What is a Network

What is a Network

There are many types of networks that provide us with different kinds of services. In the course of a day, a person might make a phone call, watch a television show, listen to the radio, look up something on the Internet, or even play a video game with someone in another country. All of these activities depend on robust, reliable networks. Networks provide the ability to connect people and equipment no matter where they are in the world. People use networks without ever thinking about how they work or what it would be like if the networks did not exist.

This picture of the airport illustrates people using networks to share information, use resources and communicate with others. There are multiple types of networks shown in this scene. How many can you find?

 Communication technology in the 1990s, and before, required separate, dedicated networks for voice, video and computer data communications. Each of these networks required a different type of device in order to access the network. Telephones, televisions, and computers used specific technologies and different dedicated network structures, to communicate. But what if people want to access all of these network services at the same time, possibly using a single device?

 New technologies create a new kind of network that delivers more than a single type of service. Unlike dedicated networks, these new converged networks are capable of delivering voice, video and data services over the same communication channel or network structure.

New products are coming to market that take advantage of the capabilities of converged information networks. People can now watch live video broadcasts on their computers, make a telephone call over the Internet, or search the Internet using a television. Converged networks make this possible.

 In this course, the term network refers to these new multi-purpose, converged information networks.

Benefits of Networking

Networks come in all sizes. They can range from simple networks consisting of two computers, to networks connecting millions of devices. Networks installed in small offices, or homes and home offices, are referred to as SOHO networks. SOHO networks enable sharing of resources, such as printers, documents, pictures and music between a few local computers.

In business, large networks can be used to advertise and sell products, order supplies, and communicate with customers. Communication over a network is usually more efficient and less expensive than traditional forms of communication, such as regular mail or long distance phone calls. Networks allow for rapid communication such as email and instant messaging, and provide consolidation, storage, and access to information on network servers.

Business and SOHO networks usually provide a shared connection to the Internet. The Internet is considered a "network of networks" because it is literally made up of thousands of networks that are connected to each other.

Here are other uses of a network and the Internet:

Sharing music and video files

Research and on-line learning

Chatting with friends

Planning vacations

Purchasing gifts and supplies

Can you think of other ways people use networks and the Internet in their daily lives?

Basic Network Components

There are many components that can be part of a network, for example personal computers, servers, networking devices, and cabling. These components can be grouped into four main categories:

Hosts

Shared peripherals

Networking devices

Networking media

The network components that people are most familiar with are hosts and shared peripherals. Hosts are devices that send and receive messages directly across the network.

 Shared peripherals are not directly connected to the network, but instead are connected to hosts. The host is then responsible for sharing the peripheral across the network. Hosts have computer software configured to enable people on the network to use the attached peripheral devices.

 The network devices, as well as networking media, are used to interconnect hosts.

 Some devices can play more than one role, depending on how they are connected. For example, a printer directly connected to a host (local printer) is a peripheral. A printer directly connected to a network device and participates directly in network communications is a host.

 Computer Roles In a Network

All computers connected to a network that participate directly in network communication are classified as hosts. Hosts can send and receive messages on the network. In modern networks, computer hosts can act as a client, a server, or both. The software installed on the computer determines which role the computer plays.

Servers are hosts that have software installed that enable them to provide information, like email or web pages, to other hosts on the network. Each service requires separate server software. For example, a host requires web server software in order to provide web services to the network.

Clients are computer hosts that have software installed that enable them to request and display the information obtained from the server. An example of client software is a web browser, like Internet Explorer.

A computer with server software can provide services simultaneously to one or many clients.

 Additionally, a single computer can run multiple types of server software. In a home or small business, it may be necessary for one computer to act as a file server, a web server, and an email server.

 A single computer can also run multiple types of client software. There must be client software for every service required. With multiple clients installed, a host can connect to multiple servers at the same time. For example, a user can check email and view a web page while instant messaging and listening to Internet radio.

Peer to Peer Networking

Client and server software usually runs on separate computers, but it is also possible for one computer to carry out both roles at the same time. In small businesses and homes, many computers function as the servers and clients on the network. This type of network is called a peer-to-peer network.

The simplest peer-to-peer network consists of two directly connected computers using a wired or wireless connection.

Multiple PCs can also be connected to create a larger peer-to-peer network but this requires a network device, such as a hub, to interconnect the computers.

