WO2013044954A1

WO2013044954A1 - Optical transmission apparatus

Info

Publication number: WO2013044954A1
Application number: PCT/EP2011/066911
Authority: WO
Inventors: Gianmarco Bruno; Anna SOSO
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2011-09-28
Filing date: 2011-09-28
Publication date: 2013-04-04

Abstract

An optical transmission system (5) comprises a first optical transmission apparatus (10) and a second optical transmission apparatus (30) connected by an optical link (11). Optical transmission apparatus (10) comprises a first transmitter (12) for transmitting traffic on a first portion of optical bandwidth of the first optical link (11) and a first receiver (13) for receiving traffic on a second, different, portion of optical bandwidth of the first optical link (11). A controller (15) is arranged to control allocation of the portions of optical bandwidth used by the first transmitter (12) and the first receiver (13) based on traffic demands in the transmit and receive directions of communication. System (5) can comprise a second optical link (21) and optical transmission apparatus (10) can comprise a second transmitter (22) for transmitting traffic on a first portion of optical bandwidth of the second optical link (21) and a second receiver (23) for receiving traffic on a second portion of optical bandwidth of the second optical link (21).

Description

OPTICAL TRANSMISSION APPARATUS

TECHNICAL FIELD

This invention relates to optical transmission apparatus and to a method of optical transmission.

BACKGROUND

Some communication systems have high peak-to-average traffic levels. One example is a system which interconnects data centres. A traffic pattern for this type of system, for a period of one day, is shown in Figure 1. The data centres can be connected by a short-reach, very high capacity, optical transmission system.

A known way to build high-capacity client side optical interfaces is to use single or multi-channel interfaces. Serial 40GbE (IEEE 802.3 bg) is an example of a single-channel interface and 100G OTU-4 over 40 km (ITU-T G.959.1 4L1-9C1F) is an example of multi-channel interface where the signal is split into four lanes.

On the other hand, line side optical interfaces usually are based on single channel transmission to maximise spectral efficiency. For example, the Optical Internetworking Forum (OIF) implementation agreement for 100G Dense Wavelength Division Multiplexed (DWDM) transmission relies on single-channel transmission with polarization multiplexed quadrature-phase-shift-keying (DP-QPSK) that i s suitable for the 50GHz DWDM grid. Multi-channel transmission is attractive for carrying higher bit rates like 400Gbit/s or lTbit/s where it is mandatory to reduce as much as possible the symbol rate.

Although it is possible to build high-capacity transmission systems, they can be over-engineered. That is, they are typically dimensioned to cope with the maximum bandwidth and, when used to service applications with a high peak-to-average traffic rate, the systems will have unused capacity for large periods of time. This results in unnecessary capital expenditure on apparatus and unnecessary operating expenditure to operate and maintain the over-engineered system.

SUMMARY

An aspect of the invention provides an optical transmission apparatus. The apparatus comprises a first port for connecting to a first optical link. The apparatus further comprises a first transmitter for transmitting traffic on a first portion of optical bandwidth of the first optical link. The apparatus further comprises a first receiver for receiving traffic on a second, different, portion of optical bandwidth of the first optical link. The apparatus further comprises a controller arranged to control allocation of the portions of optical bandwidth used by the first transmitter and the first receiver based on traffic demands in the transmit and receive directions of communication.

The optical transmission apparatus is deployed at a first node at one end of the first optical link. A corresponding apparatus is deployed at a second node at the remote end of the first optical link.

The first and second portions of bandwidth are different, i.e. they are non- overlapping (mutually exclusive) parts of the spectrum.

In an embodiment, the first transmitter and the first receiver are arranged to use a set of optical wavelength channels and the controller is arranged to allocate a sub-set of the optical wavelength channels for use by the first transmitter and to allocate a different sub-set of the optical wavelength channels for use by the first receiver.

In another embodiment, the first transmitter is arranged to modulate an optical carrier with a set of sub-carriers and the first receiver is arranged to receive a different optical carrier which has been modulated by a different set of sub-carriers, and the controller is arranged to control the number of sub-carriers used by the first transmitter and the number of sub-carriers used by the first receiver.

In an embodiment, the optical transmission apparatus further comprises a second port for connecting to a second optical link. The apparatus further comprises a second transmitter for transmitting traffic on a first portion of optical bandwidth of the second optical link. The apparatus further comprises a second receiver for receiving traffic on a second portion of optical bandwidth of the second optical link. The controller is arranged to control allocation of the portions of optical bandwidth used by the second transmitter and the second receiver based on traffic demands in the transmit and receive directions of communication.

