SE511594C2 - Method and Device for Reducing the Total Delay for ATM Traffic in a Time Multiplexed Communication Network - Google Patents

Abstract

A method and a device for reduction of the total delay of ATM traffic in a communication network where a multitude of nodes (2-4) share a common time multiplex physical medium (5) for transmitting ATM cells in an uplink direction (6). The nodes (2-4) comprise a circuit emulator (11) where the ATM cells which are to be transmitted are packed and a network terminal (8) comprising at least one PLL (27), a phase detector (26) and preferably a phase delay unit (24). The nominal frequency for the allocated time slots on the physical medium (5) is the same as the frequency for generation of the ATM cells. By using the above components the generation of ATM cells will be locked in frequency and phase to the allocated time slots in order to minimize the waiting time in the buffer (26) of the node.

Description

10 15 20 25 30 511 594 2 The delays vary when packets are waiting in a queue at highly loaded links. If all packages are treated with the same priority, then there is no possibility for time-critical packages to slip ahead in the queue.

When it comes to queuing problems, ATM has a prioritization function that makes it possible for time-critical traffic to take the lead in the queue.

In addition, an end piece of equipment may require that a certain bandwidth be allocated to the current channel, which guarantees the required capacity. It is all based on the traffic being classified as either Constant Bit Rate (CBR) Variable Bit Rate (VBR) or Available Bit Rate (ABR). An terminal equipment that intends to transmit a video or audio sequence via CBR or VBR must reserve the required bandwidth in connection with the connection being connected.

If all traffic has the same priority, then of course the problem remains. The problem becomes especially great when a narrowband CBR signal (speech) packaged in ATM cells is to be transmitted and where the physical layer only offers a fixed low transmission rate.

This is often the case in the upstream direction (traffic from users or terminal equipment towards a common point, eg a local station or a so-called “head end”) when PON (Passive Optical Network) and COAX systems based on TDM (Time Division Multiplex) are used to send ATM traffic. In addition, if the delay varies over time, this adds to the overall delay of the receiver. The more the delay varies, the larger the CDV buffer (Cell Delay Variation) is needed.

For example, when a bus is used in common by a plurality of nodes to send cells in a TDM based network in the upstream direction (from user nodes to a common node), the physical medium, here the bus, is a common resource that must 10 15 20 25 3 511 594 utilized in the best possible way. Since only one user at a time can transmit on the bus, the cells in the user nodes before they can be transmitted over it simply have to wait for the "turn" physical medium.

This problem arises in a number of uses where several users have to share a time-multiplied common physical media. The closest thing to think about is when talking (telephone) and transmitting with low constant bandwidth over an ATM connection. Another area is in cable television technology. An examination of the state of the art shows that most of the documents that emerge are located in the cable area. However, nothing shows how to minimize the above-mentioned waiting time.

US 5,546,19§ discloses a method for generating (synthesizing) a carrier for the upstream direction in a cable television system by utilizing a reference frequency from the downstream direction.

The purpose is to obtain a carrier with an exact frequency in a cost-effective way. The carrier is then used on the common medium, here a coaxial cable. However, the document says nothing about how to reduce the total delay in the system. Other produced documents, for example US 4,5S3,16l, deal with synchronization of upstream data traffic.

DISCLOSURE OF THE INVENTION The invention is to be provided by means of a device and a method in a communication network where a plurality of users / nodes share a total common abbreviated physical media, the delay in transmitting data packets generated at a fixed frequency.

The best or simplest would, of course, be if the connection had a bandwidth considerably greater than the need and that there was a dynamic bandwidth allocation function that immediately provided transmission status. when a data packet / cell is 'ready' in one of the nodes. In contrast to this ideal case, instead, the bandwidth to be shared by several nodes is usually very limited and is fully utilized for commercial traffic.

If all traffic has the same priority, it does not help to have a dynamic assignment of transmission conditions. In the case that all the nodes simultaneously have one cell ready to be transmitted, the last cell will have to wait an entire period T, where T is the time between two cells from one and the same connection (period time) when all other nodes must receive a transmission state each .

