Joined: 06 November 2004
In earlier columns I've addressed a number of networking issues that cause pain for users who bring their laptops home from work. In this column, I'll take a look at some of the top networking issues for home users. This list is by no means comprehensive or an exhaustive Top Five, but it does cover some of the problems heard regularly in the Windows XP Networking and the Web newsgroup. Each problem could be a whole column on its own, so I'll try to point you to other resources if I can't cover all the answers in this column. You'll find the Home and Small Office Networking with Windows XP home page a useful source for information.
So you finally got a high speed Internet connection and you can let that old modem gather dust. But you've got more than one computer, so how do you hook things up so that all of them can share the same connection?
There are two basic ways to share an Internet connection:
Use the Internet Connection Sharing (ICS) feature that is part of Windows XP.
Use a router (gateway) between your computers and the cable or DSL modem.
Expert Zone columnist Sharon Crawford does an excellent job of describing how to use Internet Connection Sharing in her earlier column, Internet Connection Sharing. I'll describe how to add a router to your network.
Routers, often called gateways, are a way to both isolate and connect one segment of your network from another. In the home environment, they provide a way to separate your home network from the Internet, while at the same time providing a connection point. To your cable company or DSL provider, they make your internal network appear to be a single device, so you don't need to pay extra for additional computers connected to them. Figure 1 shows what your network might look like with a router installed and a couple of computers networked.
Routers are fairly inexpensive, generally under $100 for basic ones that include a built-in four-port hub and maybe even a wireless access point. They provide a layer of isolation and protection from the Internet while simplifying the setup of your home network. The procedure below walks you through setting up a router at home. The steps make some assumptions. First, that all your computers have network cards. And that you have the network cards configured to automatically obtain an IP address, which is the default setting.
To set up a home network with a router:
Plug in your cable or DSL modem and connect it to the cable system or the phone line with DSL. Specific instructions for this are provided by the cable company or your DSL provider, if they don't actually do the installation for you.
Connect the cable/DSL modem to the wide area network (WAN) port of your router with the cable provided. If none was provided, a standard CAT5 cable should be fine.
Power up the router and wait for all the diagnostic lights to settle down.
Connect your computers to the LAN ports on the router using standard CAT5 network patch cables.
Power up or restart your computers.
If your router includes a wireless access point and you're connecting some of your computers using wireless, you'll need to configure the wireless connections now.
For more on connecting wirelessly, see my earlier column, Using a Wireless Laptop at Work and at Home, which is about connecting your work laptop to your home network—the procedures are the same. For the details about how to troubleshoot ICS, see Troubleshooting Internet Connection Sharing on Microsoft Windows XP.
Another common problem is when your network card won't connect and you've got an IP address that starts with 169.254. This can happen with either a regular network card or a wireless card, but is more common with a wireless card. The IP address that starts with 169.254 indicates that you didn't receive an IP address from a Dynamic Host Configuration Protocol (DHCP) server for some reason, probably because there is a connectivity issue. If this is a wired connection, start by checking that the status lights on the back of the network card are the proper color. See your network card documentation for what the different colours mean. If they aren't the right color, there are at least five possibilities:
The cable is bad.
The cable is not firmly connected at both ends.
The port on the hub or router is bad. Try plugging the network cable into a different port on the hub or router.
The network card is bad.
The cable is the wrong kind of network cable.
Let's examine that last possibility. You should always use CAT5, CAT5+ or CAT6 network cable. Not phone cable or other kinds of cable that may look the same. Second, there are both "straight-through" and "cross-over" cables. If you're connecting to a hub, switch, or gateway, then you need a straight-through cable. If you're connecting directly from the network card on one computer to the network card on another, you need a cross-over cable. For more information about cables, see Understanding Ethernet Cabling.
If all you want to do is connect two computers together to form a small network, a simple network configuration like that shown in Figure 2 is all you need.
If you're not using a hub, switch, or router to connect the two PCs, you need to use a cross-over cable. Since these are far less commonly used than straight-through cables and the two are not interchangeable, it's a good idea to clearly mark the cable as a cross-over cable. I like to keep one around the house for quickly connecting up two computers, but it's easy to get it confused with regular cables. So I bought my cross-over cable in a nice bright red colour.
Then, if you want to connect them both to the Internet, you need to turn on Internet Connection Sharing on one of the computers or connect them both to a router, rather than each other, as described earlier.
This isn't really a network question, but is still one of the most commonly asked questions in the networking newsgroups. Unfortunately, it almost certainly means your computer is infected with the Blaster worm or another type or worm. You need to both protectyourself from further infections and removethe worm. Most importantly, you need to set your computer up for automatic installation of security updates. The Protect Your PC site can help with that. And I strongly recommend installing Windows XP Service Pack 2—it includes an improved Windows Update, a stronger Windows Firewall, improved network protection, and helps make e-mail handling and Internet browsing more secure.
