NET 102 - Networking Essentials II
Chapter 14, Remote Connectivity; Chapter 15, Wireless
Networking; Chapter 20, Network Troubleshooting
This lesson covers three chapters, and covers several
protocols and network device naming. Objectives
important to this lesson:
- WAN technologies
- Last mile connections
- Remote connection methods
- Wireless standards
- Implementing a Wi-Fi network
- Troubleshooting wireless
- Troubleshooting tools
- Troubleshooting process
- Troubleshooting examples
author spends the first page telling us that he is going to tell us
things. The lesson seems to begin on page 382 with a discussion of telephone circuits. When Mr. Bell
first set up his company, you had to have a point-to-point connection from your
phone to another person's phone in order to call them. As time passed
improvements were made that included an operator who made the connections on
a local exchange switchboard,
then to automated switches
that removed the need for that operator. Think of American style
seven-digit phone numbers: xxx-xxxx.
You can think of a local exchange
as phones that share the same first
three digits in their numbers. Those phones would be serviced in
a single central office, where
switches made connections to phones in that exchange or connected to a different central office to pass the
call to one of their exchanges. Sounds a lot like switches and routers,
on and between LANs. Lines that connect one central office to another
are often called trunk lines.
Another improvement made along the way was to figure out
several forms of multiplexing.
Your book talks about one method, frequency
division multiplexing. On average, a human voice contains
several sound frequencies that usually fit in the range 0 to 4000 Hz.
If you pass that along as an analog signal on a copper wire, you can
probably only pass along one conversation. On the other hand, the wire
can carry a lot more data at the same time if you shift the next
conversation to a higher frequency range, like 4001 to 8000 Hz.
Of course, you don't want to sound like Alvin the chipmunk, so we
use a frequency division multiplexor
on each end of the circuit, one to send the conversation up to an
available range, and one to take every elevated conversation back down
to the human range.
The text discusses the conversion of the telephone system from
analog signals to digital signals, simplifying the situation by only
mentioning AT&T. There were other phone companies. AT&T was the
biggest, which is one of the reasons that Judge Greene broke it into
seven regional companies in the 1980s. Most of those regional operating
companies are now owned by Verizon or the new AT&T. The bottom line
for this discussion is that phone conversations typically start as analog signals, reach a central office, are converted to digital signals, are passed along trunk lines to other central offices, converted back to analog signals, and passed to a local exchange circuit.
The part of the phone system that remains analog (in most
places) is called the last mile.
It is the part that runs from the central office to a customer's
location. You might remember it by thinking of it as the last mile in
each connection that is still analog.
The text moves on to discuss some items that it warns us are
on the Network+ test.
- AT&T's method to convert an analog signal to digital is
to sample (measure) the analog
signal 8000 times per second,
and save the value as an 8-bit packet.
- 8 bits times 8000 times per second gives us 64 kb (kilobits) per second. This
signal rate is called a DS0
rate (zero, not oh).
- This digital conversion takes place at central offices for
each analog voice conversation.
- Digital signals are passed on to digital carrier lines,
like T1 and T3 lines. They have different
The chart on page 390 summarizes the discussion of T-carrier services. Know that T1 and
T3 are North American (US) standards, and the E1 and E3 are European
standards. For all four services, channels are 64 Kbps, as described
above. The author continues to call the bandwidth rates "speed". This
is not exactly correct, since individual bits are all effectively traveling at
the same speed, the speed of light through the media involved, slowed
by processing along the way. Like all measurements of bits per second,
the numbers shown are the bandwidths provided under ideal conditions.
||Channels in service
||672 (like 28 T1 lines)
|512 (like 16 E1 lines)
Page 388 has a small picture of the cable used for T1 lines. Note that it has two twisted pair circuits, and each one is shielded separately.
(LabSim says each wire is shielded, but the examples I have found on
line show a shield on each circuit.) We can refer to it as Individually Shielded Twisted Pair (ISTP) cable. ISTP has a resistance of 100 Ohms. One circuit is for transmission, the other for reception,
and there are no extra circuits. This cable connects to a CSU/DSU
(Channel Service Unit/Digital Service Unit) in your central
office. A separate line runs to your location to a CSU/DSU in your MDF
and it connects to a router.