 The main disadvantage of a peer-to-peer environment is that the performance of a host can be slowed down if it is acting as both a client and a server at the same time.

 In larger businesses, due to the potential for high amounts of network traffic, it is often necessary to have dedicated servers to support the number of service requests.

Network Topologies

In a simple network consisting of a few computers, it is easy to visualize how all of the various components connect. As networks grow, it is more difficult to keep track of the location of each component, and how each is connected to the network. Wired networks require lots of cabling and network devices to provide connectivity for all network hosts.

 When networks are installed, a physical topology map is created to record where each host is located and how it is connected to the network. The physical topology map also shows where the wiring is installed and the locations of the networking devices that connect the hosts. Icons are used to represent the actual physical devices within the topology map. It is very important to maintain and update physical topology maps to aid future installation and troubleshooting efforts.

 In addition to the physical topology map, it is sometimes necessary to also have a logical view of the network topology. A logical topology map groups hosts by how they use the network, no matter where they are physically located. Host names, addresses, group information and applications can be recorded on the logical topology map.

 The graphics illustrate the difference between logical and physical topology maps.

Source , Channel ,and Destination

The primary purpose of any network is to provide a method to communicate information. From the very earliest primitive humans to the most advanced scientists of today, sharing information with others is crucial for human advancement.

All communication begins with a message, or information, that must be sent from one individual or device to another. The methods used to send, receive and interpret messages change over time as technology advances.

 All communication methods have three elements in common. The first of these elements is the message source, or sender. Message sources are people, or electronic devices, that need to communicate a message to other individuals or devices. The second element of communication is the destination, or receiver, of the message. The destination receives the message and interprets it. A third element, called a channel, provides the pathway over which the message can travel from source to destination.

Rules of  Communication

In any conversation between two people, there are many rules, or protocols, that the two must follow in order for the message to be successfully delivered and understood. Among the protocols for successful human communication are:

Identification of sender and receiver

Agreed-upon medium or channel (face-to-face, telephone, letter, photograph)

Appropriate communication mode (spoken, written, illustrated, interactive or one-way)

Common language

Grammar and sentence structure

Speed and timing of delivery

Imagine what would happen if no protocols or rules existed to govern how people communicate with each other. Would you be able to understand them? Are you able to read the paragraph that does not follow commonly accepted protocols?

Protocols are specific to the characteristics of the source, channel and destination of the message. The rules used to communicate over one medium, like a telephone call, are not necessarily the same as communication using another medium, such as a letter.

Protocols define the details of how the message is transmitted, and delivered. This includes issues of:

Message format

Message size

Timing

Encapsulation

Encoding

Standard message pattern

 Many of the concepts and rules that make human communication reliable and understandable also apply to computer communication.

Message Encoding

One of the first steps to sending a message is encoding it. Written words, pictures, and spoken languages each use a unique set of codes, sounds, gestures, and/or symbols to represent the thoughts being shared. Encoding is the process of converting thoughts into the language, symbols, or sounds, for transmission. Decoding reverses this process in order to interpret the thought.

Imagine a person watching a sunset and then calling someone else to talk about how beautiful the sunset looks. To communicate the message, the sender must first convert, or encode, their thoughts and perceptions about the sunset into words. The words are spoken into the telephone using the sounds and inflections of spoken language that convey the message. On the other end of the telephone line, the person listening to the description, receives and decodes the sounds in order to visualize the image of the sunset described by the sender.

Encoding also occurs in computer communication. Encoding between hosts must be in an appropriate form for the medium. Messages sent across the network are first converted into bits by the sending host. Each bit is encoded into a pattern of sounds, light waves, or electrical impulses depending on the network media over which the bits are transmitted. The destination host receives and decodes the signals in order to interpret the message.

Message Formatting

When a message is sent from source to destination, it must use a specific format or structure. Message formats depend on the type of message and the channel that is used to deliver the message.

Letter writing is one of the most common forms of written human communication. For centuries, the agreed format for personal letters has not changed. In many cultures, a personal letter contains the following elements:

An identifier of the recipient

A salutation or greeting

The message content

A closing phrase

An identifier of the sender

In addition to having the correct format, most personal letters must also be enclosed, or encapsulated, in an envelope for delivery. The envelope has the address of the sender and receiver on it, each located at the proper place on the envelope. If the destination address and formatting are not correct, the letter is not delivered.