The problem is solved according to the invention by a device and a method in which the generation of ATM cells of the users is synchronized and the generation of upstream time slots on the common physical medium so that a cell is ready to be transmitted at the exact moment when it receives transmission status.

The prerequisite is that an ATM cell can be completely accommodated in a time slot.

Thus, it is required to synchronize the generation of ATM cells of the user nodes to the assigned time slots in the upstream channel. The synchronization can be realized in a narrow way, but the important thing is that the generated ATM cells come at the same rate / frequency as the time slots on the common physical medium so that the data packets can be sent immediately when they are packaged ready on said media without waiting time. The random phase position obtained between the time slot and the ATM cell generation in an initial stage can be corrected with, for example, a phase shifter and a phase detector or simply by throwing some cell and starting the next cell in the correct phase. It is assumed here that TDM is used and that in the normal case each connection has a fixed "time slot" for transmitting ATM cells. A frame can be defined as n time slots, one for each connection. A connection uses the "same" time slot in all frames. The period time for the frames is constant. Here it is also conceivable that the current of time slots is not defined in frames of the same length but only as a constant flow of time slots. However, it is important that the period time between two time slots intended for a particular connection is constant.

The benefits gained from this process are obvious. The shorter waiting time for the pre-packaged ATM cells in the user nodes helps to reduce the total delay in the network.

It is believed that the features of the invention as set forth in the appended claims are new. However, the structure, function and further advantages of the invention are best understood in the following description with the accompanying figures.

DESCRIPTION OF THE DRAWINGS Embodiments of the invention will be described below in connection with the accompanying drawings, in which; figure 1 shows schematically and simplified the upstream traffic in a PON / Coax system1 with a plurality of nodes and a common physical media according to prior art, - figure 2 shows with a block diagram an environment where the invention as below is useful, - figure 3 shows with a block diagram how a user node is constructed and a method of performing synchronization according to the invention. Figure 4 shows with a block diagram how a user node is constructed and a further method of performing synchronization according to the invention.

PREFERRED EMBODIMENT Figure 1 shows the upstream direction of a PON / Coax system with n nodes. The delay for upstream traffic from n nodes to the headend according to Fig. 1 consists mainly of two parts, the packaging in ATM cells and the waiting time before the cell can be transmitted.

To get the right background and understanding, it may be appropriate to use a dynamic form of MAC with two examples. In the first example bandwidth allocation and here some function (Medium Access Control) is used for continuous regulation and distribution of the available capacity between connections or groups of connections. The purpose is to transfer a cell as soon as it is ready, i.e. to dynamically control the bandwidth allocation. We use here: n = max number of connections t = the time it takes to transfer a cell, depends on the capacity of the connection.

T = the time between 2 cells from a connection (period time), (64kb / s speech connection gives T = 6ms) A condition for the connection's capacity to be sufficient for all n connections is thus that nxt S T or equivalent to n S T / t.

With full utilization of existing bandwidth, it therefore applies that n = T / t. l0 l5 20 25 7 511 594 The waiting time varies and the maximum waiting time occurs when all the connections have one cell ready to be sent at the same time (= n cells). The last cell then has the maximum waiting time T. however This is unlikely but the bandwidth available often limits n to such low values that the probability of a waiting time close to T is not negligible.

If we instead have a static bandwidth allocation, which we have in the inventive idea, consecutive transmission states are given to each ATM connection with a fixed time interval T1, regardless of whether there is a cell to transmit or not. The method involves a poll with the frequency fl = 1 / T1 per compound.

Sufficient capacity for the connection is obtained when T1 S T. To utilize the available capacity of the connection to the maximum, T1 must be chosen as large as possible but still not larger than T. For 64kb / s speech, T ~ 6ms. In such an asynchronous polling procedure, the waiting time is rectangularly distributed in the interval O - T1.

The conclusion is that the waiting time varies continuously for the two described methods between zero and 6ms. However, the waiting time can be affected in contrast to the packing delay which is fixed and is not controllable.

