good networking concept

7/31/2019 Good Networking Concept

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1 Introduction

Computer network is a group of two or more computers that are interconnected

to share resources (data, hardware, software...) Two computers are said to be

interconnected if they are able to exchange information.

Experts in the field of networking debate whether two computers that are connected

together using some form of communications medium constitute a network but we

will take above definition as relevant.

Network may involve other devices such as routers, switches, network printers, IP

telephones etc. so, in general, we speak about nodes of a network.

Main purpose of networks is to transfer data from source to destination. Data is

transferred by different types of signal and represented with sequence of bits (onesand zeros). Various physical media can be used for transmission of signal: coaxial

cable, twisted pair, fiber optics and air.

Suppose that we want to transfer data from host A to host B over the network. Data

may be e-mail, .mp3 file, web page etc. Furthermore that data have to

be segmented into smaller pieces of information, transferred over the network

and delivered to host B without errors.

Objective of this book is to try to explain this basic procesess in computer networks

and why they are important.

Network is organized hierarchically. Main concepts of hierarchical systems are:

concept oflayers

concept ofinterface

concept ofprotocol

concept ofheader

concept offragmentation

Communication protocol is a set of uniquely defined syntactic and semantic rules for

exchanging information between two entities that includes:

syntax of the information;

semantics of the information;

rules for the exchange of information

Entity may be computer, software, mobile phone, sensor etc.


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Communication architecture is a set of protocols that enables communication

between two entities.

We will talk about these concepts and architectures later. For now let's explain

concept of layers. The concept of layers is used to describe communication from one

node of network to another.

A conversation between two people provides a good opportunity to use a layered

approach to analyze information flow. In a conversation, each person wishing to

communicate begins by creating an idea. Then a decision is made on how to properly

communicate the idea.

For example, a person could decide to speak or sing, and what language to use.

Finally the idea is delivered. The person creates the sound which carries the message.

This process can be broken into separate layers that may be applied to all

conversations. The top layer is the idea that will be communicated. The middle layer

is the decision on how the idea is to be communicated. The bottom layer is thecreation of sound to carry the communication.

The same method of layering explains how a computer network distributes

information from a source to a destination.

We use models to describe communication in networks. Two models that will be

explained in this book are OSI model and TCP/IP model.

..

2 Network Topology

Network topology is a usually schematic description of the arrangement of a network,

including its nodes and connecting lines. There are two ways of defining network

geometry: the physical topology and the logical (or signal) topology.

Physical topology is the physical layout of the network, how the cables are

arranged and how the computers are connected.

Logical topology refers to the nature of the paths the signals follow from nodeto node.

In many instances, the logical topology is the same as the physical topology. But this

is not always the case. For example, some networks are physically laid out in a star

configuration, but they operate logically as bus networks.


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Network topology is technically a part of graph theory. Distances between nodes,

physical interconnections, transmission rates, and/or signal types may differ in two

networks and yet their topologies may be identical.

There are several common physical topologies, as described below and as shown in

the Figure 1.

Figure 1: Physical topologies

Bus - Topology in which all of the nodes of the network are connected to a common

transmission medium which has exactly two endpoints (this is the "bus", which is also

commonly referred to as the backbone, or trunk) all data that is transmitted between

nodes in the network is transmitted over this common transmission medium and is

able to be received by all nodes in the network virtually simultaneously. The two

endpoints of the common transmission medium are normally terminated with a device

called a terminator that exhibits the characteristic impedance of the transmission

medium and which dissipates or absorbs the energy that remains in the signal to

prevent the signal from being reflected or propagated back onto the transmission

medium in the opposite direction, which would cause interference with and

degradation of the signals on the transmission medium.

Star - Topology in which each of the nodes of the network is connected to a central

node with a point-to-point link in a "hub" and "spoke" fashion, the central node being

the "hub" and the nodes that are attached to the central node being the "spokes" (e.g.,

a collection of point-to-point links from the peripheral nodes that converge at a central

node) all data that is transmitted between nodes in the network is transmitted to this

central node, which is usually some type of device that then retransmits the data to

some or all of the other nodes in the network, although the central node may also be a

simple common connection point without any active device to repeat the signals.


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Ring - Topology in which each of the nodes of the network is connected to two other

nodes in the network and with the first and last nodes being connected to each other,

forming a ring all data that is transmitted between nodes in the network travels from

one node to the next node in a circular manner and the data generally flows in a single

direction only.

Mesh - Topology in which each of the nodes of the network is connected to each of

the other nodes in the network with a point-to-point link this makes it possible for

data to be simultaneously transmitted from any single node to all of the other nodes.

The physical fully connected mesh topology is generally too costly and complex for

practical networks, although the topology is used when there are only a small number

of nodes to be interconnected.

Tree (or hierarchical) - Topology in which a central "root" node (the top level of the

hierarchy) is connected to one or more other nodes that are one level lower in the

hierarchy (i.e., the second level) with a point-to-point link between each of the secondlevel nodes and the top level central "root" node, while each of the second level nodes

that are connected to the top level central "root" node will also have one or more other

nodes that are one level lower in the hierarchy (i.e., the third level) connected to it,

also with a point-to-point link, the top level central "root" node being the only node

that has no other node above it in the hierarchy the hierarchy of the tree is

symmetrical, each node in the network having a specific fixed number, f, of nodes

connected to it at the next lower level in the hierarchy, the number, f, being referred to

as the "branching factor" of the hierarchical tree.

Hybrid - The hybrid topology is a type of network topology that is composed of one

or more interconnections of two or more networks that are based upon different

physical topologies mentioned above.

3 Reference Models

Reference model is an abstract representation of the entities and relationships

involved in a problem space, and forms the conceptual basis for the development of

more concrete models and ultimately implementations. It serves as an abstract

template for the development of more specific models in a given domain, and allowsfor comparison between complying models.

It will be talked about two models: OSI model and TCP/IP model (protocol suite).


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3.1 The OSI Reference Model

The Open Systems Interconnection reference model (OSI reference model or OSI

model for short) is a layered, abstract description for communications and computer

network protocol design, developed as part of the Open Systems Interconnection

(OSI) initiative. It is also called the OSI seven layer model. The layers, described

below and shown in Figure 2 are, from bottom to top: Physical, Data Link, Network,

Transport, Session Presentation and Application layer. A layer is a collection of

related functions that provides services to the layer above it and receives service from

the layer below it. The layers are defined in such a manner so that changes in one

layer do not require changes in the other layers. By partitioning the communcation

functions into layers, the problem is much more manageable.

