Network-Extending Devices
Repeaters
operate only at physical layer -- knows signals but not meaning
regenerates signal (boost it due to attenuation)
used to extend LAN length
both attached networks must be of same type (e.g. ethernet)
Bridges
operate at physical plus data link layer
physical layer can differ on each side
MAC sublayer can differ (e.g. bridge ethernet to token ring) **
** some authors classify such devices as routers
LLC sublayer is same on each side
can be used to segment a network
can interpret frame contents (esp. addresses)
smart enough to decide whether or not to forward an incoming frame to adjacent segment (knows whether forwarding will get it to destination)
Frame Forwarding
- requires lookup table of physical addresses. How are table contents specified?
- simple bridge: addresses must be added by administrator
- learning (adaptive) bridge: builds table dyanmically (backward learning)
Assume use of learning bridge (a.k.a. Transparent Bridge)
- Backward learning:
learn the network topology by inspecting the source addresses of incoming frames.
- backward learning example: You are hiking a trail from Springfield to Republic. In a remote area along the way, you come to a unmarked fork in the trail. How can you make the right decision? You wait for another hiker to approach from one of the forks, and ask them where they came from!
- Bridge action upon frame arrival, summarized:
- If source and destination on same LAN, discard frame (keep from escaping)
- If destination in table, forward the frame.
- If destination not in table, flood.
- If source not in table, add it and its segment to table (backward learning)
- What if LAN segmented by bridges contains loop?
- Flooding of frame whose destination is not tabulated:
- infinite looping of frames
- receive duplicate frames
- Ethernet response is spanning tree
- Distributed algorithm for biulding logical tree structure over LAN segment graph (segments as nodes, bridges as edges)
- There is unique path from each segment to every other.
- ADV: no loops
- DIS: lots of traffic through root, some routes unused
- Algorithm conducted via special control frames
- can dynamically reconfigure the tree
- Token ring response is Source Routing Bridge
- Sender knows whether destination is on same LAN.
- If so, high-order bit of source address in frame header is 0.
- If on different LAN, set to 1.
- (This is unique flag: station address cannot have 1 in high bit; that bit in destination address field is reserved for indicating group or broadcast).
- If destination is different LAN, sender is responsible for specifying which route the frame is to follow!
- appended to the frame header, effectively extending it.
- Route is of the form: L1 - B1 - L2 - B2 - ... Bn-1 - Ln alternating LANs and bridges from source to destination.
- When bridge receives frame, checks high bit of source address.
- If 0, do not forward.
- If 1, try to match route LAN Li with LAN that the frame came in on.
- If Bi matches its own i.d., the bridge will forward frame to LAN Li+1, otherwise it will not forward it.
Routers
operate at physical plus data link plus network layers
are internet devices (small i is generic term)
router is special station on two or more networks
networks may be dissimilar
if dissimilar networks, router controlled by "gateway" level software (see below)
may have "global knowledge"; e.g. if not attached to destination network, can forward packet to another router which is "closer" to destination.
Gateways
operate at potentially all OSI layers.
connects dissimilar networks; each sees it as "their type"
capable of translating frames/packets between formats and operating in each protocol
Related Home Pages:
notes | CSC 465 | Peter Sanderson | Computer Science | SMSU
Last reviewed: 1 April 1998
Peter Sanderson ( pete@csc.smsu.edu )