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Ethernet is a contention media access method that allows all hosts on a network to share the
same bandwidth of a link. Ethernet is popular because it’s readily scalable, meaning that it’s
comparatively easy to integrate new technologies, such as Fast Ethernet and Gigabit Ethernet,
into an existing network infrastructure. It’s also relatively simple to implement in the first
place, and with it, troubleshooting is reasonably straightforward. Ethernet uses both Data
Link and Physical layer specifications, and this section of the chapter will give you both the
Data Link layer and Physical layer information you need to effectively implement, troubleshoot,
and maintain an Ethernet network.
Ethernet networking uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD),
a protocol that helps devices share the bandwidth evenly without having two devices transmit at
the same time on the network medium. CSMA/CD was created to overcome the problem of those
collisions that occur when packets are transmitted simultaneously from different nodes. And trust
me—good collision management is crucial, because when a node transmits in a CSMA/CD network,
all the other nodes on the network receive and examine that transmission. Only bridges and
routers can effectively prevent a transmission from propagating throughout the entire network!
So, how does the CSMA/CD protocol work? Let’s start by taking a look at Figure 1.18.
When a host wants to transmit over the network, it first checks for the presence of a digital
signal on the wire. If all is clear (no other host is transmitting), the host will then proceed
with its transmission. But it doesn’t stop there. The transmitting host constantly monitors
the wire to make sure no other hosts begin transmitting. If the host detects another signal
on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop
sending data (think busy signal). The nodes respond to that jam signal by waiting a while
before attempting to transmit again. Backoff algorithms determine when the colliding
stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting
to transmit will then timeout. Pretty clean!
All devices in the same collision domain.
All devices in the same broadcast domain.
Devices share the same bandwidth.
A B C D
32 Chapter 1 Internetworking
FIGURE 1 . 1 8 CSMA/CD
When a collision occurs on an Ethernet LAN, the following happens:
A jam signal informs all devices that a collision occurred.
The collision invokes a random backoff algorithm.
Each device on the Ethernet segment stops transmitting for a short time until the
timers expire.
All hosts have equal priority to transmit after the timers have expired.
The following are the effects of having a CSMA/CD network sustaining heavy collisions:
Delay
Low throughput
Congestion
Backoff on an 802.3 network is the retransmission delay that’s enforced when
a collision occurs. When a collision occurs, a host will resume transmission
after the forced time delay has expired. After this backoff delay period has
expired, all stations have equal priority to transmit data.
A B C D
A B C D
A B C D
A B C D
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
Collision
Jam Jam Jam Jam Jam Jam Jam Jam
Ethernet Networking 33
In the following sections, I am going to cover Ethernet in detail at both the Data Link layer
(layer 2) and the Physical layer (layer 1).
Half- and Full-Duplex Ethernet
Half-duplex Ethernet is defined in the original 802.3 Ethernet; Cisco says it uses only one wire
pair with a digital signal running in both directions on the wire. Certainly, the IEEE specifications
discuss the process of half duplex somewhat differently, but what Cisco is talking
about is a general sense of what is happening here with Ethernet.
It also uses the CSMA/CD protocol to help prevent collisions and to permit retransmitting
if a collision does occur. If a hub is attached to a switch, it must operate in half-duplex mode
because the end stations must be able to detect collisions. Half-duplex Ethernet—typically
10BaseT—is only about 30 to 40 percent efficient as Cisco sees it because a large 10BaseT
network will usually only give you 3 to 4Mbps, at most.
But full-duplex Ethernet uses two pairs of wires instead of one wire pair like half duplex.
And full duplex uses a point-to-point connection between the transmitter of the transmitting
device and the receiver of the receiving device. This means that with full-duplex data transfer,
you get a faster data transfer compared to half duplex. And because the transmitted data is
sent on a different set of wires than the received data, no collisions will occur.
The reason you don’t need to worry about collisions is because now it’s like a freeway with
multiple lanes instead of the single-lane road provided by half duplex. Full-duplex Ethernet is
supposed to offer 100 percent efficiency in both directions—for example, you can get 20Mbps
with a 10Mbps Ethernet running full duplex or 200Mbps for Fast Ethernet. But this rate is something
known as an aggregate rate, which translates as “you’re supposed to get” 100 percent
efficiency. No guarantees, in networking as in life.
