Network Devices
CPSC 330 - Fall 2003

devices are typically categorized according to layer

   +-----------+
   |application| http / domain name  --  gateway (may be software on host)
   +-----------+
        ...
   +-----------+
   | transport | tcp  / port         --  gateway (may be software on host)
   +-----------+
   |  network  | IP   / IP address   --  router, switch
   +-----------+
   | data link | Eth. / MAC address  --  network interface card, bridge, switch
   +-----------+
   |  physical | bits                --  repeater, hub, switch
   +-----------+

devices

• NIC - network interface card (adapter), plugs into computer and attaches computer to the network

• repeater - works at physical level, extends bus-like LAN segment (strengthens signals for increased distance), limit to number of nodes supported

  (repeater-connected stations are in the same collision domain for Ethernet networks)

• hub - works at physical level, central connection in star topology, all connected nodes are in the same LAN segment, more sophisticated hubs can disconnect failed nodes and monitor traffic

  (hub-connected stations are in the same collision domain for Ethernet networks)

• bridge - works at data link level with physical (MAC) addresses, connects LAN segments into a single IP subnet, typically plug and play
  • connection type
    • local - one bridge physically attached to multiple LANs (http://www.netbook.cs.purdue.edu/figures/f11_6.htm)
    • remote - one bridge per LAN communicating over subnet (http://www.netbook.cs.purdue.edu/figures/f11_7.htm)

  • protocol conversion
    • transparent - similar LANs (have same data-link protocols)
    • translation - dissimilar LANs

  • often self-learning (or adaptive) with regards to traffic and provides traffic isolation (i.e., separate collision domains)
    1. observe source MAC addresses of frames as they pass across the bridge and collect entries into a table
    2. if a destination MAC address matches a table entry, use the port value in that entry to determine if the frame should be sent to another LAN or if the frame destination address is on the same LAN (i.e., the same port) as the source address (in which case the frame is not sent across the bridge)
    3. if a destination address doesn't match a table entry, broadcast the frame (i.e., flood the frame across the attached LANs)

  • if LAN segments are interconnected in such a way that a loop exists, then spanning tree decisions must be made so that some bridges will not forward traffic

• router - works at network level with IP addresses, filters and routes IP packets, uses cost metrics in a routing table to determine the best path, separates each connected LAN into separate IP subnets, typically must be configured

• gateway - works at transport level or above, converts protocols, may be a combination of hardware and software (e.g., software on general-purpose host)

• switch - generic term - different types work at data link to network levels, more recent in design than hubs or bridges, more efficient, typically has a larger number of ports (input and output lines) than available on a hub or bridge, aimed at displacing hubs and bridges (some switches even perform router-like functions), typically must be configured, ports can be arranged into logically isolated groups called VLANs

                             +----------+
                input line ->|  switch  |-> output line
                input line ->|  fabric  |-> output line
                       ... ->|          |-> ...
                             +----------+
  

  switch fabric can be a single bus, a crossbar switch, or a memory

  store-and-forward - switch buffers the complete frame before forwarding
  cut-through - transmission starts as soon as destination address has been read

functional comparison

                      hubs   bridges   routers   switches
  traffic isolation    no      yes       yes       yes
  plug and play        yes     yes       no        yes
  optimal routing      no      no        yes       no
  cut-through          yes     no        no        yes

see also

• how LAN switches work

• how routers work

• how routing algorithms work

• MAN networks showing switches, routers, etc.

[extra info, not required]

devices are also categorized according to position in network

access/gateway
  LAN switching
    Ethernet
    FDDI
  wireless
    LAN bridges
    802.11
  edge termination
    access concentrators
      DSLAM
      VLAN gateways
    telephony
      VoIP
      VoATM
    modems
      xDSL
      ISDN
      analog
      cable

edge/server
  firewalls and VPNs
    VPN switch
    NAT router
  WAN bridge
    IP over ATM
  L3+
    network/storage caching
    SSL acceleration
  WAN packet switching
    ATM
    frame relay
    IPv4
  WAN circuit switching
    xDSL
    ISDN

core
  backbone routers
    IP
    ATM
    frame relay
  optical cross connections
    SONET
    ATM over WDM

[CPSC 330 homepage]

mark@cs.clemson.edu