The link in the last sentence goes to a Cisco article on setting up a a
WAN interface card in a Cisco router that will connect you to the
CSU/DSU. (The author implies that you won't have a CSU/DSU on your
premises. If this were so, why would we be able to buy them?
The text discusses the frames (DS1 frames) that are used on T1 lines. We are told that each frame
can contain traffic from each of the T1's 24 channels, but only 8 bits
per channel are in the frame, plus one framing bit, making 193 bits per frame. Multiply that times 8000 frames per second
and you get 1.544 megabits per second. Since each channel is given the
same amount of space per frame (one byte), we can say that it is given
8000 time slices per second to send its data. Giving a device this kind
of time sharing on a network line is called time division multiplexing, which is a second multiplexing method.
The author points out that a T1 line is leased by a customer
from a telephone/data carrier. Historically, some customers did not
need to use the entire bandwidth of a T1 line, so it is possible to
lease a portion of a T1, if the provider can lease the rest to another
customer. Pieces of a T1 line are marketed as fractional T1 service. As you can see from the chart above, T3 lines carry the equivalent of 28 T1 lines, which the text (and most people) round off to 45 Mbps.
On page 391, we turn to fiber connections from one network to another. We have seen the acronym SONET before. This time the author tells us it stands for Synchronous Optical Network, which is a fiber optic WAN standard in the United States. The European equivalent is Synchronous Digital Hierarchy (SDH). On these fiber systems, the services are measured by Optical Carrier (OC) levels. Each level is a multiple of the base rate, which is also called OC-1, which is 51.84 Mbps. (The text has a slightly different number which does not work as a numeric base.) SONET has its own signal/frame type numbers that correspond to the OC types.
SONET also uses a technology called FDDI, a fiber optic ring standard. This is an
ANSI standard, not an IEEE standard, but it makes use of the IEEE 802.2
and 802.5 standards. It is very fast, and has high capacity, making it
useful for three main applications:
- Backbones - connections to other networks that need to be
fast and wide
- Computer room networks - fast connections between critical
- High data rate LANs - such as SONET connections
FDDI uses two rings that are counter rotating. This means that
traffic travels clockwise on one ring and counterclockwise
on the other, making reconfiguration simple. If a break occurs
between two stations, the rings cross over at those stations,
turning the two rings into one, longer loop. (Mouse over the picture to see this happen when a nasty bug breaks the rings.)
The text and LabSim discuss the kind of multiplexing used on fiber lines. Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are mentioned in the text. Both sources talk about using multiple light signals that have different wavelengths simultaneously on the same fiber to increase throughput. The video on LabSim points out that light signals of different wavelengths means using light signals of different colors. Neither points out that frequency times wavelength equals the speed of light.
Since the speed of light is a constant, changing to a different
wavelength also means changing to a different frequency, which makes
the distinction between FDM and WDM a joke. They just called it by a different name because it uses fiber and light.
On page 392, the text discusses some older technologies:
- Frame relay - used for transmitting bursts of data, without
error checking; other protocols check for errors on each end of the
frame relay network; maps to the Physical and Data-Link layers; useful
for data only; can work with X.25 or ISDN, can run at 56 Kbps, T1, or
- ATM - Asynchronous Transfer Mode can be both a LAN and
WAN protocol. It maps to the first three layers of the ISO-OSI model.
It is listed in your text as another topology type, due to its unusual
- Uses 53 byte blocks called cells.
- Uses virtual channels.
- Can use most media: fiber optic, STP, or UTP
- Uses Internetworking Units (IWUs) to connect networks
- MPLS - Multiprotocol Label Switching was developed specifically to support TCP/IP connections over WANs; adds a label and other fields after the header in a frame, providing more information
- MPLS label - identifies MPLS traffic
- Cost of Service - rates the importance of the frame
- S - set to 1 if this is the first of several MPLS packets
- Time to Live - limit on the number of hops allowed
This part of the chapter ends by casually mentioning that some ISPs are changing to Gigabit Ethernet connections instead of using the technologies listed above to connect to the Internet.
