OSI MODEL

 

Physical Layer

Application

Presentation

Session

Transport

Network

Data Link

Physical

The OSI Model's layer 1, the Physical Layer, outlines the characteristics of the physical medium between networked devices--cabling, wiring, fiber strands and the air. Transmission and receipt of data from the physical medium (copper wire, fiber, radio frequencies, barbed wire, string etc.) is managed at this layer.

 

The Physical Layer receives data from the data link Layer, and transmits it to the wire. The physical layer controls the electrical and mechanical functions related to the transmission and receipt of a communications signal. It also manages the encoding and decoding of data contained within the modulated signal.

 

Note that for two devices to communicate, they must beconnected to the same type of physical medium (wiring). 802.3 Ethernet to 802.3 Ethernet, FDDI to FDDI, serial to serial etc. Two end stations using different protocols can only communicate through a multi-protocol bridge or a router.
The physical layer is responsible for two jobs:

1. Communication with the data link layer above it.
2. Fragmentation of data into frames
3. Reassembly of frames into data link Protocol Data Units.
4. Transmission and reciept of data.

It should be noted that in most modern network interface adaptors, the physical and data link functions are performed by the adaptor.

Example Physical Protocols

 

• CSMA/CD - Carrier Sense Multipl Access / Collision Detect
• CSMA/CA - Carrier Sense Multiple Access / Collision Avoid
• FDMA - Frequency Division Multiple Access
• MSK - Minimum Shift Keying
• GFMSK - Gaussian-Fitered Minimum Shift Keying
• TDMA - Time Division Multiple Access
• CDMA - Code Division Multiple Access
• B8ZS - Binary 8 Zero Substitution
• 2B1Q - 2 Binary 1 Quaternary
• PCM - Pulse Code Modulation
• QAM - Quadrature Amplitude Modulation
• PSK - Phase Shift Keying

DATA LINK LAYER

Application

Presentation

Session

Transport

Network

Data Link

Physical

The data link Layer is the second layer of the OSI model. The data link layer performs various functions depending upon the hardware protocol used, but has four primary functions:

1. COMMUNICATION with the Network layer above.
2. SEGMENTATION of upper layer datagrams (also called packets) into frames in sizes that can be handled by the communications hardware.
3. BIT ORDERING. The data link layer organizes the pattern of data bits into frames before transmission. The frame formatting issues such as stop and start bits, bit order, parity and other functions are handled here. Management of big-endian / little-endian issues are also managed at this layer.
4. COMMUNICATION with the Physical layer below

This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, physical link management, error notification, ordered delivery of frames, and flow control.
It should be noted that in most modern network interface adaptors, the Physical and Datalink functions are performed by the network interface adaptor.

EXAMPLE

Ethernet Network Adaptors uses a Media Access Control (MAC) address, which is a physical address that (somewhat) uniquely identifies a network card to the physical network. The Ethernet Adaptor's primary job at the data link layer is to transmit Ethernet frames onto the wire. It places its own MAC address in the frame, followed by the destination MAC address to which it wishes to communicate, sets various other values in the Ethernet frame (protocol type etc.) and then transmits on the wire. The CSMA/CD protocol that Ethernet uses to control access to the common network bus could be said to occur at the data link layer of the OSI model or could also be called a data link function.

NETWORK LAYER

Application

Presentation

Session

Transport

Network

Data Link

Physical

It is the network layer's job to figure out the network topology, handle routing and to prepare data for transmission. The network layer is concerned with the following primary functions:

1. Communication with the Transport layer above.
2. Encapsulation of Transport data into Network layer Protocol Data Units.
3. Management of connectivity and routing between hosts or networks.
4. Communication with the data link layer below.

