Ethernet Technologies
What is Ethernet?
Ethernet is a family of technologies that provides data-link and
physical specifications for controlling access to a shared network medium. It has emerged as the
dominant technology used in LAN networking.
Ethernet was originally developed
by Xerox in the 1970s, and operated at 2.94Mbps. The technology was
standardized as Ethernet Version 1 by
a consortium of three companies - DEC, Intel, and Xerox, collectively referred to as DIX - and further refined as Ethernet
II in 1982.
In the mid 1980s, the Institute of Electrical and Electronic Engineers (IEEE) published a
formal standard for Ethernet, defined as the
IEEE 802.3 standard. The original
802.3 Ethernet operated at 10Mbps, and successfully supplanted competing LAN
technologies, such as Token Ring.
Ethernet has several benefits over other LAN technologies:
·
Simple to install and manage
·
Inexpensive
·
Flexible and
scalable
·
Easy to interoperate between vendors
Ethernet Cabling Types
Ethernet can be deployed over three types of cabling:
·
Coaxial cabling – almost
entirely deprecated in Ethernet networking
·
Twisted-pair cabling
·
Fiber optic cabling
Coaxial cable, often abbreviated as coax, consists of a single wire surrounded by insulation, a
metallic shield, and a plastic sheath. The shield helps protect against electromagnetic interference (EMI), which
can cause attenuation, a reduction
of the strength and quality of a signal. EMI can be generated by a variety of
sources, such as florescent light ballasts, microwaves, cell phones, and radio transmitters.
Coax is commonly used to deploy
cable television to homes and businesses.
Two types of coax were used
historically in Ethernet networks
Thinnet
Thicknet
Thicknet has a wider diameter and more shielding, which supports
greater distances. However, it is less flexible than the smaller thinnet, and thus more difficult to work
with. A vampire tap is used to
physically connect devices to thicknet, while a BNC connector is used for thinnet.
Twisted-pair cable consists of two or four pairs of copper wires in
a plastic sheath. Wires in a pair twist around each other to reduce crosstalk, a form of EMI that occurs
when the signal from one wire bleeds or
interferes with a signal on another
wire. Twisted-pair is the most common Ethernet
cable.
Twisted-pair cabling can be
either shielded or unshielded. Shielded twisted- pair is
more resistant to external EMI; however, all
forms of twisted-pair suffer from greater signal attenuation than coax cable.
There are several categories of twisted-pair cable, identified by the number of twists per inch of the copper pairs:
·
Category 3 or Cat3 - three twists per inch.
·
Cat5 - five twists per inch.
·
Cat5e - five twists per inch; pairs
are also twisted around each other.
·
Cat6 – six twists per inch, with improved insulation.
An RJ45 connector is used to connect a device to a twisted-pair cable. The
layout of the wires in the connector dictates the function of the cable.
While
coax and twisted-pair cabling carry electronic
signals, fiber optics
uses light to transmit a signal. Ethernet supports two fiber specifications:
·
Singlemode
fiber – consists of a very small glass core, allowing only a single ray or mode of light to travel across it. This
greatly reduces the attenuation and dispersion of the light signal, supporting
high bandwidth over very long
distances, often measured in kilometers.
·
Multimode
fiber – consists of a larger core,
allowing multiple modes of light to
traverse it. Multimode suffers from greater dispersion than singlemode,
resulting in shorter supported distances.
Singlemode
fiber requires more precise electronics
than multimode, and thus is significantly more expensive. Multimode fiber is often used for high-speed
connectivity within a datacenter.
..........................................................................................................................................................................................
Network Topologies
A topology defines both the physical
and logical structure of a
network. Topologies come in a variety of configurations, including:
·
Bus
·
Star
·
Ring
·
Full or partial mesh
Ethernet supports two topology
types – bus and star.
Ethernet Bus Topology
In
a bus topology, all hosts share a single
physical segment (the bus or the
backbone) to communicate:
A frame sent by one host is
received by all other hosts on the
bus. However, a host will only process a
frame if it matches the destination hardware address in the data-link header.
Bus topologies are inexpensive to
implement, but are almost entirely
deprecated in Ethernet. There are several disadvantages to the bus topology:
·
Both ends of the bus must be terminated, otherwise a signal will
reflect back and cause
interference, severely degrading performance.
