WO2010115824A2

WO2010115824A2 - A switching device for terminating a passive optical network

Info

Publication number: WO2010115824A2
Application number: PCT/EP2010/054361
Authority: WO
Inventors: Sebastian Randel; Shingo Yamakawa
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens AG; Siemens Corp
Priority date: 2009-04-02
Filing date: 2010-03-31
Publication date: 2010-10-14
Anticipated expiration: 2011-10-02
Also published as: WO2010115824A3

Abstract

The invention refers to a switching device (ONT) for terminating a first, passive optical network (PON). The first, passive optical network (PON) comprises a number of optical network units (ONU) each of them being optically coupled to the switching device and each of them being coupled to at least one network device (PC). A unique identifier (LLID) within the first network is assigned to each of the number of optical network units (ONU) and an Ethernet address is assigned to each of the network devices (PC). A switching element (LS) is coupled to the number of optical network units (ONU) of the passive optical network (PON), wherein the switching element (LS) is adapted to process all network traffic within the passive optical network (PON). An Ethernet switching element (ES) is connected to a second communication network (IN) being based on the internet protocol, wherein the Ethernet switching element (ES) is adapted to process all other traffic being addressed to a device of the communication network (IN).

Description

A switching device for terminating a passive optical network

The invention relates to a switching device for terminating a passive optical network, wherein the passive optical network comprises a number of optical network units each of them being optically coupled to the switching device and each of them being coupled to at least one network device. In such a passive optical network a unique identifier within the passive optical network is assigned to each of the number of optical network units. Furthermore, an Ethernet address is assigned to each of the network devices. The invention further relates to a communication network comprising a passive opti- cal network being coupled to a switching device.

Today, passive optical networks are deployed as access networks delivering high speed data to an end-user. A passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 32 to 128. A passive optical network consists of an optical line terminal (OLT) at the service provider' s central office and a number of optical network units (ONUs) near end-users. Downstream signals are broadcasted to each premise sharing a single fiber. Upstream signals typically are combined using a multiple access protocol, usually time division multiple access (TDMA) . The optical line terminals "range" the optical network units in order to provide timeslot assignments for upstream communication.

It is intended to use passive optical network architectures also in the field of industrial automation or enterprise communication networks. These networks typically have a large number of nodes, i.e. network devices. For example, in an enterprise network several thousands of computers or some hundred nodes in a large factory may be connected to a passive optical network. In those networks a large portion of a net- work traffic is internal, i.e. sender and receiver of messages are network devices of the passive optical network. The reason for that is that in industrial automation or enterprise communication network scenarios lots of the traffic is caused by backup systems or machine-to-machine communication. However, today' s passive optical network systems are designed to support traffic being addressed to a device of a communication network being based on the internet protocol like the internet. As a result, current architectures of passive opti- cal networks are inefficient for large scale local area networks and therefore consume much power.

In a current implementation based on existing optical network and switching technology the optical line terminal (OLT) com- municates with the "outside world", e.g. via Ethernet. Therefore, the optical line terminal is connected to an Ethernet switch that not only handles traffic being addressed to the internet but also all traffic concerning the passive optical network (so-called intranet traffic) . Typically, and Ethernet switch is designed to handle a 6 byte MAC address (MAC = Media Access Control) which has an address room of 2⁴⁸ = 2.9 x 10¹⁴. The more complex the architecture of the passive optical network is the more traffic has to be handled by the Ethernet switch.

Fig. 7 shows a current implementation of a so-called optical network terminal ONT used for existing passive optical networks and switching technology. The optical network terminal ONT comprises an Ethernet switching element ES which might be realized as a level 2 switch. The Ethernet switching element is connected to a communication network IN being based on the internet protocol (internet) . In the example shown in Fig. 7 three optical line terminals OLT are connected to the Ethernet switching element ES. Each of the optical line ter- minals OLT terminates a part of a passive optical network. The components of each part of the passive optical network are coupled to an opto-electronic converter OEC of the optical line terminal OLT. As a representation of each part of the passive optical network only a distribution switch DS, e.g. a coupler or an arrayed waveguide grating, is shown. The three passive optical sub-networks are part of a superior passive optical network. The sub-networks support multiple premises, typically 32 to 128.

