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CIS 303a - Computer Architecture
Chapter 9, Computer Networks
Objectives:
This lesson discusses networks from a different perspective
than usual. Objectives
important to this lesson:
- Logical and physical topologies
- Message forwarding
- Media access control
- Network hardware
- OSI network model
- Internet architecture and protocols
- IEEE physical network standards
Concepts:
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:
Let n represent the number of nodes.
Let c represent the number of connections to
connect those nodes in a fully connected mesh.
c = 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.
- 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.
- 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
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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.)
- 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 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 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.
- 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.
- 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.
- 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.
- 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)
|
Application
(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
|
Presentation
(layer 6)
|
- Translation - bit translation, byte translation,
character code translation, file translation
- Encryption - cipher, private key, or public key
- Data cluster type: Packets
|
Session
(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)
|
Transport
(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)
|
Network
(layer 3)
|
- Addressing - network addresses. 2 methods:
- 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:
- 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:
- 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) |
Physical
(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:
- Bandwidth Usage - 2 methods:
- 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: 204.16.142.27
- 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
10.25.0.0 to a host on network 10.28.0.0. 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 |
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- 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.
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