NET 102 - Networking Essentials II

Chapter 3, Networking Devices; Chapter 4, Ethernet

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. ISO layers
3. Ethernet concepts
   

Concepts:

Review?

The way a network works can be understood in terms of this model of a network that was created by the International Organization for Standardization, called the ISO for short. (No it isn't an acronym, it is from the Greek word isos, meaning equal.) Their model is called the Open Systems Interconnect (OSI) Reference Model, hence the ISO-OSI model.

Once you understand this model, you will have a general, powerful reference for examining and comparing networks. 

The seven layers of the model are usually written in a list, numbering the top as layer seven and the bottom as layer one.

Several mnemonic sentences exist to help us remember the proper order. I recommend "Please Do Not Throw Sausage Pizza Away", because this is in the correct numeric order (bottom to top, 1 to 7). If you want one that goes from top to bottom, try "All People Studying This Need Drastic Psychotherapy".  On any certification test that covers this model, you MUST remember the correct order, the correct numbers, and the correct details for each layer.

The processes that happen in each layer communicate with the next layer. Which way is next, up or down? It depends whether data is being passed out of the stack (down) or into it (up). Typically, a computer generates a request starting at the top layer, and working down. The request is passed across the network (probably to a server) and the received request is passed up the layers. When a response is generated, the process reverses.

Chapter 3

Traffic on a network is broken into packets, smaller message units that are transmitted more easily on a network. Each packet must hold at least two addresses: that of the sender and that of the recipient. They also hold data, and numbers that tell the receiving device how to reassemble the pieces of the message. Chapter 2 is mainly about one network model, the ISO-OSI model, which is a logical (as opposed to physical) model that explains how networks handle their packets and perform other useful functions. The text only calls this the OSI, or Open Systems Interconnect model. ISO, the International Organization for Standardization, is another trade association that sets standards for the computer industry. Note that ISO is not an acronym. It is based on the Greek word isos, which means same, and stands for their goal of standardization.

The ISO-OSI model gives us a framework for discussing what happens on a network, and what happens at specific devices. So, we can start explaining the model by telling you some of the things associated with it. 

1. In the Physical layer, we pick a communications medium, which is usually UTP (unshielded twisted pair) cable, because it is inexpensive, easy to use, and it works well. The author mentions hubs in this layer. A hub can also be called a concentrator, because it is where lots of wires come together (concentrate). The author confuses the description by saying that a hub is like a telephone switchboard, which most of you have probably never seen, but Wikipedia has decent pictures. A hub is like a switchboard in that lots of wires from different devices come together there. It is also NOT like a switchboard, in that any signal sent into a hub will come out on ALL the other wires. On a telephone switchboard, like those shown on Wikipedia, a telephone operator determined what circuit you needed to be connected to, made the connection, and your signal only went on that circuit. That's why we don't use hubs any more: we use switches, which do what the operator did. 
   
   A lot of other topics are covered by the physical layer of the OSI model. In the chart below, you can see that this layer has more topics that any other. We will talk about them more as we go along.
   
   
2. The Network Interface Card (NIC) can be used as a reason to go to the Data-Link layer. Network cable connects to the NIC, which connects a computer to the network. NICs belong on the Data-Link layer because they have addresses that are hard coded (burned in) to them. This kind of address is also called a physical address, but that does not place the NIC on the Physical layer. A better name for the address is a MAC address, because the address is used for Media Access Control, which has to do with how devices share the medium. Before we can make them share, we have to tell them apart, so we use addresses. The text shows an example of a MAC address written two ways: as twelve hexadecimal characters with no breaks, and as six pairs of hexadecimal characters with hyphens between them. (Sometimes they use colons instead of hyphens.) The paired format is easier to read, and if you see a lot of them, it makes it easier to notice that the first six characters in a MAC address identify a manufacturer. (Large manufacturers have lots of six character sequences assigned to them.)
   
