Basic Networking Concept

What is a Network?

A network is simply defined as something that connects things together for

a specific purpose. The term network is used in a variety of contexts,

including telephone, television, computer, or even people networks.

A computer network connects two or more devices together to share a

nearly limitless range of information and services, including:

Ø  Documents

Ø Email and messaging                                                               

Ø Websites

Ø  Databases

Ø Music

Ø Printers and faxes

Ø Telephony and videoconferencing

Protocols are rules that govern how devices communicate and share

information across a network. Examples of protocols include:

Ø  IP – Internet Protocol

Ø  HTTP - Hyper Text Transfer Protocol

Ø   SMTP – Simple Mail Transfer Protocol

Multiple protocols often work together to facilitate end-to-end network

communication, forming protocol suites or stacks. Protocols are covered in

great detail in other guides.

Network reference models were developed to allow products from different

manufacturers to interoperate on a network. A network reference model

serves as a blueprint, detailing standards for how protocol communication

should occur.

The Open Systems Interconnect (OSI) and Department of Defense (DoD)

models are the most widely recognized reference models. Both are covered

in great detail in another guide.

Basic Network Types

Network types are often defined by function or size. The two most common

categories of networks are:

Ø  LANs (Local Area Networks)

Ø  WANs (Wide Area Networks)

A LAN is generally a high-speed network that covers a small geographic

area, usually contained within a single building or campus. A LAN is

usually under the administrative control of a single organization. Ethernet is

the most common LAN technology.

A WAN can be defined one of two ways. The book definition of a WAN is a

network that spans large geographical locations, usually to connect multiple

LANs. This is a general definition, and not always accurate.

A more practical definition of a WAN is a network that traverses a public or

commercial carrier, using one of several WAN technologies. A WAN is often

under the administrative control of several organizations (or providers), and

does not necessarily need to span large geographical distances.

A MAN (Metropolitan Area Network) is another category of network,

though the term is not prevalently used. A MAN is defined as a network that

connects LAN’s across a city-wide geographic area.

An internetwork is a general term describing multiple networks connected

together. The Internet is the largest and most well-known internetwork.

Some networks are categorized by their function, as opposed to their size. A

SAN (Storage Area Network) provides systems with high-speed, lossless

access to high-capacity storage devices.

A VPN (Virtual Private Network) allows for information to be securely

sent across a public or unsecure network, such as the Internet. Common uses of a VPN are to connect branch offices or remote users to a main office

Network Architectures

A host refers to any device that is connected to a network. A host can also

be defined as any device assigned a network address.

A host can serve one or more functions:

Ø A host can request data, often referred to as a client.

Ø  A host can provide data, often referred to as a server.

A host can both request and provide data, often referred to as a peer.

Because of these varying functions, multiple network architectures have

been developed, including:

Ø Peer-to-Peer

Ø Client/Server

Ø Mainframe/Terminal

In a basic peer-to-peer architecture, all hosts on the network can both

request and provide data and services. For example, two Windows XP

workstations configured to share files would be considered a peer-to-peer

network.

Peer-to-peer networks are very simple to configure, yet this architecture

presents several challenges. Data is difficult to manage and back-up, as it is

spread across multiple devices. Security is equally problematic, as user

accounts and permissions much be configured individually on each host.

In a client/server architecture, hosts are assigned specific roles. Clients

request data and services stored on servers. An example of a client/server

network would be Windows XP workstations accessing files off of a

Windows 2003 server.

There are several advantages to the client/server architecture. Data and

services are now centrally located on one or more servers, consolidating the management and security of that data. As a result, client/server networks can scale far larger than peer-to-peer networks.

One key disadvantage of the client/server architecture is that the server can present a single point of failure. This can be mitigated by adding

redundancy at the server layer.

Network Architectures (continued)

In a mainframe/terminal architecture, a single device (the mainframe)

stores all data and services for the network. This provides the same

advantages as a client/server architecture – centralized management and

security of data.

Additionally, the mainframe performs all processing functions for the dumb

terminals that connect to the mainframe. The dumb terminals perform no

processing whatsoever, but serve only as input and output devices into the

mainframe.

In simpler terms, the mainframe handles all thinking for the dumb terminals.

A dumb terminal typically consists of only a keyboard/mouse, a display, and an interface card into the network.

The traditional mainframe architecture is less prevalent now than in the early

history of networking. However, the similar thin-client architecture has

gained rapid popularity. A thin-client can be implemented as either a

hardware device, or software running on top of another operating system

(such as Windows or Linux).

Like dumb terminals, thin-clients require a centralized system to perform all

(or most) processing functions. User sessions are spawned and managed

completely within the server system.

