Chapter 1: Networking Basics






What is a computer network

It wasn’t long after the development of electronic computers before people needed to get information from one computer to another. I have not been able to find out when or how the first two computers were wired together in such a way that data could be sent directly from one to another. However, it is easy to speculate that the first computer network, if you can call it that, consisted of a central computer connected to two or more teleprinters. The Internet started out as a single terminal at UCLA that could connect to a single computer at Stanford University.

In the late 1970s and early 1980s the PC proliferated in the office and people needed to move data between computers and share printers. Data was moved by floppy disk. Today this is jokingly called sneakernet. Printers were shared by connecting several computers to one printer through a switchbox.

What people needed was for a user at one computer to be able to access the hard disk and printer at another computer as easily as if they were part if his or her own computer. During the 1970s Xerox developed such a system and expanded into modern networking. Xerox called their network Ethernet and it is still by far the dominant system.

Home and Small Business Networks

A home network may be as simple as two computers connected to the Internet via a broadband router. A larger home network could have a computer for each member of the family with a home theater computer and possibly a file server. They are essentially the same except for the number of computers. The needs of a small business are about the same except more computers. A small business is more likely to have a central logon server to control access to the whole network plus file servers, printer servers, networked printers, web servers and the like. However, the only difference is size. Compared to a home network a small business network just has bigger switches (where all the computers connect together) so there can be more computers. A large organization may have multiple small networks connected through routers. This is called an enterprise network. Elements of an enterprise network are not completely beyond the scope of this book but we will be concentrating on small networks here.

Types of networks

Geographic models

When working with networks you will frequently hear the terms Local Area Network (LAN) and Wide Area Network (WAN). Textbooks and certification exams focus on the advantages and disadvantages of LANs and WANs as if you need to choose between the two when designing a network. This can be confusing to someone who is not technically savvy and is trying to make an executive decision. It’s no easier for the network manager who has to explain that you can’t string an Ethernet cable from one side of the city to the other.

The difference between a LAN and a WAN is not a matter of choice. You do not weigh the advantages and disadvantages of LANs and WANs and choose the best technology for your situation. If your network is a group of computers that are close together you will string Ethernet cables between them and you will have a Local Area Network. If you have two or more computers that are too far apart to string your own cables you will use the telephone system to connect your computers. This is a Wide Area Network.
 
From these definitions you can see that a Wide Area Network can be a network that spans across the street or a network that spans around the world. Attempts have been made to define networks that are beyond the scope of a LAN but have a limited geographic scope. For example, in the late 1980s technology was developed that was suitable for networks that spanned an area the size of a city. Such a network was defined as a Metropolitan Area Network (MAN). This definition is still used occasionally, but the technology failed in the marketplace. It was too expensive to implement and telephone companies already had suitable infrastructure in place that could be leased at an affordable price. The end result is that a "MAN" now uses the same technology as a WAN so there is no difference from a network administrator's point of view.
 
There have been other attempts to define networks based on their geographic scope. One of these definitions is the Campus Area Network (CAN) for a installation such as a university campus, military base or business complex. Recently the term Personal Area Network (PAN) has appeared for such networks as Bluetooth and wireless USB. In reality, these networks are all Local Area Networks. Use of the terms CAN and PAN may be useful in defining the geographic scope of a network but are not useful for defining the technology the network administrator will use.
 
In this book LAN and WAN are not used to describe the geographic scope of a network but to describe the different technologies used to implement the types of networks.

Local Area Network (LAN)

A Local Area Network is frequently defined as a network that covers a small area, such as an office, an office suite, one floor of a building or a single building. Sometimes this definition may be extended to cover a small group of buildings but that may be called a Campus Area Network (CAN).
 
From a network administrator's point of view, the network media (the wiring) makes a LAN different from other types of networks. In the case of a Local Area Network, the owner of the network owns the media. The computers are close enough together that the network owner can install wiring or wireless access points without relying on a third party to provide connection services.
 
Examples of LANs are home networks, small business networks and segments of corporate networks.

LAN Technologies

There have been several technologies developed over the years to implement local area networks. Some of theses technologies are listed below. Technical details will be discussed later.

Current technologies used for LANs are:

Ethernet

This was the first practical LAN technology. It was developed by Xerox in the mid 1970s and released commercially in the 1980s. The speed and ease of implementation has improved over the years and Ethernet is now used on virtually all local area networks.

Arcnet

The first network technology to gain wide use. At the time of its release it was easier to implement than Ethernet. It is still has a niche for embedded systems and robotics. You will not find Arcnet in an office installation.

