METHOD FOR ESTABLISHING ALTERNATIVE ROUTES IN A TELECOMMUNICATION NETWORK

Abstract: The present invention relates to a method for establishing alternative routes in a telecommunication network, especially in case of transmission failure, and more specifically in a telecommunication network comprising transmission links for duplex operation, less expensive and more reliable than similar prior art methods, it is according to the invention suggested a method which is characterized by: arranging said network in one or more initial rings including duplex operation between the nodes included in each ring; and by arranging said nodes such as to let all time slots be transmitted in one and the same direction (OK direction) of the ring in question whilst maintaining the opposite direction (ERROR direction) as a standby or redundancy path.

Description: METHOD FOR ESTABLISHING ALTERNATIVE ROUTES IN A TELECOMMUNICATION NETWORK Field of the invention The present invention relates to a method for establishing alternative routes in a telecommunication network, especially in case of transmission failure, and more spe- cifically in a telecommunication network comprising transmission links for duplex operation. More specifically the present invention relates to a method for constructing an autonomous self healing ring in a 2 MBit/s leased line telecommunication network in order to create redundancy in the network in case of transmission failure. Technical background THE PROBLEM AREA The problem area concerns building a transmission network between digital cross-connects in such a way that alternative routes are introduced in case of transmission failures. This is desirable in order to keep the network up and running when failures occur. As described below, earlier solutions have involved expensive dual lines or slow remote management involvement. In certain applications, such as cellular networks, it is important that the solution is at low cost and that the network disturbance does not last so long as to disrupt on-going traffic. KNOWN SOLUTIONS Known solutions for providing network protection m a 2 MBit/s telecommunications network include • 1+1 protection • N+1 protection • NMS re-routing • Ring topologies 1+1 protection uses duplicated lines between nodes. Traffic is sent out identically on both lines, and the receiving node chooses tne 'best' line. The entire protection operation including detection of problems and switching of input source s performed at the receiving end Switching is autonomously and fast and does not include network management system involvement. N+1 protection adds one extra line to a group of N lines. Tne stand-by line will take over for any of the other lines in case of failure. There is no traffic on the stand-by line before the failure, and switching will have to take place at both ends . In tne case of NMS re-routing, tne network is generally built with extra capacity m the lines. In case of failure, the traffic from the failed line is re-routed on other lines. This operation is conducted by a network management system (NMS) . There are ring protection solutions that use the fact that any node m a ring may be reached m two ways . They mainly fall into two groups: • Nodes m the ring keep an updated database of all nodes m the ring with information on which time slots belong to these nodes. When a fault occurs, the ring is split and time slots are selectively transmitted on the right or left branch according to this information. • When a fault occurs, information is sent to a central management system that decides on the re-configuration of the nodes in the ring. PROBLEMS WITH KNOWN SOLUTIONS The problem with the 1+1 protection has been that this is a very costly alternative as every line segment must be duplicated. The N+1 solution is a limited solution since it can only protect a number of line segments between the same nodes. The NMS re-routing solution is generally too slow for application requiring connections to stay up. For instance, the GSM traffic in cellular networks will go down if the protection is based on NMS re-routing. The problem with existing ring solutions is that they generally are more complex and thus more expensive and subject to errors than the suggested ring solution. Also, in the case of needing to use a central management system for restoration, switching is not fast enough to be used in for instance cellular networks. Objects of the invention An object of the present invention is to provide a method for establishing alternative routes in a telecommunication network which generally is not hampered with the problems enfaced with known solutions. Another object of the present invention is to provide a method which is less complex and less expensive as well as less subject to errors than previously suggested ring solutions . Still another object of the present invention is to pro- vide a method wherein the use of a central management system for restoration is reduced to a minimum. Yet another object of the present invention is to provide a method which is autonomously self healing, and wherein the healing is accomplished in a minimum of time. Another object of the present invention is to provide a method wherein the self healing means are included in the nodes of the network. Brief summary of the invention These objects are achieved in a method as stated in the preamble, which according to the present invention is characterized by - arranging said network in one or more initial rings including duplex operation between the nodes included in each ring, and by - arranging said nodes such as to let all time slots be transmitted in one and the same direction (OK direction) of the ring in question whilst maintaining the opposite direction (ERROR direction) as a standby or redundancy path. Further objects and advantages of the present invention will appear from the following description taken in conjunction with the enclosed drawings, as well as from the enclosed patent claims. Brief disclosure of the drawings Fig. 1 is a simplified sketch illustrating an initial protected ring wherein the present invention has been lm- plemented. Fig. 2 is a simplified sketch illustrating details m one of the nodes which are tied into the ring by dropping and inserting time slots. Fig. 3 is a simplified sketch, similar to Fig. 1, illustrating the ring after switching to redundancy mode. Fig. 4 is a simplified sketch illustrating a ring con- figuration before, during and after