Biomedical Engineering Reference
In-Depth Information
network to another. Switches, which lack the intelligence of a router, are normally used in the
network backbone and at gateways, where speed is of the essence. Also called LAN switches, data
switches, and packet switches, they typically contain buffer memory to hold packets briefly until
network resources become available.
Gateways
A gateway links two networks running different protocols by functioning as a router and providing
translation and amplification of network signals. Because gateways can connect networks using
different protocols, they are slower than simple routers.
Hubs
A standard wired hub is the center of a network physically connected in a star configuration. These
hubs generally have little intelligence and serve primarily as a common connection point. However,
hubs can also be complex devices that provide bridging and routing between multiple LAN
architectures.
Wireless hubs, also known as access points, function like wired hubs but use different protocols that
provide for different levels of interoperability. With a wireless hub, a wireless LAN can be established
quickly with only a server and wireless modem cards (or PCMCIA cards for laptops). Except for the
wired connection to the Internet, there is no need to drill holes in walls and pull cables to individual
workstations.
Content Filters
A content filter is a device that prevents workstations from accessing specific types of external Web
content, such as high-bandwidth streaming video entertainment. Content filters, which can also be
implemented in software, maximize available network bandwidth for work-related content.
Firewalls
A firewall is a dedicated device or suite of programs running on a server that protects a network from
unauthorized external access. Firewalls are especially relevant in establishing collaborative intranets
that allow, for example, researchers in China to work with information in a U.S. laboratory's intranet
around the clock. A flexible firewall is one component in a system that allows external collaborators
to freely access the laboratory's internal intranet. Firewalls are typically used in conjunction with
routers, gateways, and proxy servers to limit access to internal network resources.
Modems
Modems (short for modulator/demodulator) provide connectivity between a workstation or network
with a remote network such as the Internet. Telephone modems translate digital data into analog
signals for transmission over a twisted pair telephone line and convert incoming analog data into
digital form. Telephone modems have a maximum bandwidth of about 56 Kbps. Cable modems
provide the same digital-to-analog and analog-to digital conversion as telephone modems, but they
connect to a cable TV circuit and provide a bandwidth of about 1.5 Mbps.
A wireless modem, the equivalent of a telephone modem or NIC, allows a computer to access a
wireless hub or access point through radio frequency (RF), or, less frequently, IR light. Wireless
modems are protocol-specific, in that they only work with access points following the same
communications standard.
Satellites
