Establishing connections in a multi-rate optical network

An optical transmission network comprises nodes which support a plurality of different wavelength channels and support at least a first bitrate traffic type and the second bitrate traffic type on respective wavelength channels. A connection of the second bitrate traffic type is established on an available wavelength, if the wavelength offers an acceptable quality of transmission using a first quality of transmission calculation. Alternatively, a connection of the second bitrate traffic type is established on a wavelength which is spaced, by a guard band, from wavelengths used for connections of the first bitrate traffic type, if the wavelength offers an acceptable quality of transmission using a second quality of transmission calculation. The second quality of transmission calculation is less stringent than the first quality of transmission calculation, and can ignore the effects of interference due to cross-phase modulation. The guard band is a wavelength spacing at which the interference between a connection of the first bitrate traffic type and a connection of the second bitrate traffic type is less than a predetermined amount.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National stage of International Application No. PCT/EP2010/065559, filed Oct. 15, 2010, which claims priority to EP Application No. 10176796.0, filed Sep. 15, 2010, which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to optical transmission networks, and to set-up of connections in such a network.

BACKGROUND

Optical transmission networks allow all-optical transmission between network nodes. Traffic is carried by optical wavelength channels, called lambdas, and optical switching technology, such as Wavelength Selective Switches (WSS), allow lambdas to be switched at nodes.

A control plane can be added to this kind of network to allow automated set-up of paths, tear-down of paths and traffic recovery when faults occur in the network. A possible control plane is the Generalized Multi Protocol Label Switching (GMPLS) protocol suite being developed by the Internet Engineering Task Force (IETF). The GMPLS application for optical networks is called Wavelength Switched Optical Network (WSON).

There are now a range of different transmission technologies available for connections within an optical transmission network. Connections can operate at 10 Gbit/s, 40 Gbit/s or 100 Gbit/s and there is a range of different modulation formats, such as On-Off Keying (OOK) and various phase modulation formats, which will generally be called xPSK. Connections operating at different bit-rates and modulation formats can co-exist in the same wavelength switched optical networks (WSONs). In such a multi bit rate WSON, cross-phase modulation (XPM) can be detrimental, especially when it is induced by an OOK signal on an xPSK signal at a higher bit-rate.

A current way of establishing a connection within an optical transmission network estimates a Quality of Transmission (QoT) for the proposed connection and considers a worst-case penalty for the effects of cross-phase modulation. A value of QoT that is acceptable in the worst-case scenario (i.e. when the central wavelength is occupied by a 100 Gbit/s xPSK signal and all other wavelengths by 10 Gbit/s OOK signals) assures the preservation of the lightpath when other new lightpaths are established.

A disadvantage of the current approach is that it can result in many new connections being refused because the Quality of Transmission is unacceptable under worst-case transmission conditions.

SUMMARY

A first aspect of the invention provides a method of establishing a connection of a second bitrate traffic type in an optical transmission network. The network comprises nodes connected by optical links. The nodes support a plurality of different wavelength channels on the links and support at least a first bitrate traffic type and the second bitrate traffic type on respective wavelength channels. The method comprises, at one of the nodes, receiving first information identifying wavelengths which are available on an upstream path to the node. The method further comprises receiving second information identifying wavelengths which are available on an upstream path to the node and which are spaced, by a guard band, from wavelengths used for connections of the first bitrate traffic type. The guard band is a wavelength spacing at which the interference between a connection of the first bitrate traffic type and a connection of the second bitrate traffic type is less than a predetermined amount. The method further comprises determining a quality of transmission of a wavelength in the first information using a first quality of transmission calculation. If a result of the first quality of transmission calculation is not acceptable, the method determines a quality of transmission of an available wavelength in the second information using a second quality of transmission calculation. The second quality of transmission calculation is less stringent than the first quality of transmission calculation.

Advantageously, the first bitrate traffic type is on-off key (OOK) modulated traffic at a first bitrate, such as 10G OOK traffic, and the second bitrate traffic type is phase modulated (xPSK) traffic at a second bitrate, higher than the first bitrate, such as 100G xPSK traffic.

The above method can be performed at a destination node of a connection, or at an intermediate node along a path of the connection.