·
Adding or removing hosts to the bus can be difficult.
·
The bus represents a single point of failure -
a break in the bus will affect all hosts on the segment. Such faults
are often very difficult to troubleshoot.
A bus topology is implemented
using either thinnet or thicknet coax cable.
Ethernet Star Topology
In a star
topology, each host has an individual point-to-point connection to a
centralized hub or switch:
A hub provides no intelligent forwarding whatsoever, and will always
forward every frame out every port, excluding the port originating the frame. As with a bus topology, a host will
only process a frame if it matches
the destination hardware address in the data-link header. Otherwise, it will
discard the frame.
A switch builds a hardware address table, allowing it to make intelligent forwarding
decisions based on frame (data-link) headers. A frame can then be forwarded out
only the appropriate destination
port, instead of all ports.
Adding or removing hosts is very
simple in a star topology. Also, a break in a cable will affect only that one host, and not the entire network.
There are two disadvantages to the star topology:
·
The hub or switch represents a single point of failure.
·
Equipment and cabling costs are generally
higher than in a bus topology.
However, the star is still the
dominant topology in modern Ethernet networks, due to its flexibility and
scalability. Both twisted-pair and fiber cabling can be used in a star topology.
The Ethernet Frame
An Ethernet frame contains the
following fields:
Field Length Description
Preamble
|
7 bytes
|
Synchronizes
communication
|
Start of Frame
|
1 byte
|
Signals the start of a valid frame
|
MAC Destination
|
6 bytes
|
Destination MAC address
|
MAC Source
|
6 bytes
|
Source MAC address
|
802.1Q tag
|
4 bytes
|
Optional VLAN tag
|
Ethertype or length
|
2 bytes
|
Payload type or frame size
|
Payload
|
42-1500 bytes
|
Data payload
|
CRC
|
4 bytes
|
Frame error
check
|
Interframe Gap
|
12 bytes
|
Required idle period between frames
|
The preamble is 56 bits of alternating 1s and 0s that synchronizes
communication on an Ethernet network. It is followed
by an 8-bit start of frame
delimiter (10101011) that indicates a valid frame is about to begin. The
preamble and the start of frame are not
considered part of the actual frame, or calculated as part of the total
frame size.
Ethernet uses the 48-bit MAC address for hardware addressing.
The first 24-bits of a MAC address determine the manufacturer of the network
interface, and the last 24-bits uniquely identify the host.
The destination MAC address identifies who is to receive the frame -
this can be a single host (a unicast),
a group of hosts (a multicast), or
all hosts (a broadcast). The source MAC address indentifies the host
originating the frame.
The 16-bit Ethertype/Length field provides a different function depending on
the standard - Ethernet II or 802.3. With Ethernet II, the field identifies the
type of payload in the frame (the Ethertype).
However, Ethernet II is almost entirely deprecated.
With 802.3, the field identifies
the length of the payload. The length of a frame is
important – there is both a minimum and maximum
frame size.
The Ethernet Frame (continued)
Field Length Description
Preamble
|
7 bytes
|
Synchronizes
communication
|
Start of Frame
|
1 byte
|
Signals the start of a valid frame
|
MAC Destination
|
6 bytes
|
Destination MAC address
|
MAC Source
|
6 bytes
|
Source MAC address
|
802.1Q tag
|
4 bytes
|
Optional VLAN tag
|
Ethertype or length
|
2 bytes
|
Payload type or frame size
|
Payload
|
42-1500 bytes
|
Data payload
|
CRC
|
4 bytes
|
Frame error
check
|
Interframe Gap
|
12 bytes
|
Required idle period between frames
|
The absolute minimum frame size for
Ethernet is 64 bytes (or
512 bits)
including headers. A frame that is smaller than 64 bytes will be discarded as a
runt. The
required fields in an Ethernet header add up to 18 bytes – thus, the frame payload must be a
minimum of 46 bytes, to equal the minimum 64-byte frame size. If the payload does
not meet this minimum, the payload is padded with 0 bits until the minimum is met.
Note: If the optional 4-byte 802.1Q tag is used, the Ethernet header size will total 22 bytes,
requiring a minimum payload of 42 bytes.