According to this implementation all three sub-networks are connected to the Ethernet switching element ES which not only handles traffic to or from the internet but also all intranet traffic, e.g. communication between network units within a single sub-network or between different sub-networks of the superior passive optical network. To enable this intranet communication the Ethernet switching element ES has access to a MAC address table for being able to make a translation be- tween a MAC address of each network device and a unique identifier within the passive optical network which is assigned to each of the number of optical network units each of them being coupled to at least one network device. Since the address room of an address is large the operation of the Ethernet switching element ES is comparable inefficient and power consuming.

It is therefore an object of the present invention to provide a switching device which allows a more efficient switching of network traffic in a passive optical network. It is a further object of the present invention to provide a communication network comprising a passive optical network which is more efficient and can be used with reduced power consumption.

These objects are solved by a switching device according to claim 1 and a communication network according to claim 12. Preferred embodiments are set out in the dependent claims.

According to the invention, a switching device for terminat- ing a first passive optical network is provided wherein the first, passive optical network comprises a number of optical network units each of them being optically coupled to the switching device and each of them being coupled to at least one network device. A unique identifier within the first network is assigned to each of the number of optical network units. An Ethernet address is assigned to each of the network devices. The switching device comprises a switching element to be coupled to the number of optical network units of the passive optical network, wherein the switching element is adapted to process all network traffic within the passive optical network. The switching device further comprises an Ethernet switching element to be connected to a second commu- nication network being based on the internet protocol, wherein the internet switching element is adapted to process all other traffic being addressed to a device of the communication network.

With a switching device according to the invention intranet traffic can be handled more efficiently since the switching element to be coupled to the number of optical network units is able to process them more efficient than an Ethernet switching element due to the fact that instead of MAC ad- dresses identifiers of the passive optical network are used which allows to reduce the address room to be handled.

The switching device according to the invention is based on the consideration that in a passive optical network each op- tical network unit has a unique identifier within the passive optical network which is known by the switching device. This identifier will be used to determine whether network traffic is related to the passive optical network or to the communication network being based on the internet protocol, i.e. the internet. By using this already existing identifier an easy differentiation is possible so that the handling of the traffic may be made by that switching element which is optimized for the respective communication.

According to a preferred embodiment the switching element is adapted to access a table in which for each network device an assignment between the MAC address of a network device and the identifier of the optical network unit the related net- work device is connected to is stored. In contrast to prior art applications such an "address translation or determination" is done by the switching element which is able to do that more efficiently than the Ethernet switching element.

According to a further embodiment the switching device is adapted to determine the identifier from the preamble of an Ethernet frame which is sent from a sending network device to the switching device for forwarding it to a receiving network device. Advantageously no amendments in communication protocols have to be made. Therefore, a realization of the invention can be made with low costs.

According to a further embodiment of the invention the iden- tifier is a Logical Link Identification according to the standard IEEE 802.3ah comprising an address room of at least 2¹⁵. In the IEEE 802.3ah standard the Logical Link Identification (LLID) of 2 bytes is defined. This LLID identifies each optical network unit in a single passive optical net- work. Therefore, the switching device according to the invention has to handle a reduced address room of 2¹⁵ = 327687. It thus can be realized in a much more efficient way compared to the Ethernet switch with 6 byte MAC addresses.

According to a further embodiment the switching element is adapted to forward all traffic being addressed to a device of the communication network to the Ethernet switching element. The analysis whether a data packet received by the switching device is related to intranet or internet traffic is done by the switching element which either processes the message itself or forwards it to the Ethernet switching element to proceed the communication with the second communication network.

In an alternative embodiment one indication bit of the iden- tifier is determined by the switching device as an indication whether a frame is for processing in the first, passive optical network, i.e. intranet traffic, or processing in the second communication network, i.e. internet traffic. Preferably, the switching device further comprises at least one filter located in an upstream path between a number of the optical network units and the switching element for evaluating the indication bit. In this embodiment frames relating to inter- net traffic are directly forwarded to the Ethernet switching element. All other intranet traffic is forwarded to the switching element which will proceed with processing.

The passive optical network comprises a number of distribu- tion switches to which at least one of the optical network units is connected wherein for each of the distribution switches a filter is provided and connected to. Preferably, the at least one filter is directly connected to the Ethernet switching element.

According to a further embodiment, the switching element is connected to a further Ethernet switching element which is to be connected to an internet protocol based virtual private network being administrated by the same entity as the passive optical network. The internet protocol based virtual private network can be considered as a part of the passive optical network, i.e. a kind of sub-network. However, to make the communication between network devices of the internet protocol based virtual private network the further Ethernet switching element is necessary to process incoming and outgoing packets/frames.