An advantage of the method is that a connection can be established in a multi-rate optical transmission network even when the first Quality of Transmission (QoT) calculation (e.g. a QoT calculation assuming “worst-case” transmission conditions) would reject the connection. A connection which does not offer an acceptable result for the first QoT calculation can still be used for a connection, and “guarded”, thereby preventing other connections from occupying wavelengths within a “guard band” each side of the wavelength used for the connection. These other connections can be of the type which cause the effects assumed when making the first QoT calculation, such as cross-phase modulation (XPM) between a lower bitrate traffic type (e.g. 10 Gbit/s OOK traffic) and a higher bitrate traffic type (e.g. 40G or 100G xPSK traffic).

Advantageously, if a result of the second quality of transmission calculation is satisfactory, the method further comprises signalling to nodes to select that wavelength for the connection.

Advantageously, if a result of the second quality of transmission calculation is satisfactory, the method further comprises signalling to nodes to designate the wavelength which gave the satisfactory result as a guarded wavelength.

Another aspect of the invention provides a method of establishing a connection of a second bitrate traffic type in an optical transmission network. The network comprises nodes connected by optical links. Nodes support a plurality of different wavelength channels on the links and support at least a first bitrate traffic type and the second bitrate traffic type on respective wavelength channels. An in-use wavelength can be guarded or unguarded. The method comprises, at one of the nodes, determining available wavelengths on a downstream link from the node. The method further comprises determining available wavelengths on a downstream link from the node which are spaced, by a guard band, from wavelengths used for connections of the first bitrate traffic type. The guard band is a wavelength spacing at which the interference between a connection of the first traffic type and a connection of the second traffic type is less than a predetermined amount. The method further comprises advertising the determined wavelengths to a downstream node.

Advantageously, the first bitrate traffic type is on-off key (OOK) modulated traffic at a first bitrate, such as 10G OOK traffic, and the second bitrate traffic type is phase modulated (xPSK) traffic at a second bitrate, higher than the first bitrate, such as 100G xPSK traffic.

Another aspect of the invention provides a method of establishing a connection of a first bitrate traffic type in an optical transmission network. The network comprises nodes connected by optical links. The nodes support a plurality of different wavelength channels on the links and support at least a first traffic type and the second traffic type on respective wavelength channels. The method comprises, at one of the nodes, receiving information identifying wavelengths which are available on an upstream path to the node and which are spaced, by a guard band, from guarded in-use wavelengths used for a connection of the second bitrate traffic type. The guard band is a wavelength spacing at which the interference between a connection of the first traffic type and a connection of the second traffic type is less than a predetermined amount. The method further comprises determining a quality of transmission of a wavelength in the received information.

Advantageously, the first bitrate traffic type is on-off key (OOK) modulated traffic at a first bitrate, such as 10G OOK traffic, and the second bitrate traffic type is phase modulated (xPSK) traffic at a second bitrate, higher than the first bitrate, such as 100G xPSK traffic.

The above method can be performed at a destination node of a connection, or at an intermediate node along a path of the connection.

Another aspect of the invention provides a method of establishing a connection of a first bitrate traffic type in an optical transmission network. The network comprises nodes connected by optical links. The nodes support a plurality of different wavelength channels on the links and support at least the first traffic type and a second traffic type. An in-use wavelength can be guarded or unguarded. The method comprises, at one of the nodes, determining available wavelengths on a downstream link from the node which are spaced, by a guard band, from guarded in-use wavelengths used for connections of the second bitrate traffic type. The guard band is a wavelength spacing at which the interference between a connection of the first traffic type and a connection of the second traffic type is less than a predetermined amount. The method further comprises advertising the determined wavelengths to a downstream node.

An advantage of this method is that the node does not advertise wavelengths which will interfere with guarded wavelengths to downstream nodes, thereby preventing downstream nodes from using the determined wavelengths.

Advantageously, the first bitrate traffic type is on-off key (OOK) modulated traffic at a first bitrate, such as 10G OOK traffic, and the second bitrate traffic type is phase modulated (xPSK) traffic at a second bitrate, higher than the first bitrate, such as 100G xPSK traffic.

