Broadcast Networks
On a broadcast network, such as Ethernet, any of the many systems attached to the network cable can send a message at any time; each system examines the address in each message and responds only to messages addressed to it. A problem occurs on a broadcast network when multiple systems send data at the same time, resulting in a collision of the messages on the cable. When messages collide, they can become garbled. The sending system notices the garbled message and resends it after waiting a short but random amount of time. Waiting a random amount of time helps prevent those same systems from resending the data at the same moment and experiencing yet another collision. The extra traffic that results from collisions can strain the network; if the collision rate gets too high, retransmissions may result in more collisions. Ultimately the network may become unusable.



Point-to-Point Networks
A point-to-point link does not seem like much of a network because only two endpoints are involved. However, most connections to WANs (wide area networks) go through point-to-point links, using wire cable, radio, or satellite links. The advantage of a point-to-point link is its simplicity: Because only two systems are involved, the traffic on the link is limited and well understood. A disadvantage is that each system can typically be equipped for only a small number of such links; it is impractical and costly to establish point-to-point links that connect each computer to all the rest. Point-to-point links often use serial lines and modems. The combination of a modem with a point-to-point link allows an isolated system to connect inexpensively to a larger network. The most common types of point-to-point links are the ones used to connect to the Internet. When you use DSL1 (digital subscriber line), you are using a point-to-point link to connect to the Internet. Serial lines, such as T-1, T-3, ATM links, and ISDN, are all point-to-point. Although it might seem like a point-to-point link, a cable modem is based on broadcast technology and in that way is similar to Ethernet.



Switched Networks
A switch is a device that establishes a virtual path between source and destination hosts in such a way that each path appears to be a point-to-point link, much like a railroad roundhouse. The switch creates and tears down virtual paths as hosts seek to communicate with each other. Each host thinks it has a direct point-to-point path to the host it is talking to. Contrast this approach with a broadcast network, where each host also sees traffic bound for other hosts. The advantage of a switched network over a pure point-to-point network is that each host requires only one connection: the connection to the switch. Using pure point-to-point connections, each host must have a connection to every other host. Scalability is provided by further linking switches.

Source of Information : Prentice Hall A Practical Guide to Fedora and Red Hat Enterprise Linux 5th Edition

Computers communicate over networks using unique addresses assigned by system software. A computer message, called a packet, frame, or datagram, includes the address of the destination computer and the sender’s return address. The three most common types of networks are broadcast, point-to-point, and switched. Once popular token-based networks (such as FDDI and token ring) are rarely seen anymore.

Speed is critical to the proper functioning of the Internet. Newer specifications (cat 6 and cat 7) are being standardized for 1000BaseT (1 gigabit per second, called gigabit Ethernet, or GIG-E) and faster networking. Some of the networks that form the backbone of the Internet run at speeds of almost 10 gigabits per second (OC192) to accommodate the ever-increasing demand for network services.


Network specifications
DS0
64 kilobits per second

ISDN
Two DS0 lines plus signaling (16 kilobits per second) or 128 kilobits per second

T-1
1.544 megabits per second (24 DS0 lines)

T-3
43.232 megabits per second (28 T-1s)

OC3
155 megabits per second (100 T-1s)

OC12
622 megabits per second (4 OC3s)

OC48
2.5 gigabits per seconds (4 OC12s)

OC192
9.6 gigabits per second (4 OC48s)

Source of Information : Prentice Hall A Practical Guide to Fedora and Red Hat Enterprise Linux 5th Edition

The communications facilities linking computers are continually improving, allowing faster and more economical connections. The earliest computers were unconnected stand alone systems. To transfer information from one system to another, you had to store it in some form (usually magnetic tape, paper tape, or punch cards—called IBM or Hollerith cards), carry it to a compatible system, and read it back in. A notable advance occurred when computers began to exchange data over serial lines, although the transfer rate was slow (hundreds of bits per second). People quickly invented new ways to take advantage of this computing power, such as email, news retrieval, and bulletin board services. With the speed of today’s networks, a piece of email can cross the country or even travel halfway around the world in a few seconds.

Today it would be difficult to find a computer facility that does not include a LAN to link its systems. Linux systems are typically attached to an Ethernet network. Wireless networks are also prevalent. Large computer facilities usually maintain several networks, often of different types, and almost certainly have connections to larger networks (companywide or campuswide and beyond).



Internet
The Internet is a loosely administered network of networks (an internetwork) that links computers on diverse LANs around the globe. An internet (small i) is a generic network of networks that may share some parts in common with the public Internet. It is the Internet that makes it possible to send an email message to a colleague thousands of miles away and receive a reply within minutes. A related term, intranet, refers to the networking infrastructure within a company or other institution. Intranets are usually private; access to them from external networks may be limited and carefully controlled, typically using firewalls.