The process of placing one message format (the letter) inside another message format (the envelope) is called encapsulation. De-encapsulation occurs when the process is reversed by the recipient and the letter is removed from the envelope.

A letter writer uses an accepted format to ensure that the letter is delivered and understood by the recipient. In the same way, a message that is sent over a computer network follows specific format rules for it to be delivered and processed. Just as a letter is encapsulated in an envelope for delivery, so computer messages are encapsulated. Each computer message is encapsulated in a specific format, called a frame, before it is sent over the network. A frame acts like an envelope; it provides the address of the intended destination and the address of the source host.

The format and contents of a frame are determined by the type of message being sent and the channel over which it is communicated. Messages that are not correctly formatted are not successfully delivered to or processed by the destination host.

Massage Size

Imagine what it would be like to read this course if it all appeared as one long sentence; it would not be easy to read and comprehend. When people communicate with each other, the messages that they send are usually broken into smaller parts or sentences. These sentences are limited in size to what the receiving person can process at one time. An individual conversation may be made up of many smaller sentences to ensure that each part of the message is received and understood.

Likewise, when a long message is sent from one host to another over a network, it is necessary to break the message into smaller pieces. The rules that govern the size of the pieces, or frames, communicated across the network are very strict. They can also be different, depending on the channel used. Frames that are too long or too short are not delivered.

The size restrictions of frames require the source host to break a long message into individual pieces that meet both the minimum and maximum size requirements. Each piece is encapsulated in a separate frame with the address information, and is sent over the network. At the receiving host, the messages are de-encapsulated and put back together to be processed and interpreted.

Message Timeing

One factor that affects how well a message is received and understood is timing. People use timing to determine when to speak, how fast or slow to talk, and how long to wait for a response. These are the rules of engagement.

Access Method

Access Method determines when someone is able to send a message. These timing rules are based on the environment. For example, you may be able to speak whenever you have something to say. In this environment, a person must wait until no one else is talking before speaking. If two people talk at the same time, a collision of information occurs and it is necessary for the two to back off and start again. These rules ensure communication is successful. Likewise, it is necessary for computers to define an access method. Hosts on a network need an access method to know when to begin sending messages and how to respond when errors occur.

Flow Control

Timing also effects how much information can be sent and the speed that it can be delivered. If one person speaks too quickly, it is difficult for the other person to hear and understand the message. The receiving person must ask the sender to slow down. In network communication, a sending host can transmit messages at a faster rate than the destination host can receive and process. Source and destination hosts use flow control to negotiate correct timing for successful communication.

Response Timeout

If a person asks a question and does not hear a response within an acceptable amount of time, the person assumes that no answer is coming and reacts accordingly. The person may repeat the question, or may go on with the conversation. Hosts on the network also have rules that specify how long to wait for responses and what action to take if a response timeout occurs.

Massage Patterns

Sometimes, a person wants to communicate information to a single individual. At other times, the person may need to send information to a group of people at the same time, or even to all people in the same area. A conversation between two people is an example of a one-to-one pattern of communication. When a group of recipients need to receive the same message simultaneously, a one-to-many or one-to-all message pattern is necessary.

There are also times when the sender of a message needs to be sure that the message is delivered successfully to the destination. In these cases, it is necessary for the recipient to return an acknowledgement to the sender. If no acknowledgement is required, the message pattern is referred to as unacknowledged.

Hosts on a network use similar message patterns to communicate.

A one-to-one message pattern is referred to as a unicast, meaning that there is only a single destination for the message.

When a host needs to send messages using a one-to-many pattern, it is referred to as a multicast. Multicasting is the delivery of the same message to a group of host destinations simultaneously.

If all hosts on the network need to receive the message at the same time, a broadcast is used. Broadcasting represents a one-to-all message pattern. Additionally, hosts have requirements for acknowledged versus unacknowledged messages.

Protocol use in communication

All communication, both human and computer, is governed by pre-established rules, or protocols. These protocols are determined by the characteristics of the source, channel and destination. Based on the source, channel and destination, the protocols define the details for the issues of message format, message size, timing, encapsulation, encoding and standard message pattern.