Figure 2: OSI model layers

Physical layer Concerned with transmission of unstructured bit stream over the

physical link. It invokes such parameters as signal voltage swing and bit duration. It

deals with the mechanical, electrical, procedural characteristics to establish, maintain

and deactivate the physical link

Data Link layer Provides reliable transfer of data across the physical link. It sends

blocks of data (frames) with the necessary synchronization, error control and flow

control.

Network layer Provides upper layers with independence from the data transmission

and switching technologies used to connect systems. It is responsible for establishing,

maintaining and terminating connections.


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Transport layer Provides reliable, transparent transfer of data between end points.

It provides end-to-end error recovery and flow control.

Session layer Provides the control structure for communication between

applications. It establishes, manages and terminates connections (sessions) between

cooperating applications.

Presentation layer Performs transformations on data to provide a standardized

application interface and to provide common communications services. It provides

services such as encryption, text compression and reformatting.

Application layer Provides services to the users, FTP, HTTP, TELNET, etc.

3.2 The TCP/IP Reference Model

TCP/IP originated out of the investigative research into networking protocols that theUS Department of Defense (DoD) initiated in 1969. In 1968, the DoD Advanced

Research Projects Agency (ARPA) began researching the network technology that is

called packet switching.

The original focus of this research was that the network be able to survive loss of

subnet hardware, with existing conversations not being broken off. In other words,

DoD wanted connections to remain intact as long as the source and destination nodes

were functioning, even if some of the machines or transmission lines in between were

suddenly put out of operation. The network that was initially constructed as a result of

this research to provide a communication that could function in wartime., then calledARPANET, gradually became known as the Internet. The TCP/IP protocols played an

important role in the development of the Internet. In the early 1980s, the TCP/IP

protocols were developed. In 1983, they became standard protocols for ARPANET.

Because of the history of the TCP/IP protocol suite, it's often referred to as the DoD

protocol suite or the Internet protocol suite.


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Figure 3: TCP/IP model

layers

Network Access Layer The lowest layer of the TCP/IP protocol hierarchy. It

defines how to use the network to transmit an IP datagram. Unlike higher-levelprotocols, Network Access Layer protocols must know the details of the underlying

network (its packet structure, addressing, etc.) to correctly format the data being

transmitted to comply with the network constraints. The TCP/IP Network Access

Layer can encompass the functions of all three lower layers of the OSI reference

Model (Physical, Data Link and Network layers).

As new hardware technologies appear, new Network Access protocols must be

developed so that TCP/IP networks can use the new hardware. Consequently, there

are many access protocols - one for each physical network standard.Access protocol is a set of rules that defines how the hosts access the shared medium.

Access protocol have to be simple, rational and fair for all the hosts.

Functions performed at this level include encapsulation of IP datagrams into the

frames transmitted by the network, and mapping of IP addresses to the physical

addresses used by the network. One of TCP/IP's strengths is its universal addressing

scheme. The IP address must be converted into an address that is appropriate for the

physical network over which the datagram is transmitted.

Internet layer Provides services that are roughly equivalent to the OSI Network

layer. The primary concern of the protocol at this layer is to manage the connections

across networks as information is passed from source to destination. The Internet

Protocol (IP) is the primary protocol at this layer of the TCP/IP model.

Transport layer It is designed to allow peer entities on the source and destination

hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end

transport protocols have been defined here TCP and UDP Both protocols will be

dicussed later.

Application Layer includes the OSI Session, Presentation and Application layers as

shown in the Figure 4. An application is any process that occurs above the Transport

Layer. This includes all of the processes that involve user interaction. The application

determines the presentation of the data and controls the session. There are numerous

application layer protocols in TCP/IP, including Simple Mail Transfer Protocol

(SMTP) and Post Office Protocol (POP) used for e-mail, Hyper Text Transfer

Protocol (HTTP) used for the World-Wide-Web, and File Transfer Protocol (FTP).

Most application layer protocols are associated with one or more port number. Port

numbers will be dicussed later.


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Figure 4: OSI and TCP/IP

4 Basic Definitions in Data Networks

In this chapter some basic definitions in data networks will be discussed including

types of switching, header, interface, protocol, PDU, SDU, encapsulation, network

math and metric units etc.

4.1 Circuit-Switching, Message-Switching and Packet-Switching

Circuit switching network establishes a fixed bandwidth circuit (channel) betweennodes before the users may communicate, as if the nodes were physically connected

with an electrical circuit. The bit delay is constant during the connection, as opposed

to packet switching, where packet queues may cause varying delay.

Each circuit cannot be used by other callers until the circuit is released and a new

connection is set up. Even if no communication is taking place in a dedicated circuit

then, that channel still remains unavailable to other users. Channels that are available

for new calls to be set up are said to be idle. Telephone network is example of circuit

switching system.

Virtual circuit switching is a packet switching technology that may emulate circuit

switching, in the sense that the connection is established before any packets aretransferred, and that packets are delivered in order.

Message switching was the precursor of packet switching, where messages were

routed in their entirety, one hop at a time. It was first introduced in 1961. Nowadays,

message switching systems are mostly implemented over packet-switched or circuit-

switched data networks. E-mail is example of a message switching system.


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Packet switching is a communications paradigm in which packets (discrete blocks of

data) are routed between nodes over data links shared with other traffic. The term

"packets" refers to the fact that the data stream from your computer is broken up into

packets of about 200 bytes (on average), which are then sent out onto the network.

Each packet contains a "header" with information necessary for routing the packet

from source to destination. Each packet in a data stream is independent.

The main advantage of packet-switching is that it permits "statistical multiplexing" on

the communications lines. The packets from many different sources can share a line,

allowing for very efficient use of the fixed capacity. 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").

4.2 Communication between Hosts in Packet-Switched

Networks

All communications on a network originate at a source, and are sent to a destination.

The information sent on a network is referred to as data or data packets. Let host A be

a source and let host B be a destination.

As shown in the Figure 5 real communication happens through physical medium

while logical communication between host A and host B takes place on logical link

between two peer layers. Hosts A and B communicate using peer to peer protocol of

corresponding layer.


