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Railway Communication Network Validation

 

Objective

To simulate the communication network of a metro railway line and develop a detailed traffic model to meet the system design of all of the sub-systems to make an assessment of:

  1. Traffic patterns and the required minimum circuit capacity
  2. Resulting bandwidth to satisfy the communication needs of each site covering the requirements for each sub-system
  3. Minimum 25% spare capacity, available at each site, for future expansion

Traffic load

  • Worst case scenario based on a MPEG model (used for movies)
    • Modeled for simultaneous video usage by all passengers
  • Simultaneous audio via an audio codec model

Use Case


The Rail Communication Network Created in NetSim

The overall railway communication network is composed of four rings, which was simulated and analyzed.

Traffic models were created for:

  • CCTV
  • Radio Communication
  • PA
  • Telephone
  • EPAX
  • Fare Collection
  • Supervisory Control

All traffic was modeled was unicast traffic flowing from Control Center to STATIONS per the rates provided.

Assumptions


Traffic flow in the Network

The following assumptions have been made for simulation purposes:

  1. The link error rate assumed is Bit Error Rate (BER) of 10 -6 which is higher than the typical rate of 10 -9 thereby giving us a more conservative estimate of throughput
  2. The propagation delay in the links are 5 micro seconds Packet size at the application layer is 1460 and the addition of overheads layer wise with a total packet size of 1514 bytes.

Recommendations

NetSim simulation was run during about 2 million packets and 22 million events were processed.


Increase link capacity

The result of the performed simulation showed that the worst case scenario caused traffic loaded to cross 1 Gbps and hence the original link dimensioning had to increase to factor this in.


Use multicast where possible

These results were based on simulation of unicast traffic. If the path between server and client traverses h3 router hops and h2 switch hops, the "unicast" video would consume 1.5 x n x h3 Mbps of router bandwidth, plus 1.5 x n x h2 Mbps of switch bandwidth, where n is the number of unicast clients. However in a multi cast environment, a single video stream is replicated as required by the network's multi cast routers and switches to allow an arbitrary number of clients to subscribe to the multi cast address and receive the broadcast. Within the network, the multi cast transmission would consume only 1/nth of the bandwidth of the unicast solution.