Examples of network layer protocols include:

 

• Internet Protocol
• Internet Control Message Protocol (ICMP or "ping")
• Internet Gateway Management Protocol (IGMP)
• IPX/SPX

TRANSPORT LAYER

Application

Presentation

Session

Transport

Network

Data Link

Physical

If networking software performs reliable data tranfer functions, then the detection of errors, and retransmission of data to recover those errors or lost data will occur in software managing this layer. The transport layer may use a variety of techniques such as a Cyclic Redundancy Check, windowing and acknowledgements. If data is lost or damaged it is the transport layer's responsibility to recover from that error.

1. Communicate with the Session layer above.
2. Reassemble transport Protocol Data Units into data streams
3. Reliable protocols operating at this layer will
• Detect errors and lost data
• Recover lost data
• Manage retransmission of data.
4. Segmentation of data streams into transport Protocol Data Units.
5. Communicate with the Network layer below.

Examples of transport layer protocols include:

 

• Transmission Control Protocol (Reliable)
• User Datagram Protocol (Unreliable)

SESSION LAYER

Application

Presentation

Session

Transport

Network

Data Link

Physical

The session layer tracks connections, also called sessions. The session layer should keep track of multiple file downloads requested by a particiular FTP application, or multiple telnet connections from a single terminal client, or web page retrievals from a web server.
With TCP/IP this functionality is handled by application software addressing a connection to a remote machine and using a different local port number for each connection.
The session layer performs the following functions:

1. Communication with the Presentation layer above.
2. Organize and manage one or more connections per application, between hosts.
3. Communication with the Transport layer below.

EXAMPLE

Sessions are used to keep track of individual connections to remote servers. Your web browser is an excellent example of the use of sessions.
Your web browser (an application layer object) opens a web page. That page contains text, graphics, Macromedia Flash objects and perhaps a Java applet. The graphics, the Flash object and the Java applet are all stored as separate files on the web server. To access them, a separate download must be started. Your web browser opens a separate session to the web server to download each of the individual files. The session layer keeps track of which packets and data belong to which file and keeps track of where they go (in this case, to your web browser).
In most modern Internet applications, the session, presentation and application layers are usually rolled together inside the application itself, thus, your web browser performs all functions of the session, presentation and application layers.

PRESENTATION LAYER

Application

Presentation

Session

Transport

Network

Data Link

Physical

The presentation layer handles the conversion of data between a Standards-based or platform independant formats to a format understood by the local machine. This allows for data to be transported between devices and still be understood.

The presentation layer performs the folowing functions:

1. Communication with the application layer above.
2. Translation of data conforming to cross-platform standards into formats understood by the local machine.
3. Communication with the session layer below.

Examples of Presentation Layer Functions

 

• Conversion of a Sun .RAS raster graphic to JPG.
• Conversion of ASCII to IBM EBCDIC
• Conversion of .PICT on a MAC to .jpg
• Conversion of .wav to .mp3

APPLICATION LAYER

Application

Presentation

Session

Transport

Network

Data Link

Physical

The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.

Examples of applications that utilize the network are:

 

• Telnet
• FTP
• Instant Message software (AIM, MSN, ICQ, Yahoo)
• IRC (mIRC)
• America Online (the client software)
• Microsoft Windows File Shares
• Microsoft Internet Explorer (a web browser)
• Network games of Doom, Quake, Unreal (first-person 3D shooter video games)

Real World Example

 

Due to popular demand, InetDaemon has written an operational example of how the OSI model can be used to describe what happens when you browse a web page on a web site. This page you are reading is stored on our website and you are most probably using a web browser to read this. We will use the OSI model to describe what happens when you click a link in a web page in your web browser and what happens as the web server receives, processes and responds to your request. This example assumes that you are on a local area network and that you are using an Ethernet card to communicate with the network. If you're on a dial-up modem, it will work a little differently from the data link layer down. Modem communication is a bit more complicated. We won't cover that here.