With the proposed method, most of the delay is avoided except for the part that the total packaging accounts for. The delay for these parts is thus reduced to about 6 ms. It should be added, however, that other delays sonx may occur in transmission or in queues not years included.

The description below mainly applies to the transmission of 64 kb / s coded speech, but it can also be applied to other data speeds. 10 15 20 25 'is allocated to the connection. The conditions are thus that TDM is used and that in the normal case each connection has a fixed "time slot" for transmitting ATM cells. A frame is defined as n time slots, one for each connection. A connection uses the "same" time slot in all frames. The period time for the frames is constant.

It is also assumed that the up and downstream direction. on the common medium as well as the generation of upstream CBR cells (implicitly also the reference frequency for the speech coding, normally 8 kHz) have a common synchronization source.

The waiting time can then be reduced to close to zero if two conditions are met. On the one hand, the generation of ATM cells must be synchronous with the corresponding time slots, i.e. the same number of cells as time slots per unit time. Secondly, the ATM cell must be packaged and ready for transmission just before transmission in the allotted time slot.

The latter requires either that the time (phase mode) when the ATM cell is ready for transmission relative to the used time slot can be controlled / selected or that a "suitable" free time slot can be The latter method should be avoided as it depends on other set connections.

The frequency of ATM cells is determined by the AAL used and the 8kHz signal normally used for analog / digital conversion of a speech signal, i.e. the nominal frequency / generation of cells is fixed and can not be affected. The consequence of this is that it is the nominal period time of the allocated time slots that must be adapted to the generation of ATM cells, not the other way around.

We write here about speech signals and speech signals have such a quality requirement that they are sent with CBR, so therefore it is preferably speech signals that benefit from the invention in question. However, it is not a question of any restriction to speech signals, but also other types of signals with a constant bit rate can of course be sent with advantage according to the basic idea of the invention.

When a plurality of user nodes are to send cells to a common node, the term upstream direction is used and when the common node sends to the user node, the term downstream direction is used. Given that the time slot used in the upstream channel and the ATM cell generation have the same nominal period time, the ATM cell generation can be synchronized to the upstream channel as follows: First, the frequency is synchronized, ie the period time of the generated ATM cells is adjusted / synchronized to the period time allocated. , whereby a random phase position is obtained between the time slot and the ATM cell generation. Then the phase position is adjusted so that each generated. The ATM cell is ready / pre-packaged in “just” time before it is to be sent in “its” time slot, i.e. just the moment before -or at the same time as the time slot is sent- so that it has time to arrive.

The method means that Regardless of in which node an ATM cell is generated, the phase mode is adapted to the assigned time slot.

The result is that the waiting time can be made arbitrarily short.

Figure 2 shows an example of an environment where the present invention is of great benefit. The figure shows an HPC (Hybrid-Fiber-Coax) cable TV system 1 where several subscribers / nodes 2-4 share a common physical media, here a bus 5. For the sake of simplicity, only three nodes are drawn, but it should of course be understood that the number can be much larger. 10 15 20 25 30 511 594 lo The cable network is interactive, which means that the subscribers can send information in the upstream direction 6. This embodiment provides a good example of the applicability of the invention and is intended to show a practical implementation. In reality, the invention can of course be used in connection with one-way or two-way communication in a network where a plurality of nodes / subscribers share a physically common media and where communication with a common unit takes place.

In this example, the bus 5 can consist of an ordinary coaxial cable, but often then undergoes an electrical-optical conversion in a converter 7 in the upstream direction 6.

As mentioned above, a number of subscribers or nodes 2-4 are connected to the bus 5. A node 4 has been enlarged and shown in the figure in a little more detail and we see here an example of what the connection can look like. A night terminal 8 is connected as an interface to the bus 5. The night terminal will be described in more detail below. If this is used as a cable TV system, a TV 9 is suitably connected as an external unit for receiving downstream data 10. In order to transmit data in the upstream direction 6, a circuit simulator 11 can be connected to the network terminal 8 according to the figure. The circuit simulator 11 is then used, among other things, to package the subscriber's generated data in ATM cells. To send data in the upstream direction 6, the subscriber can use a computer 12 or an ordinary telephone 13 which is then connected to the node 4. The telephone then of course requires an A / D conversion 29.

