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): 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); SONET/SDH. - primary ring interconnects stations - secondary ring = backup - Single attachment station (SAS) PCs - Dual attachment station (DAS) Servers - Concentrators DAS1 N2 DAS3 N1 DAS2 = SAS1
1.4.3 Ring Topology (contin.) 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).
1.4.4 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 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.