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Figure 5: Communication between hosts

If host A wants to send data to host B, the data must first be packaged through a

process called encapsulation. (Figure 6).

Encapsulation wraps data with the necessary protocol information before networktransit. Therefore, as the data packet moves down through the layers of the OSI

model, it receives headers, trailers, and other information.

Once the data is sent from the source, it travels through the application layer down

through the other layers. The packaging and flow of the data that is exchanged goes

through changes as the layers perform their services for end users.

Network must perform the following five conversion steps in order to encapsulate

data:

1. Build the data As a user sends text file or an e-mail message,its alphanumeric characters are converted to data that cantravel across the network.

2. Package the data for end-to-end transport The data ispackaged for transport over the network. By using segments,the transport function ensures that the hosts at both ends of thenetwork can reliably communicate.


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3. Add the network IP address to the header The data is putinto a packet or datagram that contains a packet header withsource and destination logical (IP) addresses. These addresseshelp network devices send the packets across the network alongthe best path.

4. Add the data link layer header and trailer Each networkdevice have to put the packet into a frame. The frame allowsconnection to the next directly-connected network device on thelink. Each device in the chosen network path requires framing inorder for it to connect to the next device.

5. Convert to bits for transmission The frame have to beconverted into a pattern of ones and zeros (bits) fortransmission on the physical medium. A clocking functionenables the devices to distinguish these bits as they travel

across the medium. The medium can vary along the path used.For example, web page can originate on a server, cross acampus backbone, and go out a WAN link until it reaches itsdestination on a remote LAN.

Figure 6: Encapsulation and Decapsulation

As shown in the Figure 6. host B performs inverse process called decapsulation. First

on physical layer it decodes bits and then each layer cuts its header and sends the data

packet to the upper layer. In the end we have original data sent by host A. Each pair of


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adjacent layers communicate through interface. The interface defines which

operations and services the lower layer makes available to the upper one.

PDU (Protocol Data Unit) is information that is used by peer to peer protocol.

SDU (Service Data Unit) is information that is sent through interface of pair of

adjacent layers. (Figure 7)

Figure 7: Data Unit and Header

4.3 Bandwidth, Throughput and Delay

Bandwidth (in digital systems) between two given nodes is the maximal amount of

data per unit time that can be transmitted from one node to the other. Digital

bandwidth is synonymous with bit rate and data rate.

The actual bandwidth of a network is determined by a combination of the physical

media and the technologies chosen for signaling and detecting network signals.Current information about the physics of unshielded twisted-pair (UTP) copper cable

puts the theoretical bandwidth limit at over 1 Gbps. However, in practice the actual

bandwidth is determined by the signaling methods, NICs, and other network

equipment that is chosen. Therefore, the bandwidth is not determined solely by the

limitations of the medium.

Throughput defines how much useful data can be transmitted per unit time. It is

equal to the bandwidth if there is no protocol. However, in most practical cases the

throughput is less than the bandwidth for two reasons:

protocol overhead: protocols use some bytes to transmitprotocol information. This reduces the throughput

protocol waiting times: some protocols may force you to waitfor some event.


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Delay in data networks is generally the round trip delay (also called Round Trip Time

- RTT) for a packet within the network. Network delay is composed of the following

parts:

Processing delay - time routers take to process the packet

header. Queuing delay - time the packet sits in routing queues. Transmission delay - time it takes to push the packet's bits

onto the link Propagation delay - the time taken by the front of a signal to

reach the destination. Propagation of an electromagnetic signalis the speed of light (also called celerity). It depends on thewavelength and the medium in which the signal is propagating.For example propagation of signal in twisted pair is 0.66c wherec is speed of light.

There is a certain minimum level of delay that will be experienced due to the time it

takes to transmit a packet serially through a link. Onto this is added a more variable

level of delay due to network congestion. Network delays can range from a few

milliseconds to several hundred milliseconds.

4.4 Network Math and Metric Units

Computers can only understand and use data that is in this two-state or binary format.

The ones and zeros are used to represent the two possible states of an electronic

component in a computer. 1 is represented by an ON state, and 0 is represented by an

OFF state. They are referred to as binary digits orbits. The word bit is a shortening of

the words "Binary digIT".

Bit is basic unit of (digital) information.

A binary 0 might be represented by 0 volts and binary 1 might be represented by +5

volts. Volt [V] is the standard unit of measurement for electrical potential. It defines

the "force" of electricity.

Bits and binary numbers are natural to computers and other digital devices. Humans

use the decimal number system, which is relatively simple when compared to the long

series of ones and zeros used by computers. So the binary numbers need to be

converted to decimal numbers.

Byte is a grouping of eight bits. In a computer, one byte represents a single

addressable storage location.


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Base-10 Number System (Decimal Number System) - The easiest way to

understand bits is to compare them to something we know: digits. A digit is a single

place that can hold numerical values between 0 and 9. Digits are normally combined

together in groups to create larger numbers. For example, 7 214 has four digits. It is

understood that in the number 7 214, the 4 is filling the "1s place" while the 1 is

filling the 10s place, the 2 is filling the 100s place and the 7 is filling the 1 000s place.

We could express things this way if we wanted to be explicit:

(7 * 1000) + (2 * 100) + (1 * 10) + (4 * 1) = 7000 + 200 + 10 + 4 = 7214

Another way to express it would be to use powers of 10. So:

(7 * 103) + (2 * 102) + (1 * 101) + (4 * 100) = 7000 + 200 + 10 + 4 = 7214

What we can see from this expression is that each digit is a placeholder for the next

higher power of 10, starting in the first digit with 10 raised to the power of zero.

There is nothing that forces us to have 10 different values in a digit. Our base-10

number system likely grew up because we have 10 fingers, but if we happened to

evolve to have five fingers instead, we would probably have a base-5 number system.

We can have base-n number systems, where n is number of digits used. In fact, there

are lots of good reasons to use different bases in different situations.

Base-2 Number System (Binary Number System) A binary number is composed

of only ones and zeros, for example: 1101. How do we figure out what the value of

the binary number 1101 is? We do it in the same way we did it above for 7 214 but weuse a base of 2 instead of a base of 10. So:

1101(2) = (1 * 23) + (1 * 22) + (0 * 21) + (1 * 20) = 8 + 4 + 0 + 1 = 13 (10)

We can see that in binary numbers, each bit holds the value of increasing powers of 2.