Network services
Over the past decade many network services have emerged and become standardized. On Linux and UNIX systems, special processes called daemons support such services by exchanging specialized messages with other systems over the network. Several software systems have been created to allow computers to share filesystems with one another, making it appear as though remote files are stored on local disks. Sharing remote filesystems allows users to share information without knowing where the files physically reside, without making unnecessary copies, and without learning a new set of utilities to manipulate them. Because the files appear to be stored locally, you can use standard utilities (such as cat, vim, lpr, mv, or their graphical counterparts) to work with them.

Developers have created new tools and extended existing ones to take advantage of higher network speeds and to work within more crowded networks. The rlogin, rsh, and telnet utilities, which were designed long ago, have largely been supplanted by ssh (secure shell, page 621) in recent years. The ssh utility allows a user to log in on or execute commands securely on a remote computer. Users rely on such utilities as scp and ftp to transfer files from one system to another across the network. Communication utilities, including email utilities and chat programs (e.g., talk, Internet Relay Chat [IRC], ICQ, and instant messenger [IM] programs, such as AOL’s AIM and gaim) have become so prevalent that many people with very little computer expertise use them on a daily basis to keep in touch with friends, family, and colleagues.



Intranet
An intranet is a network that connects computing resources at a school, company, or other organization but, unlike the Internet, typically restricts access to internal users. An intranet is very similar to a LAN (local area network) but is based on Internet technology. An intranet can provide database, email, and Web page access to a limited group of people, regardless of their geographic location.

The ability of an intranet to connect dissimilar machines is one of its strengths. Think of all the machines you can find on the Internet: Macintosh systems, PCs running different versions of Windows, machines running UNIX and Linux, and so on. Each of these machines can communicate via IP, a common protocol. So it is with an intranet: Dissimilar machines can all talk to one another.

Another key difference between the Internet and an intranet is that the Internet transmits only one protocol suite: IP. In contrast, an intranet can be set up to use a number of protocols, such as IP, IPX, AppleTalk, DECnet, XNS, or other protocols developed by vendors over the years. Although these protocols cannot be transmitted directly over the Internet, you can set up special gateway boxes at remote sites that tunnel or encapsulate these protocols into IP packets and then use the Internet to pass them.

You can use an extranet (also called a partner net) or a virtual private network (VPN) to improve security. These terms describe ways to connect remote sites securely to a local site, typically by using the public Internet as a carrier and employing encryption as a means of protecting data in transit.

Source of Information : Prentice Hall A Practical Guide to Fedora and Red Hat Enterprise Linux 5th Edition

Get speedier file transfers, smoother video streaming, and better network gaming with the right PC networking tools.


ON MOST HOME networks, the transfer rate of a fast ethernet connection (about 12.5 megabits per second) is the speed limit—and that’s painfully slow for some tasks. The solution? Upgrade to a gigabit network. Switching over to gigabit (1000-mbps) speeds increases potential throughput tenfold, minimizing transfer times and greatly enhancing your ability to stream high-bandwidth files to connected devices without interference. Gigabit networking is now a common feature of networking devices and shouldn’t carry a big cost premium. Most modern motherboards have gigabit functionality built in. This guide does not apply to wireless networks. The factors that constrain speeds on wireless networks are entirely unlike those that limit speeds on wired networks. Here we’ll look at how to determine whether your equipment can handle gigabit networking, and (if not) how to build a gigabit network from scratch.