Legacy technologies once used for LANs are:

Token Ring

Developed in the 1980s by IBM. Its speed and ease of implementation did not keep up with Ethernet and It fell out of favor in the 1990s.
 

FDDI

A fiber optic network developed in the 1980s for local area networks and metropolitan area networks. It was initially faster than Ethernet but never exceeded 100 Mbps. For metropolitan area networks It proved too expensive to implement. For LANs, Fast Ethernet, which was less expensive and more manageable, soon equaled FDDI in performance. Now FDDI is far outperformed by Gigabit Ethernet. It fell out of favor in the 1990s.

100VG-AnyLAN

This was a scheme proposed in the mid 1990s to implement a 100 Mbps network over voice grade cables. It was also designed to connect to and pass traffic between both Ethernet and Token Ring networks (hence "AnyLAN"). It never gained a significant market share.

You may still find some of the legacy technologies in place but it is very unlikely that you will find them in new installations.

Data rates

Modern LANs can convey data at speeds from 10 Mbps to 40 Gbps. This was originally done over copper wires where speeds improved over the years. Now LANs use fiber optic cable and wireless systems in addition to copper wires.

Copper

Copper refers to any type of electrical cable used to carry network signals, usually made of copper wiring. Copper wiring may consist of coaxial cable or twisted pair cable.
 
There are many factors that limit the data rate and the distance that a usable signal can travel over copper cable. Typically, modern copper-based Ethernet has a specified maximum cable length of 100 meters.  Here are the most common Ethernet specifications:

Ethernet

Ethernet operates at 10 Mbps over coaxial cable or Category 3 twisted pair cable (also known as Cat 3 or voice grade cable).

Fast Ethernet

Fast Ethernet achieves 100 Mbps over Cat 5 or better cable. A scheme to achieve 100 Mbps over Cat 3 cable did not gain wide acceptance.

Gigabit Ethernet

Gigabit Ethernet can achieve 1 Gbps over Cat 5e or better cable.

 2.5 and 5-Gigabit Ethernet

An emerging standard that can be implemented on Cat 5e cable.

10-Gigabit Ethernet 

10-Gigabit Ethernet can achieve speeds of 10 Gbps over Cat 6a cable. Over Cat 5e cable the distance is specified at 55 meters.

40 and 100-Gigabit Ethernet 

40 and 100-Gigabit Ethernet is currently (2015) in development.

Keep in mind that these specifications are typical and not absolute. For example, 10-Gigabit Ethernet has ranges specified from 1 meter to 80 kilometers depending on the system and cable used.

Fiber Optic

Fiber optic cables carry signals via pulses of light. Modern fiber optic cable can carry signals considerably further than copper wires. For example, Ethernet has a specified maximum cable length of 100 meters using twisted pair cable. The specified maximum length of a fiber optic Ethernet is up to 80 km (10GBASE-ZR). Ethernet, Fast Ethernet, Gigabit Ethernet and 10-Gigabit Ethernet have all been implemented over fiber optics. 40 and 100-Gigabit Ethernet are planned to be implemented on fiber optics.

Wireless

Wireless Ethernet, also known as 802.11 or WiFi, is a popular alternative to wired networking. Wireless Ethernet is usually known by its 802.11 designation.

802.11a

802.11a operates at 54 Mbps on a 5 GHz frequency band.

802.11b

802.11b operates at 11 Mbps on a 2.4 GHz band. This band is susceptible to interference from cordless telephones, baby monitors, etd.

802.11g

802.11g operates at 54 Mbps on the same band as 802.11b and can communicate with 802.11b equipment.

802.11n

802.11n typically operates at 150 Mbps (single channel) to 600 Mbps (quad channel). 802.11n can communicate with 802.11g and 802.11b equipment.

802.11ac

802.11ac was released as a standard in January 2014, although it was already in  use by that time. It provides data rates from 433 Mbps (single channel) to 6.77 Gbps (eight channels).

802.11ad

802.11ad is a standard originally promoted as a high speed wireless networking technology that supports data rates up to 7 Gbps. It remains to be seen if this will be widely adopted. Current plans are to use 802.11ad for wirless USB.

Myth:

LANs are fast compared to WANs because they are small and have fewer nodes.

LANs are faster than WANs (see below) because they don't rely on telephone networks for connectivity. High speed connectivity rivaling LANs over telephone systems would be cost-prohibitive. Just 1.5 Mbps costs hundreds of dollars per month. You can get 50 Mbps Internet plans for around $100 per month, but when used as the basis of a WAN they are actually limited to about 5 to 10 Mbps because of the slow upload speed. On the other hand, 1 Gbps LANs are becoming ubiquitous at a low cost.

