LAN Access and sharing
By Bwire sedrick
LAN-Local Area Network
A LAN is a high-speed data network that covers a
relatively small geographic area. It typically connects
workstations, personal computers, printers, servers,
and other devices.
LANs offer computer users many advantages,
including shared access to devices and applications,
file exchange between connected users, and
communication between users via electronic mail and
LAN data transmissions fall into three classifications:
unicast, multicast, and broadcast.
In a unicast transmission, a single packet is sent from
the source to a destination on a network.
A multicast transmission consists of a single data
packet that is copied and sent to a specific subset of
nodes on the network.
A broadcast transmission consists of a single data
packet that is copied and sent to all nodes on the
LAN Access Control Methods
A LAN Access method describes how the
devices access the network and share
the transmission facilities
Two primary access control methods exist
for computers to communicate with each
other over the network
Token based access
Carrier Sense Multiple Access with Collision
Token based access
Used in bus and ring network topologies (token ring)
Each computer in the network can only send its data
if it has the token. This prevents collisions that occur
when data is sent at the same time over the network
The token is a special pattern of bits/bit in a frame
that is directly detectible by each node in the network
A computer may only transmit information if it is in
possession of the token
The message is sent to all other computers in the
Usually used in a bus topology
Used in Ethernet LAN’s
Unlike the token ring, all nodes can send whenever they
have data to transmit
When a node wants to transmit information, it first “listens” to
the network. If no one is transmitting over the network, the
node begins transmission
It is however possible for two nodes to transmit
simultaneously thinking that the network is clear
When two nodes transmit at the same time, a collision
The first station to detect the collision sends a jam signal
into the network
Both nodes back off, wait for a random period of time and
Carrier Sense Multiple Access
Any of the network devices can transmit data onto the
network at will; there is no central controller
A broadcast network where all stations see all frames,
regardless of whether they represent an intended
Each station must examine received frames to determine
if the station is a destination. If so, the frame is passed to
a higher protocol layer for appropriate processing
Before sending data, stations listen to the network to see
if it is already in use. If in use, the station wishing to
transmit waits, otherwise it transmits09/24/13
Collision occurs when two stations listen for
network traffic, hear none and transmit
simultaneously damaging both
Collision Detection enables stations to
detect collisions, so they know when they
Used by Ethernet LANs
Sender send a request-to-send (RTS) frame to
receiver and indicates the time needed to
complete data transmission
Receiver send clear-to-send (CTS) frame,
indicates time to complete data transmission
and reserves channel for the sender
Sender transmits the data and receiver
responds with an ACK frame, ensuring reliable
RTS and CTS frames let other stations know of
the data transmission so that collision is
Used by 802.11 wireless LANs09/24/13
Types of LAN’s
The three most popular types of LAN’s
FDDI (Fiber Distributed Data Interface)
A network access method (or media
access method) originated by the University
of Hawaii, later adopted by Xerox Corporation
And standardized as IEEE 802.3 in the early
Most pervasive network access method in use
Most commonly implemented media access
method in new LANs09/24/13
First network to provide CSMA/CD
Developed in 1976 by Xerox PARC (Palo Alto
Research Center) in cooperation with DEC and Intel
Is a fast and reliable network solution
One of the most widely implemented LAN standards
Can provide speeds in the range of 10Mbps- 10
Used with a bus or star topology
Types of Ethernet LANs
Operates at 10 Mbps
IEEE 802.3 standard
Fast Ethernet (100Base-T)
Operates at 100 Mbps
Uses twisted pair cables
Operates at 1 Gbps
Uses fiber optic cable
10 Gbps Ethernet
Latest development of ethernet
Uses fiber optic cable
Developed to meet the increasing bandwidth needs of the LAN
IEEE 802.11 standard
Fiber-Distributed Data Interface
Fiber-Distributed Data Interface (FDDI) provides a
standard for data transmission in a local area network that
can extend in range up to 200 kilometers (124 miles).
The FDDI protocol uses as its basis the token ring
In addition to covering large geographical areas, FDDI
local area networks can support thousands of users.
As a standard underlying medium it uses optical fiber
(though it can use copper cable, in which case one can
refer to CDDI).
FDDI uses a dual-attached, counter-rotating token-ring
Fiber-Distributed Data Interface
An FDDI network contains two token rings, one for
possible backup in case the primary ring fails.
FDDI has a larger maximum-frame size than standard 100
Mbit/s ethernet, allowing better throughput.
A small number of devices (typically infrastructure devices
such as routers and concentrators rather than host
computers) connect to both rings - hence the term "dual-
Host computers then connect as single-attached devices
to the routers or concentrators.
A frame, called a token, travels
around the ring and stops at
If a node wants to transmit data,
it adds that data and the
addressing information to the
The frame continues around the
ring until it finds the destination
node, which takes the data out
of the frame.
The advantage of using this
method is that there are no
collisions of data packets.
With single ring all the devices on the network share a
single cable, and the data travels in one direction only.
With dual ring two rings allow data to be sent in both
This creates redundancy (fault tolerance), meaning that in
the event of a failure of one ring, data will still be
transmitted on the other ring.
The 802.5 standard is the Token Ring access method that
FDDI uses light instead of electricity to transmit data over
a dual ring.
In large networks there might be multiple paths linking
sender and receiver. Information may be switched as it
travels through various communication channels. There
are three typical switching techniques available for digital
• Circuit Switching (isochronous)
• Message Switching (asynchronous)
• Packet Switching
Circuit SwitchingCircuit Switching
Is a technique that directly connects the sender and the receiver
in an unbroken path.
