CIS 303a - Computer Architecture

Chapter 9, Computer Networks


This lesson discusses networks from a different perspective than usual. Objectives important to this lesson:

  1. Logical and physical topologies
  2. Message forwarding
  3. Media access control
  4. Network hardware
  5. OSI network model
  6. Internet architecture and protocols
  7. IEEE physical network standards
Chapter 9

The chapter begins with the statement that a network topology is about the devices used on a network and how they are arranged and connected. Topology is the study of shapes and configurations. Physical topology is the way a network is wired. Logical topology is the way it works, regardless of the wiring. What the text is describing on page 330 is more properly called the physical topology of the network.

Physical network configurations typically fall into one of these types:

The text discusses four of them, but only one is commonly used in wired networks..

  • Bus - A bus works in a baseband mode, which means that every transmission can be received by all devices at about the same time. This is not the same thing as a broadcast. (page 332)
    • Most devices ignore a transmission unless it is addressed to them, but in a bus/baseband environment, there can be only one transmission on the wire at any given time
  • Ring - like a daisy chain, going from one station to the next and all the way back to the server or the first node in the ring; (also shown on page 332)
    • transmissions are passed from one device to the next, each adding a delay to the time it takes to get a transmission
  • Star - starting at hubs or switches with individual cables radiating away from them for each node, like the picture on page 333
    • All devices are connected to a central node (page 334) which may be a hub, a switch, or a router. There can be any number of hubs, switches, and routers in a network: the author's use of the term central node does not mean there is only one.
      Units affected by media failure: One, unless it is the hub/switch/router, in which case all nodes are affected. This is what the text will eventually call fault tolerance.
  • Mesh - redundant connections, so the network will survive in case a few cables are broken;
    • in the image above, the Mesh example is a partial mesh
    • in the image above, the Fully Connected example is a fully redundant (fully connected) mesh (see the first example on page 331)

    • The text gives us a formula to calculate the number of connections needed to fully mesh any number of nodes. The formula below is more useful than the one in the text:

      n represent the number of nodes.
      c represent the number of connections to connect those nodes in a fully connected mesh.

      = n * (n - 1) / 2

      Example: If
      n is equal to 10 nodes, we multiply 10 (which is n) times 9 (which is n - 1), and divide by 2, which gives us 45 connections.

    • units affected by media failure: Few or none
    • commonly used in wireless networks; think of cell towers and cell phones

The author begins his discussion by showing you a mesh topology. In a fully connected mesh, every device is wired to every other device. When Mr. Bell started his company, phones were like that. You had to have a direct connection to another phone to call it. As the text points out, building a network this way becomes difficult to do and difficult to maintain. This is why other methods are used for networks of any size. In the example above, if we were to mesh connect 10 computers, each would need 9 network interfaces, and we would have to run 45 cables.

Early networks often used bus or ring topologies, but star-wired networks are the most common now.

The text uses a generic term for hubs, routers, and switches: central nodes. Some texts call central nodes concentrators,because they are points where many of your networking cables come together (concentrate) to connect to the network. Obviously, the main purpose of central nodes is to provide a connection to a network. Another way to look at it is that a central node's purpose is to provide a means to forward messages toward the devices the messages are meant for. Some devices pass along signals immediately, and others work in a store and forward mode, holding the message until a line is available, or until the message is regenerated to remove line noise. The storage time may not be long in human terms.

The text backtracks a bit on page 334, as it tries to define the difference between a Local Area Network (LAN) and a Wide Area Network (WAN).

  • A LAN may only cover one room in an office building or a school, or it may cover all the nodes in one building or several buildings. Hubs and switches are typically used to connect devices to a LAN.
  • A WAN will typically use routers and a different set of protocols to connect LANs together across greater distances, such as from one city, state, or country to another.
  • In the diagram on page 334, the text shows us four LANs in two buildings, all on the north part of a campus, and each having a single central node, probably a switch.
  • The central nodes in each of those LANs connect to another node, which might be a router or a switch, joining all the LANs into one zone. The text calls the common node connecting the LANs together a zone central node.
  • The diagram includes three zone central nodes that service three geographically separate zones. Zones may be defined by geography, as in this example, or they may be defined by business needs. It is common to have one zone for your assets that are available to the public (e.g. Internet interfaces) and another zone for secure internal assets.
  • Each of the zone central nodes connects to another central node that connects the zones to each other. The text calls that new connecting node the campus central node. By this time, you should be sick of the term "central node" or wonder strongly if there is a better word for the concept. Hang in there, and remember that each central node is simply the center from its own point of view, the center of its star.

