THE OSI MODEL AND INTERNET WORKING DEVICES.

Repeaters
• Repeat signal by regenerating, retiming and amplifying the signal
• Pass all signals between attached segments
• Not read destination addresses of data packets
• Allow for the connection of different types of media
• Extend overall LAN distance effectively by repeating signals between LAN
segments.

Bridges

Bridges are more discriminating as bridge reads the destination address of
each data frame and decides whether the destination is local or remote and only
allows packets with remote address to “cross the bridge” bridges also check the
source address of each frame and adds it to a table of known local addresses
forward-if-not-local.

Type of Bridges
• Transparent bridges (both LANs must be the same type)
• Translating bridges (multiprotocol bridges contain a format converter)
• Encapsulation bridges (bridges Ethernet & FDDI)
• Source routing bridges (source routing Token Ring to Ethernet)
• Bridge performance can be measured by the filtering rate (packets/sec) and the
forwarding rate (packets/sec)

WIDE AREA NETWORKS

Wide area networks have traditionally been considered to be those that cover
a large geographical area, require the crossing of public right-of-ways, and
rely at least in part on circuits provided by a common telephone carrier.
Typically, a WAN consists of a number of interconnected switching nodes.
Transmission from any one devices is routed through these internal nodes to the
specified destination device. These nodes (including the boundary nodes to which
the devices are connected) are not concerned with the

content of the data; rather, their purpose is to provide a switching facility
that will move the data from node to node until they reach their destination.
Traditionally, WANs have been implemented using one of two technologies; circuit
switching and packet switching. Most recently, frame relay and ATM networks have
assumed major roles. WAN technologies generally function at the lower three
layers of the OSI reference model; the physical layer, the data link layer, and
the network layer. The picture illustrates the relationship between the common
WAN technologies and the OSI model.

Point-to-point Link.

A point-to-point link provides a single, pre-established WAN communications
path from the customer premises through a carrier network, such as a telephone
company, to a remote network. Point-to-point lines are usually leased from a
carrier and thus are often called leased lines. Picture illustrates a typical
point-to-point link through a WAN.