Telephone switching equipment, for example, establishes a path
that connects the caller's telephone to the receiver's telephone by
making a physical connection.
With this type of switching technique, once a connection is
established, a dedicated path exists between both ends until the
connection is terminated.
Routing decisions must be made when the circuit is first
established, but there are no decisions made after that time.
A complete end-to-end path must exist before communication can
• The communication channel (once established) is dedicated.
•Guaranteed bandwidth (Quality of Service)
•Reliable communication - Rare packet loss, Packets are delivered in order
•Simple data routing - Forwarding based on time slot or frequency
No need to inspect a packet header for address
• Possible long wait to establish a connection, (10 seconds,
more on long- distance or international calls.) during which
no data can be transmitted.
• More expensive than any other switching techniques,
because a dedicated path is required for each connection.
• Inefficient use of the communication channel (wasted bandwidth,
because the channel is not used when the connected systems are not
With message switching there is no need to establish a
dedicated path between two stations.
When a station sends a message, the destination
address is appended to the message.
The message is then transmitted through the network,
in its entirety, from node to node.
Each node receives the entire message, stores it in its
entirety on disk, and then transmits the message to the
This type of network is called a store-and-forward
A message-switching node is typically a general-purpose computer. The
device needs sufficient secondary-storage capacity to store the incoming
messages, which could be long. A time delay is introduced using this type
of scheme due to store- and-forward time, plus the time required to find the
next node in the transmission path.
• Channel efficiency can be greater compared to circuit-
switched systems, because more devices are sharing the
• Traffic congestion can be reduced, because messages may be
temporarily stored in route.
• Message priorities can be established due to store-and-forward
• Message broadcasting can be achieved with the use of
broadcast address appended in the message.
• Message switching is not compatible with interactive
• Store-and-forward devices are expensive, because they
must have large disks to hold potentially long messages.
• In packet switching, a message is broken into small parts, called
• Each packet is tagged with appropriate source and destination
• Since packets have a strictly defined maximum length, they
can be stored in main memory instead of disk, therefore access
delay and cost are minimized.
• Also the transmission speeds, between nodes, are optimized.
• With current technology, packets are generally accepted onto
the network on a first-come, first-served basis. If the network
becomes overloaded, packets are delayed or discarded
•There are two methods of packet switching: Datagram and virtual circuit
In packet switching, the analog signal from your phone is
converted into a digital data stream. That series of digital bits
is then divided into relatively tiny clusters of bits, called
packets. Each packet has at its beginning the digital address
-- a long number -- to which it is being sent. The system
blasts out all those tiny packets, as fast as it can, and they
travel across the nation's digital backbone systems to their
destination: the telephone, or rather the telephone system, of
the person you're calling.
They do not necessarily travel together; they do not travel
sequentially. They don't even all travel via the same route.
But eventually they arrive at the right point -- that digital
address added to the front of each string of digital data --
and at their destination are reassembled into the correct
order, then converted to analog form, so your friend can
understand what you're saying.09/24/13 26
Packet Switching: Datagram
• Datagram packet switching is similar to message switching in
that each packet is a self-contained unit with complete
addressing information attached.
• This fact allows packets to take a variety of possible paths
through the network.
• So the packets, each with the same destination address, do not
follow the same route, and they may arrive out of sequence at
the exit point node (or the destination).
• Reordering is done at the destination point based on the
sequence number of the packets.
• It is possible for a packet to be destroyed if one of the nodes on
its way is crashed momentarily. Thus all its queued packets may
Packet Switching: Virtual Circuit
• In the virtual circuit approach, a preplanned route is established
before any data packets are sent.
• A logical connection is established when
a sender send a "call request packet" to the receiver and
the receiver send back an acknowledge packet "call accepted
packet" to the sender if the receiver agrees on conversational
• The conversational parameters can be maximum packet sizes,
path to be taken, and other variables necessary to establish and
maintain the conversation.
• Virtual circuits imply acknowledgements, flow control, and error
control, so virtual circuits are reliable.
• That is, they have the capability to inform upper-protocol layers
if a transmission problem occurs.
Packet Switching:Virtual Circuit
• In virtual circuit, the route between stations does not mean that
this is a dedicated path, as in circuit switching.
• A packet is still buffered at each node and queued for output over
• The difference between virtual circuit and datagram approaches:
With virtual circuit, the node does not need to make a routing
decision for each packet.
It is made only once for all packets using that virtual circuit.
Advantages of packet
• Packet switching is cost effective, because switching
devices do not need massive amount of secondary
• Packet switching offers improved delay characteristics,
because there are no long messages in the queue
(maximum packet size is fixed).
• Packet can be rerouted if there is any problem, such as,
busy or disabled links.
• Many network users can share the same channel at the
same time. Packet switching can maximize link efficiency by
making optimal use of link bandwidth.
Disadvantages of packet switching
• Protocols for packet switching are typically more complex.
• It can add some initial costs in implementation.
• If packet is lost, sender needs to retransmit the data.
• Packet-switched systems still can’t deliver the same quality as
dedicated circuits in applications requiring very little delay - like
voice conversations or moving images.
•No guaranteed bandwidth- Harder to build applications requiring
•Complex end-to-end control - Packets can be lost, corrupted or
•Delay and Congestion - No congestion control, can lead to
arbitrary delays and packet drops
Differences Between Circuit & Packet Switching
Guaranteed capacity No guarantees (best effort)
Capacity is wasted if data is
Before sending data
establishes a path
Send data immediately
All data in a single flow follow
Different packets might follow
No reordering; constant
delay; no pkt drops
Packets may be reordered,
delayed, or dropped09/24/13