ISDN Terminal Adapters
Before there were DSL or cable modem systems, there was ISDN (Integrated Services
Digital Network). ISDN was once thought to be the end-all high-speed access alternative
to accessing the Internet over a POTS line.
There are two types of ISDN services: BRI (Basic Rate Interface) and PRI (Primary
Rate Interface). BRI is the type of ISDN used for home or small office connections and PRI
is used to provide high bandwidth connections for voice and data to larger companies
and telecommunications providers.
BRI is also known as single-line service or single-user ISDN. It is configured for home
and small-business use and is typically what is referred to as ISDN. BRI connects through
a device called a terminal adapter that connects the ISDN line (provided by the phone
company) to the ILEC’s CO. From there the circuit is connected to an ISP that provides a
connection to the Internet.
The ISDN terminal adapter feeds the digital data from the PC directly to the ISDN.
The BRI service uses two bearer channels (called B channels) that carry the data signals
and one digital channel (called a D channel) that carries the control signaling and other
information about the transmission. Each of the B channels carries 64 Kbps or combines
to carry 128 Kbps. The B channel carries 16 Kbps, which combines with the D channels to
complete the BRI’s rate of 144 Kbps. ISDN is capable of transmitting both voice and data
signals over the D channels.Wireless Networking
A wireless network uses radio frequency (RF) devices to transmit and receive data be-
tween computers and peripheral devices. Because they do not require a physical cable
installation to connect nodes to the network, a wireless local area network, or a WLAN
(pronounced “W-lan”), provides a great deal of flexibility and a greatly simplified net-
work installation process. To add a new node to the network does not require more cable
to be strung through the walls. The node is simply installed with a wireless network
adapter card. A WLAN can also be used to overcome structural barriers that may block
the installation of cable in a building or area.Access Points and Network Adapters
The standard that governs wireless networking is the IEEE 802.11, also called the WI-FI
(which stands for wireless fidelity) standard. Devices that meet this standard are guaranteed
tobeinteroperablewithdevicesfromothermanufacturers.Thenetworkadaptersusedto
connect a PC to a wireless LAN are called 802.11 cards (see Figure 20-13).
The model for a wireless network is very much like that used for any Ethernet net-
work in which hubs are used to cluster workstations and to connect them to the network
backbone. In the case of a wireless network, the hub is an access point (AP). An access
point is typically connected to a conventional hard-wired computer network using a(^534) PC Hardware: A Beginner’s Guide