) user-network call control messages specified in section 4.2 of Recommendation Q.931.
The purpose of the Connection Reference is to identify the connection to which the particular message applies. The Connection Reference does not have end-end significance across the network. Sec section 4.3 of Recommendation Q.931.
Connection Reference values are assigned by the originating side of the interface for a connection. These values are unique to the originating side only within a particular network port link. The Connection Reference value is assigned at the beginning of a connection and remains fixed for the lifetime of a connection. After a connection ends, the associated Connection Reference value may be reassigned to a later connection. Two identical Connection Reference values on the same connection may be used when each value pertains to a connection originated at opposite ends of the link.
Information Element identifiers (IE) are logical groupings of parameters needed to convey information. The Information Element has its own unique identifier, length field and parameters. Each Information Element remains the same regardless of the Message Type. Usually a particular information element may be present only once in a given message.
Having described preferred embodiments of a novel constant bit rate mechanism in a contention based medium access control (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as defined by the appended claims.
Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by letters patent is set forth in the appended claims
Koperda, Francis R., Lin, Bouchung
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