That makes counting in binary pretty easy. Starting at zero and going through 16,

counting in decimal and binary looks as shown in Table 1:

Table 1: Binary Counting

Decimal Number Binary Number

0 0

1 1

2 10


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3 11

4 100

5 101

6 110

7 111

8 1000

9 1001

10 1010

11 1011

12 1100

13 1101

14 1110

15 1111

16 10000

etc.

When we look at sequence shown in Table 1, 0 and 1 are the same for decimal and

binary number systems. At the number 2, you see carrying first take place in the

binary system. If a bit is 1 and you add 1 to it the bit becomes 0 and the next bit

becomes 1. In the transition from 15 to 16 this effect rolls over through 4 bits, turning

1111 into 10000.

With one byte (8 bits), we can represent 256 values ranging from 0 to 255, as shown

in Table 2:

Table 2: 256 values with 8 bits

Decimal Number 8-bit Binary Number

0 00000000

1 00000001


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2 00000010

...

253 11111101

254 11111110

255 11111111

Generally, with n bits we can represent 2n values ranging from 0 to 2n-1.

Base-16 Number System (Hexadecimal Number System or Hex) - A number

system with a base of 16. Written using the symbols 09 and AF, or af. Its primary

purpose is to represent the binary code in a format easier for humans to read, and acts

as a form of shorthand, in which one hexadecimal digit stands in place of four bits.For example, the decimal number 157, whose binary representation is 10011101 is 9D

in hexadecimal. (9(16) =1001(2), D(16) = 1101(2))

Logical Operations Now we will provide a brief explanation of the common

logical operations AND, OR, NOT and XOR (eXclusive OR). Logical operations are

performed between two bits (except for NOT operation). Bits can be either 1 or 0, and

these operations are essential to performing network math operations.

In the "truth tables" below the p and q are input bits.

AND operation - The logical AND operation compares 2 bits and if they are both 1,

then the result is 1, otherwise, the result is 0.

Table 3: Logical AND operation

p q p AND q

0 0 0

0 1 0

1 0 0

1 1 1

OR operation - The logical OR operation compares 2 bits and if either or both bits are

1, then the result is 1, otherwise, the result is 0.


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Table 4: Logical OR operation

p q p OR q

0 0 0

0 1 1

1 0 1

1 1 1

NOT operation - The logical NOT operation simply changes the value of a single bit.

If it is a 1, the result is 0; if it is a 0, the result is 1. This operation is different in that

instead of comparing two bits, it is acting on a single bit.

Table 5: Logical NOT operation

p NOT p

0 1

1 0

XOR operation - The logical XOR (eXclusive OR) operation compares 2 bits and if

exactly one of them is 1 (i.e. if they are different values), then the result is 1;

otherwise (if the bits are the same) the result is 0

Table 6: Logical XOR operation

p q p XOR q

0 0 0

0 1 1

1 0 1

1 1 0

Metric units For measuring memory, disk, file, and database sizes, in practice, the

units have slightly different meanings then in SI system of units. Prefix kilo means

210 (1024) rather than 103 (1000) because memories are always a power of two. Thus,

a 1 KB memory contains 1024 bytes, not 1000 bytes. Similarly, a 1 MB memory


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contains 220 (1 048 576) bytes, a 1 GB memory contains 230 (1 073 741 824) bytes,

and a 1 TB database contains 240 (1 099 511 627 776) bytes. However, a 1 kbps

communication channel transmits 1000 bits per second and a 1 Gbps LAN runs at 1

000 000 000 bits/s because speeds are not powers of two. Many people tend to mix up

these two systems.

We'll use the symbols KB, MB, and GB for 210, 220, and 230 bytes, respectively, and

the symbols kbps, Mbps, and Gbps for 103, 106, and 109 bits/s, respectively.

Table 7: Units of Data Storage

Unit Definition Example

Bit (b)Binary digit, 1

or 0

On / Off; Open / Closed;

+5V / 0V

Byte (B) 8 bitsRepresent the letter "E"

as ASCII code

Kilobyte

(KB)

1 KB = 1024

BTypical Email = 2KB

Megabyte(M

B)

1 MB =1024

KB = 1 048

576 B

Floppy Disks = 1,44 MB;

Typical song in MP3

format = 5 MB

Gigabyte(GB)

1 GB = 1024

MB = 1 073

741 824 B

Typical Hard Drive = 40GB or greater

Terabyte

(TB)

1 TB = 1024

GB = 1 099

511 627 776

B

Database; Amount of

data theoretically

transmittable in optical

fiber in one second

5 Classification of Networks by Scale

In Chapter 2 we discussed classification of networks by their topology. Another

important criterion for classifying networks is their scale (physical size). As shown in

Figure 8 there are:
http://networking.layer-x.com/p020000-1.htmlhttp://networking.layer-x.com/p020000-1.html


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Personal Area Network (PAN)

Local Area Network (LAN)

Metropolitan Area Network (MAN)

Wide Area Networks (WAN)

Personal Area Network (PAN) - The interconnection of devices within the range ofan individual person, typically within a range of 10 meters. For example, a wireless

network connecting a computer with its keyboard, mouse or printer is a PAN. Also, a

PDA that controls the user's hearing aid or pacemaker fits in this category. Another

example of PAN is a Bluetooth. Typically, this kind of network could also be

interconnected without wires to the Internet or other networks.

Local Area Network (LAN) - Privately-owned networks covering a small geographic

area, like a home, office, building or group of buildings (e.g. campus). They are

widely used to connect computers in company offices and factories to share resources

(e.g., printers) and exchange information. LANs are restricted in size, which meansthat the worst-case transmission time is bounded and known in advance. Knowing this

bound makes it possible to use certain kinds of designs that would not otherwise be

possible. It also simplifies network management. Traditional LANs run at speeds of

10 Mbps to 100 Mbps, have low delay (microseconds or nanoseconds), and make very

few errors. Newer LANs operate at up to 10 Gbps.

Metropolitan Area Network (MAN) - Covers a larger geographical area than is a

LAN, ranging from several blocks of buildings to entire cities. MANs can also depend

on communications channels of moderate-to-high data rates. A MAN might be owned

and operated by a single organization, but it usually will be used by many individuals

and organizations. MANs might also be owned and operated as public utilities. They

will often provide means for internetworking of LANs. Metropolitan Area Networks

can span up to 50km, devices used are modem and wire/cable.