By default, the maximum frame size for Ethernet is 1518
bytes – 18 bytes of header
fields, and 1500 bytes of payload -
or 1522 bytes with the 802.1Q tag. A frame that is larger than the
maximum will be discarded as a giant.
With both runts and giants, the receiving host will not notify the sender that the frame was
dropped. Ethernet relies on higher-layer protocols, such as TCP, to provide
retransmission of discarded frames.
Some Ethernet
devices support jumbo frames of 9216 bytes, which provide less overhead
due to fewer frames. Jumbo frames must be
explicitly enabled on all devices in
the traffic path to prevent the frames from being dropped.
The 32-bit Cycle
Redundancy Check (CRC) field is used for error- detection. A frame with an
invalid CRC will be discarded by the receiving device. This field is a trailer, and not a header, as it follows the payload.
The 96-bit Interframe Gap is a required idle period between frame
transmissions, allowing hosts time to prepare for the next frame.
CSMA/CD and Half-Duplex Communication
Ethernet was originally developed
to support a shared media environment.
This allowed two or more hosts to use the same physical network medium.
There are two methods of communication on a shared physical medium:
·
Half-Duplex
– hosts can transmit or receive, but not simultaneously
·
Full-Duplex
– hosts can both transmit and receive simultaneously
On a half-duplex connection, Ethernet utilizes Carrier Sense Multiple Access with
Collision Detect (CSMA/CD) to control media access. Carrier sense specifies that a host will monitor the physical link,
to determine whether a carrier (or signal) is currently being transmitted.
The host will only transmit a frame
if the link is idle, and the
Interframe Gap has expired.
If two hosts
transmit a frame simultaneously, a collision
will occur. This renders the collided frames unreadable. Once a collision
is detected, both hosts will send a 32-bit
jam sequence to ensure all transmitting hosts are aware of the collision.
The collided frames are also discarded.
Both devices will then wait a random amount of time before resending
their respective frames, to reduce the likelihood of another collision. This is
controlled by a backoff timer process.
Hosts must detect a collision before a frame is finished transmitting, otherwise
CSMA/CD cannot function reliably. This is accomplished using a consistent slot time, the time required to send a
specific amount of data from one end of the network and then back, measured in bits.
A host must continue to transmit
a frame for a minimum of the slot time. In a properly
configured environment, a collision should always occur within
this slot time, as enough time has elapsed for the frame to have reached the
far end of the network and back, and thus all devices should be aware of the transmission. The slot time effectively
limits the physical length of the network – if a network segment is too long, a
host may not detect a collision within the slot time period. A collision that occurs after the slot time is referred to
as a late collision.
For 10 and 100Mbps Ethernet, the
slot time was defined as 512 bits, or 64 bytes. Note that this
is the equivalent of the minimum Ethernet
frame size of 64 bytes. The slot time actually defines this minimum. For Gigabit Ethernet, the slot
time was defined as 4096 bits.
Full-Duplex Communication
Unlike half-duplex, full-duplex Ethernet supports
simultaneously communication by providing separate transmit and receive paths. This effectively doubles the throughput of a network interface.
Full-duplex Ethernet was
formalized in IEEE 802.3x, and does not
use CSMA/CD or slot times. Collisions should never occur on a functional full- duplex link. Greater distances
are supported when using full-duplex
over half-duplex.
Full-duplex is only supported on a
point-to-point connection between two devices. Thus, a bus topology using coax
cable does not support full-duplex.
Only a connection between two hosts or between a host and a switch supports full-duplex. A host connected to a hub is limited to half-duplex. Both hubs
and half-duplex communication are mostly deprecated in modern networks.
Categories of Ethernet
The original 802.3 Ethernet
standard has evolved over time,
supporting faster transmission rates, longer distances, and newer hardware technologies. These revisions or amendments are identified by the letter appended to the standard,
such as 802.3u or 802.3z.
Major categories of Ethernet have also been organized by their speed:
·
Ethernet (10Mbps)
·
Fast Ethernet (100Mbps)
· Gigabit Ethernet
·
10 Gigabit Ethernet
The physical standards for Ethernet are
often labeled by their transmission rate, signaling type, and media type. For
example, 100baseT represents the
following:
·
The first part (100) represents the transmission rate, in Mbps.
·
The second part (base) indicates that it is a baseband transmission.
·
The last part (T) represents the physical media type (twisted-pair).
