Network Interconnection Devices; Basic Network Packet And Frame Contents - Fluke 8846A Programmer's Manual

The 10/100BaseT wiring method is very popular because the wire is inexpensive to
purchase and install, the use of a hub allows some protection from malfunctioning
network hosts, and this type of wiring is easier for network administrators to manage and
control.

Network Interconnection Devices

The following basic devices are used in networks to extend, partition, and interconnect
networks:
•
A repeater is a device used to physically isolate and extend the length of a
network segment, but it does not divide the network into subnets.
•
A hub is a device that allows communication among multiple 10/100BaseT
network hosts and allows these hosts to communicate with other Ethernet wiring.
Some hubs also perform Router functions.
•
A Bridge is used on Ethernet networks to isolate two portions of the network at
the Frame level (see "Basic Network Packet and Frame Contents" later in this
appendix). This is done to minimize transmission collisions, but the two portions
isolated by a Bridge remain on the same subnet. A Bridge also acts as a Repeater.
•
A Router is used to isolate two portions of the network into subnets, as described
in IP Addresses and Segmented Networks in this appendix. A router also acts as
a bridge. Make sure your router will route IP packets if you want to place the PC
and the Meter in different subnets.
•
A Gateway is used to connect two networks with different architecture and
protocols. For example, a Gateway could be used to connect an Ethernet to an
X.25 net running on public telephone wiring. Gateways also perform router,
bridge, and repeater functions.
The operation of the Meter and its software should be unaffected by a properly designed
network using any of these network devices. However, you must use extra care when
setting up host PCs and the Meter in a network containing multiple subnets (net
containing one or more routers and/or gateways). The IP addresses assigned to the host
PC and instruments must be carefully selected in this case. See IP Addresses and
Segmented Networks for more information on IP address assignment.
When a bridge, router, or gateway is used in a network containing the Meter and hosts, be
sure to reboot the host PC whenever an instrument is moved from one segment of the
network to another. This is required to allow the network hardware devices to initialize
bridging and routing tables.
Also, when an 8845/8846 Series instrument is used in a different subnet from the host
PC, the routers and gateways used to connect the subnets must be able to route IP packets
across the segments. The default gateway addresses on the the Meter and host PC must be
set properly as well.

Basic Network Packet and Frame Contents

Network messages consist of short (mostly less than 1 KB) chunks of data, surrounded by
header and error detection information used by the protocols. The message, together with
the protocol information, is called a "packet." When physical network information (clock
synchronization, error detection, etc.) is wrapped around the packet, it is called a "frame."
The interface hardware adds and strips off the Frame information around the packet as it
moves through a network. So, unless you're doing low level analysis of network traffic,
you never need to deal with frame information.
Digital Multimeter
Network Primer
89