NET 102 - Networking Essentials II

Chapter 1, Networking Basics; Chapter 2, Cables and Connectors

Objectives:

This lesson introduces the student to networking concepts and to network cabling. Objectives important to this lesson:

1. Basic network concepts, network types, network signaling
2. 
   

Concepts:

Chapter 1

There is a good deal of repetition in this lesson from concepts in NET 101, so the parts that are familiar should be easier. It begins with a very general discussion of communications. The lesson should mention:

• devices that send communications signals
• the signals are picked up by devices capable of retransmitting them across some medium
• the signals are sent across the medium (and maybe forwarded in several steps to a semi-final destination)
  
• the signals are received by other devices like the ones in the second bullet (such as those at a distant network)
  
• those devices distribute the signals to other communications devices that the signals were meant for
  

A definition of networking should be provided. Networking can be defined with five features:

• users sharing resources (like printers or files)
• across a common medium (like copper wire or fiber optic cable)
• by way of specific rules (like TCP/IP or other network protocols)

The discussion on pages 418 and 419 adds a bit, pointing out that a network may be set up to provide a specific purpose, like access to one's bank account.

Some commonly used three letter acronyms are used for classifying networks based on their physical size (as opposed to how many hosts they have). People also tend to make up new items for this list, regardless of the lack of need for them.

• A Personal Area Network (PAN) may cover a work area for one person, as the book states, or it may be the network formed by your phone, your Bluetooth headset, your tablet, and other personal devices. For this one to be useful, you need to connect to a larger network as well, making the designation a bit lame.
  
• A Local Area Network (LAN) covers a small area, like a building or a campus.
  
• A Metropolitan Area Network (MAN) covers a city, linking computers at various locations.
  
• A Wide Area Network (WAN) covers an area larger than a MAN. This may be a network between cities or countries.
  
• A Virtual Local Area Network (VLAN) is a subset of a LAN. It will not be physically separate from the rest of the LAN, but it will work as though is it a different LAN.

You should have discussed client-server and peer-to-peer networks previously. Let's review the ideas:

If entities on a network act as peers, then this is Peer-to-Peer Networking. If entities act in strictly defined roles, as either servers or clients, but not as peers, then this is Server-Centric (Client-Server) Networking. Most PC networks are this type.

Most networks follow a client/server model. Clients typically perform some or most of the processing on the network, while servers provide services like data storage, instead of providing all the computing power. Client/server networks are typically easier to upgrade, both on the client side and on the server side.

Let's consider physical topologies. Topology is the study of shapes and configuration. Physical topology is the way a network is wired (or wireless-ed?) together. Logical topology is the way it works, regardless of the wiring. Network configurations typically fall into one of these types:

• Bus - essentially one continuous cable for each segment, as in the examples of coaxial cable on page 423; in the image above, a line is a special case of a bus
  
  • Easy to install
  • Moderately difficult to reconfigure. This is because it is difficult to add new devices or move existing ones without sufficient room to tap into the bus.
  • Difficult to troubleshoot
  • Units affected by media failure: All
• Star - using hubs or switches with an individual cable radiating away from them for each node, like the picture at the top of page 423
  
  • Moderately easy to install
  • Easy to reconfigure
  • Easy to troubleshoot
  • Units affected by media failure: One, unless it is the hub/switch/router, in which case all nodes are affected.
• Ring - like a daisy chain, going from one station to the next and all the way back to the server
  
  • Moderately simple to install
  • More difficult to reconfigure as the number of stations increases.
  • Easy to troubleshoot
  • Units affected by media failure: All
• Mesh - redundant connections, to survive in case one cable link is broken; in the image above, the Fully Connected example is a fully redundant mesh
  
  • Difficult to install
  • Difficult to reconfigure
  • Easy to troubleshoot
  • Units affected by media failure: Few or none
• Tree - more easily understandable if you think about a collection of related networks, separated by geography, which pass signals from one region to another, then pass them for delivery within a region. How about islands, where you can't run cables from one to another whenever you want?
  
  
  
  In the image above, imagine a message originating on Niihau, passing to a network on Kauai, then to a network on Oahu, then Maui, then Hawaii, and to an observatory at the top of Mauna Kea. Each of the named nodes might have branches, but you would have to pass traffic along the main trunk to get to them, if the network was structured that way. (Don't know which island is which? Hover over the image. Or click here for more information.)
  
  
• Hybrid - combination of more than one type, such as a series of bus networks, connected together in a ring, or a combination of networks running different Network Operating Systems.
  
  • Can be Easy or Difficult to install
  • Easy to reconfigure
  • Easy to troubleshoot
  • Units affected by media failure: All, if the central hub fails. One, if a workstation fails.

Operational standards for networks have been created by many organizations, including the Institute of Electrical and Electronic Engineers (IEEE) and the American National Standards Institute (ANSI). Several are listed in most texts.

Standards are often referred to by number. Some of the IEEE 802.x standards (there are about a dozen and a half, currently) are LAN protocols.

