A computer network is basically a group of multiple networking devices connected together for data sharing. Collectively networking devices have single purpose “securely transfer data as fast as they can”. To achieve this goal every networking device has its own functionality.
Let’s have a look on some common networking devices.
Network Interface Card (NIC)
In the list of networking devices, NIC stands on first place. Without this device, networking cannot be done. This is also known as network adapter card, Ethernet Card and LAN card. NIC allows our PC to communicate with other PCs. Basically it converts data transmission technology. A PC uses parallel data transmission technology to transmit data between its internal parts while the media that connects this PC with other PCs uses serial data transmission technology. A NIC converts parallel data stream into serial data stream and vice versa serial data stream is get converted in parallel data stream.
Usually all modern PCs have integrated NICs in motherboard. NICs are also available separately. For desktop or server system they are available in adapter format which can be plugged into the available slots of motherboard. For laptop or other small size devices they available in PCMCIA (Personal Computer Memory Card International Association) card format which can be inserted in PCMCIA slots.
Types of NICs
There are two types of NICs
Media Specific :- Different types of NICs are required to connect with different types of media. For example we cannot connect wired media with wireless NIC card. Just like this, we cannot connect coaxial cable with Ethernet LAN card. We have to use the LAN card that is particularly built for the media type which we have.
Network Design Specific :- A specific network design needs a specific LAN card. For example FDDI, Token Ring and Ethernet have their own distinctive type of NICs card. They cannot use other’s NIC card.
HUB is used to connect multiple computers in a single workgroup LAN network. Typically HUBs are available with 4,8,12,24,48 ports. Based on port type, there are two types of HUB:-
Ethernet HUB :- In this type of HUB all ports have RJ-45 connectors.
Combo HUB :- In this type of HUB ports have several different types of connectors such RJ-45, BNC, and AUI.
HUBs generally have LED (light-emitting diode) indicator lights on each port to indicate the status of link, collisions, and other information.
To understand the functionality of hub let’s take an example from real life.
There are four friends who share everything. One of them finds a photo of Amitabh Bachchan. To share this with friends, he will make three photo copies from Xerox machine and give one copy to each friend. He doesn’t need a copy of photo for himself as he has the original one.
Now change the characters in this example. Replace friends with HUB’s port, photo with data signal and Xerox machine with HUB.
There is a HUB which has four ports. Ports share everything. One port received data signal from its connected device. It will make three copies of data signal from HUB and give one copy to each port. Receiver port doesn’t need a copy of data signal for itself as it has it the original version.
This is what exactly a HUB do. When a hub receives signal on its port, it repeats the signal and forwards that signal from all ports except the port on which the signal arrived.
There are two types of HUB
Passive HUB:- It forwards the data signal from all ports except the port on which signal arrived. It doesn’t interfere in data signal.
Active HUB:- It also forwards the data signal from all ports except the port on which signal arrived. But before forwarding, it improves quality of data signal by amplifying it. Due to this added features active HUB is also known as repeaters.
Usually HUB has one or more uplink ports that is used to connect it with another HUB. Right cable type is required to connect two HUBs.
Logically HUB creates a star topology where it sits in the center of the topology and all connected systems stays at the points of the star.
Physically HUB creates a bus topology where all connected systems share the same bus connection.
There are two similar devices to HUB, MAU and Patch Panel.
MAU :- MAU (Multi Access Unit) is the sibling of HUB for token ring network. The only differences between HUB and MAU are :-
- HUB is used for Ethernet Network while MAU is used for Token ring network.
- HUB creates logically star topology while MAU creates logically ring topology.
Patch Panel :- It is used to organize the UTP cables systematically. It doesn’t interfere in data signal.
HUB and Repeater works at Layer 1 (Physical layer). These devices only understand the signals. Signals received on incoming port are forwarded from all available ports.
While signals are being forwarded no other ports should receive another signals (data) form their connected devices. If other port receives signals (data) while current transmission is going on, it will create collision.
Collision is the effect of two devices sending transmissions simultaneously in Ethernet. When they meet on the physical media, the signals from each device collide and damaged.
Collision domain is the group of devices that share same collision effects over the Ethernet network.
It is a mechanism of removing collision from network. When two or more nodes simultaneously sense the wire and found no signals, and each device places it’s signal on the wire. These signals would be collide in wire and a collision will occur.