a transmission fault occurs . Detailed description of embodiments It is to be understood that the present invention has been developed m connection with 2 Mb/s transmission links which are always duplex, but it is to be understood that the general principle of the present invention can be applied m any telecommunication network comprising transmission links for duplex operation. In Fig. 1 there is illustrated an example of a ring including 4 nodes, designated A, B, C, D, respectively. Such a ring may be included as one of more rings m a network and between each node m the ring m question there is established a duplex operation. In the following it is assumed that the example of the ring illustrated m Fig. 1 is established for a 2 Mb/s transmission link. 2 MBit/s transmission links are always duplex. There is symmetrical transmission in both directions. This symmetrical nature of the 2 Mbit/s network makes it possible to set up a ring where all time slots are transmitted in one direction while the other direction is not used. The empty direction may then be used as a stand-by path. The initial ring is shown in Fig. 1. The normal direction is called the OK direction. The spare direction is called the ERROR direction. By using bits in time slot 0, it is possible to monitor each 2 Mbit/s section in the ring for both directions. A transmission fault on a section can thus be detected at both ends and re-routing of the time slots to the spare capacity may be done in both affected nodes. The fault criteria may be: LIS - Loss of Incoming Signal LFA - Loss of Frame Alignment RAI - Remote Alarm Indication For a cross-connect node, the timing information is usually taken from one of the incoming line signals. In the protected ring, timing is propagated along with the normal traffic direction. When the traffic switches, the timing will also have to switch. Other nodes are tied into the ring by dropping and in- serting time slots as shown in Fig. 2. Most of the time slots entering the node in the OK direction is fed right through the node. Time slots destined for the connected equipment are dropped off to the port where this equipment is connected. Time slots from the connected equip- ment is inserted into the OK direction thus keeping the one-way traffic in the ring. If the ring should be treated as a sub-network connecting to a central node such as the BSC in cellular networks, all time slots would be dropped towards this node. When a fault occurs on a section in the ring the two nodes in both ends of that section will be triggered by the switching criteria. The two nodes will then reconfigure so that traffic that normally would be sent out on the faulted section will be routed back onto the ERROR direction. Traffic that normally would be taken from the faulted section will instead be taken from the ERROR direction. This is shown in Fig. 3 where the fault occurred between nodes C and D. If, in Fig. 3, node D received timing information from node C, its timing source would have to change. This is trivial for node D because this node is next to the faulty section and is able to detect the switching criteria. However, had there been more nodes between nodes D and A, for example nodes DX, DY and DZ , these would also have to change timing sources . These nodes do not have access to the same fault information as have nodes D and C. To make these nodes aware of the need to switch timing source, one of the spare bits in time slot 0 is used as a timing source bit. On ports used as timing source, this bit is sent in the high state. On all other ports the bit is sent in the low state. It is not legal to use a port with the incoming timing source bit in the high state as a timing source. As node D starts using the other (non faulty) port as timing source, the timing source bit will force the node at the other end to change timing source if timing previously was taken from node D. This will ripple through all possible nodes between nodes D and A until the complete rightmost branch takes timing from node A. Clock stability requirements secure that the timing change information is transported before bit- slip will occur. In other words when the node D immediately following the faulty section starts using the other non- faulty/redundancy channel port as timing source, then the timing source bit will force the next node DX in the redundancy ERROR direction to change its timing source if timing was previously taken from said next to fault node D, which forcing will take place also in any still further nodes DY, DZ arranged between said next to fault node D and said node A being connected to a central node CN. In Fig. 4 there is illustrated another ring configuration between four other nodes R, S, T and U, wherein a transmission fault has occurred between the nodes S and T. Fig. 4 further illustrates how ports used as timing source will send one of the spear bits in time slot 0 in the high state, whereas on all other ports this bit is sent in the low state. Further, Fig. 4 illustrates that when node S, due to a transmission fault, will have to use the redundancy or error path, it will establish its output port as a timing source, i.e. especially for the closest node R, which further communicates the redundancy path to node U and node T, in which latter node T the incoming of time slots on the redundancy port thereof will set the associated timing source bit to high state. This high state will be transferred from node T to node U, and further to node R, but then via the OK path, until the correct timing configuration is established. Advantages There are several advantages to this invention. They may be summarised as follows: Usually redundancy creates largely increased costs due to duplication of transmission capacity. The suggested redundant ring only needs one extra section and the inherent spare capacity in duplex lines to protect a number of other sections and is thus less costly. The switching of time slots and timing sources are completely autonomous and does not require any interaction with a remote management system. Switching is thus rapid and restoration of connections is done with minimum interruption of traffic. Because only the two nodes in either end of the faulty section is actively involved in the switching, there is no need for additional complexity in order to synchronise other nodes in the ring with configuration information. The suggested solution is thus less complex, less expensive and more reliable.