   Computers and NICs may send signals with electricity, light, or radio waves. From there, we can turn to a new idea: frames. I already said that we break signals into packets. Well, you should know that we also collect data into usable clumps or clusters and call them by different names on different layers. On the Data Link layer, where NICs live, those clusters are called frames. Many frame types have been created over the years. For any two devices on the same network to communicate, they must send and receive frames of the same type. (Devices that connect one network to another can translate frames from one type to another.) One year I ran into several new computers that were configured with a default frame type (802.3) that was not the type our network used. Guess what? Users could not log in to the network on those computers until they were reconfigured to use Ethernet II frames. Once I diagnosed the problem, I told my staff what to do, and it was a ten minute fix for every device that had the problem.
   
   In most networks, every device on a network can see every frame that is transmitted on it. There are exceptions, especially when we start breaking networks into subnets, but in this simple example the statement is true. The point is that a frame is usually addressed to a particular NIC, because frames use MAC addresses. (They hold the MAC address of the sender and the receiver.)  Because of this, only the device whose MAC address matches a frame will process that frame. There are two exceptions to this rule. First, as the author explains, a frame sent to the broadcast address (FF-FF-FF-FF-FF-FF) of a network. will be processed by all devices. That address, by the way, is the broadcast address for frames on any network, not just a particular one. In the second case, a network admin may set the NIC on device to work in promiscuous mode, which means that it processes all frames, which is useful in monitoring activity on a network. 
   
   Regarding the broadcast MAC address, that address can be used to make a general request to all devices on a system, asking them to respond with their MAC addresses and some kind of device name. There are several systems of naming, which we will see in a later chapter.
   
   The Data Link layer used to be the only OSI layer with sublayers. (Wireless networking has caused us to add sublayers to the Physical layer as well.) The sublayers are the MAC sublayer and the LLC sublayer. Several topics that belong there:
   

   MAC sublayer Logical Topology - 2 methods: Bus - passes frames to all devices at once Ring - passes frames from one device to the next in a circular path Media Access - 3 methods: Contention - devices transmit when they need to, if the line is clear Token Passing - devices take turns transmitting Polling - devices are asked if they need to transmit Addressing - 1 method: Physical Device Address - the MAC address LLC sublayer Transmission Synchronization - 3 methods: Synchronous - devices send markers for signal timing in each conversation Asynchronous - devices send markers for signal timing in each frame Isochronous - devices use a common network timing signal Connection Services - 3 methods: Unacknowledged Connectionless - no guarantee of delivery Connection Oriented - guaranteed delivery Acknowledged Connectionless - usually point-to-point, so connection services not needed  Data cluster type: Frames

   
   
   
3. When the world was new and there were only four computers that were about to be connected to what would become the Internet, the kind of networking that only used layers 1 and 2 may have been enough. 
   
   When it was first turned on (1969), the ARPANET connected computer networks at only four locations: UCLA, Stanford University, UC Santa Barbara, and the University of Utah. When the first message was sent on it, the connection failed before the first word was completely sent. Things got better. 
   
   As soon as it became a goal to connect separate networks together, the ARPANET planners knew it would be necessary to use a method that named networksas well as the devices on them. Several methods of accomplishing this have been devised by different vendors. The method that has become dominant is the one that is used on the Internet, IP addressing.
   
   In this section about the Network layer, the author tells us that TCP and IP are only two protocols out of a much larger suite of protocols. Internet Protocol (IP) is used for an addressing scheme that includes a reference to an individual device, and to the network it is on. IP lives on the Network layer, Layer 3. On an IP network, each device (node) is known as a host, and every host must have an address. 
   
   The addresses we discuss first are actually IP version 4 addresses. (IPv6 addresses will be 16 bytes, or 128 bits long.) IP version 4 addresses are numeric addresses, stored as four bytes, which is equal to 32 bits. For example: an IP v.4 address might be 10.45.17.122. Each of the four numbers is held on one byte, which means no number can be bigger than 255. IP addresses contain two parts: one part of the address identifies the network a host is on, and the other part identifies the host itself. Every network is assigned an address which could take up 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. (It gets more complex: this is how we start.) 
   