Transmission apparatus according to an embodiment of the invention is able to flexibly allocate bandwidth on one or more optical links between one of two directions of communication, i.e. a transmit direction and a receive direction. The overall capacity of the transmission system can be lower compared to a transmission system with dedicated forward and reverse links, reducing the need to over-engineer the system. The transmission apparatus is particularly useful with traffic that has a high peak-to-average rate, traffic which is asymmetric, or traffic with a mix of both of these characteristics.

Advantageously, the controller is arranged to allocate bandwidth within a particular window of optical bandwidth (e.g. a wavelength range). Other parts of the optical bandwidth on one or more links can be allocated in a conventional manner, with bandwidth dedicated to a particular transmission direction.

The controller can be arranged to exchange control signalling with a controller at a node at a remote end of the first optical link to co-ordinate allocation of optical bandwidth. Alternatively, the controller can be arranged to operate in an autonomous manner, independently of a controller at a node at a remote end of the first optical link (11). This means that the controller does not need to exchange control signalling with a controller at a node at a remote end of the first optical link to co-ordinate allocation of bandwidth. The controller can operate according to a rule set. For example, the controller can be arranged to autonomously control the first transmitter to use a portion of bandwidth if that portion of bandwidth has been unused in the receive direction for a predetermined time period.

Embodiments of the invention are particularly useful in applications with "peaky" traffic demands, i.e. traffic with a high peak-to-average rate or applications with asymmetric traffic demands. Embodiments of the invention are particularly useful in optical transmission systems with short-reach and very high capacity.

Embodiments of the invention can increase the peak capacity of an optical connection without increasing the number of ports. Embodiments of the invention can be used with passive systems or systems with optical amplification.

Various techniques can be used to form the channels which are transmitted on the optical link, or links. The transmitter can individually modulate an optical carrier to form one of the channels. Channels can be Wavelength Division Multiplexed (WDM) or Dense Wavelength Division Multiplexed (DWDM). The transmitter can modulate a sub-carrier, or set of sub-carriers, in the electrical domain, before modulating an optical carrier. A further alternative is to use an Orthogonal Frequency Division Multiplexing (OFDM) technique to form a modulated multi-carrier signal in the electrical domain, before modulating an optical carrier.

Another aspect of the invention provides an optical transmission system comprising a first optical transmission apparatus at a first node, a second optical transmission apparatus at a second node and at least a first optical link connecting the first optical transmission apparatus and the second optical transmission apparatus.

Another aspect of the invention provides a method of optical transmission. The method comprises determining an allocation of a first portion of optical bandwidth for transmitting traffic on a first optical link and an allocation of a second, different, portion of optical bandwidth for receiving traffic on the first optical link based on traffic demands in the transmit and receive directions of communication. The method further comprises transmitting traffic on the determined portion of optical bandwidth of the first optical link. The method further comprises receiving traffic on the determined portion of optical bandwidth of the first optical link.

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. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows an example of traffic demand between a pair of data centres; Figure 2 shows a transmission system according to an embodiment of the invention;

Figures 3A-3D show examples of how bandwidth can be allocated on a transmission system according to an embodiment of the invention having a single optical link; Figure 4A-4D show examples of how bandwidth can be allocated on a transmission system according to an embodiment of the invention having a pair of optical links;

Figures 5A and 5B show examples of allocating modulation schemes to channels on a transmission system according to an embodiment of the invention;

Figures 6A-6C show examples of transmitters in the transmission apparatus; Figure 7A shows a transmission system with co-ordinated controllers;

Figure 7B shows a transmission system with autonomous controllers;

Figures 8A-8G show examples of how bandwidth can be allocated to an optical link by autonomous controllers;

Figure 9 shows a method of transmission.

DETAILED DESCRIPTION

Figure 2 shows an embodiment of a transmission system 5 comprising transmission apparatus 10 at a first node A and transmission apparatus 30 at a second node B. A first optical link 1 1 connects a port 14 of the transmission apparatus 10 at node A with a port 34 of the transmission apparatus 30 at node B . Transmission apparatus 10 comprises a transmitter 12 and a receiver 13 which connect to the port 14 and the first optical link 1 1. Similarly, a transmitter 32 and a receiver 33 of transmission apparatus 30 connect to port 34 and the first optical link 11.

In this embodiment there is also a second optical link 21 which connects a port 24 of the transmission apparatus 10 at node A with a port 44 of the transmission apparatus 30 at node B. Transmission apparatus 10 comprises a transmitter 22 and a receiver 23 of transmission apparatus 10 which connect to port 24 and the second optical link 21. Similarly, a transmitter 42 and a receiver 43 of transmission apparatus 30 connect to port 44 and the second optical link 21.