 

LAYER	EXAMPLE		FUNCTION/ACTIVITY
APPLICATION	Web Browser		A web browser such as Internet Explorer or Netscape provides the means for your computer to contact a web server and download several files that go together to produce a single web page. You can request a web page by typing in a web address (a URL) or by clicking a link in an open web page. The web browser is an APPLICATION. The web browser application gives you the means to select a web server, contact the server and request a web page. The web browser handles the process of finding the web server (the remote computer that has the web page you want stored on it) , requesting the desired web page and displaying all the files contained within the web page.
PRESENTATION	HTTP		The web browser handles PRESENTATION of the web page to the user by converting the files stored at the web server into formats used to display them on your computer. Your web browser supports varous text formats (UNICODE and ASCII), image file formats (JPG, GIF, PNG, BMP), audio file formats (WAV, MP3, AIFF) as well as Hypertext Markup Language (HTML) and one or more scripting languages (JavaScript, JScript, VBScript). Conversion of data from one format to another is the job of the PRESENTATION layer. A web browser can convert these file formats into the local formats used on the local computer for displaying images, playing sounds and displaying text; if it cannot, it often can launch an application which does understand the format. Much of the PRESENTATION layer conversions are handled in the program you're running.
SESSION			When you request a web page, a the web browser opens a TCP connection to the web server. The web server sends back the web page and closes the connection. Your web browser then parses the HTML of the web Within the are instructions written in HTML tags which tell thebrowser where to find additional files to be displayed within the web such as style sheets, sound files, images, movies, Flash files and applets. Your web browser automatically opens additional TCP connections to the webserver. Each TCP connection is a SESSION.
TRANSPORT	TCP		To communicate with a web server your computer must open a TCP connection to the web server and request a web page. The TCP connection breaks up the web page into managable chunks, lables them with numbers so they can be reassembled in the correct order and TRANSPORTS the pieces across the correct SESSION.
NETWORK	IP ARP		Internet Protocol (IP) is a NETWORK layer protocol that uses unique addresses for the web server and for your computer. IP provides the means for your computer to determine whether the web server is a local computer or a computer located somewhere on the Internet. To reach a webserver on the InternetIP protocol also allows your to figure out how to reach the Internet web server via your default gateway. Your computer creates a message addressed to the web server with your computer's return IP address. Your computer uses ARP to figure out the physical MAC address of the default gateway and then passes the data to the NETWORK layer.
DATA LINK	E T H E R N E T	LLC	Once the request from your web browser has been created it is sent to the network card. Once it reaches your network card it must be converted into a message that is sent from your computer to the default gateway which will forward the message to the Internet. At the DATA LINK layer, the web request is inserted inside a network request to the default gateway.
		MAC
PHYSICAL		CSMA/CD	The physical layer provides the means to transmit the web page request to the default gateway.

 

Keep in mind that this entire system didn't spring into existence in a day, it took decades for dozens of developments to come together to make the process of requesting, retreiving, delivering and displaying web pages possible.

 

The OSI Model and Internet Protocols

 

When you describe the way the Internet works, you don't always see all seven layers of the OSI Model as separate layers. The OSI Model is a seven layer design model and the Internet only needs a four layer model to describe how it works. If you haven't already read the OSI Model tutorial, you should go back and look at it now.

The Internet only utilizes a few of the OSI Model's seven layers. Functionality of other layers is combined in some cases or not used at all. For example, the network interface device is said to compose the Physical and Data Link layers of the OSI Model, while a web browser performs the functions of the OSI Model's Session, Presentation and Application layers.

Telnet likewise handles Session, Presentation and Application issues. A web browser or a telnet session utilize IP addresses to reach the computer they are communicating with. Every device that communicate with a local network will use a MAC address that is unique on that LAN (but only if the computing device is using an 802.x protocol). IP addresses are mapped to specific MAC addresses on a local LAN.

 

At each layer, information unique to that layer is used to identify the specific computer and specific service that is being communicated with.

 

OSI Protocol Suite

 

Just to confuse things, The ISO also outlined a suite of protocols they called "OSI". This suite was based on the ISO's OSI Design Model. Don't confuse the OSI protocols with the OSI MODEL. Just learn the OSI Model as shown in this tutorial. Very few organizations use the OSI protocol suite today.