Wide Area Networks (WAN) - Computer network that covers a large geographical

area, often a country or continent. (any network whose communications links cross

metropolitan, regional, or national boundaries). Less formally, a network that uses

routers and public communications links. Routers will be discussed later.


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Figure 8: Networks by Scale

6 Networking Devices

In addition to the hosts, LANs may use other devices to control physical access to the

shared medium, extend the maximum reach of the LAN, switch traffic, and so on.

Such hardware is in the form of NICs, repeaters, bridges, hubs, switches, routers, and

gateways.

A repeater is a network device used to regenerate a signal. It operates within the

physical layer of the OSI reference model and regenerate analog or digital signals thatare distorted by transmission loss due to attenuation. A repeater does not make

intelligent decision concerning forwarding packets. Repeater won't be disscused in

detail.

6.1 Network Interface Card (NIC)

Network Interface Cards (NIC), also known as Network Interface Units (NIU) are

chipsets on printed circuit boards that provide physical access from the node to the

LAN medium. The NIC is responsible for fragmenting the data transmission and

formatting the data packets with the necessary header and trailer. A standard IEEE

NIC contains a unique, hard-coded logical address (MAC address), which it includes

in the header of each data packet it transmits. The NIC typically has some amount of

buffer memory, which enables it to absorb some number of bits transmitted by the

associated device, form the packets, and hold them until such time as the network is

available.


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In the context of the OSI Reference Model, NICs function at the Physical and Data

Link layers. The NIC also may contain a microprocessor that can relieve the attached

device of some routine computational functions.

The NIC can take a number of forms, including a circuit board that fits into the

expansion slot of a desktop PC, a PCMCIA card, or a stand-alone device.Transceivers (transmitter/receivers) are used in LANs to receive a carrier signal and

then transmit it on its way. They are embedded in NICs.

Figure 9: Fast Ethernet NIC

6.2 Hub

Hubs can be either active or passive. Passive hubs act simply as cable-connecting

devices, while active hubs also serve as signal repeaters and are called "multiport

repeaters". The first generation of hubs (1984) acted as LAN concentrators and

repeaters, with a single internal collapsed backbone bus for connecting like LANs.

The second generation accommodated multiple LAN architectures over separate ports,

with rudimentary network management and configuration capabilities included as

well.

Multiple hubs can be interconnected with various media, depending on bandwidth

requirements and distances involved.


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Hubs operate at the Physical Layer, with a hint of the Data Link Layer of the OSI

Reference Model.

Figure 10: 4-port Ethernet Hub

6.3 Bridge

Bridges are relatively simple devices that connect LANs of the same architecture (e.g.

Ethernet to Ethernet). Bridges operate at the lower two layers of the OSI Reference

Model, providing Physical Layer and Data Link Layer connectivity. A bridge, at the

most basic level, acts simply to extend the physical reach of a LAN, passing traffic

from one LAN segment to another based on the destination MAC address. In otherwords, bridges act as LAN repeaters where specified distance limitations are

exceeded. Bridges have buffers so they can store and forward frames in the event that

the destination link is congested with traffic. Two-port bridges are the most common

configuration.

A key advantage of bridges is their inherent simplicity. As protocol-dependent

devices, they don't perform complex processes on the data frames traveling through

them, neither do they attempt to evaluate the network as a whole to make end-to-end

routing decisions. Bridges simply read the destination MAC address of the incoming

frame and forward it along its way to the next link. Bridges can be cascaded, orconnected in series, link by link. As bridges are so simple, they are inexpensive and

fast. Such bridges can support multiple LANs and LAN segments connected by

multiple media.

In contemporary networking a switch generally would be used instead of bridge.


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6.4 Switch

Switches are network devices with basic frame store-and-forward capabilities that can

support multiple simultaneous transmissions. Switches are also called "multiport

bridges" (Figure 11) . Switches have the ability to read the target MAC addresses of

the frames and forward them only to the appropriate port associated with the target

device. Device may be directly attached to the switch, or may be connected to a hub

that connects to the switch.

Switches may use several methods of operation:

Cut-Through: A cut-through switch quickly reads the MACaddress of the frame and quickly flows the frame through theswitching matrix bit by bit

Store and Forward: A store-and-forward switch temporarilybuffers, or stores, the frame as it is presented to the incomingswitch port, examining the entire frame for errors through a CRCcheck before forwarding it through the switching matrix to theoutput port. While cut-through switching is faster and lessexpensive, it carries with it the risk of the propagation of erroreddata and the resulting potential for negative impact on overallthroughput, as errored frames ultimately have to beretransmitted. Store-and-forward switching generally ispreferred over cut-through switching.

Fragment Free: A third, and less common, method isfragment-free switching, which is similar to cut-through exceptfor the fact that the switch stores the first 64 octets of the framebefore forwarding it. As most errors occur at the beginning of aframe, this approach eliminates the possibility that runt frames,that is, truncated frames, will be transmitted.

Switches operate at the Physical and Data Link Layers of the OSI Reference Model

Layers 1 and 2, respectively. Switches read the destination addresses of the frames,

filtering and forwarding as appropriate, based on MAC addresses (Layer 2 address).

Switches are fast and relatively inexpensive. Some switches make routing decisionsbased on IP addresses (Layer 3 address). Layer 3 switching involves a combination of

switching and routing. This involves more complex routing decisions that are made in

the context of the network as a whole yet not at the level of complexity that

characterizes a router.


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A switch does a great deal to reduce congestion and in a number of ways. First, a

switch can support multiple simultaneous transmissions. Second, switches serve to

segment a network through filtering, as they forward traffic only to the port associated

with the link to which the target device is connected. Thereby, that traffic does not

contribute to congestion on other links or segments. Third, a switch can be equipped

to buffer incoming frames until internal bus resources are available to process them. A

switch also can be equipped to buffer outgoing frames until such time as the link to

the next switch becomes available. Fourth, a switch can exercise a flow control

mechanism, whereby it can advise a device to stop transmitting when its buffers are in

danger of overflowing and then advise the device to resume transmission when the

pressure on resources has been relieved. Fifth, store-and-forward and fragment-free

switches variously eliminate or reduce the number of errored frames. Finally, a switch

supports full-duplex transmission, thereby eliminating data collisions associated with

CSMA in an Ethernet environment, assuming that the station is directly connected to

the switch rather than through a hub. This approach is the current best practice.

