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Expand Up @@ -207,7 +207,200 @@ The elements of this stack from the lowest to highest are as follows:

This layer maps to the ISO's Layer 5 through Layer 7, and covers the application
processes that use the network to communicate.



Networking cable
================
There are two main types of network cable in use today, namely copper and fiber-optic.

Copper
------
The most common type of network cables are what is known as "unshielded twisted
pair" cables. They use 4 sets of twisted pairs of copper, relying on the twist
with differential signaling to prevent noise and signal propagation between the
pairs. The four pairs of twisted copper wires are encased in a plastic sheath.

There are different standards for copper network cables set by the
Telecommunications Industry Association (TIA) and the International Organization
for Standardization (ISO). Both organizations use the same naming convention
("Category __") for the components, but unfortunately differ on the naming for
the cable standards. The most common reference is the TIA's, and the category
designation is usually shortened to "Cat", so you'll hear references to "Cat5"
or "Cat6" cable.

Copper Cable Standards
^^^^^^^^^^^^^^^^^^^^^^

- Category 5e ("Cat5", ISO class D)

- Category 6 ("Cat6", ISO class E)

- Category 6A ("Cat6A", ISO class Ea)

Fiber
-----
Fiber is a generic term that refers to optical transport mediums. It comes in
several types, all of which look identical but are generally incompatible.

Multimode vs Single Mode
^^^^^^^^^^^^^^^^^^^^^^^^
Single-mode fiber has a small core diameter, which only allows one (a single)
mode of light to be transmitted through the fiber. Using a single mode of light
completely eliminates the possibility of light dispersion and associated signal
loss, and so is used mainly for long-haul runs, such as the cables that run
between buildings and cities. However, since single-mode fiber can only transmit
one wavelength of light at a time, it typically involves much more expensive
light generation sources (i.e., laser diode transmitters) and is very expensive
to produce.

Multimode fiber has a larger core diameter (either 50u or 62.5u) and can
therefore carry multiple modes ("multimode") of light, which can be used to
transmit much more information during a given timeslice. The drawback is that
carrying multimode lightwaves causes light dispersion and associated signal
loss, which limits its effective distance. Multimode is a less expensive fiber
optic cable, that is typically useable with lower cost optical components. It is
very common to see it used for building intra-building backbones, and
system/switch to switch applications.

Multimode Fiber Standards
^^^^^^^^^^^^^^^^^^^^^^^^^
Multimode cables have classifications much like the copper cables discussed above; these
are known as "Optical Multimode" (OM) classes. The four designations are:

- OM1 - a "legacy" fiber class, the core being 62.5u, and cladding being 125u.
The bandwidth that can be carried ranges from 160 to 500 MHz.

- OM2 - a "legacy" fiber class, the core being 50u, and cladding being 125u.
The bandwidth that can be carried is 500 MHz.

- OM3 - a "modern" fiber class, the core being 50u, and cladding being 125u.
The bandwidth that can be carried ranges from 1500 to 2000 MHz.

- OM4 - a "modern" fiber class, the core being 50u, and cladding being 125u.
The bandwidth that can be carried ranges from 3500 to 4700 MHz.

Optical Connector Types
^^^^^^^^^^^^^^^^^^^^^^^

LC and SC connectors are the two most common type of fiber connectors you will
use. Less common connectors include ST and MPO.

LC stands for "Lucent Connector", but is also referred to as "Little Connector".
They are typically used for high-density applications, and are the type of
connector used on SFPs or XFPs. Typically the connector is packaged in a duplex
configuration with each cable side by side, and have a latch mechanism for
locking.

SC stands for "Subscriber Connector", but are also known as "Square Connector",
or "Standard Connector". This is the type of connector typically used in the
telecom industry. They have a larger form factor than the LC connectors, and can
be found in single and duplex configurations. SC connectors have a push/pull
locking mechanism, and because of this, are also colloquially known as
"Stab-and-Click" connectors.

ST stands for "Straight Tip". This connector style is secured like a BNC connector,
giving it the nickname "Stick-Twist". It is sometimes also called BFOC, which
stands for "Bayonet Fiber Optic Connector". It is fairly common for single-mode
fiber, especially in older installations.

MPO stands for "Multi-fiber Push-On". It is a connector used to carry up to 12
pairs of fiber (24 strands), most commonly used for bulk links inside datacenters.
Several companies make boxes which take an MPO connector on one side and convert
it to 12 LC connectors on the other side. This way, a large number of fibers can
be run from one area of a datacenter to another quickly and compactly.

Transceivers
^^^^^^^^^^^^

The variety in optical fiber makes for a correspondingly large variety in
optical fiber interface standards. Different interface types will impose
different requirements on the fiber used and the length of the connection.