• 802.3 - specifies the CSMA/CD access method, so this is often thought to be the Ethernet standard. More data appears below.
  802.3 was based on Ethernet, but it is a more general standard. Ethernet can be thought of as one implementation of the 802.3 standard.
• 802.5 - specifies a token passing system based on IBM's token ring standard. IBM's standard specifies a physical ring, but 802.5 does not, so we often see physical stars that are logical rings by this standard.
• 802.11 - specifies how wireless LANs work, like spread spectrum, infrared, and short range Gigahertz radio

So , what's the deal about contention? Contention systems work by letting each device try to send a message on the net as needed, contending or competing with all the other devices for the bandwidth. Two examples of methods that support such systems are CSMA/CD (Carrier Sense, Multiple Access, with Collision Detection) and CSMA/CA (Carrier Sense, Multiple Access, with Collision Avoidance). A collision occurs when two signals collide on the medium, causing signal loss. These protocols best support intermittent transmissions. Time sensitivity is good, as users do not often have to wait for media access.
In a CSMA/CD system (example: Ethernet), the collision is detected and the devices that caused it each wait a random number of seconds before sending again. This usually results in one device going ahead of the other. In a CSMA/CA system, devices can be assigned time slices or can be required to ask permission to send, avoiding collisions. Apple LocalTalk is an example of this.

Token-Passing involves passing a token, a small data frame, from station to station. When a station has the token, it is that station's turn to access the medium. Examples of this method are token ring, FDDI, and token bus. This type of media access is predictable and consistent, allowing large or small transmissions. It is not the best for time sensitive data since waits are built in, but it will support more devices than contention.

Contention is best when the load is light, token passing is better with heavier loads, and both schemes crash under too much load.

The chapter changes gears to discuss the TCP/IP protocol suite (lots of programs in it) which can run on either of the two types above. TCP/IP, in fact, is the name of two protocols: Transmission Control Protocol (TCP) and Internet Protocol (IP). They are used with several other protocols to enable some kinds of network communications. IP is used by routers, devices that find communication paths to other computers. Finding currently valid paths is necessary because any path may be available or unavailable at any given time. TCP is used to assure that messages we send are actually delivered. (There is a lot more to it, but this is an overview.) The two concepts make the Internet work: find a way, and make sure it gets there.

Another concept that the book mixes up a bit concerns equipment used on networks. A good way to think about such equipment is to answer a question: is the device used to attach something to a network, or is it used to connect networks together? It is important to know which kind of job a given device does. A previous text stated that "network connectivity devices connect individual devices to a single network", and that "internetwork connectivity devices connect multiple independent networks together to provide access to remote resources". Those are valuable statements that you should use to sort out devices. Most devices fall into one category or the other.

Short version:

• Network Interface Card (NIC) - the most common device used to directly connect a computer to a network. (It is called a NIC, not a NIC card.) Sometimes, a motherboard may have this kind of device built into it. Sometimes users connect by other methods, such as by using modems. A NIC is an example of basic hardware needed by most network devices. Contrary to the impression you may have from the text, most networked devices are not wireless.
• Hub and Switches - An advantage to UTP cable is that networks using it are usually wired as stars, which means that wire run from nodes to hubs or switches.
  The text makes a strange statement on page 439, saying that hubs are usually on large networks, switches usually on small networks. This is backwards, and no one really uses hubs any more. Why?
  A hub is a device that has several RJ-45 ports. You can plug in as many devices as you have ports, then every signal that is transmitted by any device that is plugged in to that hub will be passed on to the rest of the devices plugged in to that hub. Some hubs can retransmit (amplify) the signals, but none of them decide where to send a signal. Any incoming signal goes back out all ports except for the one the hub received the signal on. This means only one of those devices can transmit at any given time.
  A switch learns which devices are reachable on which ports by noticing the sender's MAC address on each incoming packet and making a list. If a switch knows that a message is meant for a device connected to port 7, that's the only port that signal will be sent through. All other ports are available for other traffic. This is much more efficient. It should be clear that the statement in the text is illogical and wrong.
  Switches increase the bandwidth inside a network by connecting only the devices that need to be connected.
• Routers - pass signals from one network to another. Routers use software addresses instead of hardware addresses.

Chapter 2, Cables and Connectors

Networks are often discussed in terms of models, like the ISO Model, which breaks the subject into layers, topics, and methods. The bandwidth topic can be split into two methods for using the bandwidth of a medium. It relates directly to cabling.

• Baseband - this method uses the entire bandwidth of the medium to send one signal at a time. This means one transmitter at a time, so users have to take turns or contend for access.
• Broadband - this method uses separate channels in a medium, such as frequencies, to send multiple signals simultaneously. Users have little need to wait for access in this kind of system.

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. We should consider five attributes for each type of medium to make a good choice:

• Cost - which media cost more or less than others
• Ease of installation - how easy is it to set up
• Capacity - also called bandwidth, this means how much data can be on the net at once
• Attenuation - When a signal passes along a medium, it tends to fade (attenuate) over distance. We compare media to see which ones have better (longer) attenuation rates.
• Immunity from EMI and RFI - Electromagnetic Interference happens when your medium picks up static or bad data you don't want. Radio Frequency Interference means that you are getting the interference from an actual radio signal source, not just from stray static electricity. Media that are susceptible to EMI and RFI are also susceptible to eavesdropping (also called signal capture).

