Once thought to be improbable, if not impossible, Gigabit speeds are now a reality over
twisted pair copper cabling. Many of the large network hardware
manufactures have been quick to adopt this new technology, and most are
now offering products with Gigabit UTP interfaces. Gigabit speeds have
been available over fiber optic media for several years, while Gigabit
over copper media is a relatively new phenomenon. With the thirst for
network bandwidth increasing exponentially, moving to Gigabit in network
backbone connections is not as much a choice, as it is an eventuality.
Although the need for greater bandwidth is becoming less of a question,
the type of interface you choose (copper vs. fiber) is.
In the Fall of 2000, the IEEE 802.3ab task force began working on a scheme which, by design,
would allow data to be delivered at Gigabit speeds over much of the
existing installed based of Category 5 UTP cabling. This group proposed
using an 8B/10B (8 bit user data converted to 10 bit symbols)
encoding/decoding scheme which would serve to push the center frequency
transmission below the 100MHz threshold required for category 5 copper
cables. The 802.3ab task force had the foresight to base Gigabit on
existing proven specifications, like FibreChannel (ANSI X3T11), as well
as published Ethernet standards (IEEE 802.3). This ensured that the
standard could be developed quickly and that it would provide for
compatibility with existing Ethernet and Fast Ethernet devices. The IEEE,
under 802.3ab, approved the standard for Gigabit transmission over UTP in
the Fall of 2000; now known as "1000BASE-T."
Gigabit over fiber
optic media borrowed heavily from the existing FibreChannel standards.
Again, the IEEE developed this standard via a task force which
published its work in 1998 under IEEE 802.3z (1000BASE-X). Approved and
proven, the standard for Gigabit over fiber describes high-speed
transmission of data over SX (850nm short wavelength fiber), LX (1310nm
long wavelength fiber), as well as CX (Gigabit over twinaxial copper
cabling). Fiber optic media, because of its inherently high bandwidth
carrying capability, seemed to be the most logical choice for
transmitting higher speed protocols. However, the higher cost of fiber
optic interfaces (vs. copper interfaces) and the large number of
networks with installed Category 5 cabling made development of a copper
solution for Gigabit necessary.
Using copper or
fiber interfaces on your network hardware is a choice that requires
some thought based on your unique requirements. Copper interfaces allow
you to add Gigabit to your network at a lower cost than fiber, and
theoretically, allow you to deploy it over your existing Category 5
cabling plant. However, there are technical issues with Gigabit
transmission over UTP that you should understand before you decide
which media type you will use in your Gigabit network
connection.
Gigabit speed has
become necessary in network backbones as a result of ever-increasing
thirst for bandwidth. Applications for Gigabit include:
Aggregation of
bandwidth; such as backbone connections for Fast Ethernet
switches
High-speed data
transfers between clients and server farms
Accommodating very
high bandwidth users in CAD and image editing applications
Currently, the most
common application for Gigabit is aggregation of bandwidth for backbone
connections between Fast Ethernet devices (most often switches). This
Gigabit connection is often accomplished by using a modular device that
can be installed in switches, which is available from all of the major
switch manufacturers, known as a Gbic. Gbics are relatively inexpensive
and can facilitate most Gigabit backbone connections of this type;
provided they do not exceed the maximum distance allowed by the media
and fiber optic transceiver used.
With increasing
numbers of users demanding more frequent access to storage devices and
servers, requirements for higher speed connections have become a
necessity.
The increasing
complexity of graphics used in engineering CAD software as well as
software used by graphic artists, will require that these "power users"
have access to a bigger and faster pipe.
Gigabit achieves 1Mbps
throughput by effectively transmitting 250 Mbps of data over each of four
wire pairs simultaneously in both directions; where the cumulative result
is a 1 Gbps duplex connection. The Gigabit 1000BASE-T standard was
written to accommodate both full-duplex and half-duplex operation (Shared
Ethernet regulated under CSMA/CD rules). Full-duplex is clearly the
preferred architecture, as there are some inherent problems with running
Gigabit in a shared architecture over copper, in terms of distance and
throughput.
In shared Ethernet,
an increase in speed typically equates to a decrease in distance,
because of the method in which Ethernet deals with collisions. Ethernet
devices "listen" for an opportunity to transmit on a shared wire pair.
