Definitions

• 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.
• 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.
• 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.
• 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.
• 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 .
• tne network 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) .
• NMS network management system
• 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.
• 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.
• 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.
• 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 .
• 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.
• the fault criteria may be:
• 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.
• 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.
• 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.
• one of the spare bits in time slot 0 is used as a timing source bit.
• this 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.
• 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.
• 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.
• 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.
• 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
• 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.

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• 1998
  • 1998-11-25 EP EP98960109A patent/EP1133853A1/de not_active Withdrawn

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