You use different network technologies to communicate between computers
within LANs and WANs. You may use a combination of technologies to get the
best cost-benefit and maximum efficiency from your network design. One of
the ways in which these technologies differ is the set of rules that each
uses to place data onto the network cable and remove it. This is called
access method. When data moves on the network, these various access
methods regulate the flow of network traffic.
Ethernet
Ethernet is a popular LAN technology that uses Carrier Sense Multiple Access
with Collision Detection (CSMA/CD) between clients over a variety of cable
types. Ethernet is passive, which means it requires no power source of its
own, and thus does not fail unless the cable is physically cut or improperly
terminated. Ethernet is connected by using a bus topology in which the cable
is terminated at both ends.
- Access Method
The network access method used for Ethernet is Carrier Sense
Multiple Access with Collision Detection (CSMA/CD). CSMA/CD
is a set of rules that determines how network devices respond
when two devices attempt to send data on the network
simultaneously. Transmission of data by multiple computers
simultaneoulsy over the network causes a collision. Each
computer on the network checks the cable for network traffic.
Only when a computer detects that the cable does it send
data. After data is transmitted on the cable, no other
computer can transmit data until the original data has
reached its destination and the cable is again free.
After detecting a collision, a device waits a random delay
time and then attempts to retransmit the message. If the
device detects a collision again, it waits twice as long
before trying to retransmit the message.
- Transfer Speed
Standard Ethernet, called 10baseT, supports data tranfers
of 10 Mbps over a wide range of cabling. Fast Ethernet
(100baseT) supports data transfer rates of 100 Mbps, and
Gigabit Ethernet supports data rates of 1 Gbps.
Token Ring
Token ring networks are implemented in a ring topology. The physical topology
of a token ring network is the star topology, in which all computers on the
network are physically connected to a hub. The physical ring is wired through
a hub called a multistation access unit (MSAU).
- Access Method
The access method used in a token ring network is token
passing. When the first computer on the token ring comes
online, the network generates a token. The token travels
around the ring to each computer until one of the computers
takes control of the token.
- Transfer Speed
The transfer speed in a token ring network is between 4 and
16 Mbps.
Asynchronous Transfer Mode (ATM)
Asynchronous transfer mode (ATM) is a packet-switching network that sends
fixed-length packets over LANs or WANs, instead of the variable-length
packets used in other technologies. Fixed-length packets, or cells, are data
packets that contain only basic path information, allowing switching devices
to route the packet quickly. Communication occurs over a point-to-point
system that provides a permanent and virtual data path between each
station.
Using ATM, you can send data from a main office to a remote location. The
data travels from a LAN over a digital leased line to an ATM switch and into
the ATM network. It passes through the ATM network and arrives at another ATM
switch in the destination LAN.
- Access Method
An ATM network uses the point-to-point access method. This
access method transfers fixed-length packets from one
computer to another through ATM switching equipment. The
result is technology that transmits a small, compact data
packet at a high speed.
- Transfer Speed
The transfer speed in an ATM network is between 155 and 622
Mbps.
Fiber Distributed Data Interface (FDDI)
A Fiber Distributed Data Interface (FDDI) network provides high-speed
connections for various types of networks. FDDI was designed for use with
computers that require speeds greater than the 10 Mbps available from
Ethernet or the 4 Mbps available from existing token ring architectures. An
FDDI network can support several low-capacity LANs that require a high-speed
backbone.
An FDDI network consists of two similar streams of data flowing in opposite
directions around two rings. One ring is called the primary ring and the
other is called the secondary ring. If there is a problem with the primary
ring, such as a ring failure or a cable break, the ring reconfigures itself
by transferring data to the secondary ring, which continues transmitting.
- Access Method
The access method used in an FDDI network is token passing.
A computer on an FDDI network can transmit as many packets
as it can produce within a predetermined time before
releasing the token. As soon as a computer has finished
transmitting or after a predetermined transmittal time is
up, the computer releases the token.
- Transfer Speed
The transfer speed in an FDDI network is between 155 and 622
Mbps.
Frame Relay
Frame relay is a packet-switching network that sends
variable-length packets over LANs or WANs. Variable length packets,
or frames, are data packets that contain additional addressing and error
handling information necessary for delivery.
Packet switching is a method used to send data over a WAN by dividing a large
package of data into smaller pieces (packets). These pieces are sent through
a packet switch, which sends the individual packets across the WAN using the
best route currently available. Although these packets may travel along
different paths, the receiving computer can reassemble the pieces into the
original data frame. However, you can have a permanent virtual circuit (PVC)
established, which would use the same path for all of the packets. This
allows for a faster transmission than by normal frame relay networks and
eliminates the need for packet disassembly and reassembly.
- Access Method
Frame relay uses the point-to-point access method. This
access method transfers variable-sized packets from one
computer directly to another, instead of between several
computers and peripherals.
- Transfer Speed
Frame relay allows for data transfers that is as fast as the
provider can supply over digital leased lines.