In each of the aspects above the first bitrate traffic type and the second bitrate traffic type can have the same bitrate, but different modulation formats, such as 10G OOK modulated traffic and 10G xPSK modulated traffic, although it is currently unusual for these different modulation schemes to be used at the same bitrate.

The functionality described here can be implemented in hardware, software executed by a processing apparatus, or by a combination of hardware and software. The processing apparatus can comprise a computer, a processor, a state machine, a logic array or any other suitable processing apparatus. The processing apparatus can be a general-purpose processor which executes software to cause the general-purpose processor to perform the required tasks, or the processing apparatus can be dedicated to perform the required functions. Another aspect of the invention provides machine-readable instructions (software) which, when executed by a processor, perform any of the described methods. The machine-readable instructions may be stored on an electronic memory device, hard disk, optical disk or other machine-readable storage medium. The machine-readable instructions can be downloaded to the storage medium via a network connection.

DETAILED DESCRIPTION

FIG. 1shows an optical transmission network2with nodes10. Optical transmission links5connect nodes10. Traffic is carried on links5by wavelength channels, called lambdas. Each node has an optical transceiver for optically transmitting traffic on lambdas and for optically receiving traffic on lambdas. A node10which connects to multiple links5comprises a wavelength selective-switch which is arranged to forward traffic based on a wavelength of the lambda. At a node, traffic received on a lambda on an ingress link is selectively forwarded to an egress link. A node in an optical network is typically called a Reconfigurable Optical Add Drop Multiplexer (ROADM).

Embodiments of the invention generally apply to any situation where there is a need to set-up or tear-down a connection or lightpath. The terms “connection” and “lightpath” will be used interchangeably.

FIG. 2shows one of the nodes10in the optical transmission network2ofFIG. 1. Node10connects to optical links51-54. Each link can support a set of lambdas, shown as w0-wn. Each link51-54connects to a respective optical interface31-34. A wavelength selective-switch35connects to the optical interface30of each link5.FIG. 2shows node10connecting to four links51-54, but it will be appreciated that the node10can connect to a smaller, or greater, number of links. The number of wavelength channels w0-wnsupported by each of the links51-54does not have to be equal. Bi-directional operation is supported by separate lambdas for forward and reverse direction, and advantageously separate links5are used for each direction. A wavelength selective switch (WSS)35connects to the optical interfaces31-34. Each optical interface includes at least one transceiver32for transmitting and receiving traffic on lambdas.

Each node10supports transmission and reception at multiple bitrates, such as 10 Gbit/s and 100 Gbit/s. Other possible bitrates are 2.5 Gbit/s and 40 Gbit/s. Future systems may use higher bitrates. Each node can support a range of modulation formats, such as On-Off Keying (OOK) and at least one phase modulation format. Phase modulation formats will generally be called xPSK. Possible phase modulation formats include: Differential Quadrature Phase Shift Keying (DQPSK), Dual Polarisation Quadrature Phase Shift Keying (DP-QPSK) and Quadrature Amplitude Modulation.

One way of establishing a connection in the network2is by using a distributed control plane. Node10has a control plane signalling module60for participating in control plane signalling between nodes10. A memory65stores data used by the control plane signalling module60. Module60can comprise a collection of sub-modules which perform separate functions.FIG. 2shows a module61for calculating Quality of Transmission (QoT) of a connection.

Signalling occurs between nodes10using a control plane technology such as Generalized Multi Protocol Label Switching (GMPLS). Signalling messages carry information which allows nodes10to indicate which wavelengths are available on links5between nodes10along the proposed lightpath and allows nodes to calculate a Quality of Transmission (QoT) metric for a proposed lightpath. This allows a node10to determine if a proposed lightpath will meet a required quality threshold. The signalling messages can be Resource Reservation Protocol-Traffic Engineering (RSVP-TE) messages. RFC 3473 defines a Label Set (LS) for collecting wavelength availability information.