The transmission system 5 of Figure 2 allows one, or both, of the optical links 1 1, 21 to be used in a flexible-sense manner. This means that one, or both, of the optical links 11, 21 can be used in a manner which allows traffic to be carried in either:

one direction at a time (i.e. traffic is transmitted over the link 1 1 from node A to node B or traffic is transmitted over the link 1 1 from node B to node A), or ,

6 both directions at the same time (i.e. traffic is transmitted over the link 11 from node A to node B and traffic is transmitted over the link 1 1 from node B to node A).

The amount of optical bandwidth used for each direction of transmission can be varied according to factors such as traffic demand for each of the two transmission directions.

Figure 2 shows a multiplexer/demultiplexer 16 for combining optical outputs of the transmitter 12 (e.g. a set of wavelength division multiplexed optical channels) before forwarding them over the optical link 1 1 and for demultiplexing optical wavelength channels received from the optical link 1 1. This functionality can alternatively be provided as part of the transmitter 12 itself. A circulator 19 connects the multiplexer/demultiplexer 16 to the transmitter 12 and receiver 13. The circulator forwards optical signals received from transmitter 12 to port 14 and forwards optical signals received from port 14 to receiver 13.

At node A, controller 15 controls the allocation of bandwidth to the transmission directions (A-B, B-A) on each of the links 1 1 , 21. Controller 15 also controls transmitters 12, 22 and receivers 13, 23 to transmit/receive traffic according to the allocation of bandwidth. A Transmit Data Management unit 17 is arranged to receive traffic from a traffic source (e.g. a local server or another part of a transmission network) and to forward traffic to one, or both, of the transmitters 12, 22 under the control of controller 15. A Receive Data Management unit 18 is arranged to receive traffic from one, or both, of the receivers 13, 23 and to forward traffic to a traffic sink (e.g. a local server or another part of a transmission network). Controller 15 informs the Receive Data Management unit 18 of how traffic has been allocated to receivers 13, 23. Buffer capacity can be provided in one or more of the Transmit Data Management unit 17, Receive Data Management unit 18, transmitters 12, 22 and receivers 13, 23.

At node B, controller 35 controls the allocation of bandwidth to the transmission directions (A-B, B-A) on each of the links 11, 21. Controller 35 controls transmitters 32, 42 and receivers 33, 43 to transmit/receive traffic according to the allocation of bandwidth. A Transmit Data Management unit 37 is arranged to receive traffic from a traffic source (e.g. a local server or another part of a transmission system) and to forward traffic to one, or both, of the transmitters 32, 42 under the control of controller 35. A Receive Data Management unit 38 is arranged to receive traffic from one, or both, of the receivers 33, 43 and to forward traffic to a traffic sink (e.g. a local server or another part of a transmission system). Controller 35 informs the Receive Data Management unit 38 of how traffic has been allocated to receivers 33, 43. Buffer capacity can be provided in one or more of the Transmit Data Management unit 37, Receive Data Management unit 38, transmitters 32, 42 and receivers 33, 43.

Controllers 15, 35 control allocation of bandwidth on the optical links 11, 12 to transmission directions. Controllers 15, 35 can work in a co-ordinated manner (i.e. they exchange signalling with one another to indicate allocation of bandwidth to transmission directions) or they can work in an autonomous manner (i.e. each controller 15, 35 acts independently of the other controller, using a rule or rule set to allocate bandwidth). Co-ordinated operation of controllers 15, 35 offers the greatest flexibility in allocation of bandwidth.

Advantageously, in both cases (co-ordinated controllers and autonomous controllers) the controllers 15, 35 exchange control data with upper layers. They are able to monitor the available channels for transmission and they know the rate of incoming traffic (e.g. from an input from the TX Data Management Unit 17). In case it is not possible to accommodate the traffic, a control message can be raised. A control message can be issued, for example, under the following conditions: if transmission is adapted correctly, if the bandwidth adjustment is in progress; and to indicate current available capacity per direction.

In a system with an autonomous controller 15, controller 15 is aware of which channels/frequencies are currently used for transmission by node 30 due to receivers 23 and 13 receiving optical power higher than a given threshold. Controller 15 can therefore determine which channels are available for transmission. The controller 15 receives as inputs the available frequencies, the data to be sent and can give as outputs to upper layers a message about unavailability of resources for transmission. In this way, bandwidth is negotiated by controllers 15, 35.

