basic networking

ATM

2009-07-04T00:05:00.001-07:00

Asynchronous Transfer Mode (ATM) is a network protocol that transmits data at a speed of 155 Mbps and higher. ATM works by transmitting all data in small packets of a fixed size; whereas, other protocols transfer variable length packets. ATM supports a variety of media such as video, CD-quality audio, and imaging. ATM employs a star topology, which can work with fiber optic as well as twisted pair cable.
ATM is most often used to interconnect two or more local area networks. It is also frequently used by Internet Service Providers to utilize high-speed access to the Internet for their clients. As ATM technology becomes more cost-effective, it will provide another solution for constructing faster local area networks.

FDDI

2009-07-03T23:54:00.000-07:00

Fiber Distributed Data Interface (FDDI) is a network protocol that is used primarily to interconnect two or more local area networks, often over large distances. The access method used by FDDI involves token-passing. FDDI uses a dual ring physical topology. Transmission normally occurs on one of the rings; however, if a break occurs, the system keeps information moving by automatically using portions of the second ring to create a new complete ring. A major advantage of FDDI is speed. It operates over fiber optic cable at 100 Mbps.

Token Ring

2009-07-03T23:51:00.000-07:00

Token Ring
Unlike Ethernet, Token Ring uses a ring topology whereby the data is sent from one machine to the next and so on around the ring until it ends up back where it started. It also uses a token passing protocol which means that a machine can only use the network when it has control of the Token, this ensures that there are no collisions because only one machine can use the network at any given time.
The Basics
Here is an animated GIF that shows the basic operation of a Token Ring, and below is an explanation of what is going on.

Although 16Mbps is the standard ring speed these days (and Fast Token Ring is being developed) we will consider a 4Mbps Token Ring in this tutorial to explain the basic concepts.

At the start, a free Token is circulating on the ring, this is a data frame which to all intents and purposes is an empty vessel for transporting data. To use the network, a machine first has to capture the free Token and replace the data with its own message.

In the example above, machine 1 wants to send some data to machine 4, so it first has to capture the free Token. It then writes its data and the recipient's address onto the Token (represented by the yellow flashing screen).

The packet of data is then sent to machine 2 who reads the address, realizes it is not its own, so passes it on to machine 3. Machine 3 does the same and passes the Token on to machine 4.

This time it is the correct address and so number 4 reads the message (represented by the yellow flashing screen). It cannot, however, release a free Token on to the ring, it must first send the message back to number 1 with an acknowledgement to say that it has received the data (represented by the purple flashing screen).

The receipt is then sent to machine 5 who checks the address, realizes that it is not its own and so forwards it on to the next machine in the ring, number 6.

Machine 6 does the same and forwards the data to number 1, who sent the original message.

Machine 1 recognizes the address, reads the acknowledgement from number 4 (represented by the purple flashing screen) and then releases the free Token back on to the ring ready for the next machine to use.

That's the basics of Token Ring and it shows how data is sent, received and acknowledged, but Token Ring also has a built in management and recovery system which makes it very fault tolerant. Below is a brief outline of Token Ring's self maintenance system.

Token Ring Self Maintenance
When a Token Ring network starts up, the machines all take part in a negotiation to decide who will control the ring, or become the 'Active Monitor' to give it its proper title. This is won by the machine with the highest MAC address who is participating in the contention procedure, and all other machines become 'Standby Monitors'.

The job of the Active Monitor is to make sure that none of the machines are causing problems on the network, and to re-establish the ring after a break or an error has occurred. The Active Monitor performs Ring Polling every seven seconds and ring purges when there appears to be a problem. The ring polling allows all machines on the network to find out who is participating in the ring and to learn the address of their Nearest Active Upstream Neighbour (NAUN). Ring purges reset the ring after an interruption or loss of data is reported.

Each machine knows the address of its Nearest Active Upstream Neighbour. This is an important function in a Token Ring as it updates the information required to re-establish itself when machines enter or leave the ring.

When a machine enters the ring it performs a lobe test to verify that its own connection is working properly, if it passes, it sends a voltage to the hub which operates a relay to insert it into the ring.

If a problem occurs anywhere on the ring, the machine that is immediately after the fault will cease to receive signals. If this situation continues for a short period of time it initiates a recovery procedure which assumes that its NAUN is at fault, the outcome of this procedure either removes its neighbour from the ring or it removes itself.

Token Ring Operation using a Hub

A Token Ring hub simply changes the topology from a physical ring to a star wired ring. The Token still circulates around the network and is still controlled in the same manner, however, using a hub or a switch greatly improves reliability because the hub can automatically bypass any ports that are disconnected or have a cabling fault.

Further advancements have been made in recent years with regard to Token Ring technology, such as early Token release and Token Ring switching but as this site is primarily concerned with cabling issues we will not go into any more detail here.

LocalTalk

2009-07-03T23:50:00.000-07:00

LocalTalk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. LocalTalk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established.
The LocalTalk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 Kbps.

