Concepts:The chapter begins with a very general discussion of communications on a network. The text should have mentioned:
A definition of networking is provided in the chapter. A more formal one should be provided. Networking can be defined with five features. Each is emphasized a different way in this list:
The discussion on page 322 adds a bit, with a set of common
acronyms based on how large a network is. These three letter
acronyms are used for classifying networks based on their physical
size (as opposed to how many hosts/nodes/computers they have).
People also tend to make up new items for this list, regardless of
the lack of need for them.
These designations for networks are commonly taught in classrooms, but they are not terribly important when you are creating a network. However, you need to start somewhere, and this classification is easily understood. Networks start small, then build and combine to create larger constructs. Even the Internet once connected only five locations. Now, everyone wants and needs to connect to it. The author reviews some terms that have been common for some time:
These factors apply to each of the connectivity options in table 7.1. General concepts:
Computers use electric currents and various forms of electromagnetic waves to communicate. We can class networks as being cabled (wired) or wireless, for obvious reasons. There have been several types of cable media used in networks over the years. (Follow the link to a Microsoft TechNet article about media. It is not perfect, but it is pretty good.)
For those who don't believe me about cable vs. DSL, Linus Sebastian will clarify the situation.
Systems that do not use cables between nodes fall into two groups:
Short physics lesson: In a copper wire, electrons don't actually flow from one end of the wire to the other. What happens is more like the movement of a large ripple or wave in water. Imagine a wave moving toward a shore in a lake or an ocean. Do specific water molecules make the whole trip? No. The energy of the wave is passed across a series of molecules. The energy passes across the medium. In the image on the right, floating objects at various depths illustrate that the energy of an ocean wave is more apparent near the surface, but in deep water there is little horizontal movement of the water itself. For the purists among you, I will note that the speed of
electromagnetic waves through the electrical media varies with the
nature of the conductor. It can be over
90% of c in a UTP wire, and a bit slower in coax. What's c?
The speed
of light in a vacuum. The most satisfying answer to "why c"
is that the Latin word celeritas means speed. The graphic shown here illustrates several twisted pairs of
wires. Each wire is covered with an insulator, and the two wires
in each pair are meant to be used as a circuit. These wires suffer
from crosstalk, leakage of signal. The twists help cancel
out such leaks. The graphic shows a UTP cable with eight wires in
it, making four pairs. As is typical, there is a green pair, a
blue pair, an orange pair, and a brown pair. Other color schemes
are used, but this one is very common. The wires in each pair are twisted around each other. This type of cable came in several varieties: two pair, three pair and four pair were common, but four pair is the current standard. Also, each variety may be available in grades, such as CAT 1 (Category 1, which is pretty useless on modern networks) and CAT 5 (Category 5, which has been a standard for several years). There are several such categories, and a major difference between them is the number of twists per foot in each pair. CAT 1 will have less than 5 twists per foot, CAT 5 will have 25 or more twists per foot (so it is better, and costs more). Note that the better the class of cable, the less leakage, and the more bits per second can be passed across it. Connecting a system with twisted pair wiring is easy, with the right tools and parts. In the chart below, the arrangement of wires for the standard known as TIA/EIA 568B is shown. In an alternate standard, TIA/EIA 568A, orange/white is swapped with green/white, and orange is swapped with green. It does not really matter which standard you use, as long as both ends of the cable are connected in the same way. There are two exceptions to this: a crossover cable and a rollover cable. A crossover cable is used to connect directly from one NIC to another, or from one networking device to another, such as connecting two switches. These cables are used less often, since modern devices sense what kind of devices are cabled to them and change their logic accordingly. A rollover cable is used to connect to a Cisco router's console port. Let's save that for Cisco class. Most references forget to tell you the reason you arrange wires this way instead of however you might like. Read the last two columns in the chart below. Pins 1 and 2 are used for the transmission circuit, which is why they need to be wired with two wires that are twisted around each other in the cable. Using a twisted pair of wires in a circuit reduces the amount of signal lost to other circuits (crosstalk). You need to use a real pair for each circuit that your network requires. Pins 3 and 6 are used for the reception circuit. The odd part is the 3-6 pairing, surrounding the 4-5 pairing. We wire a connector this way so that it follows a pattern of alternating stripes and solids, so a person can remember it, and because that's the way it works. Why did they decide to use the connectors and sockets this way? I don't know. Just know that this works.
Pin pairs:
When you are inserting the wires into a connector, do it with the gold contacts up so you can see the wires enter each channel in the plastic.
In the video below, the presenter has some good ideas, and some
that are not so good. He knows how to connect the wires to the
RJ-45 plugs, so let's enjoy his presentation and discuss it in
class. (Spoiler: orange white, orange, green white, blue, blue
white, green, brown white, brown)
Coaxial cable is called that because it has two conductors, one wire in center and a conductive sheath around it, that share a common axis, hence coax. Most people have seen this style of cable used with cable television. When coaxial cable was first used for networking, you could not use the same kind of cable that is used for cable TV. Improved products have allowed the same type to be used for both services. There have been many standards set by the Institute of
Electrical and Electronic Engineers (IEEE),
notably the IEEE 802.11 wireless
standards. The
text discusses some of them in more detail.