If optical fiber interfaces were incorporated directly into network equipment,
the number of models made by the manufacturer would have to be multiplied by
the number of interface standards in existence. For this reason, modern network
hardware rarely incorporates such interfaces directly. Instead, pluggable
transceiver modules are used as a layer of indirection between medium-dependent
and medium-independent interfaces. This allows a transceiver slot to be
provided supporting any desired interface standard, whether copper or fiber.
There are some limitations to this, detailed below.

Various module types have been introduced over the years:

============ ============ =========== =======
Name Introduced Speed Size
============ ============ =========== =======
GBIC 1995 1 Gb/s Large
SFP 2001 1 Gb/s Small
XENPAK 2001 10 Gb/s Large
XFP 2002 10 Gb/s Small
SFP+ 2006 10 Gb/s Small
QSFP 2006 40 Gb/s Small
CFP 2009 100 Gb/s Large
============ ============ =========== =======

There are a large number of compatibility issues with such modules. Some
compatibility issues cause problems between two ends of a link; others cause
problems between a module and its host device.

- Transceivers are not generally compatible with lower speed versions of the
same standard. A 1000BASE-T Ethernet card can interface with a 10BASE-T card,
but a 1 Gb/s fiber transceiver cannot interface with a 10 or 100 Mb/s
transceiver. In the case of fiber, this is generally due to the different
wavelengths used; but even many copper transceivers do not support lower
speeds, although some do. You should assume that any transceiver will only
support the exact interface for which it is designed unless specified
otherwise.

- Modules are only made for the speed targeted by a format. For example, SFP+
modules are only made for 10 Gb/s standards, and not for lower speeds.

- Some equipment may accept SFP modules in SFP+ slots, but this is not
universal.

- Vendor lock-in is widely practiced. Equipment may refuse to operate with the
modules made by a different manufacturer. Workarounds are generally
available, but this may complicate support or warranty arrangements.

These issues can create pathological cases. Suppose you have two switches which
should be connected to one another. One is connected via a 1 Gb/s transceiver
to fiber. The other only has SFP+ slots. If these slots also support SFP
modules, then a 1 Gb/s SFP transceiver can be used, but if they do not,
interconnection is impossible: all SFP+ modules target 10 Gb/s, and fiber
transceivers do not support lower speeds.

Twinax
^^^^^^
These cables have integrated transceivers. The cable itself is typically copper
but can be fiber.


MAC Addresses
=============
Each network interface has what is known as a MAC (Media Access Control) address.
This is a 48-bit address which identifies the card on the local network. The
addresses have several common representations, but all are composed of 12
hexadecimal digits. BSD and Linux systems typically represent addresses like this:

12:34:56:78:9A:BC

Windows uses hyphens instead of colons like this:

12-34-56-78-9A-BC

Cisco network equipment uses lowercase letters and groups of four digits separated
by dots:

1234.5678.9abc

All three of those forms represent the same address.

The first six digits (24 bits, three bytes) are what is called the OUI
(Organizationally Unique Identifier). It is a value assigned and tracked by the
IEEE which is unique per network card manufacturer. As an example, 80:86:F2 belongs
to Intel. Any MAC starting with those digits belongs to an interface manufactured by
Intel. An organization may have many OUIs, but each OUI covers only one organization.


IP Addressing
=============
Expand All @@ -231,7 +424,6 @@ hexadecimal, decimal, and octal. These are infrequently used, and will be
covered in later sections.



IPv6
----

Expand Down Expand Up @@ -260,6 +452,7 @@ of these ways:
2001:DB8::CBAD:4321:0000:0000:1234
2001:DB8:0000:CBAD:4321::1234


TCP vs UDP
==========

Expand Down Expand Up @@ -418,155 +611,3 @@ inactive and still keep its place in the table. An entry that has remained
inactive for a period of time longer than the timeout will automatically be
removed, freeing up space for a new one.

Networking cable
================
There are two main types of network cable in use today, namely copper and fiber-optic.

Copper
------
The most common type of network cables are what is known as "unshielded twisted
pair" cables. They use 4 sets of twisted pairs of copper, relying on the twist
with differential signaling to prevent noise and signal propagation between the
pairs. The four pairs of twisted copper wires are encased in a plastic sheath.

There are different standards for copper network cables set by the
Telecommunications Industry Association (TIA) and the International Organization
for Standardization (ISO). Both organizations use the same naming convention
("Category __") for the components, but unfortunately differ on the naming for
the cable standards. The most common reference is the TIA's, and the category
designation is usually shortened to "Cat", so you'll hear references to "Cat5"
or "Cat6" cable.

Copper Cable Standards
^^^^^^^^^^^^^^^^^^^^^^

- Category 5e ("Cat5", ISO class D)

- Category 6 ("Cat6", ISO class E)

- Category 6A ("Cat6A", ISO class Ea)

Fiber
-----
Fiber is a generic term that refers to optical transport mediums. It comes in
several types, all of which look identical but are generally incompatible.