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.)

• twisted pair - has been used in two types:
  • unshielded - UTP does not have an EMI resistant sheath
  • shielded - STP has an EMI resistant sheath, which can be foil or braided metal
    
• coaxial - Coax similar to that used for cable TV, and now used by cable providers for network access as well
• fiber optic - glass or plastic channels that conduct light, often red laser light

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. 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 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 common.

The wires in each pair are twisted around each other. This type of cable comes 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. A possible problem is that the wiring closets in any building are often in need of being "cleaned up". The "closet", typically on each floor of a building, contains punch-down blocks, patch panels, and hubs (or switches). Many are disorganized and messy. People who try to clean them up, however, must be careful not to disconnect circuits that are needed.

The factors for UTP:

• Cost - inexpensive
• Installation - cheap and easy
• Capacity - 1 to 100 Megabits per second (Mbps), but 10 Mbps is common in home networking. Gigabit throughput is possible if the other network equipment (network cards, switches, routers) support it
• Attenuation - nothing is perfect, so this is high (poor)
• Immunity from EMI - also poor. Recommendation: run UTP lines perpendicular to fluorescent lights to avoid a constant static buildup.

UTP cables are usually connected to devices with RJ-45 connectors. Your text does not show an RJ-45 connector (or any other) very well. In the enlarged picture on the right, note the eight gold-colored conntacts for the eight wires usually found in UTP cables. The wires are used in pairs to form two to four circuits.

RJ-11 connectors are typically used for telephone

An STP (Shielded Twisted Pair) cable is very similar to a UTP cable, except that it has a metal foil sheath around each pair of wires, or around the entire set of wires. When the sheath encloses all the wires, it resembles the ground sheath in a coaxial cable, but its purpose is to shield the cable from interference..
This cable is more expensive than unshielded cable, and is less flexible due to the stiff shielding. The shield, however, makes it more EMI resistant than UTP.

The factors for STP:

• Cost - Moderately expensive
• Installation - harder than UTP, needs special connectors (note the IBM-style Token Ring connector shown here)
• Capacity - 1 to 500 Megabits per second (Mbps) is possible, but 16 Mbps is common (for Token Ring)
• Attenuation - high (poor)
• Immunity from EMI - also poor, but not as bad as UTP.

IBM Data connectors were typically used with Shielded Twisted Pair cable on a Token Ring network.
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.

The wiring standards used for network coax have been different from those used for cable TV. This is a list of cable used through networking history:

• 50 ohm cable, available as RG-8 and RG-11. Used in Thick Ethernet, also called "Ether Hose".
• 50 ohm cable, available as RG-58. Used in Thin Ethernet
• 75 ohm cable, available as RG-59.
• 93 ohm cable, available as RG-62. Used in ARCnet.
  

The number associated with each RG specification tells you the relative size of the central conductor. Smaller numbers mean thicker wires.

• The coaxial line is essentially a single bus, going from one station to the next.
• At each end of the line, the cable has to have a terminator on it.
• At one end, it also has to be grounded.
• If using thin Ethernet, T-connectors are used.
• If using thick Ethernet, vampire taps are used. They are called vampire taps because little teeth bite into the cable (to contact the shield), and a big tooth bites deeper to contact the central conductor when you screw the clamp down.

The factors for Coax:

• Cost - Relatively low to Moderately expensive (depending on thickness of the cable)
• Installation - simple to install, hard to modify
• Capacity - high rates are possible, but 10 Mbps is common
• Attenuation - high, but less than twisted pair
• Immunity from EMI - moderate

The example on the right shows a typical T-connector with BNC fittings. The fitting on the bottom of the image might attach to a port on a NIC that looks like the barrel on either end of the top of the T. Attachment is achieved by pushing the connector onto the barrel of the port, then twisting the collar of the connector to lock onto the pin that is part of the port. In other words, it mounts like a bayonet.

The next (enlarged) picture shows a BNC connector attached to a thin Ethernet cable. Such a connector would be used to attach to one of the T-connector barrels in the photo above. The other end of the cable would run to the next node on the network.

Fiber optic can be glass or plastic, and is meant to conduct light instead of electricity. The conductor is called a waveguide, and is covered with cladding, a material to reflect the signal back into the center of the conductor. Two configurations exist. Loose configuration has a liquid filler between the outer sheath and the conductor. Tight configuration has wire or stiff fibers around the conductor to add strength to the cable.

Fiber optic comes in two modes: single mode conducts a single signal, while multi-mode conducts many signals simultaneously. You may want to know that the most common type used is 62.5 micron core with 125 micron cladding, multimode.

The factors for fiber optic:

• Cost - Expensive, mostly for installation
• Installation - difficult
• Capacity - 100 Mbps at up to 20 kilometers per segment
• Attenuation - very low
• Immunity from EMI - immune. This is light, not electricity.