Wide Area Network (WAN)

A Wide Area Network is frequently defined as a network that covers a large geographical area, such as between cities, across a country or across the world. From a network administrator's point of view, what actually makes a WAN different from other networks is that the network owner leases connection services from a third party.
 
Typically, a WAN is two or more LANs connected using a leased connection from a telephone company. Another situation that may be called a WAN is when some users connect to a LAN over the telephone system or the Internet. When individual users connect to a LAN over the Internet it is called a Virtual Private Network (VPN).

Examples of WANs are corporate networks where a company may have several offices in different cities, each with their own LAN. When these LANs are linked it creates a WAN. A company may have a central computer or LAN where offices in different cities connect for database access, etc. Government networks are similar to corporate networks. The Internet is the most famous of WANs.

Data rates

Local Area Networks are achieving speeds in the 40-Gbps range. However, the connections available through telephone companies for Wide Area Networks have maximum speeds approaching 150 Mbps, and this is at a very high cost.
 
Typical speeds of WAN connections are 1.544 Mbps for T1 connections and 2 Mbps for DSL and cable television connections. Keep in mind that the overall speed of a WAN connection is limited by the upstream speed of the connection, which is usually to ⅕ of the advertised downstream1 speed. Therefore, a 10 Mbps cable connections has about a 2 Mbps upstream speed. It doesn’t matter if you can download at 10 Mbps if the system you are downloading from can only upload at 2 Mbps.

Technologies

Wide area networks are connected via leased connections owned by telephone companies. Certification exams focus on these technologies as if they are something the average network administrator may buy and use. However, these are technologies used by telephone companies. Your responsibility as a network administrator ends where your network plugs into the telephone company's router. The following are names and brief descriptions of some of these technologies.
 
X.25
 
X.25 sends digital data over voice circuits.
 
Frame Relay
 
This is a high speed version of X.25 without error checking. Frame Relay uses dedicated low-error-rate phone lines.
 
ATM - SONET
 
Asynchronous Transfer Mode (ATM) and Synchronous Optical Network (SONET) are technologies that work together to provide very high speeds for data transmission. SONET can reach speeds of 10 Gbps. However, the end user will see only a fraction of this speed since it is being shared by many customers.
Specific Data Rates
Digital Service ("T" lines)
Telephone companies have high speed data data connections that they can parcel-out to customers. For example, a telephone company may have a system that operates at 1 Gbps. This system could theoretically service 1,000 customers where each customer has 1 Mbps. This is called “leasing bandwidth”. The actual numbers are different but this example gives the basic idea.

Telephone companies lease their bandwidth under the designation of Digital Service lines. These are are dedicated leased lines known as “T” lines to the customer. These lines are expensive but have no latency (no lag between transmitting and receiving)2 and symmetrical bandwidth (the speed is the same in both directions, upstream and downstream). These lines are billed to the customer in two parts. The first charge is for the “loop”, which is the physical connection to the telephone system. The second charge is for any extra service provided (such as Internet access).
 
Do not confuse Digital Service lines with ISDN (Integrated Service Digital Network, see Appendix 2). The designations are similar but ISDN lines are optimized for voice services. Data services are often delivered over ISDN but Digital Service and ISDN are not the same.
 
DS0
 
DS0 has the speed equivalent to one voice circuit, which is 64 kbps.
 
DS1
 
DS1 is known to consumers as T1. It operates at 1.544 Mbps and is symmetrical, meaning that it operates at 1.544 Mbps both downstream and upstream. DS1 can be divided into 24 DS0 channels. When divided, 23 channels are used for customer data and one used for control signals.
 
Do not confuse T1 with Primary Rate Interface (PRI) ISDN service. PRI is a 1.544 Mbps line that is optimized for voice services. It is not the same as a T1 line. You can get voice service on T1 and you can get data service on PRI, but they are not the same.

As of 2015, a T1 connection costs between $250 and $1,000 per month.
 
DS3
 
DS3 is known to consumers as T3. It operates at 44.736 Mbps and is symmetrical. DS3 can be divided into 28 DS1 channels or 672 DS0 channels plus extra channels for control signals

As of 2015, a T3 connection costs between $3,000 ant $12,000 per month.
 
DSL
 
DSL (Digital Subscriber Line) sends data over voice lines along with analog voice signals. It is very sensitive to distance. The farther you are from the telephone switch the lower the speed. The digital signals use audio frequencies above 5 khz. These frequencies are filtered from the telephones that share the line (only frequencies as high as 4 khz are needed for voice). Telephone companies once ran advertisements claiming that DSL didn't suffer from network congestion like cable TV connections. Since this is patently false they no-longer run such ads.
 