Why should you be aware of the ISO's OSI Protocol suite? Around 1984-5 certain people in the government started to see that network integration could be very powerful. When these government people turned to their technical people and asked them what they should do, the technical people told them to use something that follows the OSI Model to ensure that there are no problems with integration. The Government people took the words OSI and integration and turned it into a buzzword. The government next set about making sure a suite of protocols that exactly conformed to the OSI model was created. The government set the National Institute of Standards and Technology (NIST) to work writing a document outlining a government standard called the Government Open Standards Interconnect Profile or GOSIP.

 

Government Open Standards Interconnect Profile (GOSIP)

 

THE FEDERAL INFORMATION PROCESSING STANDARD (FIPS) #146: Government Open Systems Interconnection Profile (GOSIP)
National Institute of Standards and Technology (NIST), 1990
In 1990, NIST released a document called The Federal Information Processing Standard (FIPS #146) which outlined the GOSIP. All vendors were asked to comply with government offices implementing this profile starting around August of 1990.
Unfortunately, by the time the Government got around to 'standardizing' and 'implementing the GOSIP standard' the Internet had already fully implemented TCP/IP as its own standard and was using it exclusively. TCP/IP and GOSIP were NOT compatible. The government, caught with a technology that no longer matched the de-facto standard used on the Internet then mandated that the IETF and the IAB make the Internet compatible with GOSIP even though the OSI GOSIP software was not yet finished and not fully standardized even by 1990 when the FIPS #146 document was published.
If you take a look at the NIST website today, you will see that FIPS 146 / GOSIP is no longer listed, a legacy of a bygone era.
Why do we bring up GOSIP? Because when dealing with old, crusty government and military networking folks, you will hear references to the OSI protocols, FIPS 146 and GOSIP, because it takes a looong, loooooongg time for government programs and protocols to die.
Fortunately, most government offices have now fully embraced the TCP/IP suite.

 

SIMPLEX

 

Simplex communication is permanent unidirectional communication. Some of the very first serial connections between computers were simplex connections. For example, mainframes sent data to a printer and never checked to see if the printer was available or if the document printed properly since that was a human job. Simplex links are built so that the transmitter (the one talking) sends a signal and it's up to the receiving device (the listener) to figure out what was sent and to correctly do what it was told. No traffic is possible in the other direction across the same connection.

You must use connectionless protocols with simplex circuits as no acknowledgement or return traffic is possible over a simplex circuit. Satellite communication is also simplex communication. A radio signal is transmitted and it is up to the receiver to correctly determine what message has been sent and whether it arrived intact. Since televisions don't talk back to the satellites (yet), simplex communication works great in broadcast media such as radio, television and public announcement systems.

 

HALF DUPLEX

 

A half duplex link can communicate in only one direction, at a time. Two way communication is possible, but not simultaneously. Walkie-talkies and CB radios sort of mimic this behavior in that you cannot hear the other person if you are talking. Half-duplex connections are more common over electrical links. Since electricity won't flow unless you have a complete loop of wire, you need two pieces of wire between the two systems to form the loop. The first wire is used to transmit, the second wire is referred to as a common ground. Thus, the flow of electricity can be reversed over the transmitting wire, thereby reversing the path of communication. Electricity cannot flow in both directions simultaneously, so the link is half-duplex.

 

FULL DUPLEX

 

Full duplex communication is two-way communication achieved over a physical link that has the ability to communicate in both directions simultaneously. With most electrical, fiber optic, two-way radio and satellite links, this is usually achieved with more than one physical connection. Your telephone line contains two wires, one for transmit, the other for receive. This means you and your friend can both talk and listen at the same time.
Half or Full-Duplex is required for connection-oriented protocols such as TCP. A duplex circuit can be created by using two separate physical connections running in half duplex mode or simplex mode. Two way satellite communication is achived using two simplex connections.

