Saturday, August 29, 2009


ATM, which stands for Asynchronous Transfer Mode is a ”de facto standard” developed
by the ATM Forum and is a switching method of communication, which can be used in both
LANs and WANs.

ATM specifications are being written to ensure that ATM smoothly integrates numerous
existing network technologies.

Today, in many instances, separate networks are used to carry voice, data and video
information, mostly because these traffic types have different characteristics. For instance,
data traffic tends to be "bursty" while voice and video tend to be more "continuous".
With ATM, separate networks will not be required. ATM is the only technology which
from the beginning, was designed to accommodate the simultaneous transmission of data,
voice and video.

ATM is available at various speeds but the most commonly used are 25, 155 and 622 Mbps.

Fiber Data

FDDI stands for Fiber Distributed Data Interface. FDDI standard was developed by ANSI,
the American National Standards Institute. It is based on the use of double optical fiber cable
and provides for a token-passing ring configuration, operating at 100 Mbps.
FDDI is being developed to deal with the requirements of high-speed LANs, MANs and
backbone networks. Since FDDI consists of two fiber rings, primary and secondary ring,
there is good redundancy and high availability. Normally traffic only flows on the primary
ring, but if the primary ring is broken then the secondary ring is used.

Token Ring

Token Ring is mainly used to connect equipment from IBM and Novell.
In this picture you have two environments, IBM and Novell, co-existing on a single Token
Ring. Although these two environments cannot communicate with each other in this
configuration, they can still use the same Token Ring.

Token Ring Novell

A normal way for an organization to go from mainframes to more modern computers is to
use the existing Token Ring network but to change the earlier IBM devices to personal

Novell was one of the first to see this market and they are using Token Ring to connect their
servers and clients together.

In the picture you can see a typical configuration with different types of personal computers
working as Novell clients and servers.

IBM implementation

In this picture we see an IBM implementation of Token Ring. An IBM mainframe 3090
cannot directly communicate with the Token Ring. To do that it needs an NCP which is a
dedicated computer that only handles the communication between the mainframe and the
Token Ring network.

The users sitting on terminals can access the data from the mainframe through a terminal
server. There can be several thousand terminals connected to a mainframe.

Another possibility is to use mini computers such as AS/400. These mini computers can be
accessed by directly connected terminals as in the picture, or from a terminal server.

Token ring Network

Token Ring was introduced by IBM in 1987 and became their main architecture. The
standard for Token Ring from IEEE came in 1989.

Token Ring is physical star and logical ring topology. This means that you connect the
computers physically in a star configuration to the hub, but the computers still pass the
access rights with help of a token in a ring.

The bandwidths used in Token Ring are 4 or 16 Mbps.


Ethernet can be used to connect equipment from different vendors. Different protocols can
also be used at the same time on Ethernet. For example Novell’s IPX/SPX can be used
together with TCP/IP. Almost all modern computers, printers and network components can
connect to Ethernet.

In this picture you have three environments, Novell, SUN and Digital, co-existing at the
same time on a single Ethernet. Although these three environments cannot communicate with
each other in this configuration, they can still use the same Ethernet.

Sun microsystems

Sun microsystems was one of the earliest manufacturers of UNIX workstations. Sun had an
early vision that ”The network is the computer”. SUN is using Ethernet and TCP/IP as a
strategic platform. Since every UNIX workstation and UNIX server comes with an Ethernet
card and TCP/IP software, it is ready for direct connection to the network.
For the PC market, SUN has developed PCNFS software, so that a PC can communicate
with SUN equipment.

Ethernet Digital

Ethernet, as defined in IEEE 802.3 standard, can use both star and bus topology with bandwidths between 10 and 100 Mbps. Ethernet is today the most common technique used
in Local Area Networks.

Digital uses Ethernet for communication between their products. This picture represents an
early implementation by Digital. You can see that Vax computers can be accessed by VT220
terminals, through a terminal server.

LAN Technologies

This diagram shows what has happened to the development of the two most used LAN technologies today, Ethernet and Token ring. 10 Megabits Ethernet exists in two versions. Version two as specified by Digital. Intel and Xerox, is the most commonly used version and IEEE standard 802.3 which is not so commonly used. These two versions are not compatible, because the frame format differs.

Fast Ethernet, which is specified in IEEE 802.3u, offers 100 Mbps. Fast Ethernet is a modern version of Ethernet and is often used in LAN backbone networks today (that is 1999), but is still not so commonly used for clients.

Gigabit Ethernet over fiber, is specified in IEEE 802.3z, offers 1000 Mbps. Gigabit Ethernet is not so common today (that is 1999). Gigabit Ethernet is only used in LAN backbone networks because it is expensive and there is not any need today for so high bandwidth to clients.

Gigabit Ethernet over twisted pair cable, is specified in IEEE 802.3ab, offers 1000 Mbps. This standard is not fully specified today, that is in the spring 1999. Gigabit Ethernet is the future of LAN development, because Ethernet is simple, reliable and will become cheap.

Token ring as specified in IEEE 802.5 offers 4 and 16 Mbps. The use of token ring technology is diminishing even though a new standard, called high speed token ring, offering 100 Mbps, has been specified.


CSMA/CD stands for "Carrier Sense Multiple Access with Collision Detect”. CSMA/CD
is a random control access method.

