GB2361393A

GB2361393A - Transmitting packets in an optical wave division multiplex system

Info

Publication number: GB2361393A
Application number: GB0009143A
Authority: GB
Inventors: Michael O'mahony; Dimitra Simeonidou; Anna Tzanakaki; Kenneth Guild
Original assignee: ILOTRON Ltd
Current assignee: ILOTRON Ltd
Priority date: 2000-04-14
Filing date: 2000-04-14
Publication date: 2001-10-17
Anticipated expiration: 2020-04-14
Also published as: WO2001080592A2; AU4673801A; GB2361393B; JP2003531537A; WO2001080592A3; US20040037561A1; GB0009143D0; EP1281290A2

Abstract

Packets of data are transmitted over an optical transmission network by dividing the data to be transmitted into a plurality of data streams, each comprising data packets of predetermined lengths with the packet lengths of each stream differing from those of the other streams, and associating the individual packet streams on to respective wavelengths of a wavelength division multiplexed (WDM) optical signal for transmission over the network. An optical data transmission network for implementing the above method, includes optical fibres for the transmission of a digital data stream, comprising means to divide the data stream into a plurality of data streams, each of which comprises data packets of predetermined lengths with the packet lengths of each stream differing from those of the other streams, a wavelength-division multiplexer to assign the individual packet streams on to the respective wavelengths of a wavelength-divided optical signal, and means to supply the wavelength-division multiplexed optical signal to an optical fibre for transmission over the network.

Description

2361393 TRANSMISSION OF DATA This invention relates to a method of

transmitting data over an optical transmission network, as well as to such a network adapted to optimise the transmission of data.

Increasingly, the digital traffic on an optical network is in the form of Internet protocol (P) packets. To meet the demands of this rapidly increasing traffic, network operators are having to deploy dense wavelength division multiplexing (DWDIVI) equipment, so as to upgrade the capacity of the already existing optical fibre infrastructure. Further, in order that intermediate routing nodes may also handle the increase in traffic, optical packet-switched (OPS) equipment must be introduced so that the majority of the switching functions are performed in the optical domain, without the need to convert the optical signals to electronic signals which are appropriately routed before being converted up once more to optical signals.

A known problem of optical packet-switched layers is that timing conflicts occur within the routing nodes. In an attempt to resolve such timing contentions in an all-optical network, it has been proposed to introduced fixed-length fibre delay lines, so ensuring that switching can still take place in an appropriate manner.

For the above reasons, it is known to employ in an OPS network a timeslotted regime where all user data is encapsulated into fixed-length cells. Then, in order to provide time transparency for real-time applications and also to permit efficient use of the overall transmission capacity of the network, a compromise over the cell length has to be adopted.

At the present time, web applications account for approximately 75% of ail Internet traffic and generally, such applications are insensitive to packet delay variations across a network. By contrast, time-sensitive speech, audio and video Internet traffic is becoming more common and such traffic is very much more sensitive to network packet delay variations. The use of Internet Protocol version 6 (1Pv6) will gradually become more widespread and the resource reservation protocols (RSVP) of IM allow for better control of the quality of service (QoS). Consequently, as Internet traffic becomes even more heterogeneous in nature, there will be a requirement for these OoS mechanisms to be fully utilised within an OPS network.

Observation of Internet protocol traffic over a network shows that the packet sizes exhibit significant modality. It is found that nearly half the packets are 40 to 44 bytes in length, 75% are less than 522 bytes in length and almost no packets are more than 1500 bytes in length. Since the transfer connection protocol (TCP) accounts for 95% of IP traffic, this modality is primarily due to the length constraints of the TCP definitions, though the upper limit of 1500 bytes results from the maximum transmission unit (MTU) size of an Ethernetattached host.

In addition to the variable lengths of the packets, there is a large variation in the arrival times at a switching node of packets of IP traffic transmitted over an optical network. In order to reduce the probability of any given packet being lost consequent upon this variation in arrival times, it is consequently necessary to employ relatively large packet buffers at a switching node.

It is a principal aim of the present invention to enhance the transmission efficiency of an all-optical network, as well as to minimise the variation in the arrival times of transmitted packets of data.

According to one aspect of the present invention, there is provided a method of transmitting data over an optical transmission network, comprising dividing the data to be transmitted into a plurality of data streams each of which comprises data packets of predetermined lengths with the packet lengths of each stream differing from those of the other streams, and associating the individual packet streams on to respective wavelengths of a wavelength division lo multiplexed optical signal for transmission over the network.

According to a second, but closely related, aspect of this invention, there is provided an optical data transmission network including optical fibres for the transmission of a digital data stream, comprising means to divide the data stream into a plurality of data streams each of which comprises data packets of predetermined lengths with the packet lengths of each stream differing from those of the other streams, a wavelength-division multiplexor to assign the individual packet streams on to the respective wavelengths of a wavelengthdivided optical signal, and means to supply the wavelength -d ivisio n multiplexed optical signal to an optical fibre for transmission over the network.