Multimode vs Single Mode
^^^^^^^^^^^^^^^^^^^^^^^^
Single-mode fiber has a small core diameter, which only allows one (a single)
mode of light to be transmitted through the fiber. Using a single mode of light
completely eliminates the possibility of light dispersion and associated signal
loss, and so is used mainly for long-haul runs, such as the cables that run
between buildings and cities. However, since single-mode fiber can only transmit
one wavelength of light at a time, it typically involves much more expensive
light generation sources (i.e., laser diode transmitters) and is very expensive
to produce.

Multimode fiber has a larger core diameter (either 50u or 62.5u) and can
therefore carry multiple modes ("multimode") of light, which can be used to
transmit much more information during a given timeslice. The drawback is that
carrying multimode lightwaves causes light dispersion and associated signal
loss, which limits its effective distance. Multimode is a less expensive fiber
optic cable, that is typically useable with lower cost optical components. It is
very common to see it used for building intra-building backbones, and
system/switch to switch applications.

Multimode Fiber Standards
^^^^^^^^^^^^^^^^^^^^^^^^^
Multimode cables have classifications much like the copper cables discussed above; these
are known as "Optical Multimode" (OM) classes. The four designations are:

- OM1 - a "legacy" fiber class, the core being 62.5u, and cladding being 125u.
The bandwidth that can be carried ranges from 160 to 500 MHz.

- OM2 - a "legacy" fiber class, the core being 50u, and cladding being 125u.
The bandwidth that can be carried is 500 MHz.

- OM3 - a "modern" fiber class, the core being 50u, and cladding being 125u.
The bandwidth that can be carried ranges from 1500 to 2000 MHz.

- OM4 - a "modern" fiber class, the core being 50u, and cladding being 125u.
The bandwidth that can be carried ranges from 3500 to 4700 MHz.

Optical Connector Types
^^^^^^^^^^^^^^^^^^^^^^^

LC and SC connectors are the two most common type of fiber connectors you will
use.

LC stands for "Lucent Connector", but is also referred to as "Little Connector".
They are typically used for high-density applications, and are the type of
connector used on SFPs or XFPs. Typically the connector is packaged in a duplex
configuration with each cable side by side, and have a latch mechanism for
locking.

SC stands for "Subscriber Connector", but are also known as "Square Connector",
or "Standard Connector". This is the type of connector typically used in the
telecom industry. They have a larger form factor than the LC connectors, and can
be found in single and duplex configurations. SC connectors have a push/pull
locking mechanism, and because of this, are also colloquially known as
"Stab-and-Click" connectors.

Transceivers
^^^^^^^^^^^^

The variety in optical fiber makes for a correspondingly large variety in
optical fiber interface standards. Different interface types will impose
different requirements on the fiber used and the length of the connection.

If optical fiber interfaces were incorporated directly into network equipment,
the number of models made by the manufacturer would have to be multiplied by
the number of interface standards in existence. For this reason, modern network
hardware rarely incorporates such interfaces directly. Instead, pluggable
transceiver modules are used as a layer of indirection between medium-dependent
and medium-independent interfaces. This allows a transceiver slot to be
provided supporting any desired interface standard, whether copper or fiber.
There are some limitations to this, detailed below.

Various module types have been introduced over the years:

============ ============ =========== =======
Name Introduced Speed Size
============ ============ =========== =======
GBIC 1995 1 Gb/s Large
SFP 2001 1 Gb/s Small
XENPAK 2001 10 Gb/s Large
XFP 2002 10 Gb/s Small
SFP+ 2006 10 Gb/s Small
QSFP 2006 40 Gb/s Small
CFP 2009 100 Gb/s Large
============ ============ =========== =======

There are a large number of compatibility issues with such modules. Some
compatibility issues cause problems between two ends of a link; others cause
problems between a module and its host device.

- Transceivers are not generally compatible with lower speed versions of the
same standard. A 1000BASE-T Ethernet card can interface with a 10BASE-T card,
but a 1 Gb/s fiber transceiver cannot interface with a 10 or 100 Mb/s
transceiver. In the case of fiber, this is generally due to the different
wavelengths used; but even many copper transceivers do not support lower
speeds, although some do. You should assume that any transceiver will only
support the exact interface for which it is designed unless specified
otherwise.

- Modules are only made for the speed targeted by a format. For example, SFP+
modules are only made for 10 Gb/s standards, and not for lower speeds.

- Some equipment may accept SFP modules in SFP+ slots, but this is not
universal.

- Vendor lock-in is widely practiced. Equipment may refuse to operate with the
modules made by a different manufacturer. Workarounds are generally
available, but this may complicate support or warranty arrangements.

These issues can create pathological cases. Suppose you have two switches which
should be connected to one another. One is connected via a 1 Gb/s transceiver
to fiber. The other only has SFP+ slots. If these slots also support SFP
modules, then a 1 Gb/s SFP transceiver can be used, but if they do not,
interconnection is impossible: all SFP+ modules target 10 Gb/s, and fiber
transceivers do not support lower speeds.

Twinax
^^^^^^