   In the example above, the 10 (in the first byte) might be the network identifier, or it might be the 10 and the 45 (in the first two bytes) or it could be the 10, the 45,and the 17 (in the first three bytes), depending whether we are treating this network as a class A, B, or C network. Or we could treat it as a classless network, in which case it gets messy. We'll worry about that later.
   
   IP addresses, and any addresses associated with the Network layer, are logical addresses. This means they are not permanently associated with a piece of hardware like a MAC address and a NIC. A logical address is assigned to a device, by an administrator, by a user, or by a network device assigned to do so. The text shows a picture of a router on page 24, which appears to be a typical consumer device you might buy from most electronic stores. This is an example of a device that would assign an IP address to any other device that is connected to one of its switch ports. It does so because it acts like a switch (connecting devices on a small network), like a router (connecting your network to your Internet Service Provider's network), and like a Dynamic Host Configuration Protocol (DHCP) server, which is a device or program that assigns IP addresses to devices on a network. The DHCP service makes note of the MAC address of each device it gives an IP address to, to make sure it does not give out the same IP address to two currently connected devices. Giving the same address to two devices would keep at least one of them from being able to use the network.
   
   The text finally mentions the word packet, and tells you that a packet is a message unit that is used on the Network layer. In fact, the correct word for the Network layer is datagram, but the author apologizes in a marginal note on page 27, stating that he is using the word packet in a generic way. His point is that there is a Network layer message unit inside each frame. He does not explain why. I am beginning to imagine our author, a deserted building, and a pair of pliers. Okay, I feel better now. Let me explain something.
   
   Imagine the diagram below as the stack of protocols being used to send a signal out onto the Internet. 
   
• As I prepare this signal to go, I start at the Application layer, where the message is packaged by Application layer rules, then passed down to the Presentation layer.
• The Presentation layer receives the message,  repackages it as needed by its rules, keeping the information from the Application layer inside the packets it makes, then hands its packets off to the Session layer.
• The Session layer negotiates a connection with the next machine it needs to send to, which it does while it takes the received Presentation packets and repackages them as Session packets. These are handed off to the Transport layer.
• The Transport layer continues the pattern: add your magic, wrap it around the received packets, and put them all in your own message units calledsegments. The segments are handed off to the Network layer.
• The Network layer continues: it does its thing, adds IP addresses for source and destination, rewraps the segments as datagrams, and hands them to the Data Link layer.
• The Data Link layer does not change what is in the datagrams, but it adds MAC addresses for source and destination. (Some real magic happens here. If the author never gets to it, I will tell you later.) The datagrams are rewrapped as frames, and they are pushed to a network on the Physical layer.
• The Physical layer takes the frames, which are perceived as a stream of bits, moves them as needed to the next device, again and again, until the stream is processed by a NIC on a receiving machine, which may be the final destination or a router along the way.
  
  That's what happens, from layers 7 through 1, in the machine sending a message. On the final destination machine, the received message is processed through the layers from layers 1 through 7, until the message is received by a program that knows what to do with it. That is why there are IP packets inside the frames that the Network layer opens. They were put there by the Network layer processes of the sending machine. And this is why we usuallyexplain this process from the top down instead of from the bottom up.
  
  
4. Layer 4 is the Transport layer. As I have mentioned, its data units are called segments, and one of the processes of this layer is called segment development. What that means is actually simple: large messages that won't fit in one segment are broken down and the pieces are placed in two or more segments. Sometimes a message is very small, in which case the segment it is placed into would not be full. Segments are required to be full, so extra bits are generated to be used as filler.
   
   The text tells us that the segments of a larger message are given numbers so they can be reassembled at their destination. This is not unique to this layer. Any layer that packages things into packet does the same thing.
   
   The text does not mention that the TCP protocol operates on the Transport layer, which makes this layer associated with the word reliable. The author almost says this in the last sentence in this section. What he means to say is that if a packet is lost or received in a damaged state, a replacement copy of the packet is requested. This is one aspect of reliable, guaranteed delivery.
   