It will be appreciated that by adopting the method of the present invention, on any given wavelength of a wavelength division multiplexed optical signal each packet may contain the maximum, or close to the maximum, possible amount of user data, so leading to high transmission efficiencies. Moreover, by employing a fixed packet-length regime, the variation in arrival times of packets at a switching node may be greatly reduced.

When the packets are received at an optical routing node, the wavelength transporting a cell may be used to identify the length of the packet, and so also the type of traffic in that packet stream. Thus, there is maintained a simple time-slotted operation for each wavelength channel, at a routing node.

In this way, the head-of-line blocking, caused by large time-insensitive packets, may be removed and the optical network is able to offer a time-sensitive traffic channel with very low latency and a significantly smaller delay variations.

Though the method of this invention could otherwise be used, its prime application is in the transmission of Internet protocol data packets of variable length. Then, the division of the data packets into the plurality of data streams is performed on the basis of the length of those packets to produce, in each stream, fixed length packets. Further, the number of wavelengths selected for the transmission of the data packets should be such that there is an optimum utilisation of the traffic capacity across the wavelength-divided optical signal.

It will be appreciated that the method of the present invention is particularly suitable for the transmission of traffic including timesensitive data or real-time signals such as speech, audio and video traffic. Thus, the transmission method advantageously can be employed in heterogeneous traffic suitable for deployment on IM and utilising full QoS mechanisms.

In order that the invention may better be understood, it will now be described in greater detail though only by way of example, with reference to the accompanying drawing showing packet streams in both electrical and optical layers.

In the method of this invention, advantage is taken of the packet size modality of typical Internet traffic. The traffic is segregated into separate streams containing different length packets, which are then transmitted on different wavelengths of a wavelength-division multiplexed optical signal. This function may be performed in the network edge switches (NES) of the optical packet-switched network, such that a time-slotted packet stream is presented to the optical network. As shown in the accompanying drawing, in the electrical packet-switched layer, the packet traffic may consist of a number of streams 1, 2,...(N-1), N, each having variable length packets. These are then re-organised in the electricalloptical adaptation layer so as to consist of a number of packet streams each having fixed-length packets, as shown in the optical packetswitched layer. Those individual packet streams are assigned on to the separate wavelengths X,, k2....n of the optical signal transmitted over the network.

In this way, the transmitted wavelength identifies the length of a cell in a packet stream, resulting in simplification of the switching mode management. Further, the adaptation layer between the electrical and optical layers is able more efficiently to map the variable length packets on to the time-slotted optical packet-switched layer. This results in a notable reduction in the required number of optical buffers at optical routing processing points within the network.

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Claims

A method of transmitting data over an optical transmission network, comprising dividing the data to be transmitted into a plurality of data streams each of which comprises data packets of predetermined lengths with the packet lengths of each stream differing from those of the other streams, and associating the individual packet streams on to respective wavelengths of a wavelength division multiplexed optical signal for transmission over the network.
2. A method as claimed in claim 1, wherein the data to be transmitted over the optical network comprises data packets of variable length.
3. A method as claimed in claim 2, wherein the division of the data packets into separate streams is performed on the basis of the length of those packets so to produce, in each stream, packets of lengths failing within a predetermined range.
4. A method as claimed in claim 3, wherein all of the packets in any one stream are of the same length.
5. A method as claimed in any of the preceding claims, wherein the number of wavelengths employed for the transmission of the data packets is selected such that there is a substantially optimum utilisation of the traffic capacity across the wave le ngth-d ivided optical signal.
6. A method as claimed in any of the preceding claims, wherein the data to be transmitted comprises time-sensitive data the timing of which must be maintained over the network.
7. A method as claimed in claim 6, wherein the data to be transmitted comprises real-time signals.
8. A method as claimed in claim 1 and substantially as hereinbefore described with reference to accompanying drawings.
9. An optical data transmission network including optical fibres for the transmission of a digital data stream, comprising means to divide the data stream into a plurality of data streams each of which comprises data packets of predetermined lengths with the packet lengths of each stream differing from those of the other streams, a wavelength-division multiplexor to assign the individual packet streams on to the respective wavelengths of a wavelengthdivided optical signal, and means to supply the wavelength-division multiplexed optical signal to an optical fibre for transmission over the network.
10. An optical data transmission network as claimed in claim 9, wherein the network includes optical packet switched equipment to perform packet switching functions in the optical domain.
11. An optical data transmission network as claimed in claim 9 or claim 10, 15 wherein the means to divide the data stream operates to divide the data stream into a sufficient number of data packet streams that the loading across the wavelength division multiplexed optical signal substantially optimises the traffic capacity of the optical signal.
12. An optical data transmission network as claimed in claim 9 and substantially as hereinbefore described with reference to the accompanying drawings.