Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/04—Selecting arrangements for multiplex systems for time-division multiplexing
  • H04Q11/0428—Integrated services digital network, i.e. systems for transmission of different types of digitised signals, e.g. speech, data, telecentral, television signals
  • H04Q11/0478—Provisions for broadband connections
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L12/00—Data switching networks
  • H04L12/54—Store-and-forward switching systems 
  • H04L12/56—Packet switching systems
  • H04L12/5601—Transfer mode dependent, e.g. ATM
  • H04L2012/5638—Services, e.g. multimedia, GOS, QOS
  • H04L2012/5646—Cell characteristics, e.g. loss, delay, jitter, sequence integrity
  • H04L2012/5649—Cell delay or jitter
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L12/00—Data switching networks
  • H04L12/54—Store-and-forward switching systems 
  • H04L12/56—Packet switching systems
  • H04L12/5601—Transfer mode dependent, e.g. ATM
  • H04L2012/5672—Multiplexing, e.g. coding, scrambling
  • H04L2012/5674—Synchronisation, timing recovery or alignment

Definitions

• the present invention relates to reduction of a total delay of the traffic in a communication network carrying ATM traffic, wherein a multitude of users share a common physical medium.
• the reduction is particularly advantageous and useful in the use of so called CBR traffic (Constant Bit Rate) which has a low tolerance for delay in the network.
• CBR traffic Constant Bit Rate
• the reason for the delay in traditional local networks can also be expressed by observing the fact that transport of large data packages and use of a store-and-forward technique in routers and bridges takes time. First there is a delay when the user accumulates data for filling a whole data package. The same delay effect is repeated every time the package is received by a bridge or a router which has to await the entire package before it can be transmitted onwards to the next link. The delays vary when the package waits in a queue at links, having a heavy load. If all packages are given the same priority there is no possibility for a time critical package to go past the queue.
• ATM has a priority function which makes it possible for time critical traffic to go past a queue.
• a terminal can demand that a certain bandwidth is allocated for a particular channel, which guarantees a required capacity.
• the system relies on that the traffic is classified as either Constant Bit Rate (CBR) Variable Bit Rate (VBR) or Available Bit Rate (ABR) .
• CBR Constant Bit Rate
• VBR Variable Bit Rate
• ABR Available Bit Rate
• a terminal which intends to transmit a video or audio sequence via CBR or VBR must reserve the required bandwidth when the connection is set up.
• Time Division Multiplex are used for transmitting ATM traffic.
• the physical medium in this case the bus, is a common resource which must be used in the best possible manner. Since only one user at the time can transmit on the bus the cell in the user nodes must wait for "their turn" before they can be transmitted over the physical medium.
• US 5,546,199 discloses a method for generation (synthesis) of a carriers for the uplink direction in a cable television system by using a reference frequency in the downlink direction.
• the object is to provide a low-cost carrier having an exact frequency.
• the carrier is then used in the common medium, in this case a coaxial cable.
• the document does not disclose how one can reduce the total delay in the system.
• Other documents found, for example US 4,553,161 describes synchronization of uplink data traffic.
• the invention shall, using a device and a method in a communication network where a multitude (n) users/nodes share a common physical medium, reduce the total delay in transmission of data package generated at a fixed rate.
• connection had a bandwidth which was considerably larger than the need and that there was a function for dynamic bandwidth allocation which immediately generated transmission permission when a data package/cell is ready in one of the nodes.
• bandwidth which is to be shared by several nodes is usually very limited and fully used for payload. If all traffic has the same priority it will not help to have dynamic allocation of transmission permission. In the case when all nodes simultaneously have a cell ready to transmit the last cell will have to wait a whole period T, where T is the time between two cells from a particular connection (the period) when all other nodes must have a transmission permission each during that time.
• the problem is solved by a device and a method where the generation of ATM cells is synchronized at the users with the generation of uplink time slots on the common physical medium so that a cell is ready to be transmitted in exactly the same moment when it receives a transmission permission.
• the condition is that an ATM cell can be fitted in a time slot.
• the generation of ATM cells is synchronized at the user nodes with the allocated time slots in the uplink channel.
• the synchronization can be implemented in a number of ways but the important thing is that the generated ATM cells arrive at the same rate as the time slots on the common physical medium, so that the data packages can be transmitted directly when they have been packed onto said medium without delay.
• the random phase which is a result between the time slots and the ATM generation in an initial state can be corrected, for example, using a phase delay device and a phase detector or simply by not using a cell and to start the next cell in the correct phase.