 

Packets

Packets are a stream of binary octets of data of some arbitrary length. It is typically used to describe chunks of data created by software, not by hardware. Internet Protocol creates packets. The term packet is NOT synonymous with the term frame even though many people make that mistake. Information that has been broken into packets is sometimes described as having been packetized.

 

Types of Packets

 

Internet Protocol is often described as transmitting packets. Most protocols OSI Model Layer 3 communicate using packets.

 

Contents of Packets

 

Packets contain a header which contains network layer protocol data and a payload which carries user data or data from upper layer protocols.

Packet Header

The packet header contains logical addressing information, such as the source and destination IP addresses, and the payload section contains data. It also contains information about the size of the packet, the network layer protocol's options. The header may also contain a checksum field that allows reliable network protocols to detect errors in the packet.

 

Packet Payload

 

The payload section of a packet contains data from upper layer protocols. The payload section ofInternet Protocol packets contains or UDP segments.

 

ASYNCHRONOUS

 

Asynchronous communication utilizes a transmitter, a receiver and a wire without coordination about the timing of individual bits. There is no coordination between the two end points on just how long the transmiter leaves the signal at a certain level to represent a single digital . Each device uses a clock to measure out the 'length' of a bit. The transmitting device simply transmits. The receiving device has to look at the incomingsignal and figure out what it is receiving and coordinate and retime its clock to match the incoming signal.

Sending data encoded into yoursignal requires that the sender and receiver are both using the same enconding/decoding method, and know where to look in the signal to find data. Asynchronous systems do not send separate information to indicate the encoding or clocking information. The receiver must decide the clocking of the signal on it's own. This means that the receiver must decide where to look in the signal stream to find ones and zeroes, and decide for itself where each individual bit stops and starts. This information is not in the data in the signal sent from transmitting unit.

When the receiver of a signal carrying information has to derive how that signal is organized without consulting the transmitting device, it is called asynchronous communication. In short, the two ends do not always negotiate or work out the connection parameters before communicating. Asynchronous communication is more efficient when there is low loss and low error rates over the transmission medium because data is not retransmitted and no time is spent setting negotiating the connection parameters at the beginning of transmission. Asynchronous systems just transmit and let the far end station figure it out. Asynchronous is sometimes called "best effort" transmission because one side simply transmits, and the other does it's best to receive.
EXAMPLES:
Asynchronous communication is used on RS-232 based serial devices such as on an IBM-compatible computer's COM 1, 2, 3, 4 ports. Asynchronous Transfer Mode (ATM) also uses this means of communication. Your PS2 ports on your computer also use serial communication. This is the method is also used to communicate with an external modem. Asynchronous communication is also used for things like your computer's keyboard and mouse.
Think of asynchronous as a faster means of connecting, but less reliable.

 

SYNCHRONOUS

 

Synchronous systems negotiate the communication parameters at the data link layer before communication begins. Basic synchronous systems will synchronize both clocks before transmission begins, and reset their numeric counters for errors etc. More advanced systems may negotiate things like error correction and compression.
It is possible to have both sides try to synchronize the connection at the same time. Usually, there is a process to decide which end should be in control. Both sides can go through a lengthy negotiation cycle where they exchange communications parameters and status information. Once a connection is established, the transmitter sends out a signal, and the receiver sends back data regarding that transmission, and what it received. This connection negotiation process takes longer on low error-rate lines, but is highly efficient in systems where the transmission medium itself (an electric wire, radio signal or laser beam) is not particularly reliable.

 

Circuits

 

A circuit is another word for circle or loop. In computers, a circuit is continuous path along which electricity flows.
Since most computer technology uses electricity and electricity won't function unless the circuit is open, the concept of a circuit is important. For most discussions, the term circuit simply refers to an electrical loop. You see, electricity is a funny animal. It won't flow outwards unless there is a return path back to the source. This means that electricity really does flow in a circle, or 'circuit'. This concept is also used to describe other things like radio communication and fiber optics as well because they too are energy and behave much as electricity does.
There are several terms used in describing the behavior, conditon or state of a circuit.