Hardware thin-clients are generally inexpensive, with a small footprint and

low power consumption. For environments with a large number of client

devices, the thin-client architecture provides high scalability, with a lower

total cost of ownership.

The two most common thin-client protocols are:

RDP (Remote Desktop Protocol) – developed by Microsoft

 ICA (Independent Computer Architecture) – developed by Citrix

OSI Reference Model -

Network Reference Models

A computer network connects two or more devices together to share

information and services. Multiple networks connected together form an

internetwork.

Internetworking present challenges - interoperating between products from

different manufacturers requires consistent standards. Network reference

models were developed to address these challenges. A network reference

model serves as a blueprint, detailing how communication between network

devices should occur.

The two most recognized network reference models are:

Ø The Open Systems Interconnection (OSI) model

Ø The Department of Defense (DoD) model

Without the framework that network models provide, all network hardware

and software would have been proprietary. Organizations would have been

locked into a single vendor’s equipment, and global networks like the

Internet would have been impractical, if not impossible.

Network models are organized into layers, with each layer representing a

specific networking function. These functions are controlled by protocols,

which are rules that govern end-to-end communication between devices.

Protocols on one layer will interact with protocols on the layer above and

below it, forming a protocol suite or stack. The TCP/IP suite is the most

prevalent protocol suite, and is the foundation of the Internet.

A network model is not a physical entity – there is no OSI device.

Manufacturers do not always strictly adhere to a reference model’s blueprint,

and thus not every protocol fits perfectly within a single layer. Some

protocols can function across multiple layers

OSI Reference Model

The Open Systems Interconnection (OSI) model was developed by the

International Organization for Standardization (ISO), and formalized in

1984. It provided the first framework governing how information should be

sent across a network.

The OSI model consists of seven layers, each corresponding to a specific

network function:

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data-link

1 Physical

Note that the bottom layer is Layer 1. Various mnemonics make it easier to

remember the order of the OSI model’s layers:

7 Application           All          Away

6 Presentation        People      Pizza

5 Session                 Seem       Sausage

4 Transport                  To      Throw

3 Network                   Need Not

2 Data-link              Data Do

1 Physical               Processing Please

ISO further developed an entire protocol suite based on the OSI model;

however, the OSI protocol suite was never widely implemented.

The OSI model itself is now somewhat deprecated – modern protocol suites,

such as the TCP/IP suite, are difficult to fit cleanly within the OSI model’s

seven layers. This is especially true of the upper three layers.

The bottom (or lower) four layers are more clearly defined, and

terminology from those layers is still prevalently used. Many protocols and

devices are described by which lower layer they operate at.

OSI Model - The Upper Layers

The top three layers of the OSI model are often referred to as the upper

layers:

Ø  Layer-7 - Application layer

Ø  Layer-6 - Presentation layer

Ø Layer-5 - Session layer

Protocols that operate at these layers manage application-level functions,

and are generally implemented in software.

The function of the upper layers of the OSI model can be difficult to

visualize. Upper layer protocols do not always fit perfectly within a layer,

and often function across multiple layers.

OSI Model - The Application Layer

The Application layer (Layer-7) provides the interface between the user

application and the network. A web browser and an email client are

examples of user applications.

The user application itself does not reside at the Application layer - the

protocol does. The user interacts with the application, which in turn interacts

with the application protocol.

Examples of Application layer protocols include:

• FTP, via an FTP client

• HTTP, via a web browser

• POP3 and SMTP, via an email client

• Telnet

The Application layer provides a variety of functions:

• Identifies communication partners

• Determines resource availability

• Synchronizes communication

The Application layer interacts with the Presentation layer below it. As it is

the top-most layer, it does not interact with any layers above it.

OSI Model - The Presentation Layer

The Presentation layer (Layer-6) controls the formatting and syntax of user

data for the application layer. This ensures that data from the sending

application can be understood by the receiving application.

Standards have been developed for the formatting of data types, such as text,

images, audio, and video. Examples of Presentation layer formats include:

• Text - RTF, ASCII, EBCDIC

• Images - GIF, JPG, TIF

• Audio - MIDI, MP3, WAV

• Movies - MPEG, AVI, MOV

If two devices do not support the same format or syntax, the Presentation

layer can provide conversion or translation services to facilitate

communication.

Additionally, the Presentation layer can perform encryption and

compression of data, as required. However, these functions can also be

performed at lower layers as well. For example, the Network layer can

perform encryption, using IPSec.

OSI Model - The Session Layer

The Session layer (Layer-5) is responsible for establishing, maintaining,

and ultimately terminating sessions between devices. If a session is broken,

this layer can attempt to recover the session.