• TDM time division multiple access
• 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 for the frames is constant. It is also possible that the stream of time slots is not defined in frames of the same length but only as a constant flow of time slots. It is however important that the period between two time slots intended for a particular connection is constant. The advantages which are obtained using this method are clear.
• the short delay for the packed ATM cells in the user nodes contribute to reduce the total delay in the network.
• figure 1 schematically and simplified shows the uplink traffic in a PON/Coax system having a multitude of nodes and a common physical medium according to the state of the art
• FIG 2 shows a block diagram in an environment where the invention as described below is useful
• figure 3 shows a block diagram of how a user node is implemented and a method of performing synchronization according to the invention
• figure 4 shows a block diagram of how a user node is implemented and an additional method of performing synchronization according to the invention.
• Figure 1 shows the uplink direction for a PON/Coax system having n nodes .
• the delay in the uplink traffic from n nodes towards a headend according to figure 1 mainly consists of two parts, partly the packing into ATM cells and partly the delay time before the cell can be transmitted.
• dynamic bandwidth allocation is used and in this case some form of MAC function (Medium Access Control) for continuous control and allocation of the available capacity between connections or groups of connections is used.
• the object is to as soon as possible transmit a cell when it is ready, i.e. to dynamically control the bandwidth allocation.
• MAC function Medium Access Control
• n max number of connections
• t the time to transmit a cell, which depends on the capacity of the connection.
• T the time between 2 cells from a connection (the period) , (64 kb/s voice connection results in T 6 ms) .
• a condition for the capacity of the connection to be enough for all n connections is thus that nxt T or equivalently that n T/t.
• n T/t.
• the last cell then will have the maximum waiting time T. This is however not likely, but the available bandwidth often limits n to such low values that the probability for a waiting time close to T is not negligible.
• the conclusion is that the waiting time varies continuously for the two described methods between 0 and 6 ms .
• the waiting time can however be controlled as a difference from the packing delay which is fixed and not controllable.
• the description below mainly concerns transmission of 64 kb/s coded speech, but can also be used for other data rates.
• each connection makes of a use of a fixed "time slot" for transmission of 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 for the frames is constant .
• uplink and downlink direction on the common medium and the generation of uplink CBR cells have a common synchronization source.
• the waiting time can then be reduced to almost zero if two conditions are fulfilled. Firstly the generation of ATM cells has to be synchronous with the corresponding time slot, i.e. the same number of cells as time slots per time unit. Secondly the ATM cells must be packed and ready for transmission just before transmission in the allocated time slot. The latter requires that either the time (the phase) when the ATM cell is ready for transmission relative the used time slot can be controlled/ chosen or that a "suitable" free time slot can be allocated to the connection. The latter method should be avoided since it is dependent on other setup connections.
• the rate of ATM cells is determined by the AAL being used and by the 8 kHz signal which is normally used for analogue/digital conversion of a speech signal, i.e. that the nominal frequency/ generation of cells is fixed and cannot be controlled. The consequence hereof is that it is the nominal period of time for the allocated time slots that must be adapted to the generation of ATM cells, and not vice versa.
• speech signals since such high quality standards are put on them that they must be sent by CBR, it is preferably speech signals which benefit from the invention. It is however not restricted to speech signals, but other types of signals having a constant bit rate can of course be transmitted using the same basic concept of the invention.
• the term uplink direction is used and when the common node transmits towards the user node the term downlink direction is used.
• the ATM cell generation can be synchronized to the uplink channel in the following manner: First the frequency is synchronized, i.e. the period of time for the generated ATM cells is adapted/synchronized to the period of time for the allocated time slot, giving a random phase between the time slot and the ATM cell generation. Next, the phase is adjusted so that each generated ATM cell becomes ready/packed just in time before it is to be transmitted in "its" time slot, i.e. just before or at the same time as the time slot is transmitted so that it can be placed in the time slot.
• the method implies that regardless of in which node an ATM cell is generated the phase is adapted to the allocated 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 use.
• the figure shows a HPC (Hybrid-Fiber- Coax) cable television system 1 where a multitude of subscribers/nodes 2-4 share a common physical medium, in this case a bus 5.
• HPC Hybrid-Fiber- Coax
• FIG. 2 shows an example of an environment where the present invention is of great use.
