EIGRP uses a vector metric (VEIGRP) to calculate the best route for packet direct transmission. There are a total of six possible parameters deciding the distance of a communication vector metric but only four are employed in the metric [152]:
The physical link bandwidth (𝐵𝐸) scaled with respect to 10 Gbps and measured
in units of kbps.
Load - A factor measure with 255 arbitrary levels
Propagation and response time delay (𝐷𝐸) in ms.
A reliability measure with 255 arbitrary levels
𝑉𝐸𝐼𝐺𝑅𝑃 =((𝐾1𝐵𝐸+ 𝐾2𝐵𝐸 256 − 𝐿𝑜𝑎𝑑+ 𝐾3𝐷𝐸) 𝐾5 𝐾4+ 𝑅𝑒𝑙𝑖𝑎𝑏𝑖𝑙𝑖𝑡𝑦) 256 Equation 4-16 This vector metric is adjusted using the free parameters K1-K5 to suit custom network routing requirements. Often in commercial networks, congestion delay and link reliability are not considered (i.e. setting K2 = 0, K1 = K3 = 1 and considering the factor involving K5 and K5 to be unity) leading to a metric based only bandwidth and total delay [152]:
𝑉𝐸𝐼𝐺𝑅𝑃 = (𝐵𝐸+ 𝐷𝐸)256
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Retransmission problems are handled separately by the transport layer. Transport and application problems can be simplified by actively managing routing and switching together to improve the time response of safety critical applications varying K1 and K3, and using K2 for congestion control. Safety critical application packets that fall outside of the transmission window are lost since there is no point congesting the network with packets that exceed the critical time window. The delay factor becomes the important topic in this work, as packet delay in the network layer affects the response time in the application layer. Time critical networking examines the total delay comprising [152]: (a) the propagation delay – the time to traverse the medium (minimum network delay); (b) responsive delay – additional latency introduced by packet retransmission; (c) buffer delay – from the internal packet load of the router buffer; (d) congestion delay – from the number of packets in the system. The last two of these are often probabilistic when the routers are encumbered with random traffic load on their physical links. Here the Erlang C formula is used to encompass both buffer and congestion delay probability in an IP network [125].
4.7 Methodology
The concepts above were tested in a simulation study to design an effective safety critical network for maintaining high quality of service (QoS) across all safety critical information airport transmissions. Here, the particular focus was on the organisation and design of a network linking air traffic radar to the control tower using the SWIM architecture [19]. Information produced from SWIM radar can contain weather reports, air traffic plans and radar information. The initial two simulations examined the traffic
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behaviour of the recovery process of three congested physical links, where traffic congestion was severe enough to consider that the link had effectively broken down; the final simulation used the NTO and its vector metric for routing decisions to avoid such problems. The prevailing regime disconnects the physical links when there is a breakdown and this includes payload congestion or transmission time out in addition to true physical disconnection. In the current paradigm, a routing decision is based on a label distribution protocol, and network management. The first two simulations demonstrate destination based and packet based routing algorithms. Destination based routing has a simplified transmission management, but a deceitful label distribution and management protocol could shut down the link prematurely; packet based routing is genuinely a better option for networks with multiple routes but has the potential problem of infinite packet service time due to mismatched or lost sequences of individual packet transmissions. The airport SWIM simulation topology was as shown in Figure 4.13.
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Figure 4.13 Simulation topology in which the control tower is connected to the radar using the multi-path connection via three routers 1-3 in the centre that handle
network switching and routing, and routers A- D that handle application information exchange
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Multiple routing path links were formed from three central routers to create connection redundancy between the control tower and the radar. Each link had different distances, link one has the longest propagation distance route (blue), link two is the shortest (red) and link three is in the middle (green). All three links operated under 10/100Mbps Ethernet since this represented the typical speed of a radar link and sufficed given the data rate of the radar equipment. The simulation had three stages. The first test was based on the EIGRP routing algorithm with the Constraint-based Routing Label Distribution Protocol (CR-LDP) [158] on a 1 Mbps transmission application with routing service update of one hundred seconds. The single 100% intensity traffic was equally divided between three links in this test with each mid-point router having a first come first served packet queue. Each packet was of size 100 bits and 10000 such packets comprised a sample. The second simulation kept the packet size and number the same but employed a packet based policy for routing with the simplified vector metric (30) to match current EIGRP practice. The total delay had a constant propagation element with variable congestion and buffer components added to conform to the Erlang C formula by substituting an arrival rate based on [156]. The third stage of the simulation implemented an NTO with a vector metric from the air traffic radar to the controller tower.