The chapter changes topics on page 397 to talk about last mile connections. How does a home user connect? Lots of technologies exist, but your choices are limited by your location.
- Dial-up service - once very popular, still available, uses the Public Switched Telephone Network, also called Plain Old Telephone Service; requires the use of a modem (modulator/demodulator) to turn the digital signal of a computer to an analog signal for the PSTN
- The text lists several generations of modem standards by their V.x numbers. They were established by the CCITT, which was mainly French, which explains the various standards ending in bis, which means revised. Know that modems evolved from 300 bps through 14.4 kbps, 28.8 kbps, and 56.6 kbps, where they have topped out. The book has somewhat different numbers. It depends on whether you call a kilobyte 1000 bytes or 1024 bytes. Both definitions are used by the industry.
- ISDN - After three paragraphs of history, the text tells us that an Integrated Services Digital Network connection gives you a digital connection to the telephone company's digital network, eliminating the need for a modem, as such. It uses a terminal adapter instead, which you can think of as a digital modem or adapter. ISDN is limited by distance: you can't get it unless your location is within 18,000 feet of a central office that offers it.
- DSL - digital subscriber lines come in several types: symmetric, asymmetric, and very high bit rate are listed in the text. Like ISDN, you can't get this option unless you are within 18,000 feet of a central office that offers this service, which it will not do unless the telephone cable to your location is up to the task. A DSL connection requires a phone jack, a DSL modem, and a patch cable to a NIC in your computer.
- Cable modem - uses a cable modem that looks like a DSL modem, except for the coaxial jack; uses Digital Over Cable Service Interface Specification (DOCSIS) protocol
- Satellite systems - available for the most remote locations, may be one way (download only) or two way service
- Cellular WAN - the text discusses two main types: cellular modems for laptops use Mobile Data Service, typically through a cellular provider; WiMAX is also called 802.16, and is a long range wireless service (3 to 30 miles) made available in communities
- Fiber - the text is referring to fiber connections from telephone companies, as opposed to cable system fiber, both of which are available in some markets
- BPL - Broadband over Power Line is a newer technology that has not performed as well as the established methods
The text moves on, in this chapter that seems like it will never end, to remote access, which is not the same thing as just using the Internet. Remote access means accessing your organization's assets from a remote location, The methods discussed vary by their cost, their bandwidth, and their level of security. The author's list also varies in purpose from line to line:
- Dial-up to an ISP - this a about creating a dial-up connection to an Internet Service Provider, which requires a modem, and does not by itself grant access to your company's network
- Private dial-up - still using a modem, making a connection through the PSTN to some kind of server that provides gateway access to the network you are seeking; the text mentions Microsoft's Remote Access Server (RAS) as an example of a product that will allow a server to provide access through a modem connection
- Virtual Private Network - the text spends little time on this item which is a more valid way to get a secure connection; using VPN software, you get an encrypted connection to your desired network, which might be done by any of the methods above, or by using a broadband connection to access the Internet, and then your network gateway
- Dedicated connection - this method is always on, typically through a leased line from a data carrier (probably a cable or telephone company), which may be a T1 or any other grade of connection we have discussed; the author includes cable and DSL connections in this discussion
- Remote terminal - a remote terminal program lets you run a session on a remote system as though your computer were on that system; this does not belong on the list because this is a way to do something but not a way to connect to the distant network: it relies on a dial-up, Internet, or dedicated line to function
- Voice over IP (VoIP) - does not belong on this list, but the author covers three protocols that are commonly used for VoIP: Real-time Transport Protocol (RTP) defines VoIP packets, Session Initiation Protocol (SIP), and H.323 provide session set up and packet delivery services.
This chapter is about wireless networks, which most employers will expect a technician to know something about.
A wireless network typically has one or more devices that use some variety of Wi-Fi. The standards that apply to it are all contained in IEEE 802.11x, each new standard having its own letter or letters. The range of a wireless connection can be affected by walls, other devices on the same frequency, and environmental factors.