• the figure shows a HPC (Hybrid-Fiber- Coax) cable television system 1 where a multitude of subscribers/nodes 2-4 share a common physical medium, in this case a bus 5.
• a bus 5 for reasons of simplicity only three nodes are shown but it is to be understood that the number can be much greater .
• the cable television network is interactive which means that the subscribers can transmit information in the uplink direction 6.
• This embodiment gives a good example of the use of the invention and is intended to show a practical implementation. In reality the invention can of course be used in all systems with simplex or duplex communication in a network where a multitude of nodes/subscribers share a common physical medium and where communication with a common unit takes place.
• the bus 5 can be a conventional coax cable, but which often in the uplink direction 6 is electrically/optically converted in a converter 7.
• the bus 5 is as mentioned connected to a number of subscribers or nodes 2- .
• One node 4 has been enlarged and is shown in the figure in more detail and one can here see an example of how the connection can look like.
• a network terminal 8 is connected as an interface towards the bus 5. The network terminal will be described in more detail below. If this is used as a cable television network, a TV 9 is suitably connected as an external unit for reception of downlink data 10.
• a circuit emulator 11 can be connected to the network terminal 8 according to the figure .
• the circuit emulator 11 is then e.g. used for packing the data generated by the subscriber in ATM cells.
• the subscriber can use a computer 12 or an ordinary telephone 13 which then is connected to the node 4.
• the telephone then of course requires an A/D-conversion 29.
• the system has an antenna 14 for reception of the TV-signals which then are transmitted to the subscribers.
• the module 15 illustrates a head-end in a HFC network and provides the telephony function in an interface towards PSTN 16 or the like.
• the module comprises a MAC (Medium Access Control) . In this it is decided which subscriber who is allowed to transmit and when.
• modulation of the carrier can also take place. It is common to use a downlink QAM modulation and a QPSK uplink demodulation.
• the signals from the module 15 and the signals from the antenna 14 can be transmitted on the same medium 19 by means of combining them using a combiner 17.
• FIG 3 shows the network terminal 8 and the circuit emulator 11 in more detail.
• the circuit emulator 11 comprises, besides 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.
• the A/D converter 29 receives data from a telephone 13 (see figure 2) via the link 18.
• the emulator 11 is thus connected to the network terminal 8 which in turn has a connection to the bus 5.
• the starting point is always information from a phase detector 26 at the buffer 23 in the network terminal 8.
• the detector 26 detects the phase between incoming ATM cells from the transmitter 22 and the allocated time slot on the bus 5.
• a signal thus arrives which it takes some time for, let us say x ms to pack.
• a transmission permission with downlink data also arrives each x ms .
• the object is to make certain that the generated ATM cell is packed and transmitted to the buffer 23 just before transmission is to take place in the common medium 5, using the phase control of the downlink traffic.
• phase in the ATM cell generation There are many alternative ways to control the phase of the ATM cell generation.
• the alternative which is illustrated in figure 3 is when the clock in the downlink direction 10 is used as a reference for the uplink traffic 6 as well (here illustrated with the network reference 31) .
• the phase in the downlink direction 10 to the circuit emulator 11 can be adjusted using a controllable phase delay unit 24, using the information from the above mentioned phase detector 26 via the connection 28, until the uplink cells obtain a suitable phase (become ready/packed in an optimal time according to above) , relative their allocated time slots.
• This indirect method can also be used in an embodiment where the uplink ATM cells are generated in a unit integrated in the network terminal 8. In this case there is no need for a separate PLL 21 but the output signal from the phase delay 24 can be used as a clock signal.
• a control channel 30 can be used for controlling the phase from the PLL 21'. Then one must let the phase detector 26' transmit the same phase control information as above via the control channel 30 to, for example, the PLL 21' from cell generation so that the phase to be controlled to a desired position according to the above.
• This method is called a direct method since in this case necessary information is given directly to the PLL 21' of the circuit emulator.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Data Exchanges In Wide-Area Networks (AREA)

Applications Claiming Priority (3)

Publications (1)

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Family Applications (1)

Country Status (6)

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• 1997
  • 1997-12-19 SE SE9704766A patent/SE511594C2/sv not_active IP Right Cessation
• 1998
  • 1998-12-04 CN CN 98812363 patent/CN1282501A/zh active Pending
  • 1998-12-04 AU AU17931/99A patent/AU1793199A/en not_active Abandoned
  • 1998-12-04 EP EP98962772A patent/EP1040709A1/de not_active Withdrawn
  • 1998-12-04 WO PCT/SE1998/002219 patent/WO1999035878A1/en not_active Application Discontinuation
  • 1998-12-04 CA CA002314908A patent/CA2314908A1/en not_active Abandoned

Non-Patent Citations (1)

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