Sessions communication falls under one of three categories:

• Full-Duplex – simultaneous two-way communication

• Half-Duplex – two-way communication, but not simultaneous

• Simplex – one-way communication

Many modern protocol suites, such as TCP/IP, do not implement Session

layer protocols. Connection management is often controlled by lower layers,

such as the Transport layer.

The lack of true Session layer protocols can present challenges for highavailability

and failover. Reliance on lower-layer protocols for session

management offers less flexibility than a strict adherence to the OSI model.

OSI Model - The Lower Layers

The bottom four layers of the OSI model are often referred to as the lower

layers:

• Layer-4 – Transport layer

• Layer-3 – Network layer

• Layer-2 – Data-Link layer

• Layer-1 – Physical layer

Protocols that operate at these layers control the end-to-end transport of data

between devices, and are implemented in both software and hardware.

OSI Model - The Transport Layer

The Transport layer (Layer-4) does not actually send data, despite its

name. Instead, this layer is responsible for the reliable transfer of data, by

ensuring that data arrives at its destination error-free and in order.

Transport layer communication falls under two categories:

• Connection-oriented – requires that a connection with specific

agreed-upon parameters be established before data is sent.

• Connectionless – requires no connection before data is sent.

Connection-oriented protocols provide several important services:

• Segmentation and sequencing – data is segmented into smaller

pieces for transport. Each segment is assigned a sequence number, so

that the receiving device can reassemble the data on arrival.

• Connection establishment – connections are established, maintained,

and ultimately terminated between devices.

• Acknowledgments – receipt of data is confirmed through the use of

acknowledgments. Otherwise, data is retransmitted, guaranteeing

delivery.

• Flow control (or windowing) – data transfer rate is negotiated to

prevent congestion.

The TCP/IP protocol suite incorporates two Transport layer protocols:

• Transmission Control Protocol (TCP) – connection-oriented

• User Datagram Protocol (UDP) – connectionless

OSI Model - The Network Layer

The Network layer (Layer-3) controls internetwork communication, and

has two key responsibilities:

• Logical addressing – provides a unique address that identifies both

the host, and the network that host exists on.

• Routing – determines the best path to a particular destination

network, and then routes data accordingly.

Two of the most common Network layer protocols are:

• Internet Protocol (IP)

• Novell’s Internetwork Packet Exchange (IPX).

IPX is almost entirely deprecated. IP version 4 (IPv4) and IP version 6

(IPv6) are covered in nauseating detail in other guides.

OSI Model - The Data-Link Layer

While the Network layer is concerned with transporting data between

networks, the Data-Link layer (Layer-2) is responsible for transporting

data within a network.

The Data-Link layer consists of two sublayers:

• Logical Link Control (LLC) sublayer

• Media Access Control (MAC) sublayer

The LLC sublayer serves as the intermediary between the physical link and

all higher layer protocols. It ensures that protocols like IP can function

regardless of what type of physical technology is being used.

Additionally, the LLC sublayer can perform flow-control and errorchecking,

though such functions are often provided by Transport layer

protocols, such as TCP.

The MAC sublayer controls access to the physical medium, serving as

mediator if multiple devices are competing for the same physical link. Datalink

layer technologies have various methods of accomplishing this -

Ethernet uses Carrier Sense Multiple Access with Collision Detection

(CSMA/CD), and Token Ring utilizes a token.

Ethernet is covered in great detail in another guide.

OSI Model - The Data-Link Layer (continued)

The Data-link layer packages the higher-layer data into frames, so that the

data can be put onto the physical wire. This packaging process is referred to

as framing or encapsulation.

The encapsulation type will vary depending on the underlying technology.

Common Data-link layer technologies include following:

• Ethernet – the most common LAN data-link technology

• Token Ring – almost entirely deprecated

• FDDI (Fiber Distributed Data Interface)

• 802.11 Wireless

• Frame-Relay

• ATM (Asynchronous Transfer Mode)

The data-link frame contains the source and destination hardware (or

physical) address. Hardware addresses uniquely identify a host within a

network, and are often hardcoded onto physical network interfaces.

However, hardware addresses contain no mechanism for differentiating one

network from another, and can only identify a host within a network.

The most common hardware address is the Ethernet MAC address.

OSI Model - The Physical Layer

The Physical layer (Layer-1) controls the signaling and transferring of raw

bits onto the physical medium. The Physical layer is closely related to the

Data-link layer, as many technologies (such as Ethernet) contain both datalink

and physical functions.

The Physical layer provides specifications for a variety of hardware:

• Cabling

• Connectors and transceivers

• Network interface cards (NICs)

• Wireless radios

• Hubs

Physical-layer devices and topologies are covered extensively in other

guides.