Think about the path, or route, a message would have to follow to go from a node in a north campus LAN to a node in a south campus LAN. We can see on the diagram that the northern node would pass the message to its local central node, which would pass it to its zone central node, which would pass it to the campus central node, which would then pass it down to the south zone node, which would pass it to the correct south campus LAN central node, which would finally pass it to the destination node. The text asks us to consider how the nodes figure that out. At this point the text could make the answer clearer by forgetting its generic language, but it does not.

The text wanders around several concepts without giving you a context in which to understand them for several pages. Come with me for a few minutes, and I will try to make the points more clearly.

The ISO-OSI network model is a logical (as opposed to physical) model. It explains how networks handle passing information from one node to another and how they perform other useful functions. The text calls it the OSI, or Open Systems Interconnect model. ISO, the International Organization for Standardization, is a 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 OSI model has seven layers, which are used to separate the things that happen on a network into more digestible pieces.

The OSI Model's seven layers:

  • 7 - Application
  • 6 - Presentation
  • 5 - Session
  • 4 - Transport
  • 3 - Network
  • 2 - Data Link
  • 1 - Physical

So, what's in each layer? The things that a network device does must fit the topics, the operational functions, of a layer for the protocol to belong/live/run on that layer. See the list of layers and topics in the chart below. The chart shows methods, too. A topic is a thing we do on a network. A method is how we accomplish the goals of a topic.

So, the author eventually starts explaining the model by telling you some of the things associated with its layers.

  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). One of our other authors 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. Hubs don't care about the identity of a device, so they belong on the Physical layer.

    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 than any other layer. We will talk about them more as we go along.

  2. The author discusses Network Interface Cards (NICs) on page 338. They belong on 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 (page 336). Before we can make them share, we have to tell them apart, so we can use MAC addresses to do that. A MAC address is often written in one of two ways: as twelve hexadecimal characters with no breaks, or as six pairs of hexadecimal characters with hyphens or colons between them. 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.) I just checked my MAC address with the IPConfig utility, and saw that it begins 5C-26-0A. Google that, and tell me what kind of computer I am probably using. NICs, and the computers that use them, may send signals with electricity, light, or radio waves.

    All signals sent on a network are broken into numbered pieces, commonly called packets. You should also know that we also collect data into usable clumps or clusters. Each layer of the OSI model may use the word packet or may use a different word specific to that layer. The signals sent by a NIC are sent in packages 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 another frame type, 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.

    The author tells you that 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 his simple example the statement is true. His point is that a frame is usually addressed to a particular NIC, because frames use MAC addresses. (Each frame includes 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, a frame might be sent to the broadcast address (FF-FF-FF-FF-FF-FF) of a network. This one 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 administrator may set the NIC on a 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 is the only OSI layer with sublayers. They are the MAC sublayer and the LLC sublayer. The critical functions of each sublayer, are described below:
    Data-Link Layer
    • 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

    Back to the text for a bit, the author tells us on page 337 that most networks use baseband transmission, which is a Physical layer method. Baseband transmissions only work if we have only one signal on the medium at a time. Two or more signals sent at the same time will interfere with each other, causing a signal collision, which deforms the signal and makes it unusable. Three Media Access methods of overcoming this problem have been tried and found useful, polling, token passing, and contention. The text describes two versions of the only one in common use: contention.

    So, what's the deal about contention? Contention systems work by letting each device try to send a message on the network 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). These methods support intermittent transmissions better than transmissions that need to continue for a long time. 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 nanoseconds before sending again. This usually results in one device going ahead of the other.

    In a CSMA/CA system (example: wireless devices), devices can be assigned time slices or (as described in the text) they can be required to ask permission to send, avoiding collisions. In the example on page 337, wireless devices must contact a wireless access point (WAP), send a ready-to-send (RTS) signal before sending any data, and may not continue until they are sent a clear-to-send (CTS) signal from the WAP. As the name of the method implies, this does not prevent collisions, but they happen less often this way.

    NICs and switches care about hardware addresses, which puts them on the Data-Link layer. (Some switches do more that that, but that's another story.)

