Network Topologies

In computer networking, topology refers to the layout of connected devices. This article introduces the standard topologies of networking.

Topology in Network Design
Think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there. 
Network topologies are categorized into the following basic types: 
• bus 
• ring 
• star 
• tree 
• mesh
More complex networks can be built as hybrids of two or more of the above basic topologies.


Bus Topology
 
Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message. 
Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

DSL and Cable Broadband Speed Tweaks

So-called broadband speed tweaks are techniques to improve performance of DSL and cable Internet connections. Home networking enthusiasts started experimenting with cable and DSL tweaks many years ago, when broadband Internet services first became popular.

Performance tweaks for low-speed (dialup) Internet often do not work for broadband links. Additionally, while broadband speed tweaks originally focused on increasing the performance of general Web surfing, speed tweaks are now more commonly made to tune specific applications like P2P file sharing systems and games.

Limitations of Broadband Speed Tweaks
First, broadband tweaks should be made only after your network is tested and running reliably. Speed tweaks are performance optimizations only, not designed to fix installation errors or basic network configuration issues.

You should expect broadband tweaks might yield only small speed increases, and then only in certain situations. For example, a tweak to improve the performance of one online game may only benefit that title and then only initially when it is loading. Broadband tweaks may help certain applications (like games) but at the same time slow down others (like Web browsing). In general, assume any performance benefits you obtain may be on the order of 10-20% gain rather than 100-200%.

Finally, speed tweaks also can create instability on some networks. Depending on the type of equipment and Internet service you use, some tweaks will be technically incompatible and need to be avoided.

Types of Broadband Speed Tweaks
The most common broadband tweaks involve adjusting various parameters of the TCP/IP network protocol, typically:
• TCP receive window size
• Maximum Transmission Unit (MTU)
• Maximum Segment Size (MSS)
• Time-To-Live (TTL)

The Microsoft Windows Registry contains default values for TCP/IP parameters. You can apply these speed tweaks to your computers by using a Registry editor or the TCP Optimizer utility (see below) to change some of the default values on each, rebooting the computers each time. Other operating systems like Linux and Mac OS X provide alternative mechanisms to tune TCP/IP parameters.

Another common broadband tweak entails manipulating Web browser settings. For example, suppressing the download of large images saves network bandwidth that can be used instead to download other data faster.

Finally, though less common, a few speed tweaks modify settings on routers and modems. For example, TCP/IP MTU settings can be changed on a broadband router separate from individual computers on the network.



About Web Accelerators for Broadband Tweaks
Speed tweaks have been traditionally applied to the network by an administrator manually, one device at a time, but in recent years software applications have been developed to help automate and maintain tweaks.

So-called Internet download accelerators are pre-packaged software programs that automatically apply speed tweaks to a computer. Installing and running an accelerator program will automatically make the Registry, Web browser and other configuration changes. The more sophisticated applications collect information about your computers and network and apply tweaks intelligently to ensure maximum benefit.

While many Web accelerators are designed specifically for dialup networks, examples of accelerator applications useful for broadband are:
• Download Accelerator Plus
• Google Web Accelerator
• Netfury
• ONSPEED

Making Broadband Tweaks Work
Because speed tweaks can cause computer and network crashes if made improperly, test each change methodically. If possible, use a proven Web accelerator program rather than configuring tweaks manually, and test each change individually before making the next one.

To determine whether a speed tweak is working, use an Internet speed test service to measure your Internet performance before and after making a tweak. In addition, try local file transfers, Web downloads, online games, and other applications you use often to assess whether a tweak makes any noticeable difference. Do not hesitate to undo a change if you cannot observe any benefit.

Wi-Fi Hotspots

A Wi-Fi hotspot is a wireless access point that provides Internet access to network devices in public locations such as downtown centers, cafes, airports and hotels. Businesses and schools are increasingly using Wi-Fi hotspots for their internal (intranet) networks. Home wireless networks also use similar Wi-Fi technology.

Requirements to Use Wi-Fi Hotspots
Computers (and other devices) connect to hotspots using a Wi-Fi network adapter. Newer laptop computers contain built-in adapters, but most other computers do not. Wi-Fi network adapters can be purchased and installed separately. Depending on the type of computer and personal preferences, USB, PC Card, ExpressCard, or even PCI card adapters can be used.

