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188 TELETRAFFIC MODELING AND ANALYSIS

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IEEE Trans. Vehicular Technol., vol. 35, no. 3, 1986, pp. 77–92.

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7

Adaptive Network Layer

7.1 GRAPHS AND ROUTING PROTOCOLS

The most important function of the network layer is routing. A tool used in the design and analysis of routing protocols is graph theory. Networks can be represented by graphs where mobile nodes are vertices and communication links are edges. Routing protocols often use shortest path algorithms. In this section we provide a simple review of the most important principles in the field which provides a background to study the routing algorithms.

7.1.1 Elementary concepts

A graph G(V, E) is two sets of objects, vertices (or nodes), set V , and edges, set E. A graph is represented by dots or circles (vertices) interconnected by lines (edges). The magnitude of graph G is characterized by number of vertices |V| (called the order of G) and number of edges |E|, size G. The running times of algorithms are measured in terms of the order and size.

7.1.2 Directed graph

An edge e E of a directed graph is represented as an ordered pair (u, v), where u, v V. Here u is the initial vertex and v is the terminal vertex. Also assume here that u =v. An example with

V = {1, 2, 3, 4, 5, 6}, |V | = 6

E = {(1, 2), (2, 3), (2, 4), (4, 1), (4, 2), (4, 5), (4, 6)}, |E| = 7

is shown in Figure 7.1.

Advanced Wireless Networks: 4G Technologies Savo G. Glisic

C 2006 John Wiley & Sons, Ltd.

192 ADAPTIVE NETWORK LAYER

 

2

 

1

 

3

5

4

6

6

Figure 7.1 Directed graph.

7.1.3 Undirected graph

An edge e E of an undirected graph is represented as an unordered pair (u, v) = (v, u), where u, v V . Also assume that u =v. An example with

V = {1, 2, 3, 4, 5, 6}, |V | = 6

E = {(1, 2), (2, 3), (2, 4), (4, 1)}, (4, 5)(4, 6)|E| = 6

is shown in Figure 7.2.

7.1.4 Degree of a vertex

Degree of a vertex in an undirected graph is the number of edges incident on it. In a directed graph, the out degree of a vertex is the number of edges leaving it and the in degree is the number of edges entering it. In Figure 7.2 the degree of vertex 2 is 3. In Figure 7.1 the in degree of vertex 2 is 2 and the in degree of vertex 4 is 1.

 

2

 

1

 

3

5

4

6

 

Figure 7.2 Undirected graph.

GRAPHS AND ROUTING PROTOCOLS

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2

1.21

1

0.4

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2.1

 

 

4

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Figure 7.3 Weighted graphs.

7.1.5 Weighted graph

In a weighted graph each edge has an associated weight, usually given by a weight function w : E R. Weighted graphs from Figures 7.1 and 7.2 are shown in Figure 7.3. In the analysis of the routing problems, these weights represent the cost of using the link. Most of the time this cost would be delay that a packet would experience if using that link.

7.1.6 Walks and paths

A walk is a sequence of nodes (v1, v2, . . . , v L) such that {(v1, v2), (v2, v3), . . . , (v L 1, v L)} E, e.g. (V 2, V 3, V 6, V 5, V 3) in Figure 7.4. A simple path is a walk with no repeated nodes, e.g. (V 1, V 4, V 5, V 6, V 3). A cycle is a walk (v1, v2, . . . , v L) where

 

V2

3

V3

 

 

 

 

 

1

 

 

4

 

 

 

 

V1

3

 

1

V6

 

4

 

 

2

 

 

1

 

 

V4

V5

 

 

 

 

Figure 7.4 Illustration of a walk.