Before placing any signal in wire, NIC (Network Interface Card) examines wire for any existing signal. This method is known as CS (Carrier Sense).
If two NICs sense wire on exactly same time and see no signal then both will place their signals in wire. This is known as MA (Multiple Access).
If the NICs see a collision for their transmitted signals, they have to resend the signals. In this situation, each NIC that was transmitting a frame when a collision occurred creates a special signal, called a jam signal, on the wire, waits a small random time period, and examine the wire again. If there is no signal in the wire, NIC will retransmit its original signals again.
The more devices you place on a segment, the more likely you are going to experience the collisions. More devices means more random time interval, creating even more collisions, gradually slowing down a device’s access when trying to transmit the data.
If a 100Mbps HUB has 10 ports, it means each port will effectively get only 10 Mbps (one tenth of total bandwidth.) Situation can be more worse if we connect this HUB with another HUB on uplink port. Each new node will decrease the available bandwidth for other nodes in network.
Bridge solves these (bandwidth and collision) major issues of HUB.
To improve the performance, usually networks are divided in smaller segments. Bridge is used to divide a large network in smaller segments. For example a network has 70 nodes. Without segmentation all these nodes will share same collision domain that will bring down overall network performance. To run a network smoothly we should not place more than 20 nodes in a collision domain. To deal with this situation we can use Bridge. Bridge has per port collision domain. It means if a port faces collision, other ports will not effect from this collision. If we use a four ports bridge in our example network, we will get four collision domains.
Basic function of Bridge are following :-
- Break a large network in smaller segments.
- Join different media types such as UTP with fiber optic.
- Join different network architectures such as Ethernet with Token Ring.
A bridge can connect two different types of media or network architecture but it cannot connect two different types of network layer protocol such as TCP/IP or IPX. Bridge requires same network layer protocol in all segments.
There are three types of bridge:-
Local Bridge :- This bridge connects two LAN segments directly. In Ethernet Implementation it is known as Transparent bridge. In Token Ring network it is called Source-Routed bridge.
Remote Bridge :- This bridge connects with another bridge over the WAN link.
Wireless Bridge :- This bridge connects with another bridge without wiring between them.
In OSI Layer model Bridge works at physical layer and data link layer.
Bridges have following issues :-
- Bridges have limited ports.
- In bridge forward decision are made through the software which slow down overall performance of network.
- Bridges use age old technology which is not capable to fulfill the requirement of modern networks effectively.
Switch and Router solves these issues.
Just like Hub and Bridge, switch is also used to connect multiple computers together in a LAN segment. Switches available with 4,8,12,24,48,64 ports. Each switch port has a separate collision domain. Switch works at layer two in OSI Layer model. At layer two data signals are formatted in frames.
When a switch receives frame, it checks FCS (Frame checksum sequence) field in it. Switch process the frame only if it is valid. All invalided frames are automatically dropped. All valid frames are processed and forwarded to their destination MAC address.
Switch makes their switching decisions in hardware by using application specific integrated circuits (ASICs). Unlike generic processor such as we have in our PC, ASICs are specialized processors built only to perform very few particular tasks. In cisco switch ASICs has single task, switch frames blazingly fast. For example an entry level catalyst 2960 switch has frame rate of 2.7 million frames per second. Higher end switches have higher FPS rate such as Catalyst 6500 has a rate of 400 million FPS rate.
Switches support three methods of switching.
- Store and Forward
- Cut and Through
- Fragment Free
Store and Forward
This is the basic mode of switching. In this mode Switch buffers entire frame into the memory and run FCS (Frame Check Sequence) to ensure that frame is valid and not corrupted. A frame less than 64bytes and higher than 1518bytes is invalid. Only valid frames are processed and all invalid frames are automatically dropped. Among these three methods, this method has highest latency. Latency is the time taken by device in passing frame from it.
Cut and Through
Cut and Through method has lowest latency. In this method Switch only read first six bytes from frame after the preamble. These six bytes are the destination address of frame. This is the fastest method of switching. This method also process invalid frames. Only advantage of this method is speed.
This is a hybrid version of Store and Forward method and Cut and Through method. It takes goodies from both methods and makes a perfect method for switching. It checks first 64 bytes of frame for error. It processes only those frames that have first 64bytes valid. Any frame less than 64 bytes is known as runt. Runt is an invalid frame type. This method filters runt while maintaining the speed.