Public Wi-Fi hotspots normally require a paid subscription. The sign-up process involves providing credit card information online or by phone and choosing a service plan. Some service providers offer plans that work at thousands of hotspots throughout the country.

A few pieces of technical information are also required to access Wi-Fi hotspots. The network name (also called SSID) distinguishes hotspot networks from each other. Encryption keys (a long series of letters and numbers) scramble the network traffic to and from a hotspot; most businesses require these as well. Service providers supply this profile information for their hotspots.

Finding Wi-Fi Hotspots
Computers can automatically scan for hotspots within range of their wireless signal. These scans identify the network name (SSID) of the hotspot allowing the computer to initiate a connection.

Instead of using a computer to find hotspots, some people prefer to use a separate gadget called a Wi-Fi finder. These small devices scan for hotspot signals similarly to computers, and many provide some indication of signal strength to help pinpoint their exact location.

Before traveling to a far-away place, the location of Wi-Fi hotspots can be found using online wireless hotspot finder services.

Connect To Wi-Fi Hotspots
The process for connecting to a Wi-Fi hotspot works similarly on home, business and public wireless networks. With the profile (network name and encryption settings) applied on the wireless network adapter, you initiate the connection from your computer operating system (or software that was supplied with the network adapter). Paid or restricted hotspot services will require you to log in with a user name and password the first time you access the Internet.

Dangers of Wi-Fi Hotspots
Although few incidents of hotspot security issues are reported in the press, many people remain skeptical of their safety. Some caution is justified as a hacker with good technical skills can break into your computer through a hotspot and potentially access your personal data.

Taking a few basic precautions will ensure reasonable safety when using Wi-Fi hotspots. First, research the public hotspot service providers and choose only reputable ones who use strong security settings on their networks. Next, ensure you do not accidentally connect to non-preferred hotspots by checking your computer's settings. Finally, be aware of your surroundings and watch for suspicious individuals in the vicinity who may be reading your screen or even plotting to steal your computer.

Is It Legal to Use Open Access Wi-Fi Internet Connections?
Wi-Fi wireless technology simplifies the sharing of network connections between multiple computers and people. Even if you don't subscribe to Internet service, you can log on to public hotspots or to a neighbor's wireless access point to get online. However, using someone else's Internet service isn't always a good idea. It may even be illegal.
Using a public Wi-Fi hotspot is normally legal when you have the service provider's permission and follow their terms of service. These terms may include the following:
• restrictions on time of day when, or locations where, the public Internet may be accessed
• limits on the amount of network bandwidth that may be used
• restrictions on viewing of inappropriate Web sites or engaging in questionable online activities

Using a neighbor's wireless access point may not be legal even with their permission. The answer varies depending on the policies of residential Internet service providers and plans among other factors.

Many U.S. states prohibit unauthorized access to computer networks including open Wi-Fi networks. While interpretations of these laws may vary, some precedents have been set in recent years:
• 2007 - a man in Michigan was fined for using the Wi-Fi hotspot of a local cafe, from his car
• 2006 - Illinois man David Kauchak was fined for unauthorized use of a local agency's Wi-Fi access point
• 2005 - a Florida man faced felony charges for piggybacking onto a neighbor's Internet connection without their permission

Similar restrictions on using open Wi-Fi networks exist outside the U.S. as well:
• 2006 - In Singapore, a teenager received a probationary sentence for wirelessly accessing a neighbor's Internet connection without their permission.
• 2005 - In the U.K., Gregory Straszkiewicz was fined and his computer confiscated for using a local resident's Internet service illicitly

Just as entering a home or business without the owner's permission is considered trespassing (even if the doors are unlocked), likewise accessing wireless Internet connections (even open access ones) can be considered an illegal activity. At a minimum, obtain consent from the operator of any Wi-Fi access point before using their service. Read any online Terms of Service documentation carefully when signing on, and contact the owner offline if necessary to ensure compliance.