 

OPEN, Closed and SHORT CIRCUITS

An OPEN circuit is an incomplete electrical pathway. A cut in the wire, a blown fuse, anything that increases resistance to electricity somewhere in the path creates an open circuit. Note that cutting or disconnecting a portion of an electrical circuit creates an 'air gap' between the wiring or conductors. This air gap increases the electrical resistance of the circuit and breaks the electrical connection.

Closed Circuit

A closed circuit is a path which could allow electricity to flow across the circuit from an area of strong charge to an area of weak charge. It is important to remember that a closed circuit might not actually have an electrical current applied to it, but still be a closed circuit.

Short Circuit

A SHORT is when the electricity doesn't flow along the expected path. This could be because the connection is cross-connecting somewhere else, or grounding to another source of electricity. Simply put, the circuit is completing before it gets to where we actually want the electricity to flow to. This is why we talk about burnt wiring having shorted out.
Here's a simple trick to remembering the difference between the terms. Think about the classic science experiment with a battery, a light bulb, some wire and one of those funny-looking switches with the long lever and big contacts. The wires connected the battery to the switch and the bulb. When the switch lever was lifted, the circuit is open. When the switch lever is down, the circuit is closed and the lightbulb lights up. Note that if you leave the switch lever down until the battery goes completely dead, the electrical circuit is still considered a closed circuit.
If our little closed circuit is crossed with a separate piece of wire that completes the circuit before it gets to the bulb, it causes the electricity to run across a shorter path, and is called a short circuit.

 

Transmission Media

Data is transmitted over copper wires, fiber optic cable, radio and microwaves. The term 'media' is used to generically refer to the physical connectors, wires or devices used to plug things together.

 

Basic Communications Media Types

 

• Copper
• Unshielded Twisted Pair (3,5,5e,6,7)
• Shielded Twisted Pair
• Coaxial Cable (Thinnet, Thicknet)
• Heliax
• Fiber Optic
• Single-mode
• Multi-mode
• Infrared
• Radio & Microwave

COPPER

Coaxial Cabling

Coaxial cabling is used in bus-style Ethernet networks. Coaxial cable consists of a copper wire core surrounded by a plastic cladding sheathed in a wire mesh. Coaxial cable comes in two sizes which are called thinnet and thicknet.

 

Unshielded Twisted Pair (UTP)

 

If you use two pairs of wires to enable two communications circuits, one for transmit, and one for receive. If you twist the wires of each pair, you can place them much closer together. There are several grades of coaxial cable with category ratings. There are Category 3 (<10 Mbps), Category 5 (10 Mbps), Category 5e (10/100 Mbps) and Category 6 (100/1000 Mbps) versions of unshielded twisted pair.

 

Fiber Optic

 

Single Mode

Single mode fiber refers to the fact that only a single wavelength (one color of light) is transmitted over the physical medium. Typically, single mode fiber is true doped fiberglass fibers wrapped in a plastic cladding. Single mode typically has much longer reach, but a larger bend radius than multi-mode.

• Dispersion Shifted
• Non-Dispersion Shifted
• Non-Zero Dispersion-Shifted

Multi-Mode

Multi-mode fiber can carry multiple wavelengths, is made of special clear plastic materials and has a much smaller bend radius than single mode fiber. Multi-mode does not have as long a reach as single mode fiber.

• Step Index
• Graded-Index

Infra-Red

 

There are many systems today using infra-red communications. This is usually a directional infrared light signal transmitted into the air and received by nearby devices. Such systems came into use in the early 90's for use with laptops, printers and later in the 90's with cameras and handhelds.

Radio and Microwave

 

Last of all, but certainly not least are radio and microwave signals. These are the signals we think of as being radio, television and satellite, but they are now being put to use in wireless Ethernet and Bluetooth communications technologies.