It is further preferable that the Ethernet switching element and the further Ethernet switching element are connected to each other. In this case a direct communication between the second communication network and the internet protocol based virtual private network is possible. On the other hand, with the help of the further Ethernet switching element the switching element of the switching device is able to forward all traffic being addressed to a device of the communication network to the Ethernet switching element if no direct communication between the Ethernet switching element and the switching element of the switching device is realized. The invention further provides a communication network comprising a passive optical network being coupled to a switching device wherein the switching device comprises the fea- tures of a switching device as set out above.

The invention will be described more detailed by reference to the figures.

Fig. 1 shows the principle of data transmission in a passive optical network using logical link identification (LLID) .

Fig. 2 shows a known Ethernet frame structure indicating the structure of the preamble containing a Logical

Link Identification as identifier.

Fig. 3 shows a schematic view of a switching device according to the invention connected to a passive op- tical network.

Fig. 4 shows a first embodiment of a network comprising a passive optical network and a communication network being based on the internet protocol wherein a switching device according to the invention may be implemented.

Fig. 5 shows a second embodiment of a network comprising a passive optical network and a communication network being based on the internet protocol wherein a switching device according to the invention may be implemented.

Fig. 6 shows a third embodiment of a network comprising a passive optical network and a communication network being based on the internet protocol wherein a switching device according to the invention may be implemented. Fig. 7 shows an already described optical network terminal according to the prior art.

According to the invention, a switching device for terminating a passive optical network being connected to an Internet Protocol based communication network (Internet) is provided which has the functionality to process intranet and internet traffic more efficiently. This is accomplished with the help of a switching device comprising a switching element in addition to an Ethernet switching element. The switching device is adapted to process all intranet traffic through the switching element whereas all communication being addressed to devices of the internet protocol based communication net- work will be processed by the Ethernet switching element using MAC addresses. The switching element administrates all network devices of the passive optical network with the help of a unique identifier within the passive optical network being assigned to each of the number of optical network units.

The switching device according to the invention is based on the fact that in a passive optical network each optical network unit has a unique identifier which is known by the switching device. According to the invention, this identifier which is preferably a Logical Link Identification (LLID) according to standard IEEE 802.3ah will be used by the switching element to determine whether network traffic is related to the passive optical network itself (intranet traffic) or to the communication network being based on the internet pro- tocol, i.e. the internet.

If an optical line terminal (OLT) of a passive optical network broadcasts information each optical network unit ignores all packets and frames, respectively, having a different logical link identification than its assigned one. On the other hand, when an optical network unit sends a packet and frame, respectively, it adds its own logical link identification to the header for enabling data transmission within, from or to the passive optical network. This principle is shown in Fig. 1.

In this example the optical line terminal OLT broadcasts five packets or frames. In this sequence of packets or frames only a logical link identification 1, 2 or 3 is outlined to indicate to which optical network unit (ONU 1 with LLD: 1, ONU 2 with LLD: 2 or ONU N with LLD: 3) the packet/frame is determined. The optical network units ONU 1, ONU 1, ONU N are con- nected to a distribution switch DS, e.g. the coupler or an arrayed waveguide grating, to the optical line terminal OLT. The sequence of packets or frames is directed to LLIDs in the following sequence: "1, 2, 3, 2, 1". On receiving the sequence of packets the distribution switch broadcasts these packets/frames in its original sequence to each of the optical network units ONU 1, ONU 2, ONU N. When receiving the sequence of packets through each optical network unit, it is determined whether a packet or frame contains a logical link identification corresponding to the optical network unit's own logical link identification. Therefore, optical network unit ONU 1 deletes (Del) packets with LLID: =2, 3, optical network unit ONU 2 deletes (Del) packets or frames with LLID: =1, 3 and optical network unit ONU N deletes (Del) packets for frames with LLID: =1, 2.

Fig. 2 shows the structure of an Ethernet frame ETF, which in principle is known from the prior art. The Ethernet frame consists of a Preamble (8 bytes), DA (6 bytes), SA (6 bytes), Length/type (2 bytes) , OP cord (2 bytes) , Time stamp (4 bytes), Data (46 to 1500 bytes) and FCS (4 bytes) . Within the preamble 2 bytes are used for the Logical Link Identification LLID. 15 bits of the logical link identification can be used as address room. Therefore, there is an address room of 2¹⁵ = 327687. The first bit is reserved for a mode selection. The first bit of the latter (indicated as NETID) can be used as an identification bit of the Logical Link Identification to enable the switching device to determine whether a frame is for processing in the passive optical network or processing in the communication network (internet) .