Importance of Protocol

Computers, just like humans, use rules, or protocols, in order to communicate.

 Protocols are especially important on a local network. In a wired environment, a local network is defined as an area where all hosts must "speak the same language" or in computer terms "share a common protocol".

 If everyone in the same room spoke a different language they would not be able to communicate. Likewise, if devices in a local network did not use the same protocols they would not be able to communicate.

 The most common set of protocols used on local wired networks is Ethernet.

 The Ethernet protocol defines many aspects of communication over the local network, including: message format, message size, timing, encoding, and message patterns.

Standrazation of Protocols

In the early days of networking, each vendor used their own, proprietary methods of interconnecting network devices and networking protocols. Equipment from one vendor could not communicate with equipment from another.

 As networks became more widespread, standards were developed that defined rules by which network equipment from different vendors operated. Standards are beneficial to networking in many ways:

Facilitate design

Simplify product development

Promote competition

Provide consistent interconnections

Facilitate training

Provide more vendor choices for customers

 There is no official local networking standard protocol, but over time, one technology, Ethernet, has become more common than the others. It has become a de facto standard.

 The Institute of Electrical and Electronic Engineers, or IEEE (pronounced eye-triple-e), maintains the networking standards, including Ethernet and wireless standards. IEEE committees are responsible for approving and maintaining the standards for connections, media requirements and communications protocols. Each technology standard is assigned a number that refers to the committee that is responsible for approving and maintaining the standard. The committee responsible for the Ethernet standards is 802.3.

 Since the creation of Ethernet in 1973, standards have evolved for specifying faster and more flexible versions of the technology. This ability for Ethernet to improve over time is one of the main reasons that it has become so popular. Each version of Ethernet has an associated standard. For example, 802.3 100BASE-T represents the 100 Megabit Ethernet using twisted pair cable standards. The standard notation translates as:

100 is the speed in Mbps

BASE stands for baseband transmission

T stands for the type of cable, in this case, twisted pair.

 Early versions of Ethernet were relatively slow at 10 Mbps. The latest versions of Ethernet operate at 10 Gigabits per second and faster. Imagine how much faster these new versions are than the original Ethernet networks.

Physical Addressing

All communication requires a way to identify the source and destination. The source and destination in human communication are represented by names.

When a name is called, the person with that name listens to the message and responds. Other people in the room may hear the message, but they ignore it because it is not addressed to them.

On Ethernet networks, a similar method exists for identifying source and destination hosts. Each host connected to an Ethernet network is assigned a physical address which serves to identify the host on the network.

Every Ethernet network interface has a physical address assigned to it when it is manufactured. This address is known as the Media Access Control (MAC) Address. The MAC address identifies each source and destination host on the network.

Ethernet networks are cable based, meaning that a copper or fiber optic cable connects hosts and networking devices. This is the channel used for communications between the hosts.

When a host on an Ethernet network communicates, it sends frames containing its own MAC address as the source and the MAC address of the intended recipient. Any hosts that receive the frame will decode the frame and read the destination MAC address. If the destination MAC address matches the address configured on the NIC, it will process the message and store it for the host application to use. If the destination MAC address does not match the host MAC address, the NIC will ignore the message.

Ethernet Communication

The Ethernet protocol standards define many aspects of network communication including frame format, frame size, timing and encoding.

When messages are sent between hosts on an Ethernet network, the hosts format the messages into the frame layout that is specified by the standards. Frames are also referred to as Protocol Data Units (PDUs).

The format for Ethernet frames specifies the location of the destination and source MAC addresses, and additional information including:

Preamble for sequencing and timing

Start of frame delimiter

Length and type of frame

Frame check sequence to detect transmission errors

 The size of Ethernet frames is limited to a maximum of 1518 bytes and a minimum size of 64 bytes. Frames that do not match these limits are not processed by the receiving hosts. In addition to the frame formats, sizes and timing, Ethernet standards define how the bits making up the frames are encoded onto the channel. Bits are transmitted as either electrical impulses over copper cable or as light impulses over fiber optic cable.

Hierarchical Design of Ethernet Networks

Imagine how difficult communication would be if the only way to send a message to someone was to use the person's name. If there were no street addresses, cities, towns, or country boundaries, delivering a message to a specific person across the world would be nearly impossible.

