Computer device that determines the best path out of the available path for a transmission.
# 2 main Softwares used in it:
1. Operating System
2. Routing Protocol
# Router uses Logical and Physical Address to connect two or more logically separate network.
# It converts large network into logical network segments or Sub Network.
# Each Sub Network has its own Logical Address.
It looks like the separation made these Sub Network secluded but they can still access other network also and can exchange data when needed.
# The Packet contains:
1. Physical Device Address and
2. Logical Network Address.
# Router can used to connect identical networks as well as network of different types of hardware.
# Bridges are used in LAN only whereas Router used in LAN, WAN, FDDI and others.
Compare to one giant LAN It would be good to use router supported Network.
BENEFITS
1. SECURITY: LAN uses broadcast mode and so security is less there. Though only the destinational computer can read data but the data has to go through whole network till it reached to the destination that makes LAN connection vunerable in case of security. Using Router can increase the security bu making whole LAN into segement.
2. RELIABILITY: If one network goes down the router isolates the affected network and send the message through another route.
3. PERFORMANCE ENHANCEMENT: It splits the network and so the traffic on those.
4. NETWORKING RANGE: In some network cable length can’t eceed 1000 meters. A Router can used there to increase the length.
TYPES OF ROUTERS
CISCO is the major manufacturer.
1. Edge Router: Routes data between LAN and ATM network. ATM (Asynchronous Transfer Mode)
They are placed at the edge of the ISP network.
2. Subscribe Edge Router: Used in Organization.
It uses EBGP Protocol also called Extended Border Gateway Protocol.
3. Inter-provided Border Router: Interconnect the Internet Service Provider.
BGP Protoco used here.
4. Core Router: When Router is used in LAN is called Core Router.
uses BGP protocol.
5. SOHO Router: used to connect network in small geographical area.
6. Wired and Wireless Router: Used in Office.
Some Wireless Routers use Firewall functions also.
BRAND IN ROUTERS
CISO, BET GEAR, LinkSys and D-Link.
Speed: 802.11a, 802.11g, 802.11n etc.
WORKING OF ROUTERS
# It is an Inter connectivity device.
# Router visualize the network path.
# The paths may be:
Electric Network.
Transport Network or
Phone Network.
# Routing Control Plane: Part of Router architect that concerned with drawing the network map.
# Routing Forwarding Plane: Part of the Router architect that decided hat to do with the packet arriving on an inbound Interface.
# Convergence: is the process of agreement, by all routers on OPTIMAL ROUTES. When a network event cause routes to fail or become unavailable the Convergence method is used to find out the route.
ASCII vs. EBCDIC
ASCII American Standard Code for Information Interchange also known as ANSI X3.4
7 bit code uses 8th bit as most significant bit for error checking.
Text files stored in ASCII format are sometimes called ASCII files. Text editors and word processors are usually capable of storing data in ASCII format, although ASCII format is not always the default storage format. Most data files, particularly if they contain numeric data, are not stored in ASCII format. Executable programs are never stored in ASCII format.
65 – 90 : A – Z
97 – 122 : a – z
48 – 57 : 0 – 9
EBCDIC Extended Binary Coded Decimal Interchange Code (pronounced either “ehb-suh-dik” or “ehb-kuh-dik”) is a binary code for alphabetic and numeric characters that IBM developed for its larger operating systems.
# Character Set used by IBM Mainframe.
# Use Full 8 bit code so there is no parity checking bit (as in ASCII).
# Wider range of control characters than ASCII.
0000 0000 to 0011 1111 : CONTROL CHARACTER
0100 0000 to 0111 1111 : PUNCTUATION CHARACTER
1000 0000 to 1011 1111 : LOWERCASE CHARACTER
1100 0000 to 1111 1111 : UPPERCASE CHARACTER AND NUMBERS
Why EBCDIC is better than ASCII?
# Easier to use on Punched Cards.
# Include the ⊄ (cent sign) that ASCII doesn’t.
Why ASCII is better than EBCDIC?
# Writing Code; Where EBCDIC is a mess.
# Most of the World runs on ASCII.
Standard version of EBCDIC does not have [ ] \ { } ^ ~ |.
# EBCDIC’s some variants are incompatible with other variants of EBCDIC.
Documentation is very hard on EBCDIC.
Every network textbook includes a picture of the famous “seven-layer” model. Actually, this model is vague, and it does not always correspond to specific portions of specific networks. Nevertheless, it serves as a useful overview of the field.
Physical Layer
This is concerned with the nature of the physical media (metal or optical cable, free-space microwave, etc.) used to send signals, the nature of the signals themselves, and so on.
There is also the question of signal form; the signals themselves may be in the form of pure 0-1 bits, or may be in the form of certain frequencies. In addition there are questions concerning how a receiver distinguishes two bits which are adjacent in time.