Page 329 begins a section on adding a computer to an Ethernet network. (Ethernet is a kind of network invented by Bob Metcalf at Xerox PARC in 1975 Xerox invented a lot of things.) It assumes you are connecting a Windows computer to an existing network. The text gives us two pages on using a wired connection, and another two on a wireless connection. It also describes connection through a VPN (Virtual Private Network) and through a dial-up (telephone) connection. A VPN is a secure communication channel that is often used by people who need to connect to their usual network when they are traveling, working from home, or are otherwise away from their usual work location. A VPN may pass traffic across the Internet, but it can be considered as secure because all traffic passed from one end of the channel to the other is encrypted. Using a VPN provides a level of security that an unsecured data channel cannot provide. Each end of a VPN channel is called an endpoint. Dial-up connections are typically over analog phone lines, and they are painfully slow by modern expectations. As noted about other technologies, however, it sure beats not being able to connect at all. I tried to find a video that would be educational, but most just wallow in the old tech with little explanation. Don't do it, children! You'll throw away your entire life! The author reminds us that almost any kind of connection to a network can be set up on a Windows computer through the Network and Sharing Center, which is a utility in Windows, not a charitable organization. In the course of setting up a computer to function on a network, you will need to either hard code an IP address for the computer (a static address), or allow the computer to be assigned an address by a network device (a dynamic address). The purpose of an IP address is to identify each unique device on each unique network. On an IP network, each device is known as a host, and every host must have an address. The addresses we discuss are usually IP version 4 addresses. (IPv6 addresses have 16 bytes (128 bits).) IPv4 addresses are numeric addresses, stored as four bytes, which is equal to 32 bits. When we write these addresses, we usually place dots between the bytes, but the dots do not exist in the addresses as used. Dr. Andrews points out that most networks still use IPv4 addressing, so she discusses it the most. Since the TCP/IP protocol suite was invented with networking in mind, IP addresses contain two parts: one to identify the address of the network a host is on, and the other part to identify the host itself. Every network is assigned an address which, according to the original plan, could be one, two, or three bytes, depending on the class of the network (A, B, or C). The remaining byte or bytes are typically used for hosts on networks. (There are other ways to do it as well.) Each byte in an IPv4 address will hold a number in the range 0
through 255. To pass some certification tests associated with this
course, students have been expected to convert decimal
notation to binary notation and vice versa, by hand.
Pencil and paper. Really. Dr. Andrews does not go into this skill
at this time, so we will let it slide for now. Dynamic Host Configuration Protocol (DHCP) service allows us to dynamically assign IP addresses to hosts on an IP network. You need to understand that, as far as IP is concerned, "host" means any device on the network. It can also set the subnet mask, the default gateway, and the DNS server. (A subnet mask is a template that defines which part of an IP address is for the network and which part is for the host.) Three methods for assigning addresses:
The text discusses what you will see if DHCP fails. Automatic Private IP Addressing (APIPA) could be described as an error condition, if you are using DHCP on your Windows-based network. The first clue may be that a workstation cannot reach the Internet, because APIPA does not set a default gateway. What it does is to set a private IP address instead of one that matches your network scheme. It will be in the range 169.254.0.0 - 169.254.255.254. APIPA is meant to be a fallback method of assigning addresses. It will result in communications failures if some of your workstations have APIPA addresses, and others have DHCP assigned addresses. The two groups will essentially be on two different networks. The trouble they encounter is meant to be a flag that leads you to repairing the failure. If a Windows workstation is configured for DHCP, but has an address in the APIPA range, try opening a command line interface and entering two commands: ipconfig -release The first command tells the computer to let go of the lease it has (or thinks it has). The second command tells the computer to start a DHCP discover, offer, request, acknowledge sequence.
The same set of commands can be useful on devices running Linux/UNIX/osX by running the Linux version of these commands: sudo ifconfig eth0 down In a home network, the DHCP assignment is typically provided as a service on your router/WAP/cable modem. In a work setting, the service may be running on an actual server. As noted above, you can hard code an IP address to a machine. This is useful for devices like switches, routers, printers, and other devices whose IP addresses need to be known and shared with other devices. The text briefly discusses Network Interface Cards (NICs). We have talked about NICs already. Remember that a computer's NIC is where its MAC address is located and that you should check the indicator lights on a NIC to diagnose problems. A router connects two or more networks together. Its purpose is to find routes and pass traffic from one network to another. A router is also a host device on each network it is directly connected to. Network addresses are used to pass data from one network to another, but hardware addresses are usually used to pass data to hosts on the same network. In the diagram on page 345, the small office/home office router is being used as a switch, to connect four devices in a LAN. It is also connected being used as a router connecting its own network to an ISP's network, passing signals to that network through a cable or DSL modem. Many cable modems and home routers also provide Wi-Fi service, which make them wireless access points (WAPs). Your device may also provide firewall service, rejecting traffic from known bad sources. With all the things your router can do, it is reasonable to get a good one, but it is also reasonable to put some of your eggs in a different basket. The text presents a walk-through of configuring a router. Depending on what you have, your experience should be both similar and different. You will need to know the ID and password for a user the router will recognize as an administrator. If you do not, you will need to look up the default ID and password for your brand and model, press the factory reset button on the router, then log in and make the darn thing secure again. Never leave a device in a state in which factory default ID/password combinations exist. The exercise continues with configuring DHCP, reserving IP addresses (for network equipment, servers, and printers), MAC address filtering, and providing Quality of Service priority to a device or application. The text discusses the same issues as they relate to a wireless LAN (WLAN), along with some unique issues through page 364. It continues with several utilities found on most Windows workstations.
|
UTP network cables are
usually connected to devices with