Full-duplex Ethernet can be used in three situations:
With a connection from a switch to a host
With a connection from a switch to a switch
With a connection from a host to a host using a crossover cable
Full-duplex Ethernet requires a point-to-point connection when only two
nodes are present. You can run full duplex with just about any device except
a hub.
Now, if it’s capable of all that speed, why wouldn’t it deliver? Well, when a full-duplex
Ethernet port is powered on, it first connects to the remote end and then negotiates with the
other end of the Fast Ethernet link. This is called an auto-detect mechanism. This mechanism
first decides on the exchange capability, which means it checks to see if it can run at 10 or
100Mbps. It then checks to see if it can run full duplex, and if it can’t, it will run half duplex.
Remember that half-duplex Ethernet shares a collision domain and provides
a lower effective throughput than full-duplex Ethernet, which typically has a
private collision domain and a higher effective throughput.
34 Chapter 1 Internetworking
Last, remember these important points:
There are no collisions in full-duplex mode.
A dedicated switch port is required for each full-duplex node.
Both the host network card and the switch port must be capable of operating in fullduplex
mode.
Now let’s take a look at how Ethernet works at the Data Link layer.
Ethernet at the Data Link Layer
Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to
as hardware addressing or MAC addressing. Ethernet is also responsible for framing packets
received from the Network layer and preparing them for transmission on the local network
through the Ethernet contention media access method.
Ethernet Addressing
Here’s where we get into how Ethernet addressing works. It uses the MAC address burned into
each and every Ethernet NIC. The MAC, or hardware, address is a 48-bit (6-byte) address
written in a hexadecimal format.
Figure 1.19 shows the 48-bit MAC addresses and how the bits are divided.
FIGURE 1 . 1 9 Ethernet addressing using MAC addresses
The organizationally unique identifier (OUI) is assigned by the IEEE to an organization.
It’s composed of 24 bits, or 3 bytes. The organization, in turn, assigns a globally administered
address (24 bits, or 3 bytes) that is unique (supposedly, again—no guarantees) to each and
every adapter it manufactures. Look closely at the figure. The high-order bit is the Individual/
Group (I/G) bit. When it has a value of 0, we can assume that the address is the MAC address
of a device and may well appear in the source portion of the MAC header. When it is a 1, we
can assume that the address represents either a broadcast or multicast address in Ethernet or
a broadcast or functional address in Token Ring (TR) and Fiber Distributed Data Interface
(FDDI). And who really knows about FDDI?
The next bit is the global/local bit, or just G/L bit (also known as U/L, where U means universal).
When set to 0, this bit represents a globally administered address (as in administered by the
IEEE). When the bit is a 1, it represents a locally governed and administered address (as in what
DECnet used to do). The low-order 24 bits of an Ethernet address represent a locally administered
or manufacturer-assigned code. This portion commonly starts with 24 0s for the first card made
Organizationally
Unique Identifier (OUI)
(Assigned by IEEE)
24 bits 24 bits
I/G G/L Vendor assigned
47 46
Ethernet Networking 35
and continues in order until there are 24 1s for the last (16,777,216th) card made. You’ll find that
many manufacturers use these same 6 hex digits as the last 6 characters of their serial number on
the same card.
Ethernet Frames
The Data Link layer is responsible for combining bits into bytes and bytes into frames. Frames
are used at the Data Link layer to encapsulate packets handed down from the Network layer
for transmission on a type of media access.
The function of Ethernet stations is to pass data frames between each other using a group
of bits known as a MAC frame format. This provides error detection from a cyclic redundancy
check (CRC). But remember—this is error detection, not error correction. An 802.3 frame and
Ethernet_II frame are shown in Figure 1.20.
Encapsulating a frame within a different type of frame is called tunneling.
FIGURE 1 . 2 0 802.3 and Ethernet frame formats
Following are the details of the different fields in the 802.3 and Ethernet frame types:
Preamble An alternating 1,0 pattern provides a 5MHz clock at the start of each packet, which
allows the receiving devices to lock the incoming bit stream. The preamble is seven octets.