ADSL
 
Asymmetrical DSL (ADSL) is the most common type of DSL service. It is typically used by non-commercial (home) users. It has data rates of 512 kbps to 1.5 Mbps, non-symmetrical (upstream speed is typically to the speed of the downstream speed). Speed can vary for many reasons, such as network congestion.
 
SDSL
 
Symmetrical DSL (SDSL) has the same data rate both downstream and upstream. It is typically used by commercial users. Data rates are up to 1.5 Mbps.
 
IDSL
 
This is a data service using ISDN (Integrated Service Digital Network). It is used where ADSL and SDSL cannot be used due to the distance between the customer and the telephone switch. The data rate is about 150 kbps.
 
VDSL/VHDSL (Very-high-bit-rate DSL)
 
This is a high speed DSL service is commonly used for Fiber to the Curb (FTTC) installations. VDSL has data rates up to 52 Mbps downstream and 16 Mbps upstream. VDSL2 has data rates up to 100 Mbps symmetrical at distances up to 300 meters..
 
ADSL2+
 
This is also known as G.992.5. It is an emerging technology with data rates up to 24 Mbps. It is only available in limited areas.
Cable Television
Cable television companies provide Internet connectivity with downstream data rates of 20 Mbps or more. It is used by both commercial and non-commercial users. Cable Internet access is typically non-symmetrical with the upstream speed around to ⅕ of the downstream speed or less. Like DSL it can suffer from network congestion. DOCSIS 3.1 modems are planned to support 10 Gbps downstream and 1 Gbps upstream.. As of 2011, the fastest cable Internet access offered in the U.S. Is 100 Mbps.
Virtual Private Networks
A Virtual Private Network (VPN) is a way of creating a WAN using the Internet as a pathway. This can be used to connect remote workstations to LANs or to connect separate LANs together. A VPN creates what is called a private tunnel through the Internet. This means that data is sent over the Internet in such a way that it looks like a virtual Ethernet cable to the computers or routers at the ends of the connection (a very slow Ethernet connection). The data is encrypted to keep it private in case it is intercepted.

Windows Server has a VPN service available. Other programs are available for Unix. Many broadband routers are VPN-capable and can connect two LANs over the Internet to form a WAN.
Other geographic models
The following network types are really types of LANs or WANs based on the technology used. However, when networks are classified by size rather than technology, these names are often used
CAN
Campus Area Network
University campus, military base or commercial complex
 
Typically uses LAN technologies
 
May use WAN technologies on large installations (e.g. military bases)
Controller Area Network
Embedded systems such as automobiles
 
Also used for robotic networks
MAN
Metropolitan Area Network
Within a single city
 
Typically uses WAN technologies
 
Attempts were made to develop technologies that specialized in city-wide networks. One was FDDI (Fiber Distributed Data Interface) and another was DQDB (Distributed Queue Dual Bus). Both schemes failed due to the cost of implementation. Now, city-wide networks typically use the same technologies as world-wide networks, i.e. a "MAN" will use leased bandwidth supplied by a telephone company just as a WAN does. Therefore, the Metropolitan Area Network is no-longer a viable term to describe a unique type of network technology.  IEEE MAN committee disbanded around the year 2000.
Special case
Microwave and laser
 
Microwave and laser links are line-of-sight connections that can be used if there is a “straight shot” between installations. Since microwave and laser systems would be owned by the network owner, it is arguable that these technologies are LAN technologies.
PAN
Personal Area Network
Peripherals close to the computer
 
Bluetooth
 
IrDA
 
Infrared signals used by some hand-held devices. Mostly obsolete.
 
Wireless USB
SAN
Storage Area Network
A typical SAN consists of an array of hard disks aggregated to appear as a single hard drive. Computers usually access this SAN array over a fiber optic network called Fiber Channel. Portions of the SAN array can be assigned to individual computers. Those computers see their portions of the array as if they were regular hard disk drives attached directly to the motherboard. Think of a SAN as a big hard disk drive with an extra long cable that can be accessed by more than one computer. This centrally located hard disk drive offers much more flexibility in a large data center than smaller drives in individual computer cases.

It is possible to install a SAN as a service over a regular Ethernet LAN, but this would not be done in a major data center. A SAN is usually a completely separate network from the regular LAN and computers are connected to both networks.

Do not confuse SAN with NAS (Network Attached Storage). A NAS device is essentially just another computer on the network configured to share files.
Relationship models
Literature on networking usually divides network into to relationship models, peer-to-peer and client/server.
Peer-to-peer
Do not confuse peer-to-peer networks with peer-to-peer file sharing programs. Programs that share files with other users on the Internet are often called peer-to-peer networks. However, these programs do not allow direct sharing over a LAN.
 
