**2. Routing functions**

330 Telecommunications Networks – Current Status and Future Trends

described techniques, procedures and algorithms, even if explained on an example of ISL network, can be generalised and used also in other types of networks exhibiting high level of dynamics (Liu et al., 2011; Long et al., 2010; Rao & Wang, 2010, 2011). The modular approach allows easy (re)usage of presented procedures and techniques, thus, only

ISL network exhibits several useful properties which support the development of routing

• Predictability – motion of satellites around the earth is deterministic, thus the position of satellites and their connectivity can be computed in advance, taking into account the parameters of the satellite orbit and constellation. Consequently, in an ISL network only undeterministic parameters need to be monitored and distributed through the network,

• Periodicity – satellite positions and thus the configuration of the space segment, repeats with the orbit period, which is defined uniquely by the selected orbit altitude. Taking into account also the terrestrial segment, an ISL network will experience a quasi-periodic behaviour on a larger scale, defined as the smallest common integer multiple of the orbit period and the traffic intensity period, referred to as the system

• Regularity – a LEO constellation with an ISL network is characterized by a regular mesh topology, enabling routing procedures to be considered independently of the actual serving satellite (i.e. concealing the motion of satellites with respect to the earth from the routing procedure). Furthermore, the high level of node connectivity (typically between 2 and 6 links to the neighbouring nodes) provides several alternative paths

• Constant number of network nodes – routing procedures in ISL networks are based typically on the explicit knowledge of the network topology which, in the case of satellite constellation, has a constant, predefined number of network nodes in the space (satellites) and terrestrial (gateways) segments (except in the case of a node or a link

failure). This property has a direct influence on the calculation of routing tables.

The above properties are incorporated in the described routing and traffic modelling techniques and procedures. Special attention is given to properties which support the development of efficient, yet not excessively complex, adaptive routing and traffic

However, for the verification, validation and performance evaluation of algorithms, protocols, or whole telecommunication systems, the development of suitable traffic models, which serve as a vital input parameter in any simulation model, is of paramount importance. Thus, at the end of the chapter we are presenting the methodology for modelling global aggregate traffic comprising of four main modules. It can be used as a whole or only selected modules can be used for particular purposes connected with

Routing and traffic engineering on one side require good knowledge of the type of network and its characteristics and on the other side also of the type of traffic in the network. This is needed not only for adapting particular techniques, procedures and algorithms to the

particular or entire procedures can be used.

thus minimizing the signalling load.

between a given pair of satellites.

period.

engineering techniques.

simulation of particular models.

procedures. These properties include (Wood et al., 2001):

The main task of any routing is to find suitable paths for user traffic from the source node to destination node in accordance with the traffic's service requirements and the network's service restrictions. Paths should accommodate all different types of services using different optimisation metrics (e.g. delay, bandwidth, etc.). Thus, different types of traffic can be routed over different routes. Routing functionality can be in general split in four core routing functions, (i) acquiring information about the network and user traffic state, and link cost calculation, (ii) distributing the acquired information, (iii) computing routes according to the traffic state information and chosen optimization criteria, and (iv) forwarding the user traffic along the routes to the destination node.

For each of these functions, several policies exist. Generally speaking, the selection of a given policy will impact (i) the performance of the routing protocol and (ii) the cost of running the protocol. These two aspects are dual and a careful design in the routing algorithm must achieve a suitable balance between the two. The following sub-sections will discuss the four core routing functions.