Fig. 3 shows a schematic view of the switching device ONT ac- cording to the invention for terminating a passive optical network PON.

The passive optical network PON comprises a number of optical network units ONU. Each of the optical network units is opti- cally coupled to the switching device ONT. Just as an example, four optical network units are connected each to a distribution switch DS. Each distribution switch is connected to an upstream path UP and a downstream path DP being connected to a respective opto-electronic converter OEC of the switch- ing device ONT. As noted above, a unique identifier within the passive optical network, namely the Logical Link Identification LLID, is assigned to each of the number of optical network units ONU. Distribution switches DS of an upstream path and a corresponding downstream path can be realized as one component. Each of the optical network units ONU is connected to an amount of network devices PC. Each of the network devices PC has a unique MAC address.

The switching device ONT according to the invention comprises a switching element LS and a table LT connected to the switching element LS which contains for each network device PC an assignment between the MAC address of the network device PC and the Logical Link Identification (LLID) of the optical network unit ONU the related network device PC is con- nected to. The table LT therefore can be named a LLID-table and the switching element a LLID-switch. The switching device further comprises an Ethernet switching element ES which is connected to the communication network IN being based on the internet protocol (internet) . Both, the Ethernet switching element ES and the switching element LS are connected to a number of (only optional) filters LF. Each of the filter LF is connected to an opto-electronic converter OEC lying in the upstream path of the distribution switch DS. Furthermore, the switching element LS is connected to a further Ethernet switching element FES which is connected to an internet protocol based virtual private network (IN-VPN) which is administrated by the same entity as the passive optical network and therefore can be seen as a part (a kind of sub-network) of the passive optical network. The further Ethernet switching element FES has a connection to the Ethernet switching element ES.

The switching element LS has the functionality to efficiently process all intranet traffic and to forward only internet traffic to the Ethernet switch ES. With this implementation intranet traffic can be handled more efficiently since the switching element LS which processes the Logical Link Identi- fications (LLIDs) has to handle a reduced address room of 2¹⁵ = 327687 compared to the Ethernet switch with its 6 byte MAC address .

The principle of operating the switching device ONT according to the invention is as follows: It is assumed that network device PCA with IP address "192.168.1.1" and MAC-address "11- 22-33-44-55-aa" wants to transmit a package to network device PCB within the passive optical network PON. Network device PCB has an IP address "192.168.2.1" and a MAC-address "11-22- 33-44-55-bb" . A packet or frame sent from network device PCA is transmitted to the optical network unit ONU the network device PCA has coupled. The optical network unit ONU A adds its own logical link identification LLID to the header of the packet/frame. This packet is sent via the distribution switch, the opto-electrical coupler OEC, the filter LF (not needed in this example) to the switching element LS. The switching element LS determines the MAC-address and the logical link identification from the header of the packet to find out to which optical network unit ONU network device PCB is connected to. After having determined the logical link identification LLID of the optical network unit ONU B the network device PCB is connected to, the packet is forwarded in the respective downstream path DP. The distribution switch DS broadcasts the packet to all optical network units ONU which are connected to DSB. The optical network unit ONU B having the logical link identification which has been found in the header of the packet processes the packet and forwards it to the recipient network device PCB by analyzing the MAC-address of PCB from the header of the packet/frame.

As can be seen from this description the Ethernet switching element ES is not involved in processing communication relat- ing to network traffic within the passive optical network

PON. In case that the packet/frame from PCA is intended for a device of communication network IN, the switching element will forward it via FES to the Ethernet switching element for further processing.

The decision whether a packet or frame received by the switching device ONT relates to network traffic within the passive optical network or to the internet can be made by analyzing the information in the header as described above.

In an alternative, the structure of the logical link identification may be modified such that one bit, the so-called indication bit NETID, indicates whether a packet or frame is for intranet or internet traffic. The address room is thereby reduced to 2¹⁴ = 16384. In this case the optional LLID filters LF are used as shown in Fig. 2. The LLID filter LF evaluates the indication bit and forwards only internet packets or frames to the Ethernet switching element ES while intranet packets or frames are processed by the switching element LS as described above. The benefit of this configuration is that the switching element LS does not have to handle internet packets/frames any more.

In a further embodiment, not shown in the figures, the pream- ble may be extended by a further byte and thus the address room can be increased by a factor of 2⁸ = 256. The benefit of this option is an increased address room enabling larger networks. The further byte may be taken from one of the five bytes in front of the logical link identification which are used for synchronization purposes (cf. Fig. 2) .