On a peer-to-peer network computers are equal. No single computer controls access to the network. A peer-to-peer network is only suitable where security is not an issue; where all users are highly trustworthy. The operating systems on the computers are usually the same. There is no special server version of the operating system on a peer-to-peer network.
 
Myths:
 
1. Peer-to-peer networks are only suitable for networks with 10 to 15 computers.
 
This is an contrived limitation. There are no technical limitations on the number of computers on a peer-to-peer network. Windows XP will only allow eight external connections at a time on a Peer-to-Peer network (e.g. Windows XP acting as a file server). Later versions will allow up to 20 connections. Linux has no such restriction.
 
2. Peer-to-peer networks are difficult to use and manage because resources are distributed over the computers on the network.

The myth is that chaos ensues on a peer-to-peer network because the file you need could be on any of many computers. In the real world you don’t find this; it just doesn’t work. Typically, unless the network consists of only two or three computers, peer-to-peer networks are organized into dedicated workstations, file servers and print servers, much as client/server networks are.
 
3. Each user acts as his or her own administrator
 
In reality, one user who is more computer-savvy than the others will typically act as a network-wide administrator.
Client/Server
A client/server network is based on the client/server model used for computer programs. In the client/server relationship a server program waits for a client program to contact it and make a request for service. The server then fulfills that request.  A common example of the client server model is when you visit a web page. Your web browser (the client) contacts the web server and request a web page. The web server fulfills that request by sending a page back to the client. Operating systems consist of many server programs that fulfill requests from client programs.

What actually distinguishes a client/server network from a peer-to-peer network is that users must have an account on a server before they can access resources on it. Client/server networks based on Windows Server usually work on the domain model. In this model there is a logon server called a domain controller. The domain controller not only controls user access to itself but other servers can authenticate users through the domain controller. For example, if a user tries to access files on a file server, that file server will check with the domain controller to see if the user has an account before allowing the user to have access to its files.

Servers do not use special hardware. Although you probably want a high performance computer to act as a server, the only real difference between a client computer and a server computer is the software running on the machines.

Myth:

Client/server networks have specialized computers that act as file servers, print servers, etc.

This is true, except it’s often true for peer-to-peer networks too. It is not at all uncommon to find a peer-to-peer network with computers acting as dedicated file servers and print servers. It is also common to find small client/server networks where the logon server is an all in one logon server, file server, mail server print server, etc.
Host/Terminal
It is arguable that a host/terminal network is not really a network. As mentioned above, the first “networks” were based on this model. Here a single central computer does all work. Workstations are called dumb terminals because they are little more than paperless teleprinters.
 
The Host/Terminal model is essentially obsolete. However, there is an emerging technology called virtual desktop. A virtual desktop environment uses one or more powerful central computers that can emulate several real computers in software. The computer on a user’s desk will be a low-power machine called a thin client. The thin client will have remote desktop software that allows the user to see and control a remote virtual computer.
Security models
Share level security
Share level security is used for Peer-to-Peer networks. In this case each computer handles its own security. Furthermore, each shared resource (a share) has its own password. For example, if a computer has two shared folders, the user/administrator will have to set a password for each folder. In fact, the administrator may have to set two passwords for each folder, one for full access and another for read-only access. Even if the passwords for each folder are the same, they have to be manually set for each folder.

Share level security was the security model used by Windows 3.x, Windows 9x, Novell Netware Lite and a few others. Windows NT (which includes all modern versions of Windows) allows peer-to-peer sharing but doesn’t support password-protecting those shares. Share level security is is virtually unheard of today.
User level security
The security model used for client/server networks is user level security. With user level security each user must have an account on the server in question. That user’s access to the network is tied to his or her logon account. The network administrator can group users and shared resources in such a way that central control of access to resources is simplified. For example, a user may be put in the administrator’s group and thus have the same access rights and anyone else in that group, which is full administrator access.
Single sign-on
One problem, even with client/server networks, is that users may have to logon to more than one server to have full access to the network. Single sign-on (logging in once and having access to every resource on the network) is a "holy grail" in networking. Microsoft's Active Directory (discussed below) is an attempt at implementing single sign-on. However, servers other than the domain controller must be able to query the domain controller in order to know how to grant access. Many non-Microsoft products cannot do this. Unix-like systems can interact with Active Directory through Winbind and Likewise-Open. Samba, a program that integrates Unix-like systems into Windows networks, can now act as an Active Directory domain controller.

Novell uses a system called Novell Directory Service (NDS) to attempt to implement single sign-on.