Controllers 15, 35 control allocation of the portions of optical bandwidth used by the transmitters 12, 22 and the receivers 13, 23 based on traffic demands in the transmit and receive directions of communication. A traffic demand can arise from a change in the amount of traffic arriving at a TX Management unit 17, 37 at a node. Alternatively, a traffic demand can arise from a fault in one of the links in the transmission system. For example, if a fault occurs in link 1 1, at least part of the traffic on link 11 may be carried over link 21. Alternatively, a fault occurring in some ₀

o other link of the transmission network (e.g. between nodes A and B) can trigger a traffic demand over link 11 and/or link 21.

Figures 3A-3D show examples of how bandwidth can be allocated on a transmission system having a single optical link. Link 1 represents the optical link 1 1 shown in Figure 2. A maximum of four channels CI -C4 are used in this system. The maximum number of channels shown in Figure 3 is an example, and an actual system can have a higher or lower number of channels. A solid line indicates that a channel is used for the transmission direction from node A to node B (i.e. transmitted by node A, received by node B) and a dashed line indicates that the channel is used for the transmission direction from node B to node A (i.e. transmitted by node B, received by node A).

In Figure 3 A two channels C I, C2 on the link are used for the transmission direction A-B and two channels C3, C4 on the link are used for the transmission direction B-A. This can be a nominal (default) operational state.

In Figure 3B three channels C1-C3 on the link are used for the transmission direction A-B and one channel C4 on the link is used for the transmission direction B- A. This allows the transmission system to provide greater capacity in the transmission direction A-B compared to the transmission direction B-A.

In Figure 3C, only two of the channels are in use.

In Figure 3D, all four channels C1-C4 on the link are used for the transmission direction A-B. This allows the transmission system to provide maximum capacity in the transmission direction A-B and can be used at a time when there is no demand in the transmission direction B-A.

Advantageously, all channels allocated to a particular transmission direction are contiguous in frequency, i.e. all channels allocated to one direction form a block, and all channels allocated to the opposite direction form another block. However, more generally, channels allocated to a particular direction do not have to be contiguous in frequency/wavelength. In each of the examples, the channels allocated to the transmission direction (A-B) are different (i.e. non-overlapping, mutually exclusive) to the channels allocated to the transmission direction (B-A).

Figures 4A-4D show examples of how bandwidth can be allocated on a transmission system having two optical links. Link 1 represents the optical link 1 1 shown in Figure 2 and link 2 represents the optical link 21 shown in Figure 2. A maximum of four channels C1-C4 are used on each of the links in this system. The maximum number of channels is an example, and an actual system can have a higher or lower number of channels. In this example the total number of channels used on link 1 is the same as the total number of channels used on link 2, but the number can differ. The same notation is used to indicate transmission direction.

In Figure 4A all channels on link 1 are used for the transmission direction A-B and all channels on link 2 are used for the transmission direction B-A. This can be a nominal (default) operational state. In general, interference is minimised when a link is used for a single transmission direction.

Figure 4B shows another operational state of the optical links. As in Figure 4A, all of the optical channels on link 1 are used for the transmission direction A-B. The optical channels CI, C2 are used for the transmission direction B-A and the optical channels C3, C4 are used for the transmission direction A-B. Link 2 now simultaneously carries traffic in two transmission directions using different optical channels. This allows the transmission system to provide greater capacity in the transmission direction A-B compared to the transmission direction B-A. For example, if each channel has a capacity of lOOGbit/s, the arrangement of Figure 4B provides a traffic capacity of 600 Gbit/s in the direction A-B and a traffic capacity of 200 Gbit/s in the direction B-A.

Figure 4C shows another operational state of the optical links. Each of the optical links is used to carry traffic in both transmission directions. On optical link 1 the optical channels CI, C2 are used for the transmission direction B-A and the optical channels C3, C4 are used for the transmission direction A-B. On optical link 2 the optical channels CI, C2 are used for the transmission direction A-B and the optical channels C3, C4 are used for the transmission direction B-A.

Figure 4D shows another operational state of the optical links. On optical link 1 the optical channels CI and C3 are used for the transmission direction A-B and the optical channels C2 and C4 are used for the transmission direction B-A. The optical channels allocated to each direction do not have to be contiguous in frequency.

In the examples described above the bandwidth on the optical links that is allocated to one of the transmission directions comprises a set of optical channels Cl- C4 which are offset in frequency/wavelength from one another. The set of optical channels can be implemented in various ways. In one embodiment, each of the optical channels C1-C4 is individually generated by modulating an optical carrier with traffic. In another embodiment, a set of electrical sub-carriers are modulated with traffic in the electrical domain and then an optical carrier is modulated with the set of electrical carriers. In this embodiment, a channel represents one of the sub-carriers. In a system using sub-carriers, the total number of sub-carriers (and hence channels) is likely to be much larger than the simple example that has been illustrated, and will typically have a number of sub-carriers of the order of tens or hundreds of sub-carriers.