The cost of switches has dropped dramatically in recent years to the point that they

often compete effectively against hubs. But switch costs are sensitive to factors such

as the type and speed of the transmission media interfaces, the number and speed of

the ports, the number and size of the buffers, the number and speed of the internal

buses, the complexity of the internal switching matrix, and the complexity of the

switching or routing logic.

Figure 11: 48-port Fast Ethernet Cisco Switch

6.5 Router

Routers are highly intelligent network devices that can support connectivity between

both like and disparate LANs and can provide access to various WANs, such asFrame Relay, IP and ISDN. (Figure 12)

Routers typically operate at the bottom three layers of the OSI model using the

Physical Layer, Data Link Layer, and Network Layer to provide connectivity,

addressing, and switching. Routers also have the capability to operate at all seven

layers of the OSI Reference Model, if so equipped.


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In addition to supporting filtering and encapsulation, routers route traffic based on a

high level of intelligence that enables them to consider the network as a whole. This is

in stark contrast to bridges, hubs, and switches, which view the network simply on a

link-by-link basis. (Note: Layer 3 switches cross the line into the routing domain,

although at a relatively simple level.) Routing considerations might include

destination address, payload type, packet priority level, least-cost route, minimum

route delay, minimum route distance, and route congestion level. Routers also are

self-learning, as they can communicate their existence to other devices and can learn

of the existence of new routers, nodes, and LAN segments. Routers constantly

monitor the condition of the network as a whole, thereby dynamically adapting to

changes in the condition of the network from edge to edge. Routers are multiport

devices with high-speed ports running at rates up to 155 Mbps or more and with high-

speed internal buses that can be on the order of 1 Gbps in the aggregate. Additionally,

routers typically provide some level of redundancy so they are less susceptible to

catastrophic failure.

Routers are unique in their ability to route data based on programmable network

policy. Policy-based routers can provide various levels of service based on factors

such as the identification of the user, the terminal and the type of payload. From one

edge of the network to the other, an edge router can select the most appropriate path

through the various switches or routers positioned in the core. An important part of

this process often is that of dividing the network into multiple subnets . Users

associated with a subnet may be afforded access to only a limited subset of network

resources in the form of sites, links, hosts, files, databases, and applications. In

addition to being limited in terms of access to such a resource, users of another subnetmay be prevented from receiving data from it. In effect, even the very existence of

those resources is masked from view. Creation of such isolated subnets may serve for

reasons of security or simply as a means of avoiding unnecessary congestion.

Routers are available that sense a network failure and reestablish the connection via

an alternate means, which may include a totally different network and service.

Figure 12: Cisco Router


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6.6 Gateway

Gateways can perform all of the functions of switches and routers as well as

accomplish protocol conversion at all seven layers of the OSI Reference Model. (As

shown in Figure 13). Generally consisting of software residing in a host computer

equivalent in processing power to a midrange or mainframe, gateway technology is

expensive but highly functional.

Protocol conversion, rather than encapsulation, can serve to fully convert from

Ethernet to Token Ring or any other standard or proprietary protocol. Additionally,

protocol conversion can address higher layers of the OSI model, perhaps through

Layer 7, the Application Layer. As the process of protocol conversion is complex,

gateways tend to operate rather slowly as compared to switches and routers. As a

result, they impose additional latency on packet traffic and may create bottlenecks of

congestion during periods of peak usage. In a large and complex network routers tend

to be positioned at the edges of the network where they can be used to full advantage.

Therefore, they make complex and time-consuming decisions and invoke complex

and time-consuming processes only where required. Switches tend to be positioned

within the core of the network because they can operate with greater speed.

Figure 13: Networking devices and OSI model

Now we can conclude that switch creates separate collision domains and router

creates separate broadcast domains. All ports of hub are one collision domain.


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A collision domain is defined as a single CSMA/CD network segment in which there

will be a collision if two computers attached to the system both transmit at the same

time.

A broadcast domain is a logical division of a computer network, in which all nodes

can reach each other by broadcast at the data link layer. (Figure 14)

Figure 14: Collision Domain and Broadcast Domain

7 The Physical Layer

In this chapter we will look at the lowest layer of the OSI model - the physical layer.

It defines connector and interface specifications, as well as the medium (cable)

requirements. Its purpose is to transport a raw bit stream from one host to another.


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Electrical, mechanical, functional, and procedural specifications are provided for

sending a bit stream on a computer network.

We will cover two kinds of transmission media: guided (copper wire and fiber optics)

and wireless (terrestrial radio).

7.1 Twisted Pair

One of the oldest and still most common transmission media is twisted pair. As shown

in Figure 15 a twisted pair consists of two insulated copper wires, typically about 1

mm thick. The wires are twisted together in a helical form. Twisting is done because

two parallel wires constitute a fine antenna. When the wires are twisted, the waves

from different twists cancel out, so the wire radiates less effectively.

Twisted pairs can be used for transmitting either analog or digital signals. The

bandwidth depends on the thickness of the wire and the distance traveled, but severalMb/s can be achieved for a few kilometers in many cases. The frequency range of

twisted-pair cables is approximately 0 to 1 MHz. Due to their adequate properties and

low cost, twisted pairs are widely used and are likely to remain so for years to come.

Twisted pair cables are often shielded in attempt to prevent electromagnetic

interference. Because the shielding is made of metal, it may also serve as a ground.

However, usually a shielded or a screened twisted pair cable has a special grounding

wire added called a drain wire. This shielding can be applied to individual pairs, or to

the collection of pairs. When shielding is applied to the collection of pairs, this is

referred to as screening. The shielding must be grounded for the shielding to work. Incontrast to FTP (foiled twisted pair) and STP (shielded twisted pair) cabling, UTP

(unshielded twisted pair) cable is not surrounded by any shielding. It is the primary

wire type for telephone usage and computer networking, especially as patch cables.

UTP comes in several varieties:

Category 3: Was the earliest successful implementation of UTP.Its primarily used for voice and lower-speed data applications.Its rated for a maximum of 10 Mbps.

Category 4: Never achieved the popularity of Cat 3 or Cat 5.Its primarily used for voice and lower-speed data at a maximumof 16 Mbps.

Category 5: As Fast Ethernet became a standard, Cat 5became the basis for most high-speed data implementations.Cat 5 runs at a maximum of 100 Mbps.

