Abstract:
A telecommunications network comprising first and second access equipment on one side of the network and third and fourth access equipment on another side of the network, a respective communication link is provided between the first and the third access equipment and between the second and fourth access equipment, and further, a communication link is provided between the first and the second access equipment, and between the third and fourth access equipment, each access equipment providing an interface for communication with the network, and each access equipment is configured to be capable of causing a change in the path followed by at least some of the traffic conveyed over the communication links.

Description:
This application is the U.S. national phase of International Application No. PCT/EP2008/061922 filed 9 Sep. 2008 which designated the U.S. and claims priority to GB Patent Application No. 0718946.7 filed 28 Sep. 2007 and EP Patent Application No. 07120565 filed 13 Nov. 2007, the entire contents of each of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates generally to telecommunication networks. 
     BACKGROUND 
     Known transport networks use circuit oriented technologies (eg Synchronous Digital Hierarchy (SDH) and more recently wavelength switching) and include sophisticated restoration schemes, including double path protections (eg Sub-Network Connection Protection (SNCP)), bandwidth sharing restorations (eg supported by control planes Automatically Switched Transport Network (ASTN)/Generalised Multi-Protocol Label Switching (GMPLS)) and even dual ended restorations (eg International patent application WO 2005/081435 A1). 
     A major application of transport networks is to provide support to a packet network interconnection (eg backbone Internet Protocol (IP) routers). In this application, it is known to engineer the IP network already with its own resilience schemes. These generally account for the totality of reliability of IP backbone networks, and in transport networks unprotected connections are used. 
     SUMMARY 
     According to a first aspect of the technology described in this application there is provided a telecommunications network comprising first and second access equipment on one side of the network and third and fourth access equipment on another side of the network. A respective communication link is provided between the first and the third access equipment and between the second and fourth access equipment. Further, a communication link is provided between the first and the second access equipment, and between the third and fourth access equipment. Each access equipment provides an interface for communication with the network, and each access equipment is configured to be capable of causing a change in the path followed by at least some of the traffic over the communication links. 
     In one non-limiting, example embodiment, a restoration scheme is implemented in the transport network with integrated packet switching functions which allows an improved integration with a client packet network and in particular an IP backbone interconnection. This advantageously results in there being no single point of failure. Also, since emulation of independent connections is realised, this advantageously avoids changes having to be made on the IP routers of the client packet network. 
     According to another aspect of the technology described in this application, there is provided a method of controlling traffic in the telecommunications network of the first aspect. The method comprises at least one of the access equipments altering the path followed by at least some of the traffic over the links. 
     According to another aspect of the technology described in this application, there is provided network access equipment for providing an interface to a telecommunications network, the equipment comprising three interface ports and further comprising a switch arrangement The switch arrangement is configured, in use, to cause traffic input at a first interface port to be output from a third interface port instead of being output at a second interface port. 
     The network access equipment may comprise a processor, for example an interface card, to control the switch arrangement in accordance with instructions stored in a memory. 
     In one non-limiting, example embodiment, restoration techniques on a circuit oriented transport network are combined with packet flow forwarding and multiplexing on the access devices interfacing a client packet network. The restoration mechanisms may be supported either by an automatic control plane or by a combination of embedded signalling and management functions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a network; 
         FIG. 2  is a block diagram of a node of the network of  FIG. 1  in a first condition; 
         FIG. 3  is a block diagram of the node of  FIG. 2  in a second condition; and 
         FIGS. 4 ,  5 ,  6  and  7  are further schematic representations of the network of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference initially to  FIG. 1  there is shown a telecommunications network  1  in the form of a transport network and comprising first and second access devices A 1  and A 2  on one side of the network and third and fourth access devices B 1  and B 2  on the other side of the network. Each network device comprises an interface card. The access devices A 1  and B 1 , and A 2  and B 2 , are connected by respective (long-distance) communication connections  7  and  8 . Furthermore, access devices A 1  and A 2 , and B 1  and B 2 , are connected by a respective communication connection  9  and  10 . Routers  2 ,  3 ,  4  and  5  are connected to the access devices A 1 , A 2 , B 1  and B 2  respectively. The routers form part of respective IP backbone networks. Each access device provides an interface to the network  1  for the respective router to allow traffic to be sent and/or received to/from another router on the other side of the network  1 . 
     The connection  8  between interconnected routers is the restoration connection (represented by a dashed line), so that it can be implemented only during a fault condition and use shared resources. 
       FIG. 3  shows the packet forwarding capabilities of the access device A 1 . However, each of the access devices A 2 , B 1  and B 2  has identical capabilities. Each access device comprises three interface ports: C towards the respective client router, R towards the transport connection to the core router and D towards the transport connection to its (corresponding) access device on that side of the network  1 . 
     If interface R is disabled (either performing a restoration role in normal conditions or when the connection is faulty), the packet forwarding functions are between C and D, as shown in  FIG. 2 , in a DET or detour mode. 
     If interface R is enabled (working connection in normal conditions or activated restoration during a fault), packets are forwarded between C and R and between D and R, as in  FIG. 3  in a MUX or multiplexing mode. Packets sent from C to R and D to R must be tagged to distinguish between C and D. Packets sent from R to C and D must be inspected and have the tag removed which was added by the (remote) access device, to decide if the packet must be forwarded to C or D. 
     Each access device A 1 , A 2 , B 1  and B 2  each comprises a monitoring arrangement (for example circuit alarm detection or Operations, Administration and Management (OAM)) to monitor the alarm status of the transport connections  7 ,  8 ,  9  and  10  that originate from them. 
     Counterpart access devices (eg A 1  and A 2 ) must communicate (eg on the overhead information of the transport connection between them) the alarm status of the long (R) distance connection (eg A 1  must communicate to A 2  the status of the connection  7  from A 1  to B 1 ). The connections  9  and  10  between each two counterpart access devices are used also to monitor the status of the access device itself. 
     Status monitoring using the connections,  7 ,  8 ,  9  and  10  can be achieved in various ways. One way comprises each device periodically, or on demand, issuing a status signal. Should such a signal not be received or be indicative of a fault having been detected, then the appropriate re-routing can be effected. 
     If one of the following conditions is met, the restoration connection  8  must be activated:
     1. the (working) connection  7  is faulty,   2. the connection  9  or  10  between (corresponding) access devices is faulty, or   3. the access device originating the working transport connection  7  is faulty.   