Ethernet communication is baseband, which dedicates the entire capacity of the medium to one signal
or channel. In broadband, multiple signals or channels can share
the same link, through the use of modulation (usually frequency modulation).
Ethernet (10 Mbps)
Ethernet is now a somewhat generic term, describing the entire family of technologies. However,
Ethernet traditionally referred to the original 802.3 standard, which operated
at 10 Mbps. Ethernet supports coax,
twisted-pair, and fiber cabling. Ethernet over twisted-pair uses two of the four pairs.
Common Ethernet physical
standards include:
IEEE Standard Physical Standard Cable Type Max Speed Max Cable Length
802.3a
|
10base2
|
Coaxial (thinnet)
|
10 Mbps
|
185 meters
|
802.3
|
10base5
|
Coaxial (thicknet)
|
10 Mbps
|
500 meters
|
802.3i
|
10baseT
|
Twisted-pair
|
10 Mbps
|
100 meters
|
802.3j
|
10baseF
|
Fiber
|
10 Mbps
|
2000 meters
|
Both 10baseT and 10baseF support
full-duplex operation, effectively doubling the bandwidth to 20 Mbps. Remember,
only a connection between two hosts or
between a host and a switch support full-duplex. The
maximum distance of an Ethernet segment can be extended through the use of a repeater. A hub or a switch can also serve as a repeater.
Fast Ethernet (100 Mbps)
In 1995, the IEEE formalized 802.3u, a 100 Mbps revision of Ethernet that became known as Fast Ethernet. Fast Ethernet supports
both twisted-pair copper and fiber cabling, and supports both half-duplex and full-duplex.
Common Fast Ethernet physical
standards include:
IEEE Standard Physical Standard Cable Type Max Speed Max Cable Length
802.3u
|
100baseTX
|
Twisted-pair
|
100 Mbps
|
100 meters
|
802.3u
|
100baseT4
|
Twisted-pair
|
100 Mbps
|
100 meters
|
802.3u
|
100baseFX
|
Multimode fiber
|
100 Mbps
|
400-2000 meters
|
802.3u
|
100baseSX
|
Multimode fiber
|
100 Mbps
|
500 meters
|
100baseT4 was never widely
implemented, and only supported half-duplex operation. 100baseTX is the
dominant Fast Ethernet physical standard.
100baseTX uses two of the four pairs in a twisted-pair
cable, and requires Category 5 cable for reliable performance.
Speed and Duplex Autonegotiation
Fast Ethernet is
backwards-compatible with the original Ethernet
standard. A device that supports both Ethernet and Fast Ethernet is often referred to as a 10/100 device.
Fast Ethernet also introduced the
ability to autonegotiate both the
speed and duplex of an interface. Autonegotiation will attempt to use the fastest
speed available, and will attempt to use full-duplex if both devices support it.
Speed and duplex can also be hardcoded,
preventing negotiation.
The configuration must be consistent on both sides of the
connection. Either both sides must
be configured to autonegotiate, or both sides must be hardcoded with identical settings. Otherwise a duplex mismatch error can occur.
For example, if a workstation’s
NIC is configured to autonegotiate,
and the switch interface is hardcoded for 100Mbps and full-duplex, then a
duplex mismatch will occur. The workstation’s NIC will sense the correct speed of 100Mbps, but will not detect the
correct duplex and will default to half- duplex.
If the duplex is mismatched,
collisions will occur. Because the full-duplex side of the connection does not
utilize CSMA/CD, performance is severely degraded.
These issues can be difficult to troubleshoot, as the network connection will
still function, but will be excruciatingly slow.
When autonegotiation was first
developed, manufacturers did not always adhere to the same standard. This
resulted in frequent mismatch issues, and a sentiment of distrust towards autonegotiation.
Though modern network hardware has
alleviated most of the incompatibility, many administrators are still skeptical
of autonegotiation and choose to hardcode all connections. Another common practice is to hardcode server and
datacenter connections, but to allow user devices to autonegotiate.
Gigabit Ethernet, covered in the
next section, provided several enhancements to autonegotiation, such as
hardware flow control. Most
manufacturers recommend autonegotiation on
Gigabit Ethernet interfaces as a best practice.