 On an Ethernet network, the host MAC address is similar to a person's name. A MAC address indicates the individual identity of a specific host, but it does not indicate where on the network the host is located. If all hosts on the Internet (over 400 million of them) were each identified by only their unique MAC address, imagine how difficult it would be to locate a single one.

Additionally, Ethernet technology generates a large amount of broadcast traffic in order for hosts to communicate. Broadcasts are sent to all hosts within a single network. Broadcasts consume bandwidth and slow network performance. What would happen if the millions of hosts attached to the Internet were all in one Ethernet network and were using broadcasts?

 For these two reasons, large Ethernet networks consisting of many hosts are not efficient. It is better to divide larger networks into smaller, more manageable pieces. One way to divide larger networks is to use a hierarchical design model.

In networking, hierarchical design is used to group devices into multiple networks that are organized in a layered approach. It consists of smaller, more manageable groups that allow local traffic to remain local. Only traffic that is destined for other networks is moved to a higher layer.

A hierarchical, layered design provides increased efficiency, optimization of function, and increased speed. It allows the network to scale as required because additional local networks can be added without impacting the performance of the existing ones.

 The hierarchical design has three basic layers:

Access Layer - to provide connections to hosts in a local Ethernet network.

Distribution Layer - to interconnect the smaller local networks.

Core Layer - a high-speed connection between distribution layer devices.

 With this new hierarchical design, there is a need for a logical addressing scheme that can identify the location of a host. This is the Internet Protocol (IP) addressing scheme.

Logical Addressing

A person's name usually does not change. A person's address on the other hand, relates to where they live and can change. On a host, the MAC address does not change; it is physically assigned to the host NIC and is known as the physical address. The physical address remains the same regardless of where the host is placed on the network.

 The IP address is similar to the address of a person. It is known as a logical address because it is assigned logically based on where the host is located. The IP address, or network address, is assigned to each host by a network administrator based on the local network.

IP addresses contain two parts. One part identifies the local network. The network portion of the IP address will be the same for all hosts connected to the same local network. The second part of the IP address identifies the individual host. Within the same local network, the host portion of the IP address is unique to each host.

 Both the physical MAC and logical IP addresses are required for a computer to communicate on a hierarchical network, just like both the name and address of a person are required to send a letter

Access and Distribution Layers and Devices

IP traffic is managed based on the characteristics and devices associated with each of the three layers: Access, Distribution and Core. The IP address is used to determine if traffic should remain local or be moved up through the layers of the hierarchical network.

Access Layer

The Access Layer provides a connection point for end user devices to the network and allows multiple hosts to connect to other hosts through a network device, usually a hub or switch. Typically, all devices within a single Access Layer will have the same network portion of the IP address.

 If a message is destined for a local host, based on the network portion of the IP address, the message remains local. If it is destined for a different network, it is passed up to the Distribution Layer. Hubs and switches provide the connection to the Distribution Layer devices, usually a router.

 Distribution Layer

 The Distribution Layer provides a connection point for separate networks and controls the flow of information between the networks. It typically contains more powerful switches than the Access Layer as well as routers for routing between networks. Distribution Layer devices control the type and amount of traffic that flows from the Access Layer to the Core Layer.

Core Layer

The Core Layer is a high-speed backbone layer with redundant (backup) connections. It is responsible for transporting large amounts of data between multiple end networks. Core Layer devices typically include very powerful, high-speed switches and routers. The main goal of the Core Layer is to transport data quickly.

 Hubs, switches, and routers are discussed in more detail in the next two sections.

Access Layer

The Access Layer is the most basic level of the network. It is the part of the network in which people gain access to other hosts and to shared files and printers. The Access Layer is composed of host devices, as well as the first line of networking devices to which they are attached.

Networking devices enable us to connect many hosts with each other and also provide those hosts access to services offered over the network. Unlike the simple network consisting of two hosts connected by a single cable, in the Access Layer, each host is connected to a networking device. This type of connectivity is shown in the graphic.

Within an Ethernet network, each host is able to connect directly to an Access Layer networking device using a point-to-point cable. These cables are manufactured to meet specific Ethernet standards. Each cable is plugged into a host NIC and then into a port on the networking device. There are several types of networking devices that can be used to connect hosts at the Access Layer, including Ethernet hubs and switches.