If a device detects that no other devices are transmitting, it deems it
safe to send its data. Collisions occur if two devices on the same
network attempt to transmit at the same time. These collisions, if not
too frequent, are perfectly normal and easily dealt with by the
protocol (under the provision of CSMA/CD - Carrier Sense Multiple
Access/Collision Detection - part of the IEEE 802.3 standard).
In Ethernet, the
smallest packet size allowed is 64 bytes (8 bits per byte = 512 bits).
The purpose of establishing a minimum packet size was to ensure that a
station could detect collisions at the furthest point of the network,
allowing the CSMS/CD portion of the protocol to deal with it
appropriately (referred to as the 512 bit-time rule). As speed
increases by factors of 10 (10 Mbps to 100 Mbps to 1 Gbps), the
distance that you can transmit and still properly detect collisions is
decreased by a factor of 10. Consequently, at Gigabit speeds in a
shared Ethernet environment, you are limited to about 20 meters over
UTP.
The Gigabit standard
addresses this distance limitation issue by a method known as "carrier
extension." Carrier extension effectively increases the packet size to
512 bytes (4096 bits), by adding "extension symbols" to increase the
size of the packet to a size that can be detected by all devices on a
Gigabit link up to 100 meters away. The end device then strips this
additional data or "extension symbols" off when it is received. The
problem is that increasing the packet size (adding 448 bytes of
extension symbols) means that you have actually decreased the
throughput to about 100 Mbps Fast Ethernet speed. (Sending larger
amounts of data down a larger pipe nets you no significant
gain.)
To deal with this
reduction in throughput, a method known as "packet bursting" is used in
conjunction with carrier extension. Packet bursting improves the
efficiency of carrier extension by decreasing the inter-packet gap when
multiple packets are transmitted. (Reducing the amount of data you send
down a larger pipe nets you a nominal gain.) However, even when both
methods are used, throughput in half-duplex Gigabit remains hindered
and never achieves full 1 Gbps speed. The bottom line is that
half-duplex is possible but not recommended in Gigabit
environments.
Carrier detection
and packet bursting are not required in a full-duplex Gigabit
environment.
Theoretically, IEEE
802.3ab intended to make use of much of the existing Category 5 cabling
by enabling 1000BASE-T to operate at the 100 MHz rating of CAT 5 UTP.
The cabling system used to support 1000BASE-T requires four pairs of
Category 5 balanced cabling with nominal impedance of 100 ohms as
required in the TIA/EIA 568-A standard. The demands placed on a
Category 5 cabling plant to support Gigabit speed may surpass the
ability of much of the installed base of Category 5 cable to support it
reliably. To make certain that a given Category 5 cabling plant is able
to support 1000BASE-T, IEEE developed additional requirements.
In addition to the
requirements stated in EIA/TIA 568A for Category 5 cabling,
additional requirements were added (Annex 40A) with further
requirements for the 1000BASE-T installations. The transmission
parameters in Annex 40A call out additional requirements for
insertion loss, delay, characteristic impedance, return loss, and
most importantly, NEXT/ELFEXT (Near-end cross talk/equal level
far-end cross talk). Cross-talk is simply electrical interference to
each of the individual transmitters caused by noise from the other
three transmitters on a segment (NEXT) or interference to each
receiver caused by the three adjacent transmitters (ELFEXT).
Effectively, much of the installed Category 5 UTP, because it was
installed prior to the publication of Annex 40A, and therefore, not
tested to meet its requirements, may not support 1000BASE-T. To
provide a safety margin, some network hardware manufacturers
recommend that Category 5e cabling be used in 1000BASE-T
installations. Category 5e installation requirements include testing
parameters for return loss, NEXT, ELFEXT, etc., with a built in
margin for error sufficient to support 1000BASE-T (designed with
1000BASE-T in mind).
The cost benefit
of being able to deploy Gigabit over the existing Category 5 cabling
plant is enticing, but can be somewhat diminished, since it is
believed that many existing Category 5 cabling plants will not
support 1000BASE-T. Additionally, the cost of installing, testing,
and certifying Category 5E or Category 6 cabling (including cables,
cords & connectors) exceeds that of Category 5 UTP. However, the
lower cost of 1000BASE-T (copper) devices, when compared to
1000BASE-X (fiber) devices, tends to offset this to some degree. You
are also limited to 100 meters with UTP in 1000BASE-T, whereas you
can transmit up to 550 meters across multimode fiber (see Table 1)
with additional headroom for more bandwidth in the future.
