1.4 Network Topologies - curs/Prez/En/ARI2014-03-141.4 Network Topologies A network topology = physical arrangement of the various network elements and ... bus; ring; star; mesh; ... the tree topology combines the P2P and the star topologies;

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  • 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.

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