   
5. Layer 5 is the Session layer, which the text explains as being useful when any device is doing more than one thing at a time on the network. Have you ever had two browser windows open at once? When you click something in one of those windows (or tabs), how does the computer know where to put the response to that click? Each of those windows is assigned a different session ID, which is used in any requests that are sent from it. This assignment of session IDs takes place for other kinds of connections as well, for any program that establishes a connection to a service across a network.
   
   
6. Layer 6 is the Presentation layer, which our author seems to think does nothing, since all files are stored in common formats in the 21st century. I think the author and I had different teachers for this course. Files can still be stored by different methods on mainframes as opposed to PC based servers, bytes can still be sent across a wire most significant digit first or last, and most importantly files can be encrypted. Encryption services live on the Presentation layer.
   
   
7. The Application layer is layer 7, the top layer in the OSI model. The author makes the point that this layer is about the network interfaces that exist so that application programs can use network services, like file service, print services, and message services. 
   

You should know some things about hubs, switches, and routers.

Think about it like this. A switch is a networking device: it allows hosts to connect to your network. A router is an internetworking device: is allows your network to connect to another network. A hub does a job that is similar to what a switch does, but it does not allow more than one transmission at a time across all the devices connected to it.

• 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, called a Source Address Table (SAT). 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 switches increase the bandwidth inside a network by connecting only the devices that need to be connected at any given moment.

LabSim videos discuss the idea that a router receives a frame from its local network, unwraps the frame and examines the Network layer information to determine whether this message has to stay on this network or be passed on to another one. We discussed this last week. The router clears the MAC addresses from the frame, reads the information in the IP addresses, writes what needs to be written in the frame, and passes the frame to another router, or a switch if the network address is local.

Let's continue with a discussion of routing tables. A router keeps a table that is really a set of rules. It says what to do with incoming packets, based on the network addresses in those packets. The rules in this table can be set set automatically, set by an administrator, set by a protocol, or can be set by all three methods. Each line has a rule that describes some kind of packets, and a route to use for packets that are described by that rule. an example in a previous text was  meant to explain a routing table used by a home router. That router only referenced two routes on three rules. The table shown in the text had three lines and four columns. It looked like this:

To understand the table you need to know several things:

• A router automatically puts a rule in its routing table for every network it is attached to. This is what happened on the first two lines.
• The first line represents the home network this router is part of. It says, "If I see a message for an address on network 10.12.14.0/24, I do not need to hand off to another router, and I will pass it as a frame to my LAN port."
  If the message being examined is not described by this rule, the next rule is evaluated.
• The second line represents the ISP network that this router is part of. It says, "If I see a message for an address on network 76.30.4.0/23, I do not need to hand off to another router, and I will pass it as a frame to my WAN port."
  If the message being examined is not described by this rule, the next rule is evaluated.
• The third line has a new meaning for 0.0.0.0. The rule means, "If I see a message for an address on any network, I will pass it to the gateway router on my ISP's network, as a frame to my WAN port." 
  This rule is only evaluated if the message being considered did not meet the requirements of either the first or second rule.

The first two rules are placed in the table automatically when those two networks are detected. They essentially say to pass along any message for hosts on thosenetworks on those networks. No other router is mentioned in the Gateway column for those rules. There is no next hop, because the message is already being handled by a router on the correct network. On the third rule, a catch all filter is used: for a host on any network, the next hop is my ISP's gateway, which is on the network connected to my WAN port. This is the router's default rule: unless I have already told you otherwise, do this with any packet you see.