Claims: P a t e n t c l a i m s 1. Method for establishing alternative routes in a telecommunication network, especially in case of trans- mission failure, and more specifically in a telecommunication network comprising 2 Mbit/s transmission links and nodes with G.704 framing for duplex operation, wherein said network is arranged in one or more initial rings including duplex operation between the nodes included in each ring, c h a r a c t e r i z e d by - arranging said nodes, due to the symmetrical nature of the 2 Mbit/s network, such as to let all time slots be transmitted in one and the same direction (OK direction) of the ring in question whilst maintaining the opposite direction (ERROR direction) as a standby or redundancy path until alerted by a specific time slot reflecting the occurrence of a fault. 2. Method as claimed in claim 1, c h a r a c t e r i z e d by providing a time slot, preferably time slot 0, with alertable means which are adapted to monitor each section in the ring in question for both directions, said alerting means being defined by the bit configuration of said time slot. 3. Method as claimed in claim 1 or 2 , c h a r a c t e r i z e d i n that each node is provided with detection means adapted for detecting a sec- tion transmission fault both at the incoming or at the outgoing ports thereof, i.e. allowing detection of a transmission fault on a section at both ends of said section . 4. Method as claimed in any of the claims 1-3, c h a r a c t e r i z e d by letting inter alia the following fault criteria determine the re-routing of the time slots: LIS - Loss of Incoming Signal LFA - Loss of Frame Alignment RAI - Remote Alarm Indication 5. Method as claimed in any of the preceding claims , c h a r a c t e r i z e d i n that during normal and initial ring operation the timing information will preferably be taken from one of the incoming signals, i.e. signals being propagated along the normal (OK) direction. 6. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that when a fault occurs on a section in the ring the two nodes at the end of that faulty section will be triggered by one or more appropriate switching criteria, for thereby reconfiguring the two nodes in question for routing the traffic which should have been sent via the faulty section to its des- tination node or nodes via the duplex channel in the opposite (ERROR) direction. 7. Method as claimed in any of the preceding claims , c h a r a c t e r i z e d i n that when a fault occurs in a section between two nodes then the node immediately following the faulty section in the previous normal (OK) direction will have its timing source changed upon detection of the appropriate fault occurrence switching cri- teria, and that any further node or nodes being arranged between said immediately following node and a node (A) being connected to a central node (CN) will also have its or their timing sources changed. 8. Method as claimed in claim 7, c h a r a c t e r i z e d i n that one of the spare bits in time slot 0 is used as a timing source bit and that on ports being used as a timing source this bit will be given a first state, preferably the high state, whereas all other ports will have the same spare bit sent in the opposite state, i.e. preferably the low state. 9. Method as claimed in claim 8, c h a r a c t e r i z e d i n that when the node (D) immediately following the faulty section starts using the other non-faulty/redundancy channel port as timing source, then the timing source bit will force the next node (DX) in the redundancy (ERROR) direction to change its timing source if timing was previously taken from said next to fault node (D) , which forcing will take place also in any still further nodes (DY, DZ) arranged between said next to fault node (D) and said node (A) be- ing connected to a central node (CN) . 10. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that the transport of the timing change information sent or rippled through nodes involved in a redundancy (ERROR) path, i.e. receiving signals thereon but transmitting signals on the remaining normal (OK) path, is adapted in relation to clock stability requirement so as to avoid the occurrence of bit slip. 11. Method as claimed in any of the preceding claims , c h a r a c t e r i z e d i n that the method is ap- plied in an autonomous manner, without any interaction with a remote management system (RMS) . 12. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that the method is applied as an autonomous self healing ring, especially in a 2 Mb/s leased line telecommunication network so as to create redundancy pathways therein in case of transmission failure.

Inventors: MOEN MORTEN, SCHUMANN-OLSEN REIDAR, MOEN, MORTEN, SCHUMANN-OLSEN, REIDAR
Applicants: ERICSSON TELEFON AB L M, MOEN MORTEN, SCHUMANN OLSEN REIDAR, MOEN, MORTEN, SCHUMANN-OLSEN, REIDAR, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1998
Main classification: H04L12
IPC codes: H04J3, H04L12, H04Q11