Category 5e: With the need for higher speeds, Gigabit Ethernethas become the new replacement for Fast Ethernet. To make it


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work, Cat 5e extends the life of Cat 5 cable. It can run at amaximum of 1,000 Mbps.

Category 6: Cat 5e can run at gigabit speeds, but with 10-Gigabit Ethernet on the horizon, Cat 5e has stretched the Cat 5standard to its limits. Cat 6 can currently run at 1,000 Mbps (1Gbps). The Category 6 specification was released for publicationvery recently, however as designed, Category 6 cabling will beable to support speeds up to at least 10 Gbps.

Nowdays Cat 5e and Cat 6 should be used.

Figure 15: UTP pairs (cable)

UTP connector - The standard connector for UTP cabling is an RJ-45 connector.

This is a plastic connector that looks like a large telephone-style connector (see Figure

16). A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered

Jack.

Figure 16: RJ-45

connector


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7.2 UTP Cable Termination Standards EIA/TIA 568A and EIA/TIA

568B

In 1985 many companies from the telecommunications industry, becoming concerned

about the lack of a third party premises cabling standard and their governing body the

CCIA (Computer Communications Industry Association) requested that the EIA

(Electronics Industry Association) develop this standard.

The first draft of the standard wasnt released until July of 1991 this was given the

name EIA/TIA-568. The new standard provided backward compatibility for phones

that used two pairs instead of just one enabling them to operate on pairs 1 and 2. Later

in 1991 a Technical Systems Bulletin (TSB-36) was released with references to

category 4 and 5 cables. Twelve months later TSB-40 was published addressing

higher speed UTP for hardware connecting, this was revised in January of 1994 to

include RJ-45 modular jacks and fly leads. At this time EIA/TIA-568 was also revisedand renamed EIA/TIA 568A, the existing AT&T standard 258A was included and

referred to as EIA/TIA 568B. As both these standards were popular and widely used

they were both adopted into the International Standards titled Generic Cabling for

Customer Premises Cabling (ISO/IEC 11801:1995).

By looking at the specifications shown in Figure 17 we see that the only difference is

that the green and orange pairs are terminated to different pins, there is no difference

as to what signal is used on what pin, only what colour wire is terminated onto it.

Technically the standards are the same, they operate in the same manner and neither

one is technically superior to another when used in Ethernet applications.


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Figure 17: EIA/TIA 568A and 568B

7.3 Types of Cables and Connecting a Networking Devices

Straight-Through Cable - Four-pair, eight-wire, straight-through cable, which meansthat the color of wire on Pin 1 on one end of the cable is the same as that of Pin 1 on

the other end. Pin 2 is the same as Pin 2, and so on. The cable is wired to either

EIA/TIA T568B or T568A standards for 10BASE-T Ethernet, which determines what

color wire is on each pin.

Crossover Cable - A crossover cable means that the second and third pairs on one

end of the cable will be reversed on the other end. The pin-outs are T568A on one end

and T568B on the other end. All 8 conductors (wires) should be terminated with RJ-

45 modular connectors. Crossover cable conforms to the structured cabling standards.

If the crossover cable is used between switches, it's considered to be part of the"vertical" cabling. Vertical cabling is also called backbone cabling. A crossover cable

can be used as a backbone cable to connect two or more switches in a LAN, or to

connect two isolated hosts to create a mini-LAN. This will allow the connection of

two hosts or a server and a host without the need for a hub between them. This can be

very helpful for testing and training. To connect more than two hosts, a switch is

needed.

Rollover Cable - A 4-pair "rollover" cable. This type of cable is typically 3.05 m long

but can be as long as 7.62 m. A rollover cable can be used to connect a host or dumb

terminal to the console port on the back of a router or switch. Both ends of the cablehave RJ-45 connectors on them. One end plugs directly into the RJ-45 console

management port on the back of the router or switch. Plug the other end into an RJ-

45-to-DB9 terminal adapter. This adapter converts the RJ-45 to a 9-pin female D

connector for attachment to the PC or dumb terminal serial (COM) port. A DB25

terminal adapter is also available to connect with a PC or dumb terminal. This adapter

uses a 25 pin connector. Figure 18 shows a rollover console cable kit.


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Figure 18: Rollover Console Cable Kit

Cable is called a rollover because the pins on one end are all reversed on the other end

as though one end of the cable was rotated or rolled over. Figure 19 shows how to

connect networking devices.


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Figure 19: Connecting a Networking Devices

7.4 Fiber Optics

An optical transmission system has three key components: the light source, thetransmission medium, and the detector. Conventionally, a pulse of light indicates a 1

bit and the absence of light indicates a 0 bit. The transmission medium is an ultra-thin

fiber of glass or plastic. The detector generates an electrical pulse when light falls on

it. By attaching a light source to one end of an optical fiber and a detector to the other,

we have a unidirectional data transmission system that accepts an electrical signal,

converts and transmits it by light pulses, and then reconverts the output to an electrical

signal at the receiving end. Higher bandwidth links can be achieved using optical

fibers. One of the best substances used to make optical fibers is ultrapure fused silica.

These fibers are more expensive than regular glass fibers. Plastic fibers are normally

used for short-distance links where higher losses are tolerable.

Optical fiber links are used in all types of networks, LAN and WAN. The frequency

range of fiber optics is approximately 180 THz to 330 THz. There are two types of

fiber optics cables:

Multimode fiber


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Single-mode fiber

Multimode fiber - Light rays can only enter the core if their angle is inside the

numerical aperture of the fiber. Once the rays have entered the core of the fiber, there

are a limited number of optical paths that a light ray can follow through the fiber.

These optical paths are called modes. If the diameter of the core of the fiber is largeenough so that there are many paths that light can take through the fiber, the fiber is

called "multimode" fiber. Single-mode fiber has a much smaller core that only allows

light rays to travel along one mode inside the fiber.

Fiber-optic cable used for networking consists of two glass fibers encased in separate

sheaths. One fiber carries transmitted data from host A to host B. The second fiber

carries data from host B to host A. The fibers are similar to two one-way streets going

in opposite directions. This provides a full-duplex communication link. Fiber-optic

circuits use one fiber strand to transmit and one to receive. Typically, these two fiber

cables will be in a single outer jacket until they reach the point at which connectors

are attached.