Unfortunately, this is a more common problem than anyone would like. If you're used to connecting your computers using Windows 95, Windows 98, or Windows Me, you'll find things a bit different with Windows XP. The single biggest cause of a disappearing Windows XP machine on the network is probably a computer browsing issue as described in my column on Troubleshooting Home Network Issues.
Although several things can cause a browsing problem, there's usually a pretty simple workaround. If you know the name of the share on the computer you want to connect to and you know the IP address of that computer, you can connect to it directly without ever having to actually "see" the computer on the network. For example, if I want to connect from the computer in the kitchen to the My Documents folder on my home office PC, all I need to know is the IP address of that home office PC. That's easy enough to find out. On your home office PC:
Click Start, point to All Programs, point to Accessories, and then click Command Prompt.
Type ipconfig as shown in Figure 3, and then press Enter.
Your IP address is listed. In this example, it's 192.168.50.100, but yours may well be different.
Now that you've got the IP address of the computer you want to connect to, go back to the kitchen computer. There are graphical ways to make the connection, but probably the easiest way is to use another command prompt. Assuming that you've already shared your home office My Documents folder as "CharlieDocs," you'd connect it to a Windows-assigned drive letter by following these steps:
Open a command prompt.
Type net use * \\192.168.50.100\CharlieDocs as shown in Figure 4, and then press Enter.
That's it, now drive Z is connected to my home office computer and I never had to worry about being able to actually see it in Windows Explorer or My Network Places at all. Obviously, your share point probably isn't called CharlieDocs, and your IP address is different than mine, so change the commands accordingly.
For more information about computer browsing, see Computer Browsing for SOHO Networks with Microsoft Windows and Troubleshooting Computer Browsing on SOHO Networks with Microsoft Windows.
For more information about file sharing, see File and Printer Sharing with Microsoft Windows and Troubleshooting File and Printer Sharing in Microsoft Windows XP.
If you use Windows XP or Windows XP Tablet PC Edition and have a networking or Tablet PC topic you'd like to see me cover, feel free to write me at Charlie@mvps.org. I especially want to hear what you think about your Tablet PC and what interesting ways you've found that a Tablet PC makes your work better.
It's impossible for me to acknowledge or answer individual e-mail messages and I can't provide individual technical support via e-mail. I do regularly participate in the Windows XP Tablet PC Edition newsgroup and the Microsoft Services for UNIX newsgroup and look forward to seeing you there.
Joined: 14 September 2004
Bus, ring, star, and all the rest
In networking, the term topology refers to the layout of connected devices on a network. This article introduces the standard topologies of computer networking.
Topology in Network Design
One can think of a topology as a network's "shape" ... (see below)
|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, but it would be highly unlikely to find an actual ring topology there. |
Network topologies are categorized into the following basic types:
Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, like the Internet, employ mesh routing.
Topologies remain an important part of network design theory. You can probably build a home or small business network without understanding the difference between a bus design and a star design, but understanding the concepts behind these gives you a deeper understanding of important elements like hubs, broadcasts, ports, and routes.
Joined: 14 September 2004
2. data link
OSI designates the application, presentation, and session layers as "upper" layers. Generally speaking, software in these layers performs application-specific functions like data formatting, encryption, and connection management. Examples of upper layer technologies in the OSI model are HTTP, SSL and NFS.
The remaining lower layers provide more primitive network-specific functions like routing, addressing, and flow control. Examples of lower layer technologies in the OSI model are TCP, IP, and Ethernet.
The layered approach in the OSI model offers several advantages to system implementers. By separating the design into logical smaller pieces, vendors can more easily solve network design problems through divide-and-conquer. A product from one vendor that implements OSI Layer 2 functionality, for example, will be much more likely to interoperate with another vendor's OSI Layer 3 product because both vendors are following the model. Finally, the OSI model makes network designs more extensible. New protocols and other network services are generally easier to add to a layered architecture than to a monolithic one.
Joined: 14 September 2004
What is a topology?
A topology refers to the manner in which the cable is run to individual workstations on the network. The dictionary defines topology as: the configurations formed by the connections between devices on a local area network (LAN) or between two or more LANs
There are three basic network topologies (not counting variations thereon): the bus, the star, and the ring.
It is important to make a distinction between a topology and an architecture. A topology is concerned with the physical arrangement of the network components. In contrast, an architecture addresses the components themselves and how a system is structured (cable access methods, lower level protocols, topology, etc.). An example of an architecture is 10baseT Ethernet which typically uses the start topology.
NE SPT, p. 34
What is a bus topology?
A bus topology connects each computer (node) to a single segment trunk. A 'trunk' is a communication line, typically coax cable, that is referred to as the 'bus.' The signal travels from one end of the bus to the other. A terminator is required at each end to absorb the signal so it does not reflect back across the bus.
FAQ 9FAQ 9 In a bus topology, signals are broadcast to all stations. Each computer checks the address on the signal (data frame) as it passes along the bus. If the signal's address matches that of the computer, the computer processes the signal. If the address doesn't match, the computer takes no action and the signal travels on down the bus.