In the examples shown in Figures 3 and 4, a maximum of four channels can be flexibly allocated to transmission direction (A-B) or transmission direction B-A within the spectral window allocated to this particular transmission interface. Other channels can be transmitted or received on the same links 11, 21 by other transmission interfaces occupying other parts of the usable spectrum of the links. The other transmission interfaces can be dedicated to a particular transmission direction, or they can be other instances of the flexible interface.

Selecting constellation size

In any of the transmission schemes, the transmitter and receiver at each of the transmission apparatus 10, 30 can be capable of using a range of different modulation schemes. There are different ways of operating. Firstly, it is possible to allocate a predetermined (fixed) modulation scheme to each one of the optical channels. An example is shown in Figure 5 A. Channel CI is allocated a first modulation scheme (e.g. 16QAM), channels C2 and C3 are allocated a second modulation scheme (e.g. QPSK) and channel C4 is allocated a third modulation scheme (e.g. BPSK). This may be advantageous if fixed constellations give advantages in term of more cost-effective hardware, etc.

Secondly, it is possible to allocate one of the possible modulation schemes on- demand to each one of the channels. An example is shown in Figure 5B. For each of the channels C1-C4 it is possible for the transmitter and receiver at each end node A, B to use one of three possible modulation schemes: 16QAM, QPSK and BPSK. This allows a finer granularity in the bandwidth allocation. The number of modulation schemes supported per channel does not have to be equal, as shown in Figure 5B. For example, channel C4 may only support one or two possible modulation schemes.

Modulation schemes, and corresponding apparatus is described in ITU-T Recommendation G.supplement 39 (12/2008) "Optical system design and engineering considerations", at page 17. As described above, there are various ways in which the bandwidth can be allocated. Some possible schemes will now be described.

Multi carrier IM-DD

In the simplest arrangement, the transmission apparatus 10, 30 at each node comprises a transmitter 12, 32 which can operate at N separate wavelengths. A schematic is shown in Figure 6A. The transmitter 12, 32 can comprise an optical source, or optical sources 61, which are capable of outputting optical carriers at N separate wavelengths (λΐ- λΝ). If a channel is required in the transmit direction, the transmitter 12, 32 modulates an optical carrier at the wavelength of the channel with data 62. For example, this can be achieved by intensity-modulation (IM) of the carrier signal by Non Return-to-Zero ( RZ) coding/modulation. The receiver 33 , 13 at the remote node is configured to receive the modulated signal . The receiver 33 , 13 demodulates the signal to retrieve the data. For an intensity modulated signal, the receiver can use direct detection. If a channel is not required for the transmit or receive direction, the transmitter 12, 32 and the receiver 33, 13 can be powered down to save energy. Multi-carrier IM-DD systems have an advantage of simplicity, but offer limited granularity in bandwidth allocation. SCM RF-assisted implementation

A more advanced implementation consists in moving from IM-DD to more complex modulation formats and coherent detection to increase spectral efficiency and increase tolerance to linear impairments . RF-assisted transmission (sub-carrier multiplexing or SCM) concept can be used to keep minimal the number of optical components whilst providing multi-carrier transmission with complex modulation schemes. In this case, the transmitter in Figure 2 still uses one only laser for modulation, another for detection, only one modulator at the transmit side. The number of sub-carriers (that defines the granularity) is determined in the RF domain.

Figure 6B shows a simplified schematic of the transmitter 12. If a particular channel is required in the transmit direction, the transmitter 12, 32 modulates a sub- carrier 63 representing that channel (in the electrical domain) with data 62. One or multiple sub-carriers can be modulated with data, depending on the number of channels required in the transmit direction. The modulated sub-carriers 64 then modulate an optical carrier at a wavelength Xc output by an optical source 65. The number of sub-carriers in use at any time can be varied. The receiver 33, 13 at the remote node is configured to receive the modulated signal on each of the channels. The receiver 33, 13 demodulates the signal to retrieve the data.

The problem of transmission penalty from counter-propagating channels is well known from the study of single-fiber optical systems. Penalty is highest when channels traveling in opposite direction share the same frequency, and can be reduced to an acceptable level by proper channel spacing.