Until the connectors are attached, there is no need for shielding, because no light

escapes when it is inside a fiber. There are no crosstalk issues with fiber. It is very

common to see multiple fiber pairs encased in the same cable. One cable can contain 2

to 48 or more separate fibers. Fiber can carry many more bits per second and carry

them farther than UTP can.

Usually, five parts make up each fiber-optic cable. The parts are the core, the

cladding, a buffer, a strength material, and an outer jacket.

The core is the light transmission element at the center of the optical fiber. All the

light signals travel through the core. A core is typically glass made from a

combination of silicon dioxide and other elements. Multimode uses a type of glass,

called graded index glass for its core. This glass has a lower index of refraction

towards the outer edge of the core. The outer area of the core is less optically dense

than the center and light can go faster in the outer part of the core. This design is used

because a light ray following a mode that goes straight down the center of the core

does not have as far to travel as a ray following a mode that bounces around in the

fiber. All rays should arrive at the end of the fiber together. Then the receiver at the

end of the fiber receives a strong flash of light rather than a long, dim pulse.

Surrounding the core is the cladding. Cladding is also made of silica but with a lower

index of refraction than the core. Light rays traveling through the fiber core reflect off

this core-to-cladding interface as they move through the fiber by total reflection.

Standard multimode fiber-optic cable is the most common type of fiber-optic cable


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used in LANs. A standard multimode fiber-optic cable uses an optical fiber with either

a 62.5 or a 50m core and a 125m diameter cladding. This is commonly designated

as 62.5/125 or 50/125 micron optical fiber.

Surrounding the cladding is a buffer material that is usually plastic. The buffer

material helps shield the core and cladding from damage. There are two basic cabledesigns. They are the loose-tube and the tight-buffered cable designs. Most of the

fiber used in LANs is tight-buffered multimode cable. Tight-buffered cables have the

buffering material that surrounds the cladding in direct contact with the cladding. The

most practical difference between the two designs is the applications for which they

are used. Loose-tube cable is primarily used for outside-building installations, while

tight-buffered cable is used inside buildings. The strength material surrounds the

buffer, preventing the fiber cable from being stretched when installers pull it. The

material used is often Kevlar, the same material used to produce bulletproof vests.

The final element is the outer jacket. The outer jacket surrounds the cable to protectthe fiber against abrasion, solvents, and other contaminants. The color of the outer

jacket of multimode fiber is usually orange.

Infrared Light Emitting Diodes (LEDs) types of light source usually used with

multimode fiber. LEDs are cheap to build and require somewhat less safety concerns

than lasers. However, LEDs cannot transmit light over cable as far as the lasers.

Multimode fiber (62.5/125) can carry data distances of up to 2 km.

Single-mode fiber - Consists of the same parts as multimode. The outer jacket of

single-mode fiber is usually yellow. The major difference between multimode andsingle-mode fiber is that single-mode allows only one mode of light to propagate

through the smaller, fiber-optic core. The single-mode core is eight to ten m in

diameter. Nine-micron cores are the most common. A 9/125 marking on the jacket of

the single-mode fiber indicates that the core fiber has a diameter of 9 microns and the

surrounding cladding is 125 m in diameter.

An infrared laser is used as the light source in single-mode fiber. The ray of light it

generates enters the core at a 90-degree angle. The data carrying light ray pulses in

single-mode fiber are essentially transmitted in a straight line right down the middle

of the core. This greatly increases both the speed and the distance that data can be

transmitted.

Single-mode fiber is capable of higher bandwidth and greater cable run distances than

multimode fiber. Single-mode fiber can carry LAN data up to 3 km. Although this

distance is considered a standard, newer technologies have increased this distance.

Multimode is only capable of carrying up to 2 km. Lasers and single-mode fibers are


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more expensive than LEDs and multimode fiber. Because of these characteristics,

single-mode fiber is often used for inter-building connectivity. Multimode and single-

mode fibers are shown in Figure 20.

Warming: The laser light used with single-mode has a longer wavelength than can be

seen. The laser can seriously damage eyes. Do not look at the near end of a fiberthat is connected to a device at the far end. Do not look into the transmit port on a

NIC, switch, or router. Remember to keep protective covers over the ends of fiber and

inserted into the fiber-optic ports of switches and routers. Be very careful!

Figure 20: Multimode and Single-mode Fiber

7.5 Wireless Links and Transmission

Nowdays there are people who need to be on-line all the time. For these mobile users,

twisted pair and fiber optics are of no use. They need to get data for their laptop or

wristwatch computers without being tethered to the terrestrial communication

infrastructure. For these users, wireless communication is the answer. Computer

networks can take advantage of the wireless infrastructure where physical wirescannot be laid out.

Some people believe that the future holds only two kinds of communication: fiber and

wireless. All nonmobile computers, telephones, printers, and so on will use fiber, and

all mobile ones will use wireless.


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One of the key challenges in wireless networking is the efficient utilization of the

available transmission spectrum. Because the frequency spectrum available for

wireless communication is limited, frequencies must be reused within the same

geographic area. The spectrum used for wireless communications typically ranges up

to several GHz. Security is also a concern in wireless networks. The open-air interface

makes it difficult to prevent snooping.

The link-level design techniques involve making trade-offs among the various

parameters relevant to the link layer. The optimum design would involve the use of

minimum bandwidth and transmit power while maintaining a high data rate, low

latency, and low bit error rates (BER). These design challenges must be achieved in

the presence of channel imperfections, such as interference.

Wireless links use devices as an antenna for transmitting signals through vacuum,

space, air, or substances. Electromagnetic waves can be propagated through the first

three, as well as through water and wood. The frequency range depends on the type ofsubstance.

Radio waves are easy to generate, can travel long distances, and can penetrate

buildings easily, so they are widely used for communication, both indoors and

outdoors. Radio waves are omnidirectional, meaning that they travel in all directions

from the source, so the transmitter and receiver don't have to be carefully aligned

physically.

The properties of radio waves are frequency dependent. At low frequencies, radio

waves pass through obstacles well, but the power falls off sharply with distance fromthe source, roughly as 1/r2 in air. At high frequencies, radio waves tend to travel in

straight lines and bounce off obstacles. They are also absorbed by rain. At all

frequencies, radio waves are subject to interference from motors electrical equipment.

Due to radio's ability to travel long distances, interference between users is a problem.

For this reason governments tightly license the use of radio transmitters.

good networking concept

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