A major issue is the form of the medium, both in terms of the materials it uses and its topology. A basic wired Ethernet, for example, consists of cable conducting electrical signals; the connections could also be wireless. More complicated networks, including Ethernet, may consist of more than one cable, with all of them connected via a hub. The latter has become common even at the household level.
Data Link Layer
For example, in an Ethernet, this layer is concerned with ensuring that two network stations connected to the same cable do not try to access the line at the same time. For this reason the Ethernet operation is an example of what is called a Medium Access Control (MAC) Protocol.
Here is an overview of how the Ethernet MAC protocol works, using a “listen before talk” approach. When a network node has a message ready to send, it first senses the cable to see if any node is currently sending. If so, it generates a random backoff time, waiting this amount of time before trying again. If the node does not “hear” any other node sending, it will go ahead and send.
There is a small chance that another node actually had been sending but due to signal propagation delay the transmission had not yet reached the first node. In that case a collision will occur, destroying both messages. Both nodes will sense the collision, and again wait random amounts of time before trying again.
This layer also does the setting up of frames of bits (i.e. sets of consecutive bits sent along the wire), which not only include the message itself but also information such as (say, in the Ethernet case) the Ethernet ID number of the destination machine.
Messages may be broken up into pieces before being sent. This may be handled at the transport level (see below), but may also be done at the data link level.
Network Layer
This is the routing layer. Questions addressed in this layer include: If in our example Sushant wants to send a message to Shweta, how is that accomplished? Obviously its first step is to send the message to her mail; how does Sushant know this? How can alternate routes be found if traffic congestion occurs?
Transport Layer
Suppose Sushant’s message to Shweta consists of a large file transfer, say 100 megabytes. This transfer will take a long time (by network standards), and we certainly don’t want it to monopolize the network during that time.
We also must deal with the fact that the buffer space at Shweta won’t be large enough to deal with a 100 megabyte message. Also, one 100-megabyte message would have a sizable probability of having at least one bit in error, and if so, we would have to retransmit the entire message!
So, the file transfer must be done in pieces. But we don’t want to burden the Sushant with the task of breaking up the 100 megabytes into pieces, nor do we want to burden the Shweta with the reassembly of the messages. Instead, the network software (again, typically in the OS) should provide these services, which it does at the transport layer, as for example is the case with TCP.
Session Layer
This layer is concerned with management of a session, i.e. the duration of a connection between two network nodes. The word connection here does not mean something physical, but rather refers to an agreement between two nodes that some chunks of data with some relation to each other will be exchanged for some time.
Actually, TCP does this in some senses, as does the socket interface to TCP, which is very much like the interfaces for reading or writing a file.
Presentation Layer
This layer deals with such matters as translating between character codes, if the source uses one and the destination the other.
In the old days, this could mean ASCII at one end and EBCDIC on the other end. Today, though, it could mean for example two different coding systems for Chinese characters, Big 5 and GB.
Application Layer
You can write programs at the application layer yourself, and of course you use many programs written by
others, such as ftp, Web browsers, e-mail utilities, and so on.
There are three ways for transmitting data from one point to another as shown in Figure.
1. Simplex : In simplex mode the communication can take place in one direction. The receiver receives the signal from the transmitting device. In this mode the flow of information is Uni-directional. Hence it is rarely used for data communication.
2. Half-duplex : In half-duplex mode the communication channel is used in both directions, but only in one direction at a time. Thus a half-duplex line can alternately send and receive data.
3. Full-duplex: In full duplex the communication channel is used in both directions at the same time. Use of full-duplex line improves the efficiency as the line turnaround time required in half-duplex arrangement is eliminated. Example of this mode of transmission is the telephone line.
Data is transmitted from one point to another point by means of electrical signals that may be in digital and analog form. So one should know the fundamental difference between analog and digital signals. In analog signal the transmission power varies over a continuous range with respect to sound, light and radio waves.
On the other hand, a digital signal may assume only discrete set of values within a given range. Examples are computer and computer related equipment. Analog signal is measured in Volts and its frequency is in Hertz (Hz). A digital signal is a sequence of voltage represented in binary form. When digital data are to be sent over an analog form the digital signal must be converted to analog form. So the technique by which a digital signal is converted to analog form is known as modulation. And the reverse process, that is the conversion of analog signal to its digital form, is known as demodulation. The device, which converts digital signal into analog, and the reverse, is known as modem.
Asynchronous and Synchronous Transmission
Data transmission through a medium can be either asynchronous or synchronous. In asynchronous transmission data is transmitted character by character as you go on typing on a keyboard. Hence there is irregular gaps between characters. However, it is cheaper to implement, as you do not have to save the data before sending. On the other hand, in the synchronous mode, the saved data is transmitted block by block. Each block can contain many characters. Synchronous transmission is well suited for remote communication between a computer and related devices like card reader and printers.
