The author begins chapter 4 with a history lesson that does not mean much without a context. This chapter is about Ethernet, a set of rules for running a network that was invented at Xerox PARC, their Palo Alto Research Center. Most histories give credit to Bob Metcalf as the inventor.Let's discuss some facts about the subject that relate to the chapter.
Back in my notes for NET 101, I gave the students a definition of networking. Networking can be defined with five features:
Somewhere along the way, you are expected to learn that networks typically act like client-server or peer-to-peer networks. Let's review those ideas:
If entities on a network act as peers, sharing all their local resources with each other, then this is Peer-to-Peer Networking.
If entities act in strictly defined roles, as either servers or clients, but not as peers, then this is Server-Centric (Client-Server) Networking. Most PC networks are this type because peer-to-peer does not work well if you have more than ten hosts..
Most networks follow a client/server model. Clients typically perform some or most of the processing on the network, while servers provide services like data storage, instead of providing all the computing power. Client/server networks are typically easier to upgrade, both on the client side and on the server side.
The text move on to physical topologies. Topology is the study of shapes and configuration. Physical topology is the way a network is wired (or wireless-ed?) together. Logical topology is the way it works, regardless of the wiring. Network configurations typically fall into one of these types:
The author has mentioned hubs several times, and uses the term generically most places in the text. Think of a hub as being like a power strip, but instead of devices pulling power from it they are sending network signals into it, so the other devices plugged into it can see those signals.
On page 68, the author starts describing what actually goes in an Ethernet frame. He tells us it has seven parts. We talked about four of them in the last lesson when he described a generic frame. Let's look at the seven pieces of an Ethernet frame:
On page 71, the author gets around to discussing a topic that belongs to the MAC sublayer of the Data-Link layer of the OSI model: media access. He has mentions a couple of times that if you were using a star-wired network, concentrating the connections with hubs, only one device at a time can transmit. This is because any signal going into a hub comes back out on all the ports, except the one it came in on. Sort of like turning on a water spigot that connects to three different sprinklers: they all spray water. That being the case in early Ethernet implementations, there had to be a way to make the devices wait until no one else was transmitting.
There are three classic methods. The author only discusses one, which is the only one used in most networks. To be complete, these are the methods:
Media Access - 3 methods:
Many operational standards have been created by the Institute of Electrical and Electronic Engineers (IEEE) or the American National Standards Institute (ANSI). Standards are often referred to by a number. Some of the IEEE 802.x standards (there are about a dozen and a half, currently) are LAN protocols. 802.3 specifies the CSMA/CD access method, so this is often thought to be the Ethernet standard. 802.3 was based on Ethernet, but it is a more general standard. Ethernet can be thought of as one implementation of the 802.3 standard.
So , what's the deal about contention? Contention systems work by letting each device try to
send a message on the net as needed, contending or competing
with all the other devices for the bandwidth. Two examples of methods
that support such systems are CSMA/CD (Carrier Sense,
Multiple Access, with Collision Detection) and CSMA/CA (Carrier
Sense, Multiple Access, with Collision Avoidance). A collision
occurs when two signals collide (are sent at the same time) on the
medium, causing signal loss. These protocols best support intermittent
transmissions. Time sensitivity is good, as users do not often have to
wait for media access.
The author discusses collision
domains on page 72. A collision
domain is a circuit/channel on which a collision can occur. If you only
had one device on a given circuit, you would think there would be no
danger of a collision, but there still is because devices send and
receive. This is usually not a problem, because transmissions are
typically short. It becomes a problem when you have too many devices on
a single collision domain, so the goal is often increase the number of
collision domains, reducing the number of devices on each one, which
improves throughput. In a star-wired network, each device wired to a switch is usually considered to be in its own collision domain.
On page 73, the text discusses 10BaseT networks, and it puzzles me that the author persists in not defining the term. He finally tells you that 10BaseT means 10 Mbps, baseband transmission, and UTP cable. Baseband means the cable carries one signal at a time, which is the cause of the collision domain problem.
Although many 10BaseT LANs were put together with Cat 3 (category 3) UTP cable, no one would do so now with cable less than Cat 5. A 10BaseT network uses UTP cable, and RJ-45 connectors. Only four of the eight wires in the cable are actually used by the network. In the wiring table below, it would be the orange and green circuits that are actually used.