A further embodiment uses an OFDM transmitter 12, 32 and OFDM receiver 13, 33. Figure 6C shows a simplified schematic of the transmitter 12. As in Figure 6B, a set of data inputs 62 are received. OFDM module 66 forms a set of modulated carriers 67, each carrier modulated with data 62. OFDM module 66 is implemented as digital signal processing (DSP) performed by a processor. The set of modulated carriers are converted to the time-domain by an Inverse Fast Fourier Transform (IFFT) processing stage and then used to modulated an optical carrier at wavelength Xc output by an optical source 65. The number of sub-carriers in use by the transmitter at any time can be varied to vary the overall bandwidth allocated to the transmit direction. This embodiment can allow an even further granularity in the bandwidth allocation because the number of sub-carriers and their modulation format is determined by processing in the electrical domain. The sub-carriers do not have to be generated by signal sources, but only exist as part of the DSP performed by the processor. The receiver 13 , 33 operates in a complementary manner to the transmitter described above. The number of sub-carriers in use by the receiver 13, 33 at any time can be varied to vary the overall bandwidth allocated to the receive direction. Control protocols

As explained above, controllers 15, 35 at each node control allocation of bandwidth on the optical links 1 1, 12 to transmission directions.

The controllers 15, 35 can work in a co-ordinated manner, as shown in Figure 7A. Controllers 1 5, 35 can exchange signalling with one another to co-ordinate allocation of bandwidth on the links 1 1, 21. The signalling can be sent along a path 71 which uses the link 1 1. It is possible to carry signalling between controllers 15, 35 using a control channel which is carried as part of the overhead information, such as the one in the LCAS protocol (ITU-T G.7042). A possible disadvantage of this scheme is that it may be mandatory to maintain a minimum of one channel per direction, losing some flexibility in the bandwidth allocation. Alternatively, the signalling can be sent along a path 72 which uses an external infrastructure. The signalling channel can be a dedicated channel within the DCN, such as Ethernet, Optical Supervisory Channel (OSC) or GCC connection.

Alternatively, the controllers 15, 35 can work in an autonomous manner, as shown in Figure 7B. Each controller 15, 35 acts independently of the other controller.

In an embodiment, a channel numbering scheme can be used to identify channels, with both controllers 15, 35 using the same channel numbering scheme. Controller 15 causes transmitters 12, 22 to use channels in one particular order (e.g. starting from the lowest channel number, and occupying channels in increasing channel number). Controller 35 causes transmitters 32, 42 to use channels in the opposite order (e.g. starting from the highest channel number, and occupying channels in decreasing channel number). This avoids allocation conflicts as long as the maximum number of channels required is less than the maximum number of channels available. For each channel allocated to the transmit direction, a corresponding receiver at the other node is controlled to receive the allocated channel.

Figures 8A-8G show a sequence of operational states of a single optical link. Figure 8A shows a starting condition with all of the channels C1-C4 available (not in use). In Figure 8B a channel is required in each of the transmission directions. Channel CI is allocated by node A for the direction A-B and channel C4 is allocated by node B for the direction B-A. Figures 8C and 8D show two possible options for the next step. In Figure 8C a further channel is required in each of the transmission directions. Channel C2 is allocated by node A for the direction A-B and channel C3 is allocated by node B for the direction B-A. In Figure 8D a further channel is required only in the transmission direction A-B. Channel C2 is allocated by node A for the direction A-B, leaving one free channel C3. In Figure 8E direction B-A stops transmitting on channels C3 and C4 (if Figure 8E follows Figure 8C), or stops transmitting on channel C4 (if Figure 8E follows Figure 8D). This leaves two free channels C3, C4. Next, a further channel is required in the transmission direction A-B. Channel C3 is allocated by node A for the direction A-B, leaving one free channel C4. In one embodiment, this remaining channel C4 can only be allocated to the transmission direction B-A, to reserve some capacity in the direction B-A.

An autonomous scheme of the type shown in Figures 8A-8G can avoid the need of control signalling between nodes using the rules: 1. an available channel can be used by a controller that needs it, until at least one channel is available. A predetermined time period shall expire before the usage of an additional channel.

2. the last channel can be used only by the transmission direction that has fewest channels currently in use (e.g. in Figure 8G channel C4 can only be used for the direction B-A, and not for the direction A-B).

Due to the manner in which channels are allocated (starting at C I and working upwards in numerical order for the direction A-B; starting at C4 and working downwards in numerical order for the direction B-A) there are no conflicts in allocation. Due to rule 1, the process is sufficiently slow to allow the controllers at each of nodes A, B know the general status of the link, i.e. if there is an available channel or not. Due to rule 2, conflicts in the usage of the last channels are avoided.