Serial vs. Parallel

The two most basic types of communication are serial and parallel. They are so common that even the cabling bears the name serial cable and parallel cable. Since electricity behaves according to the laws of physics, it is impossible to get the electrical signal to go any faster. There are two ways to get the data from one place to the other faster. The first is to squish the data bits tighter together (leave less distance between them when they travel down the wire). The second way is to transmit more bits simultaneously.
Keep in mind that the information below is very general and not exactly correct from an engineering standpoint. We're just focusing on getting you to understand concepts here.

 

SERIAL

When information is sent across one wire, one data bit at a time, its called serial. Every computer on the face of the earth has some form of serial communications connector on it, whether internally or externally. Most people are familliar with the 'D' shaped 9-pin connector on the back of thier computer. This is a serial connector. The typical 9-pin 'D' shaped connector on the back of your computer uses 2 loops of wire (1 in each direction) for data communication, plus additional wires to control the flow of information. However, in any given direction, data is still flowing over a single wire.

 

PARALLEL

Instead of squishing bits together, bits are sent over more wires simultaneously. In the case of an 25-pin parallel port, you have eight data-carrying wires so that eight bits can be sent simultaneously. Because there are 8 wires to carry the data, the data finishes being transferred eight times faster than a serial connection.

 

Point to Point, Multi-point and Broadcast

 

There are several types of communication, partly determined by the method of connecting the hosts on the network

.

POINT TO POINT

This is used in serial communication. These types of connections are assumed to be transmitting solely to the station at the opposite end. Because there is only one device at either end of the connection, no addresses are necessary.
Examples:

• EIA/TIA 232
• EIA/TIA 449
• X.25
• Frame Relay
• T-carrier
• OC3 - OC192

POINT TO MULTI-POINT (Non-Broadcast)

Point to multipoint connects one station to several other stations. In non-broadcast systems, the communication is replicated to all the remote stations. Only specific selected stations hear the replicated communication.
Examples:

• Frame Relay
• ATM

POINT TO MULTI-POINT (Broadcast)

Broadcast multipoint protocols are characterized by a physical medum that connects to all machines and where all communication is heard by all stations.
Examples:

• Token Ring
• FDDI
• Ethernet

Peer-to-Peer

 

Peer-to-Peer is a term used in networking to refer to a network built on a set of stand alone computers. Each computer is autonomous and it's functionality is independant of all other systems. Network addresses are often configured manually, and any settings for directory shares, printers and other services are unique to, and accessable from each sysetem. All access, logons, services and policies are provided to the users of that computer only. There is no centralized management in the network whatsoever. With no central database for logins and access, each user will have a separate login for each machine. This is fine for small networks, but does not allow them to grow larger (scale) very well.

In peer to peer networks, every single machine must have a separate account for each human who wants to use the machine. Worse, each machine can potentially have a different human administrator. This makes maintaining corporate-wide policies impossible, as each administrator can do whatever they want with their machine. To make things even more nightmareish, an administrator will often have to run round to each computer individually and log in locally in order to make changes. This keeps the admin running all day long, typically in circles and never rarely anything accomplished.

However, if only a few machines exist, and each user can be trusted, this model can often be more productive.

Client Server Model

The term "Client-Server model" has several uses but all are based around the same concept of using a client system to access a centralized server in order to use a service.
When a network protocol is described as being 'client-server', we mean that the application runs as a service on a server, and a client version of the application is used to access the service.
The term 'client-server network' is one where a centralized server provides network services to other clients on the network. Such a network is described as a 'domain' in Linux, Macintosh, Unix, and Windows operating systems.

 

Connection Oriented vs. Connectionless

The terms connection oriented and connectionless are descriptive words used to describe different kinds of communication.

 

Connection-Oriented:

means that when devices communicate, they perform handshaking to set up an end-to-end connection. The handshaking process may be as simple as syncrhonization such as in the transport layer protocol TCP, or as complex as negotiating communications parameters as with a modem.
Connection-Oriented systems can only work in bi-directional communications environments. To negotiate a connection, both sides must be able to communicate with each other. This will not work in a unidirectional environment.