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
instead of however you might like. Read the last two columns in the
chart below. Pins 1 and 2 are
used for the transmission
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 the reception 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.
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
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
The text describes two kinds of Ethernet, both of them old standards:
The text continues with a section about networking equipment.
Now, a problem: a bridging loop. First, you need to know that redundant bridges can be put between segments, in case one breaks. A bridging loop can be created when packets pass endlessly from one segment to the next across the two bridges. It can also happen if the bridges generate a broadcast storm of new packets. An example: Assume two segments are connected by two bridges. A frame is generated on Segment A from workstation W1, and hits both bridges. Both bridges copy the frame, learn that W1 is on Segment A, and both forward the frame to Segment B. However, each bridge will receive the copy that the other bridge forwards to Segment B. This will cause the bridges to update their databases to show workstation W1 as being on Segment B, and they will forward each these frames back to Segment A. Then the process repeats, again and again and again. This is not good.
To avoid the bridging loop problem, IEEE (Institute of Electrical and Electronics Engineers) standard 802.1d gives us the spanning tree protocol. This says that in each redundant pair of bridges, one is the designated bridge, and the other is the backup bridge. The backup bridge in each pair does not forward any frames unless it determines that the designated bridge is down. Switches also use this protocol to avoid bridging loop errors.
Chapter 5 continues the discussion of Ethernet networks with
several standards that followed 10BaseT. The chapter mentions
100BaseT4, which used four circuits of a Cat 3 cable, as a contender
that was not successful. These are the first two standards that it
discusses more fully:
The text discusses some of the process to change from one network type to another. Changing your cable may not be necessary, if your existing cable is compatible. Changing all the NICs you own may be necessary, as well as changing all your switches, routers, and hubs that are not compatible with the new standard. Any time you send signals through a circuit that includes a slower device your throughput will downgrade to that lower bandwidth. This principle applies to any major change of network equipment.
In the discussion of 100BaseFX, the text brings up reasons to use fiber optic cable instead of UTP. It does not mention the most obvious reason: greater bandwidth. In this example, the bandwidth is the same as 100BaseT. There are, however, three differences mentioned. Fiber optic has a longer segment length (2000 meters in this case, fiber optic is immune to electromagnetic interference (UTP is not), and fiber optic is much more resistant to eavesdropping.The text also mentions an idea the belongs on the session layer of the OSI model: three types of sessions (simplex, half duplex, and duplex) but does not define them all.
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.
The next topic covers several types of Gigabit Ethernet, which is more fun to say than 1000-Base-Whatever. The most useful part of the discussion is the chart on page 96, which is more useful when paired with the comments on page 95.
The text discusses the various connectors used with fiber
optic cable. This site
presents pictures and descriptions of many more types than the text.
As you can see on the Fiber Optic Association guide (link above), there are adapters that can allow you to connect from one of these fiber technologies to another.
When connecting equipment with different kinds of jacks, it is important to keep track of which end of a cable goes where. For example, if you have an ST-LC cable, the LC end may be color coded yellow on one strand and white on the other. The yellow strand is referred to as A, the white strand as B. On the other end of that same cable, the colors may be red and black. Which is which? Red is the A strand, so it is the other end of the yellow strand. Black is the B strand, so it is the other end of the white strand. In the chart below, the Column A represents two ends of the same cable. Column B represents two ends of the other cable.
The text on that web site also lists several adapters that will allow you to connect fiber to other cable types: UTP, STP, and coaxial cable.
The next performance level described is 10 Gigabit
Ethernet (10 GbE),
which is obviously 10 gigabits per second throughput. The text tells us
this is a newer technology. The wikipedia article behind the link in
the last sentence shows that several years later, there are still many
versions striving to be standards. The text tells us that there are two
copper wire standards, the web article lists four. This area pretty
volatile to use as exam material. Note the standards listed on page 98.
Several end in R, while others end in W. The W is a WAN (Wide Area
Network) indicator, the R is an indicator of a LAN (Local Area Network)
standard. Standards that end in W are also for connecting to SONET equipment, which is an
ANSI standard for optical systems.