Start Frame Delimiter (SFD)/Synch The SFD is one octet (synch). The SFD is 10101011,
where the last pair of 1s allows the receiver to come into the alternating 1,0 pattern somewhere
in the middle and still sync up and detect the beginning of the data.
Destination Address (DA) This transmits a 48-bit value using the least significant bit (LSB)
first. The DA is used by receiving stations to determine whether an incoming packet is
Preamble
8 bytes
Preamble
8 bytes
DA
6 bytes
SA
6 bytes
Length
2 bytes Data FCS
DA
6 bytes
SA
6 bytes
Type
2 bytes Data FCS
4 bytes
Ethernet_II
802.3_Ethernet
36 Chapter 1 Internetworking
addressed to a particular node. The destination address can be an individual address or a
broadcast or multicast MAC address. Remember that a broadcast is all 1s (or Fs in hex) and
is sent to all devices but a multicast is sent only to a similar subset of nodes on a network.
Source Address (SA) The SA is a 48-bit MAC address used to identify the transmitting
device, and it is transmitted LSB first. Broadcast and multicast address formats are illegal
within the SA field.
Length or Type 802.3 uses a Length field, but the Ethernet frame uses a Type field to identify
the Network layer protocol. 802.3 cannot identify the upper-layer protocol and must be
used with a proprietary LAN—IPX, for example.
Data This is a packet sent down to the Data Link layer from the Network layer. The size can
vary from 64 to 1500 bytes.
Frame Check Sequence (FCS) FCS is a field at the end of the frame that’s used to store
the CRC.
Let’s pause here for a minute and take a look at some frames caught on our trusty OmniPeek
network analyzer. You can see that the frame below has only three fields: Destination, Source,
and Type (shown as Protocol Type on this analyzer):
Destination: 00:60:f5:00:1f:27
Source: 00:60:f5:00:1f:2c
Protocol Type: 08-00 IP
This is an Ethernet_II frame. Notice that the type field is IP, or 08-00 (mostly just referred to
as 0x800) in hexadecimal.
The next frame has the same fields, so it must be an Ethernet_II frame too:
Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast
Source: 02:07:01:22:de:a4
Protocol Type: 08-00 IP
Did you notice that this frame was a broadcast? You can tell because the destination hardware
address is all 1s in binary, or all Fs in hexadecimal.
Let’s take a look at one more Ethernet_II frame. You can see that the Ethernet frame is the
same Ethernet_II frame we use with the IPv4 routed protocol but the type field has 0x86dd when
we are carrying IPv6 data, and when we have IPv4 data, we use 0x0800 in the protocol field:
Destination: IPv6-Neighbor-Discovery_00:01:00:03 (33:33:00:01:00:03)
Source: Aopen_3e:7f:dd (00:01:80:3e:7f:dd)
Type: IPv6 (0x86dd)
This is the beauty of the Ethernet_II frame. Because of the protocol field, we can run any
Network layer routed protocol and it will carry the data because it can identify the Network
layer protocol.
Ethernet Networking 37
Ethernet at the Physical Layer
Ethernet was first implemented by a group called DIX (Digital, Intel, and Xerox). They created
and implemented the first Ethernet LAN specification, which the IEEE used to create the IEEE
802.3 Committee. This was a 10Mbps network that ran on coax and then eventually twistedpair
and fiber physical media.
The IEEE extended the 802.3 Committee to two new committees known as 802.3u (Fast
Ethernet) and 802.3ab (Gigabit Ethernet on category 5) and then finally 802.3ae (10Gbps
over fiber and coax).
Figure 1.21 shows the IEEE 802.3 and original Ethernet Physical layer specifications.
When designing your LAN, it’s really important to understand the different types of Ethernet
media available to you. Sure, it would be great to run Gigabit Ethernet to each desktop and
10Gbps between switches, and although this might happen one day, justifying the cost of that
network today would be pretty difficult. But if you mix and match the different types of Ethernet
media methods currently available, you can come up with a cost-effective network solution
that works great.