It will be appreciated that the above scheme and suggested rules are one possible implementation for autonomous operation, and that the controllers at nodes A, B could operate in a different manner.

In a transmission system with two optical links, one way of autonomous operation is to nominally allocate a first of the links (link 1) to the direction A-B and a second of the links (link 2) to the direction B-A. A controller at node A can allocate channels on link 1 for the direction A-B in increasing numerical order, starting from channel CI . Similarly, a controller at node B can allocate channels for the direction B- A on link 2 in increasing numerical order, starting from channel CI . If the controller at node A wants to use a channel on link 2 for the direction A-B, it starts with the highest numbered channel on link 2, and works in a decreasing numerical order. Similarly, if the controller at node B wants to use a channel on link 1 for the direction B-A, it starts with the highest numbered channel on link 1, and works in a decreasing numerical order. A rule can be set such that a controller at node A can use a maximum number of channels on link 2 and a controller at node B can use a maximum number of channels on link 1.

Advantageously, the transmission system 5 is able to provide transport of time dependent traffic. Advantageously, bandwidth changes occur on a slower timescale than the time required to implement a change in allocation of bandwidth.

Another embodiment of autonomous controller operation will now be described. Each controller 15, 35 implements a rule, or set of rules, to allocate bandwidth on optical links 11, 12. Controllers 15, 35 cause a transmitter 12, 22, 32, 42 to begin using a channel and waiting for the controller at the other node to recognize that the channel is in use. A quantity T can be defined, which represents the maximum round-trip time of traffic between the two end nodes A, B. With a maximum reach of 600km,

T = 2 * 1.5 * 6e5m/3e8m/s = 6ms

After waiting for this time, a controller is allowed to use a new channel as soon as traffic exceeds the currently allocated channel bandwidth, providing there are at least two unused channels available.

When there is only one channel available, then the following is needed to know if one node needing more bandwidth is allowed to use the last available channel. The node that wishes to transmit more bandwidth monitors (for a time sufficient to cancel differences if also other node is doing the same measurement) the rate transmitted and received in currently configured channels:

If both directions are wire speed (i.e. completely use the available bandwidth in currently configured channels) it can assume that the other node will also need the last available channel. Under these conditions, the node can use the last available channel only if it has less channels configured with respect to the other node.

If instead it is the only node that is transmitting at wire speed, it can occupy the last available channel.

One direction that has no channels is assumed as wire speed to avoid conflicts in the usage of the last channel in this case. This means that a minimum of one channel is associated to each direction.

There are three possible ways in which controllers can operate: co-ordinated operation using a conventional photonic DCN; co-ordinated operation using channel overhead for signalling; autonomous operation. The advantages and possible disadvantages of each scheme are summarized below:

Co-ordinated operation using photonic DCN (Eth/OSC/GCC)

Needs an external infrastructure (maybe not available). Depending on the network structure, the communication time can be long (longer reaction time if protection is required). Complete flexibility of channel allocation (i.e. the bandwidth can be all used in one direction only).

Usage of channel overhead for Control Signalling ₁ ,

16

No need of additional infrastructures. Fast communication (fast reaction, suitable for protection). Does not allow complete flexibility of channel allocation as a minimum of one channel is needed for each direction.

Autonomous (Local algorithm)

Figure 9 shows a method of optical transmission. At step 101 the method determines an allocation of a first portion of optical bandwidth for transmitting traffic on a first optical link and an allocation of a first portion of optical bandwidth for receiving traffic on the first optical link based on traffic demands in the transmit and receive directions of communication. In a system with a second optical link and second set of transmitters and receivers, the method further comprises a step 102 of determining an allocation of a first portion of optical bandwidth for transmitting traffic on a second optical link and an allocation of a first portion of optical bandwidth for receiving traffic on the second optical link. At step 104 the method transmits traffic on the determined portion of optical bandwidth of the first optical link. At step 105 the method receives traffic on the determined portion of optical bandwidth of the first optical link. In a system with a second optical link and second set of transmitters and receivers, the method further comprises a step 106 of transmitting traffic on the determined portion of optical bandwidth of the second optical link and a step 107 of receiving traffic on the determined portion of optical bandwidth of the second optical link.

Modifications and other embodiments of the disclosed invention will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. Optical transmission apparatus comprising:

a first port for connecting to a first optical link;

a first transmitter for transmitting traffic on a first portion of optical bandwidth of the first optical link;

a first receiver for receiving traffic on a second, different, portion of optical bandwidth of the first optical link;

a controller arranged to control allocation of the portions of optical bandwidth used by the first transmitter and the first receiver based on traffic demands in the transmit and receive directions of communication.