  3. When it was first turned on (1969), the ARPANET (early version of the Internet) 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 networks as 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 the section about the Network layer, the author tells us that TCP and IP are two protocols out of a much larger suite of protocols, generally called the TCP/IP suite. 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, every host must have an address, and so must its network.

    The addresses we discuss first are actually IP version 4 addresses. IP version 4 addresses are numeric addresses, stored as four bytes, which is equal to 32 bits. (IPv6 addresses are 16 bytes, or 128 bits long.) For example: an IP v.4 address might be 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 network identifier might be the 10 in the first byte, 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 on 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. Routers are the devices most associated with the Network layer. They can be typical consumer devices you might buy from most electronic stores, or much more complicated devices used on large networks. A router designed for home use typically includes several ports for connecting local devices, which allows it to act like a switch. Even a low end router can assign an IP address to any other device that is connected to one of its switch ports. It acts like a switch (connecting devices to 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.

    On page 341, the text mentions that a Wireless Access Point connects a wireless network to a wired network. Thinking about it that way, you should see that the WAP acts like a home router: it can allow devices to connect to its network (like a switch), it can assign IP addresses to those devices (like a DHCP server), and it can connect its network to another network, which is the primary function of a router.

    The text also mentions that routers exchange information with each other about the paths to other networks that they know. How they do this and how often the do it vary with the protocol being used by the routers. For instance, they may send their entire routing tables (their lists of known routes) to each other, or may send only changes to those tables, on a fixed schedule or as changes occur.

    Imagine the diagram below as the stack of protocols being used to send a signal out to the Internet.
    • As I prepare this signal to go, I start at the Application layer, where the message is created in the application I am using, 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 called segments. 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 usually explain this process from the top down instead of from the bottom up.

  4. Layer 4 is the Transport layer. Its data units are called segments, and one of the processes of this layer is called segment development. Segments are all the same size. 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 in would not be full. Segments are required to be full, so extra bits are generated to be used as filler.

    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 packets does the same thing.

    The text does not mention at this time that the TCP protocol operates on the Transport layer, which makes this layer associated with the word reliable. 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 supports devices that do 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. The text incorrectly states that the Session layer is where encryption takes place, which is critical for secure purchases and data transfers. Different kinds of encryption can take place on other layers, but typically they do not belong on this one.

  6. Layer 6 is the Presentation layer, which manages differences between file types, character codes, and other differences from one system to another.  Files can be stored by different methods on mainframes as opposed to PC based servers, bytes can 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 in the OSI model.

  7. The Application layer is layer 7, the top layer in the OSI model. This layer is about the network interfaces that exist so that application programs can use network services, like file services, print services, and message services.

In the chart below, the first column shows you another model, the Department of Defense model, and how its four layers relate to the OSI model.

  • The DoD model is called the Internet model by some texts, and may appear on the Network+ certification test.
  • Your text is the first book I have seen that discusses a new version of this model, which it calls the TCP/IP model. The name is not new, but the change in the number of layers is new. If you search long and hard, you may find references to this updated version of the model, which now has five layers.
  • The original DoD model had four layers, but the newer TCP/IP model has split the old Network Access layer into the Physical and Network Interface layers. They correspond to the Physical and Data-Link layers of the OSI model, which gives the TCP/IP model five layers. (Maybe in a few years it will catch up to the OSI model.)
DoD name
(and TCP/IP name)
OSI Layer name
Topics & Methods
DoD: Process/Application layer
(TCP/IP: Application layer)
(layer 7)

  • Network Services
    • File services
    • Print services
    • Message services
    • Application services
    • Database services
  • Service Advertisement - how services become known
  • Service Use - how services are obtained
  • Data cluster type: Messages
(layer 6)