FIGURE 1 . 2 1 Ethernet Physical layer specifications
The Electronic Industries Association and the newer Telecommunications Industry Alliance
(EIA/TIA) is the standards body that creates the Physical layer specifications for Ethernet.
The EIA/TIA specifies that Ethernet use a registered jack (RJ) connector with a 4 5 wiring
sequence on unshielded twisted-pair (UTP) cabling (RJ45). However, the industry is moving
toward calling this just an 8-pin modular connector.
Each Ethernet cable type that is specified by the EIA/TIA has inherent attenuation, which is
defined as the loss of signal strength as it travels the length of a cable and is measured in decibels
(dB). The cabling used in corporate and home markets is measured in categories. A higher-quality
cable will have a higher-rated category and lower attenuation. For example, category 5 is better
than category 3 because category 5 cables have more wire twists per foot and therefore less
crosstalk. Crosstalk is the unwanted signal interference from adjacent pairs in the cable.
Here are the original IEEE 802.3 standards:
10Base2 10Mbps, baseband technology, up to 185 meters in length. Known as thinnet and
can support up to 30 workstations on a single segment. Uses a physical and logical bus with
Attachment Unit Interface (AUI) connectors. The 10 means 10Mbps, Base means baseband
technology (which is a signaling method for communication on the network), and the 2 means
Data Link
(MAC layer)
Physical
Ethernet
802.3
10Base2
10Base5
10BaseT
10BaseF
100BaseTX
100BaseFX
100BaseT4
38 Chapter 1 Internetworking
almost 200 meters. 10Base2 Ethernet cards use use BNC and T-connectors to connect to a network.
(BNC stands for British Naval Connector, Bayonet Neill Concelman, or Bayonet Nut.)
10Base5 10Mbps, baseband technology, up to 500 meters in length. Known as thicknet.
Uses a physical and logical bus with AUI connectors. Up to 2,500 meters with repeaters and
1,024 users for all segments.
10BaseT 10Mbps using category 3 UTP wiring. Unlike with the 10Base2 and 10Base5 networks,
each device must connect into a hub or switch, and you can have only one host per segment
or wire. Uses an RJ45 connector (8-pin modular connector) with a physical star topology
and a logical bus.
Each of the 802.3 standards defines an AUI, which allows a one-bit-at-a-time transfer to the
Physical layer from the Data Link media access method. This allows the MAC to remain constant
but means the Physical layer can support any existing and new technologies. The original
AUI interface was a 15-pin connector, which allowed a transceiver (transmitter/receiver) that
provided a 15-pin-to-twisted-pair conversion.
The thing is, the AUI interface cannot support 100Mbps Ethernet because of the high frequencies
involved. So 100BaseT needed a new interface, and the 802.3u specifications created
one called the Media Independent Interface (MII), which provides 100Mbps throughput. The
MII uses a nibble, defined as 4 bits. Gigabit Ethernet uses a Gigabit Media Independent Interface
(GMII) and transmits 8 bits at a time.
802.3u (Fast Ethernet) is compatible with 802.3 Ethernet because they share the same physical
characteristics. Fast Ethernet and Ethernet use the same maximum transmission unit (MTU), use
the same MAC mechanisms, and preserve the frame format that is used by 10BaseT Ethernet. Basically,
Fast Ethernet is just based on an extension to the IEEE 802.3 specification, except that it
offers a speed increase of 10 times that of 10BaseT.
Here are the expanded IEEE Ethernet 802.3 standards:
100BaseTX (IEEE 802.3u) EIA/TIA category 5, 6, or 7 UTP two-pair wiring. One user per
segment; up to 100 meters long. It uses an RJ45 connector with a physical star topology and
a logical bus.
100BaseFX (IEEE 802.3u) Uses fiber cabling 62.5/125-micron multimode fiber. Point-topoint
topology; up to 412 meters long. It uses an ST or SC connector, which are media-interface
connectors.
1000BaseCX (IEEE 802.3z) Copper twisted-pair called twinax (a balanced coaxial pair)
that can only run up to 25 meters.
1000BaseT (IEEE 802.3ab) Category 5, four-pair UTP wiring up to 100 meters long.
