1.4 Network Topologies

� A network topology = physical arrangement of the various network elements and

their interactions (links, nodes, etc.); (topology = graph).

�A network topology (redundancy) influences its performances.

� Types of network topologies (as graphs, topologies differ only in the way nodes

and arcs are arranged):and arcs are arranged):

� point-to-point;

� bus;

� ring;

� star;

� mesh;

� mixed.

� Nodes in a network topology may be interconnecting devices or terminals, while

arcs may be physical connections (direct or indirect) or logical.

1.4.1 Point-to-point Topology (P2P)

� the simplest topology, with the minimum redundancy (redundancy = 0, i.e., there

is no spare/backup link) → reduced reliability;

�Advantage: simple management;

� use:

� in “long-haul” networks (when redundancy is expensive);

� in “networks” with 2 terminals, over small distances (where the redundancy is useless).

1.4.2 Bus Topology

� A linear topology = a multipoint communications link (in LAN – Ethernet, rare

use nowadays).

�Advantages:• network homogeny;

• low cost media and couplings;

• no medium access control (MAC - LAN) – implemented in nodes.

� Disadvantages:• MAC problems = collisions, if random access is used;

• a cable interruption of a segment → 2 unusable segments (redundancy = 0).

1.4.3 Ring Topology

� All nodes are connected in a succesive manner (in pairs), a loop closes with last

node connecting the first node;

� Each node relays the message received on one port to the other port, copying it if

is the destination;

� The message can be eliminated from loop (ring) by the source node or by another

node (= ring monitor);

� a MAC mechanism can be used, based on a token, i.e., a transmission permit; such� a MAC mechanism can be used, based on a token, i.e., a transmission permit; such

a network is called Token – ring;

1.4.3 Ring Topology (contin.)

� A short-circuit mechanism is introduced at each node, in order to avoid the ring

interruption in case of a node failure;

� A double ring solution (one ring for each transmission way; ex.: FDDI) →

advantage: close the loop to form a new ring, from 2 broken rings;

� Use:

• LAN – Token Ring (IEEE 802.5) and FDDI (IEEE 802.6);• LAN – Token Ring (IEEE 802.5) and FDDI (IEEE 802.6);

• SONET/SDH.

N1

N2

- primary ring – interconnects stations

- secondary ring = backup

- Single attachment station (SAS) – PCs

- Dual attachment station (DAS) – Servers

- Concentrators

DAS1

DAS2

= SAS1DAS3

�Advantages :• no need for central management.

� Disadvantages :• redundancy = 0 (for a single ring) or redundancy = 1 (double ring);

• sensitive to interruptions (short-circuits) failures;

• network capacity increase (new stations) → high propagation delays (larger ring).

1.4.3 Ring Topology (contin.)

1.4.4 Star Topology

� A central node (CN) is used to connect each terminal node; in fact, each node is

physically connected to CN by a point-to-point link;

� The information transfer:

� point-to-point (P2P), when CN = switch (layer 2);

� point-to-multipoint (PTMP), when CN = HUB – Host-Unit Broadcast (layer 1).

� Used in LAN (IEEE 802.3): 10BASE-T Ethernet, Fast Ehernet, and Gigabit� Used in LAN (IEEE 802.3): 10BASE-T Ethernet, Fast Ehernet, and Gigabit

Ehernet.

1.4.4 Star Topology (contin.)

�Advantages :

• almost all software is concentrated in CN; terminal nodes need a simple SW;

• easy net extension (if there are enough spare ports in CN);

• facile installation and management;

• a link / equipment failure eliminates only one node, the rest are operational;

• cheap and fast switches → development;

• increased redundancy for extended stars.

� Disadvantages :

• a bottleneck (congestion) due to intense traffic → CN failure;

• the net reliability depends mostly on CN;

• net capacity is limited by the number of the available ports in CN .

1.4.5 Mesh Topology

� (full-)mesh topology = most complex topology, having the maximum redundancy

(i.e., at least two links are available between any pair of nodes in the network (one

direct link and one indirect link);

� Use: “core” networks (ex. WDM).

�Advantages:• a larke number of links (redundancy) → high reliability; for N nodes → • a larke number of links (redundancy) → high reliability; for N nodes →

N (N-1) / 2 links

� Disadvantage : • high costs (links/cables and devices).

1.4.6 Mixed (Hybrid) Topologies

� a network extension of elementary topologies (any of the above);

� A mixed net is composed by a core (backbone) net, which interconnects other

networks.

� Examples of mixed topologies:

� the tree topology combines the P2P and the star topologies;

� a full mesh core + stars for access networks.� a full mesh core + stars for access networks.