     The restoration transport connection  8  can be activated according to any suitable scheme or protocol. Some examples are:
         pre-planned restoration—the restoration path and the resources it uses (during the fault) are defined (and stored in each access device) at the time of connection definition; resources can only be shared with restoration paths whose working path does not share common points of failure;   on-the-fly restoration with a distributed control plane— the restoration path is computed by the access device that originates the path after fault detection; the access device must maintain an updated database of the network topology and operational status of the other access devices and communication links; and   optical restoration with shared wavelengths—a pre-planned scheme can be applied to an all-optical network; the shared resources are represented by the wavelengths used by the restoration light paths.       

     Alternatively the access devices may be arranged to communicate operational status information with (separate) control equipment, and the control equipment is operative to control the access devices accordingly by way of control signals. 
     Various examples are now provided of how the above described switching functionality of the access devices A 1 , A 2 , B 1  and B 2  is used to advantageously re-route traffic in the event of a fault or a congestion condition. 
     Reference is now made to  FIG. 4  in which thickened lines show the traffic flow under normal conditions. Traffic of both router paths is multiplexed on the working connection  7  by the access devices A 1  and Bl. The multiplexing is feasible because each router pair will transport half of the traffic. If one of the routers fails, its traffic will drop to zero, while the operational router pair will carry the total traffic, the sum of traffic between A 1  and B 1  remaining constant. 
       FIG. 5  shows how the traffic flow is re-distributed when a fault  20  affects the working connection  7  between A 1  and Bl. Switching within the access devices causes traffic to follow a path comprising connections  8 ,  9  and  10 . 
       FIG. 6  shows the traffic flow when the connection  9  between the two access devices A 1  and B 1  is broken at  22 . Both working and restoration connections  7  and  8  are simultaneously active carrying traffic. In the situation of there being no fault on any of the routers, each router carries half of the total traffic. 
       FIG. 7  shows the traffic flow in the case of a fault  21  on access device A 1 . If the fault is on the access device originating the working connection  7 , the restoration connection  8  is activated. If however the fault is on an access device originating the restoration connection  8 , no restoration is activated and the restoration bandwidth is free for other restorations on the network. The routers  3  and  5  adjust their traffic according with the changed topology and the connected pair will carry the total traffic. 
     Although it is assumed that the sum of traffic sent by the two routers A 1  and B 1  never exceeds the capacity of a single interface towards the transport network  1 , it is good practice to define within each router a response for the anomalous condition where this condition is not satisfied. 
     The congestion condition occurs when the access device in MUX mode detects congestion (eg queue overflow or queue crossing a certain threshold). This condition can be communicated to the counterpart access device, that is supposed to be in DET mode in which the R connection is not activated. 
     The determination of the congestion condition from the access device in MUX mode to the access device in DET mode can trigger the activation of the restoration connection  8 . In this way, extra transport capacity is allocated and the congestion condition is removed. Both access devices (eg A 1  and A 2 ) are then in MUX mode. 
     The above described embodiments result in numerous advantages over known router interconnection arrangements. These include increasing savings in the restoration resources which is achieved by shared restoration arrangements and also enhancing survivability, ie resilience to combinations of multiple faults. 
     It is also to be noted that all functionality of a conventional router interconnection scheme is preserved in the illustrated embodiment.