The system has an antenna 14 for receiving the TV signals which are then transmitted to the subscribers. The module 15 is intended to illustrate a head-end of an HPC network and for the telephony function forms an interface to PSTN 16 or the like. The module contains MAC (Medium Access Control). Here it is determined which subscriber 10 15 20 25 30 11 511 594 send and when. In the same module 15, modulation of the carrier can also take place. Typically, a QAM modulation is downstream and a QPSK demodulation is upstream. The signals from the module 15 and the signals from the antenna 14 can be transmitted on the same media 19 by combining them by means of a combiner 17.

Figure 3 shows the network terminal 8 and the circuit emulator 11 in more detail. The circuit emulator 11 comprises, in addition to an ATM cell receiver 20 and a PLL 21 (Phase Locked Loop), also a transmitter 22 where the ATM cells are generated and an A / D converter 29.

In this exemplary embodiment, the A / D converter 29 receives data from a telephone 13 (see Fig. 2) via the link 18. The emulator 11 is thus connected to the night terminal 8, which in turn has a connection to the bus 5.

When the phase mode of the generated digital speech signal is then adjusted without the introduction of delay so that each generated ATM cell must be ready for transmission in the optimal time so that the waiting time in a buffer 23 in the night terminal 8 is minimal, the starting point is. always information from a phase detector 26 at the buffer 23 in the night terminal 8. The detector 26 senses the phase position between incoming ATM cells from the transmitter 22 and the assigned incoming time slot on the bus 5. In the circuit according to the solution thus a signal which takes a certain time, let us say x msek to pack. A transmission state with downstream data also occurs every xzte msec. It is therefore important to use phase control of the downstream traffic to ensure that the generated ATM cell is pre-packaged and Transmitted to the buffer 23 just to be transmitted to the common medium 5.

There are many alternative ways to regulate the phase mode of ATM cell generation. The alternative illustrated in Figure 3 is when the clock in the downstream direction 10 is used as a reference also for the upstream traffic 6 (here illustrated with the network reference 31). Then, with the information from the above-mentioned phase detector 26 via the connection 28, the phase in the downstream direction 10 to the circuit emulator 11 can be adjusted with a controllable phase shifter 24 until the upstream cells get a suitable phase position (ie ready / pre-packaged in optimal time as above) relative to allocated time slots. This is because the traffic in the downstream direction module 25 in clocks is phase shifted in such a way that the PLL 21 in the circuit emulator 11 preferably the A / D converter 29 so that the cells can be packaged in the transmitter 22 and that the transfer to the buffer 26 is complete just before the transmission state comes out for transmission in a time slot on the common physical medium.

This is called an indirect method because the circuit simulator 11 does not "see" that the phase is changing. It only follows the phase in the downstream direction. This indirect method is also conceivable in an embodiment where the upstream ATM cells are generated in a unit integrated in the night terminal 8. In this case a separate PLL 21 is not needed but the output signal from the phase shifter 24 can be used as a clock signal.

In another embodiment illustrated here in Figure 4, a control channel 30 may be used to control the phase position from the PLL 21 ".

Then the phase detector 26 'must be allowed to transmit via the control channel 30 the same phase adjustment information as in the above example to the PLL 21' for the cell generation so that the phase can be adjusted to the desired position as above. This method can be called a direct method because here the necessary information is given directly to the circuit emulator PLL 21 'The methods shown above are not limited to the preferred embodiments shown' but can, of course, be used in all cases where ATM cells from many in The room's distributed sources are to be transported via a common TDM based physical medium, such as coax, fiber or radio. It should also be pointed out that the embodiments in Figures 3 and 4 only show possibilities of realizing the invention as such. The emphasis and core of the invention is on making sure to match the generation of ATM cells to the respective allocated time slot to minimize the waiting time for the cells.

