NET 101 - Networking Essentials I

Chapter 10, Communications and Networks

Objectives:

This lesson discusses networks in general and some of their functions. Objectives important to this lesson:

1. Purposes of networks
2. Network classifications
3. Physical and logical topologies
4. Common types based on access method or protocol
   
5. Network devices
   
   
   Concepts related to Student Learning Objectives
6. IP addressing again
   

Concepts:

There is a good deal of repetition in this chapter, so the parts that are familiar should be easier. It begins with a very general discussion of communications, that I expect is meant to build toward some of the material later in the chapter. The text talks about:

• 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
  

Back in chapter 1, I gave you a definition of networking. 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.

We have discussed client-server and peer-to-peer networks briefly. 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.

The text moves on to 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.

It gets a little harder to understand on page 425, where the discussion moves on to operational standards, many of which have been created by the Institute of Electrical and Electronic Engineers (IEEE) or the American National Standards Institute (ANSI). Several are listed, but they do not have a common purpose. Page 425, for example discusses Ethernets and Token Rings.

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 directlyy 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.
• Hubs 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 statement 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.
  

Assignment 1: Consider the ten situations listed on page 460. Form a group. Pick 3 of the problems. Research and discuss possible solutions. Write up a solution for each of your three problems. Explain why you chose it, and where you obtained the information about the solution. This is a group assignment.

We had a lot of misunderstanding about the IP address problems from last week. We will do some exercises in class, and there will be new problems assigned for this week.