- 802.11 - the first IEEE wireless networking standard; specifies a Wireless Access Point (WAP) that serves as router, switch, bridge, or other needed device, and is the point of entry to your network for wireless hosts
Using a WAP can be called using wireless networking in infrastructure mode, making the WAP part of an existing network
ad hoc (also called peer-to-peer) wireless networking does not use a WAP: it allows direct connection between two or more devices in a mesh network that may also be called an Independent Basis Service Set (IBSS) which uses a random number as its temporary network ID
- SSID - a service set identifier is a label that is typically broadcast by a WAP, to tell wireless devices that the WAP's access services are available; the text lists two types: a BSSID (Basic) and an ESSID (Extended); a BSSID is used if you have one WAP, and ESSID is used if you have multiple WAPs
- Spread spectrum - Wi-Fi calls for devices to to use multiple frequencies either one at a time with frequency hopping (FHSS) or several at once with direct sequence (DSSS)
- CSMA/CA - Wi-Fi devices cannot detect wireless collisions, so they can't use CSMA/CD; they use collision avoidance instead
- 802.11b - first widely used standard, frequency is 2.4 GHz, range 300 feet, 11 Mbps
- 802.11a - not a widely used standard, frequency is 5.0 GHz, range 150 feet, 54 Mbps
- 802.11g - widely used, can use two channels to transmit, frequency is 2.4 GHz, range 300 feet, 54 Mbps; can downgrade to use a or b if needed
- 802.11n - widely used, frequency is 2.4 GHz, range 300 to 1000 feet, 100 Mbps, uses multiple antennas for multiple connections
Wireless security starts out similar to security on a regular LAN.
- you can limit access to devices with known MAC addresses
- you can require users to authenticate through a RADIUS server
- transmissions can be encrypted
- Wired Equivalent Privacy (WEP) is no longer considered secure
- Wi-Fi Protected Access (WPA) - uses TKIP encryption, which is also hackable
- Wi-Fi Protected Access 2 (WPA2) - uses AES encryption, which is a better product
- Wi-Fi Protected Access 2 - Enterprise (WPA2E) - uses WPA2 and a RADIUS server; solutions that use a RADIUS server set the network password on that server, not on the WAP
The text mentions a feature in this section that is not a part of the security discussion. Power over Ethernet (PoE) is just a feature that some WAPs have, providing power to them over a wired Ethernet connection instead of by the usual electrical cord. The power must be supplied by the network switch that the WAP is connected to, which means it must support PoE as well.
The topic changes to setting up a Wi-Fi network. It is more likely that Wi-Fi will only be one part of a larger network, but the discussion supports both ideas.
- Determine what other wireless networks or RFI sources would affect your new network. Microwave ovens are notorious for producing 2.4 GHz radiation.
- Install Wi-Fi hardware and software on devices that need it and do not yet have it.
- Depending on the kind of network you are creating:
- If you are creating an ad hoc network, you must establish an SSID that all the devices will use, assign unique IP addresses to the participating devices, enable ad hoc networking on the NICs, and turn on File and Printer sharing on all hosts.
- If you are creating an infrastructure wireless network, you need to place access points, configure them, and configure the hosts that will use them.
The text spends the rest of the chapter discussing the setup of a infrastructure network, and troubleshooting Wi-Fi. This leads logically to chapter 20, which is about network troubleshooting.
The last chapter in the text is about troubleshooting. The author warns us to expect lots of troubleshooting and repair questions on the Network+ test, so you will want to learn all the tools and techniques that you can in this chapter.