Only one computer can 'talk' on a network at a time. A media access method called CSMA/CD is used to handle the collisions that occur when two signals are placed on the wire at the same time.
The bus topology is passive. In other words, the computers on the bus simply 'listen' for a signal; they are not responsible for moving the signal along.
A bus topology is normally implemented with coaxial cable.
NE SPT, pp. 35-39
What is the difference between a regular bus and a local bus?
|In a regular bus, each computer is attached to the cable segment (called a backbone) by means of a drop cable (a shorter cable connecting the computer to the backbone)||In a local bus, each computer is attached directly to the backbone in a daisy-chain configuration by means of a "T" connector. Peer-to-peer networks are often configured as a local bus.|
What are the advantages and disadvantages of the bus topology?
Advantages of bus topology:
Easy to implement and extend Well suited for temporary networks that must be set up in a hurry Typically the least cheapest topology to implementDisadvantages of bus topology:
- Failure of one station does not affect others
Difficult to administer/troubleshoot Limited cable length and number of stations A cable break can disable the entire network; no redundancy Maintenance costs may be higher in the long run
- Performance degrades as additional computers are added
NE SPT, p. 50
What are the key features of a star topology?
All of the stations in a star topology are connected to a central unit called a hub.
The hub offers a common connection for all stations on the network. Each station has its own direct cable connection to the hub. In most cases, this means more cable is required than for a bus topology. However, this makes adding or moving computers a relatively easy task; simply plug them into a cable outlet on the wall.
If a cable is cut, it only affects the computer that was attached to it. This eliminates the single point of failure problem associated with the bus topology. (Unless, of course, the hub itself goes down.)
Star topologies are normally implemented using twisted pair cable, specifically unshielded twisted pair (UTP). The star topology is probably the most common form of network topology currently in use.
NE SPT, p. 41
What are the advantages and disadvantages of a star topology?
Advantages of star topology:
Easy to add new stations Easy to monitor and troubleshootDisadvantages of ring topology:
- Can accommodate different wiring
Failure of hub cripples attached stations
- More cable required
NE SPT, p. 50; 1-10
What are the key features of a ring topology?
A ring topology consists of a set of stations connected serially by cable. In other words, it's a circle or ring of computers. There are no terminated ends to the cable; the signal travels around the circle in a clockwise direction.
Note that while this topology functions logically as ring, it is physically wired as a star (see FAQ 9). The central connector is not called a hub but a Multistation Access Unit or MAU. (Don't confuse a Token Ring MAU with a 'Media Adapter Unit' which is actually a transceiver.)
Under the ring concept, a signal is transferred sequentially via a "token" from one station to the next. When a station wants to transmit, it "grabs" the token, attaches data and an address to it, and then sends it around the ring. The token travels along the ring until it reaches the destination address. The receiving computer acknowledges receipt with a return message to the sender. The sender then releases the token for use by another computer.
Each station on the ring has equal access but only one station can talk at a time.
In contrast to the 'passive' topology of the bus, the ring employs an 'active' topology. Each station repeats or 'boosts' the signal before passing it on to the next station.
Rings are normally implemented using twisted pair or fiber-optic cable.
NE SPT, p. 42-43, 228, & 272-278
What are the advantages and disadvantages of a ring topology?
Advantages of ring topology:
Growth of system has minimal impact on performanceDisadvantages of ring topology:
- All stations have equal access
Most expensive topology Failure of one computer may impact others
NE SPT, p. 50
Why is a ring topology wired as a star?
A ring topology has the same outward appearance as a star; all the stations are individually connected to a central location. In the star topology the device at the center is called a hub. In a ring topology, the center is called a MAU.
While they look the same, a closer examination reveals that the ring actually consists of a continuous circuit. Signals are passed along the circuit and accessed by stations in sequence. In a star topology the signal is split and sent out simultaneously to all stations.
The diagram below illustrates the continuous circuit of a ring.
NE SPT, p. 280
What is a counterrotating ring?
A counterrotating ring is a ring topology that consists of two rings transmitting in opposite directions. The intent is to provide fault tolerance in the form of redundancy in the event of a cable failure. If one ring goes, the data can flow across to the other path, thereby preserving the ring.
Can you 'mix' topologies?
Yes, you can mix various topologies on the same network.
One very common example is a large Ethernet network with multiple hubs. Usually the hubs are located on different floors in a building or perhaps outside in another building. Each hub is wired in the typical star configuration. However, the hubs are connected together along a bus, typically referred to as a 'backbone.' The backbone between hubs might consist of fiber optic cable while the workstations are wired to each individual hub with UTP (unshielded twisted pair) cable.
NE SPT, pp. 48-49
What are the costs considerations for choosing a topology?
The following factors should be considered when choosing a topology:
Installation Maintenance and troubleshooting Expected growth Distances Infrastructure
- Existing network
As a general rule, a bus topology is the cheapest to install, but may be more expensive to maintain because it does not provide for redundancy
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