  • Translation - bit translation, byte translation, character code translation, file translation
  • Encryption - cipher, private key, or public key
  • Data cluster type: Packets
(layer 5)
  • Dialog Control - simplex, half-duplex and duplex
  • Session Administration - connection establishment, data transfer, and connection release
  • Data cluster type: Packets
DoD:Host-to-Host layer
(TCP/IP: Transport layer)
(layer 4)
  • Address/name Resolution
  • Addressing
  • Segment Development - breaking large messages into segments,
    combining small messages into segments
  • Connection Services
  • Data cluster type: Segments
DoD: Internet layer
(TCP/IP: Internet layer)
(layer 3)
  • Addressing - network addresses. 2 methods:
    • Logical Network
    • Service
  • Switching - route creation for packets, messages and circuits. 3 methods:
    • Packet switching
    • Message switching
    • Circuit switching
  • Route Discovery - finding a route. 2 methods:
    • Distance vector
    • Link-state
  • Route Selection - choosing a route. 2 methods:
    • Static
    • Dynamic
  • Connection Services - flow control, error control and packet sequence control. 3 methods:
    • Network-layer flow control
    • Error control
    • Packet sequence control
  • Data cluster type: Datagrams
DoD: Network Access layer
(TCP/IP: Network Interface layer)
Data Link
(layer 2)

  • MAC sublayer
    • Logical Topology - 2 methods:
      • Bus
      • Ring
    • Media Access - 3 methods:
      • Contention
      • Token Passing
      • Polling
    • Addressing - 1 method:
      • Physical Device Address - the MAC address
  • LLC sublayer
    • Transmission Synchronization - 3 methods:
      • Synchronous
      • Asynchronous
      • Isochronous
    • Connection Services - 3 methods:
      • Unacknowledged Connectionless
      • Connection Oriented
      • Acknowledged Connectionless
  • Data cluster type: Frames
DoD: Network Access layer; same layer as box above this one
(TCP/IP: Physical layer)
(layer 1)

  • Connection Type - 2 methods:
    • Point-to-Point
    • Multipoint
  • Physical Topology - 5 methods:
    • Bus
    • Ring
    • Star
    • Mesh
    • Cellular
  • Digital Signaling - 2 methods:
    • Current State
    • State Transition
  • Analog Signaling - 2 methods:
    • Current State
    • State Transition
  • Bit Synchronization - 2 methods:
    • Synchronous
    • Asynchronous
  • Bandwidth Usage - 2 methods:
    • Baseband
    • Broadband
  • Multiplexing - 3 methods:
    • Frequency Division
    • Time Division
    • Statistical Time Division
  • No data clusters, just bits

The text moves on to discuss some protocols and IP addresses.

On page 345, the author tells us that the Internet Protocol lives on the Internet layer of the DoD and TCP/IP models, so it lives on the Network layer of the ISO model.

  • IP has two main purposes: addressing and routing.
  • IP is a connectionless protocol: it does not guarantee delivery of packets.
  • IP supports routing (finding paths to networks), fragmentation (breaking data into numbered pieces), and reassembly (reassembling the pieces into usable data).
  • The text describes IP addresses in both IP version 4 (IPv4) and IP version 6 (IPv6).
  • IPv4 addresses are stored in four bytes (32 bits). Like all bytes, each one can hold a number from 0 to 255, inclusive. This is the reason that IPv4 addresses are typically written as four numbers, separated by dots. A classic certification question will ask you to recognize the fact that none of the four numbers can be above 255. Example of the dotted decimal notation:
  • IPv6 addresses are stored in sixteen bytes (128 bits). You may see them written as eight groups of four hexadecimal characters, with the groups separated by colons. Example of the colon hex notation: fe80:0000:0000:0000:355d:847c:1eb4:4943
  • The text points out that if the colon hex address contains one or more consecutive groups that only contain zeros, you can use a double colon to stand for them. The reader needs to understand that the real address must be filled with a sufficient number of zeros to make thirty two characters.
    Example: the address above in abbreviated colon hex notation is fe80::355d:847c:1eb4:4943
  • If the hex notation seems weird, remember that a byte holds eight bits, and the largest number it can hold is 255, expressed in decimal notation. 255 is FF when expressed in hexadecimal notation. Each pair of hex characters in the IPv6 address fills one byte. The classic exam question for this kind of address asks you to recognize that the valid characters in a hexadecimal phrase are 0 through 9 and A through F. No letter past F is valid.

The Transmission Control Protocol (TCP) lives on the Transport layer in the OSI and TCP/IP models, the Host-to-Host layer in the DoD model.

  • The main purpose of TCP is to confirm delivery of good packets by replacing those that are damaged or lost.
  • TCP is a connection-oriented protocol: it guarantees delivery of packets. Most texts associate the word reliable with TCP and the Transport layer.
  • Devices connected by TCP will typically send acknowledgement (ACK) or negative acknowledgement (NAK) signals to show they have received or not received packets.