Gigabit Ethernet
Gigabit Ethernet operates at
1000 Mbps, and supports both twisted-pair (802.3ab)
and fiber cabling (802.3z).
Gigabit over twisted-pair uses all four
pairs, and requires Category 5e cable for reliable performance.
Gigabit Ethernet is
backwards-compatible with the original Ethernet
and Fast Ethernet. A device that supports all three is often referred to as a 10/100/1000 device. Gigabit Ethernet supports both half-duplex or full-
duplex operation. Full-duplex Gigabit Ethernet effectively provides 2000 Mbps
of throughput.
Common Gigabit Ethernet physical
standards include:
IEEE Standard Physical Standard Cable Type Max Speed Max Cable Length
802.3ab
|
1000baseT
|
Twisted-pair
|
1 Gbps
|
100 meters
|
802.3z
|
1000baseSX
|
Multimode fiber
|
1 Gbps
|
500 meters
|
802.3z
|
1000baseLX
|
Multimode fiber
|
1 Gbps
|
500 meters
|
802.3z
|
1000baseLX
|
Singlemode
fiber
|
1 Gbps
|
Several kilometers
|
In modern
network equipment, Gigabit Ethernet has replaced
both Ethernet and Fast Ethernet.
10 Gigabit Ethernet
10 Gigabit
Ethernet operates at 10000 Mbps, and supports both twisted-pair (802.3an) and fiber cabling (802.3ae). 10 Gigabit over twisted-pair
uses all four pairs, and requires
Category 6 cable for reliable performance.
Common
Gigabit Ethernet physical standards include:
IEEEStandard Physical Standard Cable Type Max Speed Max Cable Length
802.3an
|
10Gbase-T
|
Twisted-pair
|
10 Gbps
|
100 meters
|
802.3ae
|
10Gbase-SR
|
Multimode fiber
|
10 Gbps
|
300 meters
|
802.3ae
|
10Gbase-LR
|
Singlemode
fiber
|
10 Gbps
|
Several kilometers
|
10 Gigabit
Ethernet is usually used for high-speed connectivity within a datacenter, and
is predominantly deployed over fiber.
Twisted-Pair Cabling Overview
A typical twisted-pair cable
consists of four pairs of copper
wires, for a total of eight wires.
Each side of the cable is terminated using an RJ45
connector, which has eight pins. When
the connector is crimped onto the
cable, these pins make contact with each wire.
The wires themselves are assigned
a color to distinguish them. The
color is dictated by the cabling standard - TIA/EIA-568B is the current standard:
Color Pin#
White Orange 1
Orange 2
White Green 3
Blue 4
White Blue 5
Green 6
White Brown 7
Brown 8
Each wire is assigned a specific
purpose. For example, both Ethernet and Fast Ethernet use two wires to
transmit, and two wires to receive
data, while the other four pins remain unused.
The
layout or pinout of the wires in the RJ45 connector dictates the function
of the cable. There are three common types of twisted-pair cable:
·
Straight-through cable
·
Crossover cable
·
Rollover cable
The network interface type determines when to use each cable:
· Medium Dependent Interface (MDI)
·
Medium Dependent Interface
with Crossover (MDIX)
Host interfaces are generally
MDI, while hub or switch interfaces
are typically MDIX.
Twisted-Pair Cabling – Straight-Through Cable
A
straight-through cable is used in
the following circumstances:
·
From a host to a hub – MDI to MDIX
·
From a host to a switch - MDI to MDIX
·
From a router to a hub - MDI to MDIX
·
From a router to a switch - MDI to MDIX
Essentially, a straight-through
cable is used to connect any device to a hub or switch, except for another hub or switch. The
hub or switch provides the crossover (or
MDIX) function to connect transmit
pins to receive pins.
The pinout on each end of a
straight-through cable must be identical.
The TIA/EIA-568B standard for a straight-through cable is as follows:
Pin#
1
|
Connector 1
White Orange
|
------------------------
|
Connector 2
White Orange
|
Pin#
1
|
2
|
Orange
|
------------------------
|
Orange
|
2
|
3
|
White Green
|
------------------------
|
White Green
|
3
|
4
|
Blue
|
------------------------
|
Blue
|
4
|
5
|
White Blue
|
------------------------
|
White Blue
|
5
|
6
|
Green
|
------------------------
|
Green
|
6
|
7
|
White Brown
|
------------------------
|
White Brown
|
7
|
8
|
Brown
|
------------------------
|
Brown
|
8
|
A straight-through cable is often
referred to as a patch cable.