Table 1
Ethernet
10BASE-T/FL
Fast Ethernet
100BASE-TX/FX
Gigabit Ethernet
1000BASE xx
Data Rate
10 Mbps
100
Mbps
1 Gbps
Category 5
(UTP)
100 m
100 m
100 m
STP/Coax
500 m
100 m
25 m
Multimode
Fiber
62.5 micron
2 km
412 m
half-duplex
2 km full-duplex
220 m**
Multimode
Fiber
50 micron
2 km
412 m
half-duplex
2 km full-duplex
550 m**
Single
Mode Fiber
20 km
long-haul: 80 km
20 km
long-haul: 80 km
5 km
long-haul: 65 km
Repeats per Segment
3
2
1
**Dependant on
modal bandwidth of the fiber and wavelength of the optics
used.
With the advent of
Fast Ethernet (100 Mbps), a means of establishing backwards
compatibility with existing Ethernet (10 Mbps) devices had to be
established. The IEEE, when they released the 802.3u standard several
years ago, addressed this with a method known as "auto-negotiation".
Auto-negotiation in Fast Ethernet can be viewed as a language that
10/100 (referred to sometimes as N-way or dual-speed) devices speak to
establish at what speed and in what mode the link should operate. Fast
Ethernet auto-negotiation "advertises" its capabilities to the device
it is connected to in terms of speed (10 or 100 Mbps), and mode (full
or half-duplex) of operation. Based on the capabilities of the device
it is communicating with, it will establish the most appropriate
link.
In 1000BASE-T,
auto-negotiation has four elements, rather than two, as in Fast
Ethernet. Because it has to be backwards compatible with other
Ethernet devices, it too must be able to establish speed (10, 100 or
1000 Mbps) and mode of operation (full-or half-duplex). Additionally,
1000BASE-T requires that auto-negotiation establish a Master-Slave
relationship between the two devices, as well as establish if both
devices are "pause" enabled. Typically, a multi-port device (switch)
will assume the role of Master and single-port devices (servers, end
stations, etc.) will be the slave unit. The 1000BASE-T protocol has a
means of arbitrating which device will assume a given role
(master/slave). The purpose of this Master/Slave relationship is to
determine which device will provide the clock for the link.
Additionally, because of the high transmission speed of Gigabit,
there is the possibility that the device being transmitted to may be
overwhelmed with data. Pause is a means by which the device receiving
the data can signal to the transmitting device to stop transmitting
momentarily so that it can "catch up."
Auto-negotiation
does not currently exist for Gigabit devices with fiber interfaces
due to differences in transmission wavelengths (850nm vs. 1300nm)
between Ethernet, Fast Ethernet and Gigabit.
If you are going to incorporate Gigabit Ethernet into your network
there is a strong possibility that you will need to implement some of
your Gigabit connections over fiber optic media. You will most likely
be doing this because of the distance limitations of 100 meters over
copper media (UTP). But what distances can you achieve using fiber
optic cabling? The answer to that question is: it depends.
With the increase
of transmission speed to 1 Gbps, the bandwidth carrying capacity of
the fiber optic media you are using will come into question. The
"Modal Bandwidth" is a measure of the information carrying capacity
of the given medium and is expressed in MHz per kilometer. In optical
fiber, bits of data are represented by pulses of light. Each pulse of
light will disperse, or spread, over time and distance as it travels
through the fiber optic cable. As these bits of data disburse, they
eventually begin to overlap with each other and distort the data that
is being transmitted. A multimode fiber optic cable's data carrying
capacity is ultimately determined by its "dispersion
characteristics."
There are two
types of dispersion: intermodal dispersion and chromatic dispersion.
Intermodal dispersion occurs because, in multimode fiber, light can
travel in multiple modes or paths, which can ultimately arrive at
different times at the receive end of the fiber. Chromatic dispersion
is caused by the slight variations in wavelengths of light that are
transmitted over the fiber. Different wavelengths of light travel at
different speeds, and over a given distance will eventually begin to
overlap. Intermodal dispersion is not a factor in single mode fiber.