User Datagram Protocol (UDP) also lives on the Transport (Host-to-Host) layer, but it is connectionless, so it is faster than TCP. Connectionless protocols send their data but they do not check to see if the intended receiver got the data. It is mostly used for jobs that use direct connections between devices, for work with other protocols that have their own error checking methods, and for jobs that send to multiple devices, such as broadcasts.

The text is a bit confusing on page 351, talking about IP addresses and hardware addresses. Think about it like this: in a very small network, as long as you didn't need to pass signals to other networks, the bottom two layers of the OSI or TCP/IP models would be enough, We know the name of each device in those layers, and we can pass signals to them as needed. We need the next layer to find other networks, because MAC addresses only identify individual devices, not networks. The text almost tells you that we use IP addresses when we pass traffic from one network to another, but we change to hardware addressing when the traffic enters the network where the intended recipient lives.

Let's walk through an example of what happens and why it happens, using the OSI model to understand what happens and where.

Routers pass signals from one network to another. Routers use software addresses instead of hardware addresses. This makes them independent of protocols used at lower layers. Almost. Example: a transmission is sent from a host on network to a host on network It could travel along several different routes. What happens is like this:

  • The Network Layer header of the outgoing message has a place to write information about the sender and the intended receiver. We are talking about IP addresses. The sender's IP address is saved in the Network Layer header, along with the IP address for the recipient. This data stays in the Network Layer header until the intended recipient breaks down the header.
    Layer Source info Destination info
    Network layer Sender's IP Receiver's IP
    Data Link layer    

  • The Data Link Layer header also has a place to write down the address of the sender and the receiver, the difference being that this layer uses MAC addresses. Since the intended recipient is not on the sender's network, the sending station sets the Data Link Layer address of the recipient to the MAC address of the router (default gateway) on its network, and sends the message as a frame to that router. If necessary, an ARP (Address Resolution Protocol) broadcast signal is sent, to determine the MAC address of the default gateway router.
    Layer Source info Destination info
    Network layer Sender's IP Receiver's IP
    Data Link layer Sender's MAC Default Gateway MAC

  • The router on the sender's network gets the frame, erases the sender and recipient addresses in the Data Link Layer (the green layer), and decides on a route to the recipient's network (which is written on the header of the Network layer, remember?). The next router in a logical chain is selected. If necessary, ARP is used to find the MAC address of the next router. The next router's MAC address is written in the Data Link Layer header as the "recipient", and the current router's MAC address is written to the Data Link Layer header as the "sender". The frame is forwarded to the next router.
    Layer Source info Destination info
    Network layer Sender's IP Receiver's IP
    Data Link layer Default Gateway MAC Next router's MAC

  • The process in the step above is repeated until a router on the intended recipient's network gets the frame. Then, the final router's MAC information and the receiver's MAC information are written to the Data Link Layer header, and the frame is delivered, where it is unpacked and handed to the IP protocol on the Network layer, and up the stack of layers.
    Layer Source info Destination info
    Network layer Sender's IP Receiver's IP
    Data Link layer Final router's MAC Receiver's MAC

In case that wasn't enough, the author closes the chapter with a list of IEEE standards, and a discussion of some of the important ones.

  • 802.1 - specifies Media Access Control methods
  • 802.2 - specifies the use of headers and frames, supporting the LLC sublayer of the Data-Link layer.
  • 802.3 - specifies the CSMA/CD access method, so this is often thought to be the Ethernet standard.
  • 802.11 - specifies how wireless LANs work; this includes the letter variations of this standard
  • 802.15 - standard for wireless personal area networks
  • 802.16 - standard for Broadband wireless; meant to be WiMAX

As you may know, revisions and extensions to standards are noted with letters appended to the standard number. For example, 802.11a, b, and g are wireless methods in common use, while 802.11n and 802.11ac are newer standards we mentioned last week.

The text mentions in a sidebar that WiMAX is IEEE standard 802.16, actually a set of standards that communities may follow to provide wireless access to their Metropolitan Area Networks. WiMAX 2 will apparently be developed as IEEE standard 802.16m.

On page 355, the text discusses several Ethernet standards. It gives the impression that Ethernet has always used UTP cable, which is not true. There have been standards for coaxial cable, UTP, STP, and fiber optic cable. Higher grades of UTP are the most popular, due to cost, ease of use, and performance.