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):
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;
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
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;
LAN Token Ring (IEEE 802.5) and FDDI (IEEE 802.6); LAN Token Ring (IEEE 802.5) and FDDI (IEEE 802.6);
- primary ring interconnects stations
- secondary ring = backup
- Single attachment station (SAS) PCs
- Dual attachment station (DAS) Servers
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
1.4.4 Star Topology (contin.)
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.
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
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.