 

Connectionless :

means that no effort is made to set up a dedicated end-to-end connection.
Connectionless communication is usually achieved by transmitting information in one direction, from source to destination without checking to see if the destination is still there, or if it is prepared to receive the information. When there is little interferance, and plenty of speed available, these systems work fine. In environments where there is difficulty transmitting to the destination, information may have to be re-transmitted several times before the complete message is received.

Walkie-talkies, or Citizens Band radios are a good examples of connectionless communication. You speak into the mike, and the radio transmitter sends out your signal. If the person receiving you doesn't understand you, there's nothing his radio can do to correct things, the receiver must send you a message back to repeat your last message.
IP, UDP, ICMP, DNS, TFTP and SNMP are examples of connectionless protocols in use on the Internet.

 

Reliable vs. Unreliable

The terms reliable and unreliable don't refer to whether it works or not. It refers to whether something is done to guarantee

 

RELIABLE

End stations running reliable protocols will work together to verify the transmission of data to ensure accuracy and integrity of the data. A reliable system will set up a connection and verify that: all data transmitted is controlled in an orderly fashion, is received in the correct order and is intact. Reliable protocols work best over physical medium that loses data, and is prone to errors. The error correction, ordering and verification mechanisms require overhead in the data packets and increase the total ammount of bandwidth required to transmit data. Transmission Control Protocol (TCP) is a typical reliable protocol. TCP often usually adds an average of 42-63 bytes of overhead to datagrams. For a Telnet connection which transmits each keystroke individually, this is horribly inefficient because up to 64 bytes of data are transmitted to communicate just 1 byte of useful information.

 

UNRELIABLE

Unreliable protocols make no effort to set up a connection, they don't check to see if the data was received and usually don't make any provisions for recovering from errors or lost data. Unreliable protocols work best over physical medium with low loss and low error rates. User Datagram Protocol (UDP) is an example of an unreliable protocol. UDP makes no provisions for verifying whether data arrived or is intact. However, UDP adds a minimum of overhead when compared to TCP and is thus much faster for data transfers over high quality physical links that are high speed and exhibit little or no errors in communication.

 

Cyclic Redundancy Check

 

A Cyclic Redundancy Check is a method of detecting errors in a block of data by performing a math calculation on a stream of data in such a way that the final calculated CRC value is a relatively unique value and unlikely to occur with a different stream of data. Thus, the CRC value can be used to verify the integrity of the data being stored or transmitted.
The CRC value is guaranteed to be sufficiently unique that any pattern of data that differs by even a single bit has almost no chance of producing the same CRC value. Thus, it becomes possible to detect whether a block of data was corrupted in the process of being transferred. CRC checking is very fast because it performs a mathematical calculation as opposed to a lookup or crosscheck. It does not guarantee 100% accuracy, but it is sufficient for most applications.

CRC checking is used in Telecom circuits over microwave and dedicated lines and is used in ATM, Digital Signalling, Ethernet, FDDI, Token Ring and many other protocols. computer systems perform internal CRC checking when accessing large blocks of data from hard disks or transferring data over serial connections or network cards.

 

802.1Q

 

The IEEE's 802.1Q standard was developed to address the problem of how to break large networks into smaller parts so broadcast and multicast traffic wouldn't grab more bandwidth than necessary. The standard also helps provide a higher level of security between segments of internal networks.
The 802.1Q specification establishes a standard method for inserting virtual LAN (VLAN) membership information into Ethernet frames.
A VLAN is an administratively configured LAN or broadcast domain. Instead of going to the wiring closet to move a cable to a different LAN, network administrators can accomplish this task remotely by configuring a port on an 802.1Q-compliant switch to belong to a different VLAN. The ability to move endstations to different broadcast domains by setting membership profiles for each port on centrally managed switches is one of the main advantages of 802.1Q VLANs.