As nodes10support a range of different bitrates and modulation formats, there can be situations where signals of different bitrate and/or different modulation format travel along the same link5, i.e. co-propagate. A form of interference, called cross-phase modulation (XPM), can be induced by one connection on another connection. Cross-phase modulation is tolerable under any one of the following conditions: when induced among connections at the same bit rate; when induced on connections using an OOK modulation format by connections using an xPSK modulation format; and when induced on connections using an xPSK modulation format by connections using an xPSK modulation format. Cross-phase modulation is problematic when induced by a lower bitrate OOK connection on an xPSK connection at a higher bit-rate, such as a 10G OOK connection on a 100G connection, a 10G OOK connection on a 40G connection, a 2.5G OOK connection on a 40G or 100G connection. Connections can follow different routes across the network2, and therefore the co-propagation can last for only one hop between nodes, or a larger number of hops.

At a destination node of a proposed connection, a signalling module60computes a Quality of Transmission (QoT) of a possible path across the network2using a particular wavelength. Typically, the calculation is for a worst-case scenario, where adjacent wavelengths carry connections which use interfering modulation formats at different bitrates.

In embodiments of the invention, a guard band can be provided between a lower bitrate connection and a higher bitrate connection.FIG. 3shows a guard band between a lower bitrate OOK connection and a higher bitrate xPSK connection. The Guard band (GB) is defined as the number of free wavelengths between a 100 Gbit/s connection and a 10 Gbit/s connection for which XPM effects are negligible on Bit Error Rate (BER), or within some acceptable threshold value, selected by the operator. The Appendix gives an example of how to compute BER in the Appendix. A higher bitrate connection is called guarded if it must be separated by at least the guard band GB from all lower bitrate OOK connections. A higher bitrate connection is called unguarded if it can be established without the need for a guard band GB between that connection and lower bitrate OOK connections. The higher bitrate can be 100 Gbit/s and the lower bitrate can be 10 Gbit/s.

Embodiments of the invention use a Secondary Label Set (SLS), in addition to the existing label set, when signalling between nodes10. The SLS can be carried as an object within GMPLS signalling messages, such as an extension to an RSVP-TE message, and can have the same structure as a LS. The SLS can be carried within the same message as the LS, or a separate message. LS is used in a method according to an embodiment of the invention to gather wavelength availability information to set up 10 Gbit/s lightpaths and 100 Gbit/s lightpaths such that 100 Gbit/s lightpaths have acceptable QoT in the worst-case scenario. SLS is used in a method according to an embodiment of the invention to gather wavelength availability information to set up 100 Gbit/s lightpaths under conditions where there is a guard band separating interfering lightpaths. Set up of connections of different bitrates will now be described.

FIG. 4shows a method performed at a source node (steps202,204) of a proposed connection and at any intermediate nodes (steps200,202,204) along the path of the proposed connection. Step200only applies to intermediate nodes. At step200the node receives a control plane signalling message which carries information about available wavelengths on the upstream path to the node. The information can be carried as a GMPLS Label Set (LS). No Secondary Label Set (SLS) objects are required to establish a 10G lightpath. The signalling message can be an RSVP-TE Path message.

At step202the node determines available wavelengths on the outgoing link from the node. A wavelength is considered available if it is not yet in use by an existing connection and if it is spaced, by more than a guard band GB, from a guarded in-use wavelength used for a connection of a higher bitrate traffic type (e.g. 100 Gbit/s). For a source node, the node creates a LS carrying the set of available wavelengths. For an intermediate node, the node receives, at step200, a LS identifying a set of available wavelengths on the upstream path. The node updates the set of wavelengths received in the LS received at step200. The node removes any wavelengths listed in the received LS which are not available on the outgoing link. Stated another way, the intermediate node determines if the wavelengths listed in the received LS are available on the outgoing link, and updates the LS. At step204the node sends the Path message to the downstream node along the path.

FIG. 5shows the use of LS at an intermediate node10during the set-up of a 10G lightpath. A LS101is received at the intermediate node10. LS101advertises the wavelengths that are available in the upstream path. The node performs the method described above to determine a LS102that can be advertised to downstream nodes along the path of the proposed connection. On the outgoing link56, wavelengths w0and w3are already in use: w0is being used to carry a 100G guarded lightpath and w3is being used to carry a 100G unguarded lightpath. These wavelengths are removed from the set of possible wavelengths that can be used on the outgoing link56. On the outgoing link56, wavelength w0is a guarded 100G wavelength, with a guard band GB value=1. Therefore, wavelength w1is also removed from the set of possible wavelengths that can be used on the outgoing link56. Label Set LS102has one entry: w2. Label Set LS102is advertised to nodes10located downstream along the path of the proposed connection.