Summary
Wi-Fi hotspots are becoming an increasingly common form of Internet access. Connecting to a hotspot requires a wireless network adapter, knowledge of the profile information of that hotspot, and sometimes a subscription to a paid service. Computers and Wi-Fi finder gadgets both are capable of scanning the nearby area for Wi-Fi hotspots, and several online services allow you find far-away hotspot locations. Whether using a home, business or public hotspot, the connection process remains essentially the same. Likewise, as with any wireless network, security issues for Wi-Fi hotspots need to be managed.

Connect Two Home Computers for File Sharing

The simplest kind of home network contains exactly two computers. You can use this kind of network to share files, a printer or another peripheral device, and even an Internet connection. To connect two computers for sharing these and other network resources, consider the options described below.

Connecting Two Computers Directly With Cable
The traditional method to network two computers involves making a dedicated link by plugging one cable into the two systems. Several alternatives exist for networking two computers in this manner:
• Ethernet crossover cable 
• Null modem serial cable or parallel peripheral cable 
• Special-purpose USB cables 

Ethernet - Of the above choices, the Ethernet method is preferred as it supports a reliable, high-speed connection with minimal configuration required. Additionally, Ethernet technology offers the most general-purpose solution, allowing networks with more than two computers to be built fairly easily later. If one of your computers possesses an Ethernet adapter but the other has USB, an Ethernet crossover cable can still be used by first plugging a USB-to-Ethernet converter unit into the computer's USB port.

Serial and parallel - This type of cabling, called Direct Cable Connection (DCC) when using Microsoft Windows, offers lower performance but offers the same basic functionality as Ethernet cables. You may prefer this option if you have such cables readily available and network speed is not a concern. Serial and parallel cables are never used to network more than two computers. 

USB - Ordinary USB cables must not be used to connect two computers directly to each other. Attempting to do so can electrically damage the computers! However, special USB cables designed for direct connection exist that can be used safely. You may prefer this option over others if your computers lack functional Ethernet network adapters. 

To make dedicated connections with Ethernet, USB, serial or parallel cables requires 
1. each computer have a functioning network interface with an external jack for the cable, and
2. the network settings on each computer appropriately configured 

One phone line or power cord cannot be used to directly connect two computers to each other for networking.

Network Bandwidth and Latency

What Is Network Latency?Bandwidth is just one element of what a person perceives as the speed of a network. Latency is another element that contributes to network speed. The term latency refers to any of several kinds of delays typically incurred in processing of network data. A so-called low latency network connection is one that generally experiences small delay times, while a high latency connection generally suffers from long delays.Latency vs. BandwidthAlthough the theoretical peak bandwidth of a network connection is fixed according to the technology used, the actual bandwidth you will obtain varies over time and is affected by high latencies. Excessive latency creates bottlenecks that prevent data from filling the network pipe, thus decreasing effective bandwidth. The impact of latency on network bandwidth can be temporary (lasting a few seconds) or persistent (constant) depending on the source of the delays.Latency of Satellite Internet ServiceSatellite Internet service illustrates the difference between latency and bandwidth on computer networks. Satellite Internet connections possess both high bandwidth and high latency. When loading a Web page, for example, most satellite users can observe a noticeable delay from the time they enter a Web address to the time the page begins loading. This high latency is due primarily to propagation delay as the request message travels at the speed of light to the distant satellite station and back to the home network. Once the messages arrive on Earth, however, the page loads quickly like on other high-bandwidth Internet connections (DSL or cable). Besides propagation delays, latency also may also involve transmission delays (properties of the physical medium) and processing delays (such as passing through proxy servers or making network hops on the Internet).Measuring Network LatencyNetwork tools like ping tests and traceroute measure latency by determining the time it takes a given network packet to travel from source to destination and back, the so-called round-trip time. Round-trip time is not the only way to specify latency, but it is the most common. On DSL or cable Internet connections, latencies of less than 100 milliseconds (ms) are typical and less than 25 ms desired. Satellite Internet connections, on the other hand, average 500 ms or higher latency.SummaryTwo key elements of network performance are bandwidth and latency. The average person is more familiar with the concept of bandwidth as that is the one advertised by manufacturers of network equipment. However, latency matters equally to the end user experience as the behavior of satellite Internet connections illustrates. Businesses use the term Quality of Service (QoS) to refer to measuring and maintaining consistent performance on a network by managing both bandwidth and latency in a coordinated fashion.