- open circuit - a wire in a cable does not connect to the connector on its end; test with a cable tester
- short circuit - one or more wires connect to another wire in the cable; test with a cable tester
- wire map problem - the wires are not attached to the correct pins in the connector; test with a cable tester
- crosstalk - signals bleed over from one circuit to another; test with a cable tester
- noise - static or false electrical impulses are added, possibly from a bad connection job; test with a cable tester
- impedance mismatch - wrong cable type used, or a broken cable; test with a cable certifier
- no function at all - broken wire in the cable; test with a time domain reflectometer, or replace the cable
- intermittent problems with performance or failure - suspect a heat or power problem; test with a voltage event recorder or a temperature monitor or both
Some tools are mentioned that are meant for installation instead of problem diagnosis:
- cable stripper - for removing outer jackets
- snips - for cutting cable
- punch down tool - for connecting wire to punch down block (66 or 110); know that a 110 blade has a notch (45 degree angle) and a 66 blade does not
- crimpers - for putting RJ-11 or RJ-45 connectors on horizontal, vertical, and patch cabling
Other tools are used mainly for diagnosis:
- multimeter - used to test voltage, continuity, and impedance (aka, resistance)
- tone probe and tone generator - used to find the cable you are looking for when they are not labeled (often called by the brand name Fox and Hound); send a signal on one end of a cable, then use the probe to find the other end
- butt set, lineman's handset - used to test signals on a line in a punch down block, or elsewhere on a telephone circuit
The discussion moves on to software tools, one of which was confusingly mentioned in the section above. Before we get to that one, there is a list of command line commands that tell you useful information:
- tracert, traceroute - the command takes one argument, which can be an IP address or a domain name; it returns information on each hop along the route being used, such as address and time to reach that hop
- ipconfig, ifconfig - discussed in an earlier chapter, used to learn current configuration of network ports, and to release or renew DHCP leases
- ping - used to send an ICMP packet to determine whether a host is running; also provided DNS resolution if you feed it a domain name instead of an IP address
- arp - can be used to learn the pairs of IP addresses and MAC addresses that a device knows: enter arp -a
- nslookup, dig - used to resolve domain names to IP addresses, if a DNS server is available and responding, which is what you test with them
- hostname - limited but useful command to learn what name is assigned to the device you are operating
- mtr - stands for my traceroute, which does not run on Windows devices; know that it runs until you stop it
- route print, netstat -r - either command will show the local device's routing table on most devices
- nbtstat - used for Windows devices, when using WINS; the text recommends using nbtstat -n to see the list of NetBIOS names your device knows
- netstat - displays a list of current processes having network connections; if you give it an IP address as an argument, it displays the list of the device at that address; runs until you stop it with ctrl-c
The utility that was mentioned in the earlier part of the chapter is sometimes called a protocol analyzer, a packet analyzer, or a sniffer. There are several available for purchase and for free. The author recommends Wireshark, which is a fine product. Be aware that using it (or any sniffer) on a network without the permission of its administrators is a serious offense. Do not use it except on your own network or one where you have permission.
Another general utility is a port scanner, which is often used by hackers to determine what ports are open on what devices. The text mentions Nmap as being the most famous example.
Most of you may have used a throughput tester, whether you called it that or not, to test the bandwidth of a system's Internet connection. The text mentions Speed Test at speakeasy.net, which I have used several times, along with several others.
The author begins a discussion of general troubleshooting on page 564, proposing that a good general plan is the best place to start. CompTIA seems to believe this, too. There are numerous pages on the web talking about a CompTIA troubleshooting plan. The outline of a plan is on page 565. I am not finding it in CompTIA's exam objectives, but the author has repeated this plan in a marginal note on page 569, so let's assume it is important.
- Identify the problem - the author takes the attitude that the person with the problem probably does not understand what is really wrong; this does happen, but it is not a rule; listen to the caller/user and form your own opinion, don't assume that the caller is wrong
- gather information
- identity symptoms
- question users
- determine what has changed since it last worked
- Establish a theory of probable cause: "question the obvious", which means to look for simple fixes like turning the device on
- Test the theory to determine cause
- If the theory is right, plan a resolution
- If the theory is wrong, form a new theory and loop this step
- Establish a plan of action to resolve the problem (Didn't he just say to do this?)
- Implement and test the solution, or escalate if you don't have authority
- Verify functionality/return to service
- Document what happened, what you did, and the result
The chapter ends with several troubleshooting scenarios. Read through pages 569 through 574. We will discuss these pages in class.