FIG. 6shows a method performed at a destination node of a proposed connection. At step210the destination node receives a signalling message from an upstream node along the path of the proposed connection. The message includes a Label Set (LS) identifying available wavelengths. At step212the destination node calculates QoT at one of the wavelengths advertised in LS, assuming worst-case conditions. Step214determines if the QoT meets a threshold representing an acceptable QoT. If the QoT is acceptable, the method selects that wavelength for the connection at step216. If the QoT is not acceptable, the method proceeds to step220and selects another of the wavelengths advertised in the LS and returns to step212. The calculation of QoT can use any suitable algorithm. If none of the wavelengths advertised in the LS offers an acceptable QoT (steps214,218), the method exits at step222and a connection cannot be set up. The method performed at upstream nodes ensures that any of the set of wavelengths carried in the LS, as received at the destination node, will cause an acceptable level of interference to 100G lightpaths.

FIG. 7shows a method performed at a source node (steps302-306) of a proposed connection and at any intermediate nodes (steps300-306) along the path of the proposed connection. Step300only applies to intermediate nodes. The node receives a control plane signalling message which carries information about available wavelengths on the upstream path. Both Label Set (LS) and Secondary Label Set (SLS) objects are carried in the signalling message used to establish a lightpath.

At step302the node determines available wavelengths on the outgoing link. A wavelength is considered available if it is not yet in use by an existing connection. For a source node, the node creates a LS carrying the set of available wavelengths. For an intermediate node, the node updates the set of wavelengths received in the LS received at step300. Stated another way, the intermediate node determines if the wavelengths listed in the received LS are available on the outgoing link, and updates the LS. At step306the node sends the Path message to the downstream node. Step304determines available wavelengths on the outgoing link based on lower bitrate interfering connections. A wavelength is considered available if it is not yet in use by an existing connection and if it is spaced, by more than a guard band GB, from a wavelength used for an existing connection of a lower bitrate interfering traffic type (e.g. 10 Gbit/s OOK traffic).

FIG. 8shows the use of LS and SLS at an intermediate node10during the set-up of a 100G lightpath. A LS111and a SLS112are received at the intermediate node10. LS111advertises the wavelengths that are available in the upstream path, calculated under worst-case QoT conditions. SLS112advertises the wavelengths that are available in the upstream path, which can be used with a less stringent QoT threshold, as they are suitably separated from interfering lightpaths. The node performs the method described above to determine a LS113and a SLS114that can be advertised to downstream nodes along the path of the proposed connection. On the outgoing link56, wavelength w1is already in use. Firstly, wavelength w1is removed from both LS113and SLS114. Next, the method considers 10G lightpaths that could interfere. With a guard band GB=1, wavelengths w0and w2are removed from SLS114. Label Set LS113has three entries: w0, w2, w3and Label Set SLS114has one entry: w3. LS113and SLS114are advertised to nodes10located downstream along the path of the proposed connection.

FIGS. 9 and 10show a method performed at a destination node of a proposed connection. At step310the destination node receives a signalling message from an upstream node along the path of the proposed connection. The message includes a Label Set (LS) and a Secondary Label Set (SLS) identifying available wavelengths. At step312the destination node determines QoT at one of the wavelengths advertised in LS using a first QoT calculation which assumes worst-case conditions. Step314determines if the QoT meets a threshold representing an acceptable QoT. If the QoT is acceptable, the method selects that wavelength for the connection at step316. If the QoT is not acceptable, the method proceeds to step320and selects another of the wavelengths advertised in the LS and returns to step312. The calculation of QoT can use any suitable algorithm, and the determination of whether QoT is acceptable at step314assumes the worst-case condition, i.e. a set of 10G OOK signals occupying neighbouring lambdas. If none of the wavelengths advertised in the LS offers an acceptable QoT (steps314,318), the method proceeds to step322. Step322determines QoT for one of the wavelengths advertised in SLS using a second QoT calculation. The determination at step322is less stringent than the QoT calculation used at step312, as the wavelengths advertised in SLS are guarded from interfering lightpaths. Therefore, the calculation at step322does not need to consider XPM effects. If the QoT is acceptable, the method proceeds to “A” and the steps shown inFIG. 10. If the QoT is not acceptable, the method proceeds to step330and selects another of the wavelengths advertised in the SLS and returns to step322. If none of the wavelengths advertised in the SLS offers an acceptable QoT (steps324,328), the method exits at step332and a connection cannot be set up.