The CSMA/CD access method is used as the access control method in Ethernet and is defined in a standard from IEEE. The CSMA/CD algorithm is quite simple and the efficiency for an ordinary Ethernet is about 65%. This means that the effective bandwidth for a 10 Mbps Ethernet is about 6.5 Mbps. The rest is lost, mainly due to collisions.

Before one host will transmit it must ”listen” on the medium whether or not another host is
transmitting. If the medium is ”quiet” the host can send its data. The term "Carrier Sense" indicates that a host listens before it transmits.

"Multiple Access" means that many hosts can be connected to the network and all hosts
have the same right to transmit.

With CSMA/CD, it occasionally happens that two hosts send their packets at the same
time. This will make a collision on the network. The information about the collision is
detected by all the other hosts on the network. This is called "Collision Detect". If a host
detects a collision it will wait a random period of time before it tries to transmit again.

Access Methods

A characteristic common to all Local Area Networks is that multiple hosts have to share access to a single physical transmission medium. Several methods can be employed to control the sharing of access to the transmission medium. The various access control methods can be characterized by where in the network the transmission control function is performed. An access method can use following forms of transmission control:

1. Random control
With random control any host can transmit and permission is not required. A host may
check the medium to see if it is free before beginning to transmit.

2. Distributed control
With distributed control only one host at a time has the right to transmit and that right is
passed from host to host. This is usually done by passing on a small piece of data called a
token. The host that has the token, is the one that has the right to transmit.


IEEE standards for LANs describe different types of transmission media. It could be cable,
fiber or wireless.

Cables typically come in two flavors: twisted pair cables or coaxial cables.

Twisted pair cables
A twisted-wire consists of two insulated strands of copper wire that have been braided.
Often a number of twisted-wire pairs are grouped together into a twisted pair cable. Twisted
pair cables are used both for data communication and telephony.
In the picture the twisted pair cables would typically be used in the star topology in the
middle, that is between the hub and the connected hosts.

Coaxial cables
Coaxial cables consist of a central conducting copper core that is surrounded by insulating
material. The insulation is surrounded by a second conducting layer, which can consist of
either a braided wire mesh or a solid sleeve. In the picture, the coaxial cable would typically
be used for the bus network seen on the top.

Optical fiber:
Optical fibers can be used to carry data signals in the form of modulated light beams with
high bandwidth. An optical fiber consists of an extremely thin cylinder of glass, called the
core, surrounded by a concentric layer of glass. In the picture, the optical fiber would
typically be used for the backbone network.

Different types of radio LANs are available on the market. This is an expensive type of
LAN technique. In the picture, wireless connection is used between the two hosts with
antennas. Wireless LAN connections are often used in old historical buildings where you are
not allowed to install cables.


The topology of a network concerns the physical configuration of the devices and the cables
that connect them.

Three principle topologies are used for local area networks:

1. Bus network
On the bus network all connected hosts are sharing the same cable. All the hosts must use
the same communication speed and every host ”hears” all traffic on the cable.

2. Ring network
In the ring topology all hosts are connected into a ring. Every host in the ring receives all data
that is passing. If the data has another destination address, the host will re-transmit the data
into the ring. The data will continue to travel in this way until it reaches the destination host.

3. Star network
A star configuration includes a central controller which could be a hub or a switch. Every
host is directly connected to a port on the central controller.

The History of Local Area Networks, LAN

In the mid 70's Robert Metcalf and David Boggs at Xerox experimented with communication
between the computers. This became the first implementation of Ethernet.

In 1982, the second version of Ethernet was implemented by Digital, Intel and Xerox. This is
the version of Ethernet that is still in use today.

In the mid 80's the first PC-networks started to appear. Network components such as
bridges and routers were now available on the market.

The normal bandwidth of the Local Area Network today is 10 Mbps.

In the near future we will see higher bandwidths, such as 100 to 1000 Mbps.


Assignment Lanka Tag Cloud
Computer Networks The History of Local Area Networks, LAN, The Topologies of a Networks, LANs describe different types of transmission Medias, Local Area Networks Access Methods, Carrier Sense Multiple Access with Collision Detect, Development of LAN Technologies. LAN -Token Ring, LAN Ethernet Digital, LAN - Ethernet Sun microsystems, LAN - Ethernet Mixed Environment, LAN - Token Ring was introduced by IBM LAN - IBM implementation of Token Ring, Token Ring Novell, LAN Token Ring - in a mixed environment, LAN - Fiber Distributed Data Interface, LAN - ATM, LAN Components, LAN Switching Methods, Virtual Local Area Network, Port based VLAN, Mac based VLAN, Protocol based VLAN, User Base VLAN, PC networks Components, PC networks Shared resources, PC Network operating systems, PC networks Novell Netware, PC networks Windows NT, PC networks IBM LAN Server Computer Programming Languages HTML Language, The Generations of Programming Languages, Different types of High Level Languages, Different types of High Level Languages Disadvantages
Computer Networks - IBM LAN Server, Windows NT Networks, Novell Netware, Network operating systems, Networks Shared, Networks Components, User Base, Protocol based, Mac based, Port based, VLAN, LAN Switching, LAN Components, ATM, Fiber Data, Token Ring, Token Ring Novell, IBM implementation, Ethernet, Sun microsystems, Ethernet Digital, Token passing, LAN Technologies, CSMA/CD, Access Methods, Transmission, Networks, The History of Local Area Networks, LAN