In addition to the size of a network and the relationship between the
nodes and server, another distinguishing feature among LANs is the topology--the
of the cables that connect the nodes of the network. There are three basic
topologies: bus, star, and ring. Network designers consider a number of factors
in determining which topology, or combination of topologies, to Among the
factors considered are the type of computers currently the type of cabling
currently in place (if any), the cost of the components and services required to
implement the network, the distance between and the speed with which data must
travel around the network.
Bus Topology
A bus network, like the bus of a computer itself, is a single conduit to
which I the network nodes and peripheral devices are attached (see Figure 7.13).
Nodes on one type of bus network, Ethernet, transmit data at any time, of any
data being sent by other nodes. If one set of data happens to collide with
another set of data transmitted by other nodes--that is, if two nodes try to
send data at the same time--each node waits a small, random amount of time and
then attempts to retransmit the data.
Although the bus topology is one of the most common, it has inherent
disadvantages. Keeping data transmissions from colliding requires extra
circuitry and software, and a broken connection can bring down (or "crash") all
or part of the network, rendering it inoperable so that users cannot share data
and peripherals until the connection is repaired.
The Star Topology
A star network places a hub in the center of the network nodes. Groups of
data are routed through the central hub, (which may also be a switch or a
router,) to their destinations. This scheme has
an advantage in that the hub monitors traffic and prevents collisions, and a
broken connection does not affect the rest of the network. If you lose the hub,
however, the entire network goes down. Figure 7.14 shows the star topology.
Networks that have a theatrical star
topology often actually do not have a physical star
topology.
The Ring Topology
The ring topology connects the nodes of the network in a circular chain in
which each node is connected to the next. The final node in the chain connects
to the first to complete the ring, as shown in Figure 7.15. With this
methodology, each node examines data that is sent through the ring. If the data
is not addressed to the node examining it, that node passes it along to the next
node in the ring. The ring topology has a substantial advantage over the bus
topology. There's no danger of collisions because data always flows in one
direction. One drawback to the ring, however, is that if a connection is broken,
the entire network goes down.
The ring topology with the token ring
technology was perhaps the very best for a LAN but because
neither was marketed very well ring topology networks are
almost non-existent now.
NETWORK MEDIA AND HARDWARE
No matter what their structure, all networks rely on media to link their
nodes and/or servers together. You may recall that when referring to data
storage, the term media refers to materials for storing data, such as magnetic
:disks and tape. In network communications, however, media refers to the wires,
cables, and other means by which data travels from its source to its
destination. The most common media for data communication are twisted-pair wire,
coaxial cable, fiber-optic cable, and wireless links.
Twisted-Pair Wire
Twisted-pair wire normally consists of four or eight cop-per strands of
wire, individually insulated in plastic, then twisted around each other in
braided pairs and bound together in another layer of plastic insulation. (There
were originally just two wires, rather than four or eight, hence the name
twisted-pair.) Except for the plastic coating, nothing shields this type of
wire from outside interference, so it is also called unshielded twisted-pair (UTP)
wire. Some twisted-pair wire is further encased in a metal sheath and therefore
is called shielded twisted-pair (STP) wire.
Indoor wiring for telephones uses twisted-pair wire, so twisted-pair is often
called telephone wire. Because it was readily available and inexpensive,
telephone wire gained early favor as a conduit for data communications. Today,
however, some twisted-pair wire used for communication is made to more demanding
specifications than voice-grade telephone wire.
Sometimes network media are compared by the amount of data they can transmit
each second. The difference between the highest and lowest frequencies of a
transmission channel is known as bandwidth. As more users transmit data over a
network, the bandwidth reduces, thereby slowing down all transmissions.
Bandwidth is expressed in cycles per second (hertz) or in bits per second.
Twisted-pair wire was once considered a low-band-width media, but networks based
on twisted-pair wires now support transmission speeds up to 150 megabits per
second (Mbps), and even faster speeds are on the horizon.
Coaxial Cable
Coaxial cable, sometimes called coax (pronounced "co-axe"), is widely used for
cable TV and is used in some net-works (however, the connectors are different
for TV and networks). There are two conductors in coaxial cable. One is a single
wire in the center of the cable, and the other is a wire mesh shield that
surrounds the first wire with an insulator in between.