2. Optical transmission apparatus according to claim 1 wherein the controller is arranged to control the size of the portion of optical bandwidth used by the first transmitter between 0 and 100% and to control the size of the portion of optical bandwidth used by the first receiver between 0 and 100%.

3. Optical transmission apparatus according to claim 1 or 2 wherein the first transmitter and the first receiver are capable of using a set of channels and the controller is arranged to control allocation of the portions of optical bandwidth used by the first transmitter and the first receiver by allocating different channels for use by the first transmitter and the first receiver.

4. Optical transmission apparatus according to claim 3 wherein the first transmitter and the first receiver are capable of using a plurality of different modulation schemes and, for each channel in the set of channels, there is a predetermined modulation scheme for that channel.

5. Optical transmission apparatus according to claim 3 wherein the first transmitter is capable of modulating a channel using one of a range of possible modulation schemes and the controller is arranged to control the first transmitter to use a particular one of the modulation schemes to modulate the channel. 1 o

6. Optical transmission apparatus according to claim 4 or 5 wherein the modulation schemes vary in constellation size.

7. Optical transmission apparatus according to any one of the preceding claims further comprising one of:

the first transmitter and the first receiver are arranged to use a set of optical wavelength channels and the controller is arranged to allocate a sub-set of the optical wavelength channels for use by the first transmitter and to allocate a different sub-set of the optical wavelength channels for use by the first receiver;

the first transmitter is arranged to modulate an optical carrier with a set of sub- carriers and the first receiver is arranged to receive a different optical carrier which has been modulated by a different set of sub-carriers, and the controller is arranged to control the number of sub-carriers used by the first transmitter and the number of sub- carriers used by the first receiver.

8. Optical transmission apparatus according to any one of the preceding claims further comprising:

a second port for connecting to a second optical link;

a second transmitter for transmitting traffic on a first portion of optical bandwidth of the second optical link;

a second receiver for receiving traffic on a second portion of optical bandwidth of the second optical link;

and wherein the controller is arranged to control allocation of the portions of optical bandwidth used by the second transmitter and the second receiver based on traffic demands in the transmit and receive directions of communication.

9. Optical transmission apparatus according to any one of the preceding claims wherein the controller is arranged to exchange control signalling with a controller at a node at a remote end of the first optical link to co-ordinate use of optical bandwidth.

10. Optical transmission apparatus according to any one of the preceding claims wherein the controller is arranged to operate in an autonomous manner, independently of a controller at a node at a remote end of the first optical link.

11. An optical transmission system comprising:

a first optical transmission apparatus according to any one of the preceding claims at a first node;

a second optical transmission apparatus according to any one of the preceding claims at a second node; and

at least a first optical link connecting the first optical transmission apparatus and the second optical transmission apparatus.

12. A method of optical transmission comprising:

determining an allocation of a first portion of optical bandwidth for transmitting traffic on a first optical link and an allocation of a second, different, portion of optical bandwidth for receiving traffic on the first optical link based on traffic demands in the transmit and receive directions of communication;

transmitting traffic on the determined portion of optical bandwidth of the first optical link; and

receiving traffic on the determined portion of optical bandwidth of the first optical link.

13. A method according to claim 12 wherein the step of determining an allocation of a first portion of optical bandwidth for transmitting traffic on a first optical link and an allocation of a second portion of optical bandwidth for receiving traffic on the first optical link portion of bandwidth comprises one of:

determining a number of optical wavelength channels;

determining a number of sub-carriers used to modulate an optical carrier.

14. A method according to claim 13 wherein the step of determining a first portion of optical bandwidth for transmitting traffic on a first optical link and a second portion of optical bandwidth for receiving traffic on the first optical link portion of bandwidth comprises one of:

determining a sub-set of a set of optical wavelength channels for use by the first transmitter and a different sub-set of the set of optical wavelength channels for use by the first receiver; determining a number of sub-carriers in a set of sub-carriers used by the transmitter to modulate an optical carrier and a number of sub-carriers in a set of sub- carriers used by the receiver.

15. A method according to any one of claims 12 to 14 further comprising:

determining an allocation of a first portion of optical bandwidth for transmitting traffic on a second optical link and an allocation of a second portion of optical bandwidth for receiving traffic on the second optical link based on traffic demands in the transmit and receive directions of communication;

transmitting traffic on the determined portion of optical bandwidth of the second optical link; and

receiving traffic on the determined portion of optical bandwidth of the second optical link .

16. A machine-readable medium carrying instructions which, when executed by a processor, cause the processor to perform the method of any one of claims 12 to 15.