Ethernet

2009-07-03T23:48:00.000-07:00

Ethernet is developed by DIX (Digital, Intel and Xerox) in the 1970s. In 1980 the IEEE 802.3 standard was released. The Ethernet protocol is by far the most widely used. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear. Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, the delay caused by collisions and retransmitting is very small and does not normally effects the speed of transmission on the network. The Ethernet protocol allows for linear bus, star, or tree topologies. Data can be transmitted over wireless access points, twisted pair, coaxial, or fiber optic cable at a speed of 10 Mbps.

OK, lets begin the lesson. Ethernet uses a protocol called CSMA/CD, this stands for Carrier Sense, Multiple Access with Collision Detection. To understand what this means lets separate the three parts.

Carrier Sense - When a device connected to an Ethernet network wants to send data it first checks to make sure it has a carrier on which to send its data (usually a piece of copper cable connected to a hub or another machine).

Multiple Access - This means that all machines on the network are free to use the network whenever they like so long as no one else is transmitting.

Collision Detection - A means of ensuring that when two machines start to transmit data simultaneously, that the resultant corrupted data is discarded, and re-transmissions are generated at differing time intervals.

Here are some animated GIF's to help explain basic Ethernet operation, below each one is a description of what is happening.
If you want to start an animation from the beginning hit your browsers refresh button.

The Basic Ethernet Bus

This is a coax based Ethernet network where all machines are daisy chained using RG58 coaxial cable (sometime referred to as Thin Ethernet or Thin-net).

Machine 2 wants to send a message to machine 4, but first it 'listens' to make sure no one else is using the network.

If it is all clear it starts to transmit its data on to the network (represented by the yellow flashing screens). Each packet of data contains the destination address, the senders address, and of course the data to be transmitted.

The signal moves down the cable and is received by every machine on the network but because it is only addressed to number 4, the other machines ignore it.

Machine 4 then sends a message back to number 2 acknowledging receipt of the data (represented by the purple flashing screens).

But what happens when two machines try to transmit at the same time? . . . . . a collision occurs, and each machine has to 'back off' for a random period of time before re-trying.

For the sake of simplicity I have omitted the acknowledgement transmissions from the rest of the animation's on this page.

Collisions

This animation starts with machine 2 and machine 5 both trying to transmit simultaneously.

The resulting collision destroys both signals and each machine knows this has happened because they do not 'hear' their own transmission within a given period of time (this time period is the propagation delay and is equivalent to the time it takes for a signal to travel to the furthest part of the network and back again).

Both machines then wait for a random period of time before re-trying. On small networks this all happens so quickly that it is virtually unnoticeable, however, as more and more machines are added to a network the number of collisions rises dramatically and eventually results in slow network response. Time to buy a switch!!!

The exact number of machines that a single Ethernet segment can handle depends upon the applications being used, but it is generally considered that between 40 and 70 users are the limit before network speed is compromised.

Using a Hub

An Ethernet hub changes the topology from a 'bus' to a 'star wired bus', here's how it works.

Again, machine 1 is transmitting data to machine 4, but this time the signal travels in and out of the hub to each of the other machines.

As you can see, it is still possible for collisions to occur but hubs have the advantage of centralised wiring, and they can automatically bypass any ports that are disconnected or have a cabling fault. This makes the network much more fault tolerant than a coax based system where disconnecting a single connection will bring the whole network down.

Using a Switch

To overcome the problem of collisions and other effects on network speed, a switch is used.

With a switch, machines can transmit simultaneously, in this case 1 & 5 first, and then 2 & 4. As you can see, the switch reads the destination addresses and 'switches' the signals directly to the recipients without broadcasting to all of the machines on the network.

This 'point to point' switching alleviates the problems associated with collisions and considerably improves network speed.

In the real world however, one or more of these machines will be servers, and as most network traffic is between the clients and a server a serious bottle neck can occur. The answer to this problem is to make server connections faster than the clients. The normal solution is to have the client machines on 100Mbs ports and the servers on 1000Mbs ports (Gigabit Ethernet). This ten to one ratio is usually adequate because not all of the clients will need to access the servers at the same time.

protocols

2009-07-03T23:09:00.000-07:00

What is a Protocol?

A protocol is a set of rules that governs the communications between computers on a network. These rules include guidelines that regulate the following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer. Common Protocols The most common protocols are:

What is computer Network?

2009-07-03T21:54:00.000-07:00

A computer network is a group of interconnected computers which able to communicate to each other.

Networking is the practice of linking two or more computing devices together for the purpose of sharing data. Networks are built with a mix of computer hardware and computer software.

Most networks use electrical wires of some special kind to convey signals and data between computers. However, numerous types of networking media, including wireless broadcast technologies and fiber optic cables can support networked connection as well. In other words, you can get from here to there in many ways on modern networks.

The basic computer network hardware for example are NIC (network interface card),repeater, hub, switch, and router.
To make computer can communicate to another, we need some protocols.

Let's see how a computer connect to another in a simple delineation.

on above figure, two computer are connected with a transmission line, e.g a cable.
the red line on that figure is represent two wire which actually connect the two of them using NIC.

See, the transmit line of the first computer connected to receive line on other computer.