All hosts on an IP network keep routing tables as well. These commands will show a device's routing table, providing it is running one of these three operating systems:

Most texts spend a great many words telling you that all the possible IPv4 addresses have been assigned. Even so, once upon a time, if you were setting up a network you would make an application to IANA to get some. Now, it is easier: you set up a private address network, and use Network Address Translation to connect to the Internet through your ISP. Variants of NAT:

• basic NAT - each device on your network could be assigned a specific public address to use; the drawback to this is that it does not provide any economy: you still need as many public addresses as you have private addresses; might be called Source NAT, Destination NAT, Static NAT, Dynamic NAT, or pooled NAT
• Port Address Translation (PAT) - in this version, your private addresseees all share one public address, but each is assigned a port number so the routers can tell which device the responses are for; this works for sessions that originate inside your network, but not for sessions that originate outside it
• Port Forwarding - for incoming traffic, requests are examined and mapped to predetermined local addresses (e.g. a request for a web page goes to your web server); specific ports can be used in URLs to route traffic more automatically

In the chart below, the arrangement of wires for the standard known as TIA/EIA 568B is shown. In an alternate standard, the TIA/EIA 568A standard, 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. (See below.) A crossover cable is used to connect directly from one NIC to another, or from one networking device to another. A rollover cable is used to connect to a Cisco router's console port.

Most references forget to tell you the reason you do it 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 thereception 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 is how it works.

These two standards are illustrated in the text on page 76. These illustrations are kind of backwards. This is how you might see the wires if you were looking at the connector from the side with the clip on it, which no one would do, because it is much easier to see them from the other side.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.

UTP cables are usually connected to devices with RJ-45 connectors. In the enlarged picture on the right, note the eight gold-colored connections for the eight wires usually found in UTP cables. Note also the clamp in the connector that grabs the cable where it is covered by its outer insulator. 

If you insert the eight wires into the connector and the outer insulator does not extend past the clamp, pull the wires back out, trimthem as needed and try inserting again. (I like the scissors on a Swiss army knife.) If everything is in the right spot, then you can carefully put the connector into a crimper and squeeze hard. 

The insulation shown in the graphics above should NOT be stripped back on these wires.

Straight-through (standard) cable If you are making a straight-through cable (to run from a workstation to a hub or switch) connect both ends as listed above and shown on the right. Insert the wires into the RJ-45 connector, then crimp with the crimping tool. (There will be no spaces between the wires when they are inserted into the RJ-45 connector. Space is used here to make the color pattern more readable.)	End 1 568B
	End 2 568B
Crossover cable If making a crossover cable (to run directly from one NIC to another) swap the orange and green circuitson one end only: put orange/white on 3, orange on 6, green/white on 1, and green on 2. Insert the wires asshown on the right, then crimp. (This second configuration is actually EIA/TIA 568A.)	End 1 568B
	End 2 568A
Rollover cable Now, for something completely different, if you are making a rollover cable (to run from a workstation to anolder Cisco router), prepare the cable like a standard cable, both ends in the same configuration.  Before crimping the second end, roll the cable (or the RJ-45 connector) over, 180 degrees. That will make pin 1 on one end of the cable connect to pin 8 on the other end, pin 2 to pin 7, pin 3 to pin 6, and pin 4 to pin 5. If you don't want to think about rolling anything over, insert the wires as shown on the right, then crimp. This cable is used with an adapter to connect to a Cisco router's console port. NOTE: A rollover cable can also have an RJ-45 on one end and an RS-232 DB-9 connector on the other end. This is useful for connecting to a laptop/server/PC that has an open serial port but no open NIC port.	End 1 568B
	End 2 568B, RJ-45 turned over

• Simplex - this is communication in one direction, more like a monolog than a dialog. It is like a public speech or a television transmission.
• Half-Duplex - this is a dialog that can flow both directions, but only one direction at a time. After one side transmits, the channel has to be "reversed" for the other side to transmit. It is like CB or Ham radio, using only one frequency at a time and taking turns.
• Full-Duplex - this is a dialog in which both sides can transmit and receive at the same time. It is like a telephone conversation, in which both sides have a live speaker and microphone.

Usually, NICs negotiate the level of service for this when a session is established. You can hard code a NIC to use only one type of conversation, but this is usually a way to make fail rather than a way to make it work better.