LAN Access and sharing Methods

ENAMBy 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 other applications.

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LAN Transmission Methods

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 network.

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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 Detection (CSMA/CD)04/07/23

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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 network

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CSMA/CD 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 occurs 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

then re-transmit

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Carrier Sense Multiple Access(CSMA)

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 destination

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

Carrier Sense 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 transmits

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CSMA/Collision Detection (CSMA/CD)

Collision occurs when two stations listen for network traffic, hear none and transmit simultaneously damaging both transmissions

Collision Detection enables stations to detect collisions, so they know when they must retransmit

Used by Ethernet LANs

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CSMA/Collision Avoidance (CSMA/CA)

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 transmission

RTS and CTS frames let other stations know of the data transmission so that collision is avoided

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Types of LAN’s The three most popular types of

LAN’s are: Token ring Ethernet FDDI (Fiber Distributed Data

Interface)

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Ethernet Operations Ethernet

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 1980s Ethernet is:

Most pervasive network access method in use Most commonly implemented media access

method in new LANs

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Ethernet 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 Gbps Used with a bus or star topology

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Types of Ethernet LANs 10Base-T

Operates at 10 Mbps IEEE 802.3 standard

Fast Ethernet (100Base-T) Operates at 100 Mbps Uses twisted pair cables

Gigabit Ethernet 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 market

Wireless Ethernet IEEE 802.11 standard Operates at around 2.4 Gbps

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Fiber-Distributed Data Interface (FDDI)

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 protocol.

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 topology.

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Fiber-Distributed Data Interface (FDDI)

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-attached".

Host computers then connect as single-attached devices to the routers or concentrators.

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Ring Topology

A frame, called a token, travels around the ring and stops at each node.

If a node wants to transmit data, it adds that data and the addressing information to the frame.

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.04/07/23

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Ring Topology 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 directions.

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 is used.

FDDI uses light instead of electricity to transmit

data over a dual ring. 1704/07/23

Switching Techniques

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 traffic.

• Circuit Switching (isochronous) • Message Switching (asynchronous)• Packet Switching

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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 take place.

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Circuit switchingAdvantages:

• 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 (multiplexing)No need to inspect a packet header for address

Disadvantages: • 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 using it.04/07/23 20

Message Switching 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 next node.

This type of network is called a store-and-forward network.

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Message Switching

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.

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Message SwitchingAdvantages:

• Channel efficiency can be greater compared to circuit- switched systems, because more devices are sharing the channel. • Traffic congestion can be reduced, because messages may be temporarily stored in route. • Message priorities can be established due to store-and-forward technique. • Message broadcasting can be achieved with the use of broadcast address appended in the message. Disadvantages

• Message switching is not compatible with interactive applications. • Store-and-forward devices are expensive, because they must have large disks to hold potentially long messages.

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Packet Switching• In packet switching, a message is broken into small parts, called packets. • Each packet is tagged with appropriate source and destination addresses. • 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 (``dropped'').

•There are two methods of packet switching: Datagram and virtual circuit

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Packet switching 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.

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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 be lost.

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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 parameters.• 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.

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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 a line. • 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.

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Advantages of packet switching

• Packet switching is cost effective, because switching devices do not need massive amount of secondary storage. • 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.

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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 QoS•Complex end-to-end control - Packets can be lost, corrupted or delivered out-of-order•Delay and Congestion - No congestion control, can lead to arbitrary delays and packet drops

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Differences Between Circuit & Packet Switching

Circuit-switching Packet-Switching

Guaranteed capacity No guarantees (best effort)

Capacity is wasted if data is bursty

More efficient

Before sending data establishes a path

Send data immediately

All data in a single flow follow one path

Different packets might follow different paths

No reordering; constant delay; no pkt drops

Packets may be reordered, delayed, or dropped

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