Claims (10)

10 15 20 25 30 511 594 14 PATENT REQUIREMENTS A method for reducing delay for ATM traffic in a communication network when a plurality of nodes (2-4) generating ASM cells use a common time division multiplexed physical media (5) for transmission, characterized in that the period time for the generation of ATM cells in said nodes (2-4) and the time slots of the physical medium are synchronized with respect to both frequency cells in the nodes (2-4) and phase so that they are pre-packaged at the same time as, or just before, transmission in a time slot allocated to each node. Method for reducing delay for ATM traffic according to claim 1, characterized in that the synchronization is realized by synchronizing the period time for the generation of ATM cells to the period time of the allocated time slots on the upstream channel of the common physical medium (6) by clocking. in the downstream direction (10) is also used as the synchronization source for the upstream direction. A method for reducing delay for ATM traffic according to claim 1 or 2, characterized in that the phase position when the ATM cells will be ready for transmission relative to their respective allocated time slots is identified by a phase detector (26) sensing the phase position between incoming ATM cells from a transmitter (22) and the assigned time slot on the bus (5). A method for reducing delay for ATM traffic according to claim 3, characterized in that a phase shifter (24) adjusts the phase of the downstream clock so that the ATM cells generated in the transmitter (22) are pre-packed and transferred to The buffer (23) so that the waiting time there becomes minimal before they are to be transmitted on the common medium (5). Method for reducing delay for ATM traffic according to claim 3, characterized in that a control channel (30) from the phase detector (26 '), with information about the phase position between ATM cells incoming to the phase detector (26') from a transmitter (22) and the assigned time slot on the bus (5), controls a PLL (21 ') in communication with the transmitter (22) so that the ATM cells generated in the transmitter are pre-packaged and transmitted to the buffer (23) transmitted on the common the medium (5). Method for reducing delay for ATM traffic in a communication network when a plurality of nodes (2-4) generating ATM cells use a common time division multiplexed physical media (5) for transmission, characterized in that the period time for the generation of ATM cells in said nodes are obtained by the clock (2-4) in the downstream direction (10) of the physical medium (5) and that the time slots of the physical medium in the upstream direction (6) are controlled by the same clock, whereby a random phase position is obtained between the generation of a cell and its allocated time slot in the upstream direction and that this phase position is adjusted so that each generated ATM cell is pre-packaged at the same time as or just before it is to be sent in the time slot allocated for this particular connection. A device for reducing delay for ATM traffic in a communication network comprising a plurality of nodes (2-4), and connected thereto, a common time-multiplexed physical media (5) on which upstream (6) and downstream traffic (10) is transmitted , so that the waiting time there becomes minimal before they should) the upstream traffic (6) generated in said nodes (2-4), characterized in that the nodes (2-4) comprise at least one night terminal (8). ) connected to said common physical media (5) and at least one module (for example a circuit emulator (11)) where the generation of ATM cells takes place, the generation being adapted so that the ATM cells are pre-packaged and ready for transmission at the same time as or just before transmission with a time slot therefore allocated to the common physical medium (5). Device for reducing delay for ATM traffic in a communication network comprising a plurality of nodes (2-4), according to claim 7, characterized in that the nodes comprise means for controlling the phase position for when the ATM cell is to be ready for transmission. relative to the allocated time slot. Device for reducing delay for ATM traffic in a communication network comprising a plurality of nodes (2-4), according to claim 8, characterized in that these means comprise at least one phase detector (26) and a phase shifter (24) which controls the phase for data in the downstream direction (10) to influence the ATM cell generation in the circuit emulator so that one (11) cell is prepackaged at the same time as or just before transmission in a therefore allocated time slot in the upstream direction of the physical medium. Device for reducing delay for ATM traffic in a communication network comprising a plurality of nodes (2-4), according to claim 8, characterized in that these means comprise a phase detector (26 ') and from this a control channel (30) for transmitting phase information to, a PLL (21 ') connected to a transmitter (22), for controlling ATM cells generated in the transmitter (22) 17 511 5i94 so that the phase mode when a cell is pre-packaged occurs at the same time as or just before transmission in a time slot therefore allocated to the physical medium.