Twisted-Pair Cabling – Crossover Cable
A crossover cable is used
in the following circumstances:
·
From a host to a host – MDI to MDI
·
From a hub to a hub - MDIX to MDIX
·
From a switch to a switch - MDIX to MDIX
·
From a hub to a switch - MDIX to MDIX
·
From a router to a router - MDI to MDI
Remember that a hub or a switch
will provide the crossover function. However, when connecting a host directly
to another host (MDI to MDI), the crossover function must be provided by a
crossover cable.
A crossover cable is often
required to uplink a hub to another hub, or to uplink a switch to another
switch. This is because the crossover is performed twice, once on each hub or switch (MDIX to MDIX), negating the crossover.
Modern devices can now automatically detect whether the
crossover function is required, negating the need for a crossover cable. This
functionality is referred to as Auto-MDIX,
and is now standard with Gigabit Ethernet, which uses all eight wires to both
transmit and receive. Auto- MDIX
requires that autonegotiation be enabled.
To create a crossover cable, the
transmit pins must be swapped with the receive pins on one end of the cable:
·
Pins 1 and 3
·
Pins 2 and 6
Pin#
1
|
Connector 1
White Orange
|
------------------------
|
Connector 2
White Green
|
Pin#
3
|
2
|
Orange
|
------------------------
|
Green
|
6
|
3
|
White Green
|
------------------------
|
White Orange
|
1
|
4
|
Blue
|
------------------------
|
Blue
|
4
|
5
|
White Blue
|
------------------------
|
White Blue
|
5
|
6
|
Green
|
------------------------
|
Orange
|
2
|
7
|
White Brown
|
------------------------
|
White Brown
|
7
|
8
|
Brown
|
------------------------
|
Brown
|
8
|
Note that the
Orange and Green pins have been swapped on
Connector 2. The first connector is using the TIA/EIA-568B standard, while the
second connector is using the TIA/EIA-568A standard.
Twisted-Pair – Rollover Cable
A rollover cable is used to connect a workstation
or laptop into a Cisco device’s console or auxiliary port, for
management purposes. A rollover cable is often referred to as a console
cable, and its sheathing is usually flat and light-blue in color.
To create a rollover cable, the
pins are completely reversed on one end of the cable:
Pin#
1
|
Connector 1
White Orange
|
------------------------
|
Connector 2
Brown
|
Pin#
8
|
2
|
Orange
|
------------------------
|
White Brown
|
7
|
3
|
White Green
|
------------------------
|
Green
|
6
|
4
|
Blue
|
------------------------
|
White Blue
|
5
|
5
|
White Blue
|
------------------------
|
Blue
|
4
|
6
|
Green
|
------------------------
|
White Green
|
3
|
7
|
White Brown
|
------------------------
|
Orange
|
2
|
8
|
Brown
|
------------------------
|
White Orange
|
1
|
Rollover cables can be used to
configure Cisco routers, switches, and firewalls.
Power over Ethernet (PoE)
Power over Ethernet (PoE) allows both data and power to be sent
across the same twisted-pair cable, eliminating the need to provide separate power connections. This
is especially useful in areas where installing
separate power might be expensive or difficult.
PoE can be used to power many devices, including:
·
Voice over IP (VoIP) phones
·
Security cameras
·
Wireless access points
·
Thin clients
PoE was originally formalized as 802.3af, which can provide roughly 13W
of power to a device. 802.3at further
enhanced PoE, supporting 25W or more power to a device.
Ethernet, Fast Ethernet, and Gigabit Ethernet all support PoE.
Power can be sent across either the unused
pairs in a cable, or the data transmission pairs, which is referred to as phantom power. Gigabit Ethernet
requires the phantom power method, as it uses all eight wires in a twisted-pair cable.
The device that provides power is referred to as the Power Source Equipment (PSE). PoE can be supplied using an external power injector, though each
powered device requires a separate power injector.
More commonly, an 802.3af-compliant network switch is
used to provide power to many devices simultaneously. The power supplies in the switch must be large
enough to support both the switch itself, and the devices it is powering.
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