Identify Your Network
Do you already have a gigabit network? Th e Windows desktop doesn’t indicate whether you’ve acquired this superspeedy networking feature. And since many factors influence network transfer speeds, your gigabit network might crawl at a data transfer rate of less than 10 mbps for various reasons. One requirement of gigabit networking is that all connected devices be connected via a gigabit port. In addition, they must be connected to one another with network cables that can handle the bandwidth. For devices such as your router, a gaming console, or an external storage device, the easiest way to discover whether they support fast Ethernet (10/100 mbps) or gigabit ethernet (10/100/1000 mbps) is to check the devices’ specifications in their online descriptions or accompanying manuals. Look for a mention of either “gigabit networking” or “1000 Mbps.” Your PC’s motherboard is a critical component of the gigabit network. If your system came to you prebuilt or if you don’t remember relevant details about your rig’s motherboard, don’t worry. In Windows, click Start and select Run (for more-modern versions of the OS, move your cursor to the search box and left -click). Type ncpa.cpl and press . The Network Connections window should pop up. Right-click the network connection that’s listed as your Local Area Connection (LAN), and left -click Properties.
Click the big Configure button located to the right of the listing for your network controller. In the new window that appears, open the Advanced tab and scroll down until you find a property labeled ‘Connection Type’ or ‘Speed’. Left -click it and click the Value field to the right. Scroll up and down through this list of options, looking for anything that starts with a value of ‘1000’ or anything that refers to network speeds in ‘Gbps’. If all you see are ‘100’ values and speeds designated in ‘Mbps’, your motherboard’s built-in Ethernet controller tops out at fast-ethernet speeds. But you can still upgrade your PC to gigabit networking by installing a third-party gigabit ethernet card. If all of the devices on your network do support gigabit functionality, great! If you add a slower, fast-ethernet device to a gigabit-ready hub, transfer speeds will crawl only when you access that particular device—a slow device connected to a router won’t poison the rest. Obviously, if you directly connect a gigabit-ready PC to a fast-ethernet device such as a network-attached storage (NAS) box, you’ll get only fast-ethernet speeds. Also, consider your cables. A typical category 5 (Cat 5) cable supports gigabit ethernet, but it’s worthwhile to invest in Cat 5e cables if you are building a gigabit network from scratch. Plain old Cat 5 cabling is now considered obsolete, and Cat 5e cabling meets more-rigorous specifications, allowing it to do a better job than Cat 5 cabling can of minimizing electromagnetic interference. On the other hand, bumping up your cabling to a classification higher than Cat 5e may not benefit your network speeds; for example, Cat 6 cabling doesn’t dramatically improve speed. To see what kind of cable you have, check the cable’s side: The spec should be printed somewhere along the length of the cord.


Test Your Network
If your parts are in order and the cables are connected, you’ll want to fire up your gigabit network so that you can check its performance. But first you need to confirm that the drivers and firmware related to your network-oriented devices (motherboard, router, NAS box, and so on) are up-to-date. Suppose that you are planning to connect your PC to a gigabit NAS box via a single router. At this point you need to make sure that you are running the latest firmware for your NAS box and your router, and either the latest firmware and drivers for your motherboard or the most recent drivers for your discrete gigabit network card, depending on how you’ve set up your system. All too often, a device may not work as intended out of the box. Head over to the manufacturer’s Web site to grab the latest drivers and firmware updates; then run the accompanying driver setup program or follow the related instructions for flashing your device. The process isn’t difficult (see find.pcworld.com/63936). Fire up your network devices and use the helpful LAN Speed Test utility (www.totusoft.com/Products) to gauge the speeds that your gigabit network is attaining. After launching the tool, click the Start Test button and browse to a folder on a connected network device. Enter a size for your test file (1GB should do the trick), and the program will begin to track the read and write speeds of transfers between your system and the target device. Of course, you won’t get the maximum 125-mbps connection that a gigabit network theoretically supports. Ultimately, the speed of the storage devices doing the reading or writing—be they magnetic hard drives or flash-based storage—will limit your network’s performance. For a hard drive, relevant factors include the physical speed of the drive itself and the location where the drive writes the data on its physical platters. For a solid-state drive (SSD), the
performance you get depends on whether the drive uses faster single-level cell flash memory or slower multilevel cell flash memory, and on whether you’re reading or writing to the drive. Unless it uses a RAM drive, or an array of hard drives or SSDs, your network won’t reach the 125-mbps limit for gigabit networking. Nevertheless, you can realistically expect to achieve speeds of at least 40 to 50 mbps, which is four times as fast as the realworld speed of a typical fast-ethernet connection. Though gigabit networking might not be the Star Trek transporter of LANbased file transfers, the performance improvement that it offers over a typical fast-ethernet connection amply compensates for the time this setup process requires.

Source of Information : PC World December 2009

Most network administrators would care less of what applications are put into play. In fact, most of them would rather address the firewall and network security issues before sitting down on the actual workstations individually and check which applications should be enabled and which should go.

Such is a common practice in companies that place a lot of emphasis in making sure that their workstations adhere towards policies. Further it is a good way to ensure that no untoward infections stemming from browsing or accessing files that are not permitted on the network. Source such as floppy drives or links that anyone could get from browsing can be entirely hazardous and apparently this is where a good clamp down has to be done.

But among their priorities, this is perhaps the last of their priorities. It remains that most network administrators would do well to focus on what comes in. They can do their part by limiting the access from a remote destination within the intranet but like all viruses, they will always find a way to ruin desktop programming or workstations.

Add to the fact that users could care less of what happens to their workstation as they know that the IT or MIS people are always there to rescue them. Such may be called a petty belief but as long as it works, nothing is bound to stop them from doing so. That is perhaps one reason why most administrators have a hard time managing networks. People fail to go deeper on how to safeguard them.