1000BaseSX (IEEE 802.3z) MMF using 62.5- and 50-micron core; uses an 850 nano-meter
laser and can go up to 220 meters with 62.5-micron, 550 meters with 50-micron.
1000BaseLX (IEEE 802.3z) Single-mode fiber that uses a 9-micron core and 1300 nano-meter
laser and can go from 3 kilometers up to 10 kilometers.
Ethernet Cabling 39
If you want to implement an Ethernet network medium that is not susceptible
to electromagnetic interference (EMI) and voltage potential differences, fiberoptic
cable provides a more secure, long-distance cable that is not susceptible
to EMI at high speeds.
Ethernet Cabling
Ethernet cabling is an important discussion, especially if you are planning on taking the Cisco
exams. Three types of Ethernet cables are available:
Straight-through cable
Crossover cable
Rolled cable
We will look at each in the following sections.
Straight-Through Cable
The straight-through cable is used to connect the following:
Host to switch or hub
Router to switch or hub
Four wires are used in straight-through cable to connect Ethernet devices. It is relatively simple
to create this type; Figure 1.22 shows the four wires used in a straight-through Ethernet cable.
FIGURE 1 . 2 2 Straight-through Ethernet cable
Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6 to 6
and you’ll be up and networking in no time. However, remember that this would be an Ethernet-
only cable and wouldn’t work with voice, Token Ring, Integrated Services Digital Network
(ISDN), and so on.
Hub/Switch
1
2
3
6
Host
1
2
3
6
40 Chapter 1 Internetworking
Crossover Cable
The crossover cable can be used to connect the following:
Switch to switch
Hub to hub
Host to host
Hub to switch
Router direct to host
The same four wires are used in this cable as in the straight-through cable; we just connect different
pins together. Figure 1.23 shows how the four wires are used in a crossover Ethernet cable.
Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect pins 1 to 3 and
2 to 6 on each side of the cable.
FIGURE 1 . 2 3 Crossover Ethernet cable
Rolled Cable
Although rolled cable isn’t used to connect any Ethernet connections together, you can use a
rolled Ethernet cable to connect a host to a router console serial communication (com) port.
If you have a Cisco router or switch, you would use this cable to connect your PC running
HyperTerminal to the Cisco hardware. Eight wires are used in this cable to connect serial
devices, although not all eight are used to send information, just as in Ethernet networking.
Figure 1.24 shows the eight wires used in a rolled cable.
FIGURE 1 . 2 4 Rolled Ethernet cable
Hub/Switch Hub/Switch
1 1
2 2
3 3
6 6
Host
1
2
3
4
7
5
6
8
Router/Switch
1
2
3
4
7
5
6
8
Ethernet Cabling 41
These are probably the easiest cables to make because you just cut the end off on one side
of a straight-through cable, turn it over, and put it back on (with a new connector, of course).
Once you have the correct cable connected from your PC to the Cisco router or switch, you
can start HyperTerminal to create a console connection and configure the device. Set the configuration
as follows:
1. Open HyperTerminal and enter a name for the connection. It is irrelevant what you name
it, but I always just use Cisco. Then click OK.
2. Choose the communications port—either COM1 or COM2, whichever is open on your PC.
3. Now set the port settings. The default values (2400bps and no flow control hardware)
will not work; you must set the port settings as shown in Figure 1.25.
Notice that the bit rate is now set to 9600 and the flow control is set to None. At this point,
you can click OK and press the Enter key and you should be connected to your Cisco device
console port.
We’ve taken a look at the various RJ45 unshielded twisted-pair (UTP) cables. Keeping this
in mind, what cable is used between the switches in Figure 1.26?
In order for host A to ping host B, you need a crossover cable to connect the two switches
together. But what types of cables are used in the network shown in Figure 1.27?
In Figure 1.27, there are a variety of cables in use. For the connection between the switches,
we’d obviously use a crossover cable like we saw in Figure 1.23. The trouble is, we have a console
connection that uses a rolled cable. Plus, the connection from the router to the switch is a
straight-through cable, as is true for the hosts to the switches. Keep in mind that if we had a serial
connection (which we don’t), it would be a V.35 that we’d use to connect us to a WAN.
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