Coaxial cable can carry more data than older types of twisted-pair wiring,
and it is less susceptible to interference from other wiring. However, it is
also more expensive and has become less popular as twisted-pair technology has
improved. Two types of coaxial cable are used with networks: thick and thin.
Thick coax is the older standard and is seldom installed in new networks.
Fiber-Optic Cable
A fiber-optic cable is a thin strand of glass that transmits pulsating
beams of light rather than electric frequencies (see Figure 7.18). When one end
of the strand is exposed to light, the strand carries the light all the way to
the other end--bending around corners with only a minute loss of energy along
the way. Because light travels at a much higher frequency than electrical
signals, fiber-optic cable can easily carry data at more than a billion bits per
second usually 1300 Mbps. Fiber-optic cable is also immune to the
electromagnetic interference that is a problem for copper wire.
The disadvantage of fiber-optic cable is that it is more expensive than
twisted-pair and coax, and it is more difficult to install because it does not
bend around corners as easily. As costs have come down, however, fiber-optic
cable has become increasingly popular, and it is now revolutionizing a number of
communications industries. Telephone and cable television companies, especially,
have been moving from twisted-pair wire and coaxial cables to fiber-optic
cables.
The Network Interface Card, Network Protocols,
and Cabling Specifications
Cables or radio waves may link a network together, but each computer on
the network still needs hardware to control the flow of data. The device that
per-forms this function is the network interface card (NIC). The NIC is a type
of expansion board--a printed circuit board that fits into one of the computer's
expansion slots and provides a port on the back of the PC to which the network
cable attaches. The computer also requires network software, which tells the
computer how to use the NIC.
Another specification of a network is to determine the network technology or
cabling equipment used to create a LAN. The most common types of net-work
technology include Ethernet (which also includes Fast Ethernet), Token Ring, and
ARCnet. Each of these is designed for a certain kind of network topology and has
certain standard features.
The original implementations of Ethernet, which used coaxial cable, were
called 10Base-5 and 10Base-2. The most popular implementation of Ethernet
currently is called 10Base-T. It uses a star topology and twisted-pair wires and
can achieve transmission speeds up to 10 Mbps.
100Base-T, also known as Fast Ethernet, is available using the same media
and topology as Ethernet, but different network interface cards are used to
achieve speeds of up to 100 Mbps. The 3COM EtherLink XL 10/100 adapter is an
example of a network interface card used to achieve 100 Mbps LAN speeds.
Hewlett-Packard's 100Base-VG competes with 100Base-T. Still other
implementations of Ethernet are pushing transmission speeds even higher.
IBM's network technology is the Token Ring. The controlling hardware in a
Token Ring network transmits an electronic token--a small set of data that
includes an address--to each node on the network many times each second. If the
token is not currently in use, a computer can copy data into the token and set
the address where the data should be sent. The token then continues around the
ring. Each computer along the way looks at the address until the token reaches
the computer with the address that was recorded in the token. The receiving
computer then copies the contents of the token and resets the token status to
empty.
Token Ring networks have the advantage of data traveling through the ring in
one direction in a controlled manner. With this approach, data cannot collide,
so a complex scheme like CSMA/CD is not necessary. However, the network hardware
is not cheap; Token-Ring adapter cards can cost as much as five times more than
other types of network adapters. Token Ring networks once operated at either 4
or 16 Mbits per second, hut as with Ethernet, new technology has pushed the
transmission rate up to 100 Mbits per second.
ARCnet (Attached Resource Computer network) has both a topology and
networking technology all its own. ARCnet uses either twisted-pair wire or coaxial cable, and the star topology is formed with hubs attached to the
network.
Most of the networking terms you have seen so far--with the exception of
the protocols discussed in the previous section--have referred to hardware. As
with every other part of the computer system, however, there must be software to
control tile hardware. The group of programs that manages the resources on the
net-work is often called the network operating system, or NOS.
The most popular NOS, NetWare, from Novell, can be used to run networks with
different protocols, including Ethernet, Token Ring, and ARCnet. NetWare also
includes support for various hardware platforms, such as Mac, PC, and UNIX nodes
and servers.
One of NetWare's competitors, Banyan's VINES, offers similar flexibility.
Some other NOSs, such as Windows NT Server, DECNet, LANtastic, and AppleShare
are designed to implement specific network protocols on specific types of
machines.
The Ring Topology