FIG. 10shows steps performed after finding a wavelength, advertised in SLS, which gives an acceptable QoT. At step340, the node selects that wavelength for the connection. Step342signals to nodes along the path of the connection to select the wavelength. The signalling can be a RSVP-TE Resv message. At step344, the node informs nodes along the path that the wavelength is to be treated as a guarded lightpath. This is because the wavelength was deemed acceptable at steps322,324on condition that the wavelength is suitably guarded from lightpaths that can cause XPM. Step324can use a flag in the RSVP-TE message, e.g. set flag=1 to indicate “guarded” and set flag=0 to indicate “unguarded”. In this way, each node receiving the Resv message also becomes aware of traversed guarded or unguarded lightpaths.

Referring again toFIG. 5, the connection established on w0and the connection established on w3share the same outgoing link56. However, they follow different upstream paths (not shown) in the network. Thus, their QoT is different. The connection established on w0has a QoT such that, if worst-case scenario is considered, the lightpath has unacceptable QoT. If the guard band is used, XPM effects are negligible and the QoT is acceptable. Thus, the connection can only be established with a guard band and is called a guarded connection. The connection established on w3has acceptable QoT in the worst-case scenario and therefore does not need a guard band. This allows better wavelength usage.

Advantageously, connections carrying 10G and unguarded 100G traffic are allocated wavelengths at lower end of wavelength range (“first-fit”), and connections carrying guarded 100G traffic are allocated wavelengths at the upper end of the wavelength range (“last-fit”). This allows a better utilisation of wavelength resources as connections carrying guarded 100G traffic are grouped at neighbouring wavelengths, thus minimising usage of the guard band. This results in a reduced number of unused wavelengths.

The methods described above use a value of guard band (GB). The value of GB can be derived during network installation. Typically, GB is a conservative value, which is valid for each connection. The value of GB can be updated during the life of the network, as changes occur to the network.

The methods described above can be performed by module60at a node, as shown inFIG. 2, or by a plurality of separate modules which each perform at least one of the individual steps of the method.

It has been described how the methods ofFIGS. 6 and 9are performed at destination nodes. These methods can also be performed at an intermediate node along a path of a connection in a type of system which performs QoT calculations at intermediate nodes.

In the embodiments described above certain values of bitrate (10G, 100G) have been used as examples of the first bitrate traffic type and the second bitrate traffic type. It will be appreciated that the invention can be applied to other bitrates.

The methods described above can offer improved utilisation of the network resources across a range of traffic loads.

Appendix

This Appendix gives a detailed example of how to calculate the guard band. Firstly, only a 100G lightpath is considered. There will not be any XPM effects induced in this signal. BER for the signal is evaluated and called E. Then, the same 100G lightpath is considered with a 10G lightpath following the same network path and occupying the adjacent wavelength channel. In this case, XPM is experienced, and BER of the 100G lightpath is calculated (called E0, where E0>E). Then, n free wavelengths are considered between the 10 and 100G lightpaths, and BER Encomputed until En=E. GB is the maxall paths(n).

In the following calculation, a DP-QPSK 100G signal is considered. The signal is obtained by multiplexing two polarisations (i.e. 50G per polarisation). Coherent detection is assumed, with electronic post-processing at the receiver compensating for the effects of Polarisation Mode Dispersion and Chromatic Dispersion. Guard band and worst-case penalty can be found by computing BER, with the following equations:

Ik(x): ordered-k modified Bessel's function of the first kind

OSNR|dB: the OSNR of the 100 Gbit/s signal considering both polarizations

σ2NL, the variance of the non-linear phase noise, is given by the following contributions:
σ2NL=σ2SPM+σ2XPM
where σ2SPMis contribution due to self phase modulation and σ2XPMis contribution due to XPM):

P: interfering OOK power

T: OOK bit time