In simple language modem is a device that is used to connect with internet. Technically it is a device which enables digital data transmission to be transmitted over telecommunication lines. A Telco company uses entirely different data transmission technology from the technology that a PC uses for data transmission. A modem understands both technologies. It converts the technology that a PC uses in the technology which a Telco company understand. It enables communication between PC (Known as DTE) and Telco company’s office (Known as DCE).
There are two types of connection line between DCE and DTE
Analog connection line
An existing telephone or cable TV network line that uses analog signals (sound waves) for transportation. Instead of supporting Internet, these lines were primarily installed for their respective requirements.
Digital connection Line
A separate connection line between DTE and DCE. Since it is installed primarily for internet, it uses digital signals for data transportation.
For analog connection line we have to use analog modem and for digital line we need to use digital modem.
Analog modem converts analog signal in digital signal and vice versa.
There are two types of analog modem; internal and external.
Internal modem is available as interface card for desktop and as PCMCIA card for laptop . We need to install it on available slot of motherboard. In comparison with external modem these are inexpensive. As these modems usage computer’s CPU for data encoding and decoding. We have to purchase these modem separately.
External modem is a separate device that has its own CPU and memory. Usually Telco company provide this with connection. Depending on subscription it may be free or chargeable.
Instead of signal conversion, digital modem performs modulation known as line coding. Line coding is used to modulate the digital signal in such a way that they can be transmitted over the digital line. DSL, ADSL and ISDN modem are the examples of digital modems.
DTE (Data Terminal Equipment) is a device (usually a router or PC) that converts data frame into signals and reconvert received signals in data frame. DTE device communicates with DCE device.
DCE (Data circuit terminating equipment) is a device (usually modem, CSU/DSU or Frame Relay switch) that provides clock rate and synchronization.
A CSU/DSU (Channel Service Unit/Data Service Unit) is a device that converts data signal between LAN network and WAN network. LAN network and WAN network uses separate communication technology. A CSU/DSU understands both technologies. DSL and cable modems are the example of CSU/DSU.
Router is a layer three device which forwards data packet from one logical network segment to another. Router forwards packets on the bases of their destination address. For this router keeps record of the path that packets can use as they move across the network. These records are maintained in a database table known as routing table. Routing table can be built statically or dynamically.
Basically routers are used :-
- To connect different network segments.
- To connect different network protocols such as IP and IPX.
- To connect several smaller networks into a large network (known as internetwork)
- To break a large network in smaller networks (Known as subnet usually created to improve the performance or manageability)
- To connect two different media types such as UTP and fiber optical.
- To connect two different network architectures such as token ring and Ethernet.
- To connect LAN network with Telco company’s office (Known as DTE device).
- To access DSL services (known as DSL Router).
Brouters are the combination of router and bridge. It can be used as a bridge or router. Brouters are the earlier implementation of the routers.
At layer two it’s a fairly expensive device. which cost more than other high end switches that work much faster than it. At layer three it has a lot of complexity. Due to these drawbacks it is rarely used. Gradually it has been replaced by high end switch at layer 2 and by router at layer three.
Gateway is used to forward the packets which are intended for remote network from local network. Till host is configured with default gateway address, every packet should have default gateway address. A default gateway address is the address of gateway device. If packet does not find its destination address in local network then it would take the help of gateway device to find the destination address in remote network. A gateway device knows the path of remote destination address. If require, it also change the encapsulation of packet so it can travel in other network to get its destination address.
Examples of Gateway
Email Gateway :- Translate SMTP e-mail in standard X.400 format before forwarding.
GSNW Gateway :- Allow windows clients to access resources from NetWare server.
PAD Gateway :- Provides connectivity between LAN network and X.25 network.
Transceivers (media converters)
Transceiver is a small device that has capability of receiving and sending both analog and digital signals. Usually it is inbuilt in network interface card. But it is also available separately in market. It detects the type of signal from network wire and converts the passing signal to match with it. For example a transceiver is attached with a device that transmit signal in digital from. Now suppose this device is connected with the network wire that uses analog form for data transmission. In this case transceiver will convert digital signal in analog signal before placing them in network wire.
Proxy can be a dedicate device or software. Proxy is used to hide the internal network from external world. If we use proxy then there would be no direct communication between internal network and external network. All communication will go through the proxy. External computer will be able to access only proxy. Thus Proxy makes tampering with an internal system from the external network more difficult.