194 ADAPTIVE NETWORK LAYER

B

 

D

 

 

D

 

 

 

A

AA

 

 

CC

C

B

C

B

(a)

 

(b)

Figure 7.5 Complete graphs: (a) [V nodes and V (V 1) edges] 3 nodes and 3 × 2 edges;

(b) [V nodes and V (V 1)/2 edges] 4 nodes and 4 × 3/2 edges.

v1 = v L with no other nodes repeated and L > 3, e.g. (V 1, V 2, V 3, V 5, V 4, V 1). A graph is called cyclic if it contains a cycle; otherwise it is called acyclic. A complete graph is an undirected/directed graph in which every pair of vertices is adjacent. An example is given in Figure 7.5. If (u, v) is an edge in a graph G, we say that vertex v is adjacent to vertex u.

7.1.7 Connected graphs

An undirected graph is connected if you can get from any node to any other by following a sequence of edges or any two nodes are connected by a path, as shown in Figure 7.6. A directed graph is strongly connected if there is a directed path from any node to any other node. A graph is sparse if |E| ≈ |V |. A graph is dense if |E| ≈ |V |2.

A bipartite graph is an undirected graph G = (V, E) in which V can be partitioned into two sets, V 1 and V 2, such that (u, v) E implies either u V 1 and v V 2 or v V 1 and u V 2, see Figure 7.7.

C F

C

B D

B

E

(a)

 

 

 

AA

C

A

D

 

(b)

 

 

Figure 7.6 Connected graphs.

 

GRAPHS AND ROUTING PROTOCOLS

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V 1

V 2

 

u1

V1

u2

V2

u3

V3

u4

Figure 7.7 Bipartite graph.

7.1.8 Trees

Let G = (V, E) be an undirected graph. The following statements are equivalent:

(1)G is a tree;

(2)any two vertices in G are connected by unique simple path;

(3)G is connected, but if any edge is removed from E, the resulting graph is disconnected;

(4)G is connected, and |E| = |V | − 1;

(5)G is acyclic, and |E| = |V | − 1;

(6)G is acyclic, but if any edge is added to E, the resulting graph contains a cycle.

For an illustration see Figure 7.8.

7.1.9 Spanning tree

A tree (T ) is said to span G = (V, E) if T = (V, E ) and E E. For the graph shown Figure 7.4, two possible spanning trees are shown in Figure 7.9. Given connected graph G with real-valued edge weights ce, a minimum spanning tree (MST) is a spanning tree of G whose sum of edge weights is minimized.

196 ADAPTIVE NETWORK LAYER

 

A

 

 

B

C

G

D

E

 

H

F

Figure 7.8 Tree.

V2

V3

 

 

V1

 

V6

 

V4

V5

V2

V3

 

V1

 

V6

 

 

V4

V5

Figure 7.9 Spanning trees.

7.1.10 MST computation

7.1.10.1 Prim’s algorithm

Select an arbitrary node as the initial tree (T ). Augment T in an iterative fashion by adding the outgoing edge (u, v), (i.e. u T and v G T ) with minimum cost (i.e. weight).

GRAPHS AND ROUTING PROTOCOLS

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w(T)=100

 

 

 

10

 

 

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Figure 7.10 Minimum spanning tree.

The algorithm stops after |V | − 1 iterations. Computational complexity = O(|V |2). An illustration of the algorithm is given in Figure 7.11.

7.1.10.2 Kruskal’s algorithm

Select the edge e E of minimum weight E = {e}. Continue to add the edge e E E of minimum weight that, when added to E , does not form a cycle. Computational complexity = O(|E|xlog|E|). An illustration of the algorithm is given in Figure 7.12.

7.1.10.3 Distributed algorithms

For these algorithms each node does not need complete knowledge of the topology. The MST is created in a distributed manner. The algorithm starts with one or more fragments consisting of single nodes. Each fragment selects its minimum weight outgoing edge and, using control messaging fragments, coordinates to merge with a neighboring fragment over its minimum weight outgoing edge.The algorithm can produce an MST in O(|V |x|V |) time provided that the edge weights are unique. If these weights are not unique, the algorithm still works by using the nodes IDs to break ties between edges with equal weight.The algorithm requires O(|V |xlog|V |) + |E|) message overhead. An illustration of the distributed algorithm is given in Figure 7.13.