Figures 4 to 6 illustrate three different types of networks which can provide high bandwidth to all users where the switching device according to the invention can be used advantageously.

Fig. 4 shows a so-called single wavelength network which uses splitters (coupler C) as distribution switch DS. The splitter is a passive optical equipment. The switching device ONT, a coupler and the optical network unit are connected by a full- duplex optical fiber. The switching device is used as the core switch which is connected to the internet IN. Optical network units ONU that have the functionality of converting electrical to optical signals and vice versa are attached as level 2-swiches S, which are connected to a number of network devices PC.

Fig. 5 shows a so-called multi-wavelength network without splitters. As distribution switches DS arrayed waveguide gratings (AWG) are used instead of splitters. However, wavelength division multiplexing (WDM) which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths is used between the switching device

ONT and the arrayed waveguide gratings so that each level 2- switch can receive a single wavelength. The data for the same sub-network device-network is forwarded by a level 2-switch S.

Fig. 6 shows a so-called multi-wavelength network with splitters which is called fiber to the desk (FTTD) . The FTTD can support a high bandwidth because multiple wavelengths are used. The switching device ONT is used as a core switch and arrayed waveguide gratings AWG are used as distribution switch and passive optical couplers C as access switches. As a result, this network is an all passive optical enterprise network. Between the network device and the arrayed waveguide gratings wavelength division multiplexing is used. In contrast to the embodiments shown in figures 4 and 5 the optical network units are directly coupled to a single network device PC.

Claims

1. A switching device (ONT) for terminating a first, passive optical network (PON) wherein the first, passive optical net- work (PON) comprises a number of optical network units (ONU) each of them being optically coupled to the switching device and each of them being coupled to at least one network device (PC) , wherein a unique identifier (LLID) within the first network is assigned to each of the number of optical network units (ONU) and an Ethernet address is assigned to each of the network devices (PC), comprising: a switching element (LS) to be coupled to the number of optical network units (ONU) of the passive optical network (PON) , wherein the switching element (LS) is adapted to process all network traffic within the passive optical network (PON) ; and an Ethernet switching element (ES) to be connected to a second communication network (IN) being based on the internet protocol, wherein the Ethernet switching ele- ment (ES) is adapted to process all other traffic being addressed to a device of the communication network (IN) .

2. The switching device according to claim 1, wherein the switching element (LS) is adapted to access a table (LT) in which for each network device an assignment between the MAC address of a network device (PC) and the identifier of the optical network unit (ONU) the related network device is connected to is stored.

3. The switching device according to claim 1 or 2, wherein the switching device (ONT) is adapted to determine the identifier (LLID) from the preamble of an Ethernet frame which is sent from a sending network device (PCA) to the switching device for forwarding it to a receiving network device (PCB) .

4. The switching device according to one of the preceding claims, wherein the identifier (LLID) is a Logical Link Iden- tification according to standard IEEE 802.3ah comprising an address room of at least 2¹⁵.

5. The switching device according to one of the preceding claims, wherein the switching element (LS) is adapted to forward all traffic being addressed to a device of the communication network (IN) to the Ethernet switching element (ES) .

6. The switching device according to one of the claims 1 to 4, wherein one indication bit (NETID) of the identifier

(LLID) is determined by the switching device as an indication whether a frame is for processing in the first, passive optical network (PON) or processing in the second communication network.

7. The switching device according to claim 6, wherein it further comprises at least one filter (LF) located in an upstream path (UP) between a number of the optical network units (ONU) and the switching element (LS) for evaluating the indication bit (NETID) .

8. The switching device according to claim 7, wherein the passive optical network (PON) comprises a number of distribution switches (C) to which at least one of the optical net- work units (ONU) is connected wherein for each of the distribution switches a filter (LF) is provided and connected to.

9. The switching device according to claim 7 or 8, wherein the at least one filter (LF) is directly connected to the Ethernet switching element (ES) .

10. The switching device according to one of the preceding claims, wherein the switching element (LS) is connected to a further Ethernet switching element (FES) which is to be con- nected to an Internet protocol based virtual private network (IN-VPN) being administrated by the same entity as the passive optical network.

11. The switching device according to claim 9, wherein the Ethernet switching element (ES) and the further Ethernet switching element (FES) are connected to each other.

12. Communication network comprising a passive optical network (PON) being coupled to a switching device (ONT) wherein the switching device (ONT) comprises the features of one of the preceding claims.