Chromatic dispersion affects all fiber but it affects single mode
fiber to a lesser degree.
Figure 1: Intermodal & Chromatic Dispersion
To truly
understand the distance that you can transmit data at Gigabit speeds
over multimode fiber you must take into account the modal bandwidth
of the fiber. Typically, modal bandwidth specifications are provided
by the fiber manufacturer. Table 2 offers some general guidelines for
determining approximate distances that can be achieved over a Gigabit
fiber link.
Table 2: Typical Modal Bandwidth and Corresponding Distances for
Gigabit Transmission Over Fiber Optic Media
100BASE-SX
Fiber Type
Modal Bandwidth
at 850nm
Range
62.5
micro m
160
MHz/km
220 m
62.5
micro m
200
MHz/km
275 m
50
micro m
400
MHz/km
500 m
50
micro m
500
MHz/km
550 m
100BASE-LX
Fiber Type
Modal Bandwidth
at 1300nm
Range
62.5
micro m
500
MHz/km
550 m
50
micro m
400
MHz/km
550 m
50
micro m
500
MHz/km
550 m
10
micro m single mode
N/A
5,000 m
Mixed Gigabit Copper and Fiber Environments
(back to top)
As mentioned earlier, there is a strong possibility that many people
implementing Gigabit in their networks will be transmitting over both
copper and fiber optic media. The lower cost potential of UTP copper
interfaces will compel people to deploy 1000BASE-T in some
applications, while the requirement to extend distances beyond the 100
meter limitation of copper will require that they deploy 1000BASE-SX/LX
in other applications. There are several options available for those
who face this mixed-medium challenge. Options include:
Purchasing
separate network devices; some with copper, some with fiber
interfaces
Purchasing a
network device that has a slide-in-module option that offers a
choice of copper or fiber (Gbic) interfaces
Using an
external device such as a media converter
In case you
haven't looked recently, Gigabit Ethernet devices are not among the
least expensive pieces of network hardware available. Although they
may be worth their weight in gold when network bandwidth reaches
critical mass, the cost can be imposing. Being a relatively new
technology, the configuration options for Gigabit devices may be
difficult to match to your specific needs. Effectively, you may end
up purchasing more Gigabit ports than you require for your
application, and at a time when the cost of the technology is at a
premium. (As the technology matures, prices will eventually
decrease.)
There is always
the option of purchasing a Gigabit device that offers a
slide-in-module option for adding either copper or fiber modules.
These devices typically offer a single port for an uplink module
that can be populated with a fiber Gbic or copper UTP card. The
intent is, most often, to use this as a backbone connection, and is
most commonly populated with a fiber Gbic slide-in-card. This is a
good choice if all you require is a single Gigabit backbone
connection. However, there are two issues that may require
additional consideration. Although you have the flexibility of
"modularity" with the slide-in-card, the remaining ports will
likely be of a "fixed" configuration. Should you need to change the
type of media on any of these fixed ports you, will need to employ
an external device, such as a media converter, or purchase a new
network device altogether. You need to also consider that not all
Gigabit network devices offer a modular uplink port, and if they
do, it is usually at a premium.
There is yet
another option that offers the flexibility of being able to add any
port configuration (copper or fiber) you require where and when you
need it. This device can allow you to take advantage of the lower
cost of purchasing a switch with copper Gigabit interfaces and add
Gigabit fiber links only where you need them. The device is known
as a "Media Converter," and can be used to change one media type
(copper or fiber) to another media type (copper or fiber) to
facilitate the transition between two disparate media types. There
are media converters that will allow you to convert 1000BASE-T to
1000BASE-SX/LX or to convert 1000BASE-SX multimode to 1000BASE-LX
single mode port-by-port as the need arises. Converters can be
added to a Gigabit link without disturbing the communication over
other links on the device. Media converters are also relatively
inexpensive and can be managed via an SNMP GUI interface.
Long Haul Options - Extending the Reach of your LAN
(back to top)
An option that is currently unique to media converters is the
concept of "Long Haul" for extending fiber connections up to 65
kilometers away. Longer distances can be achieved across a Gigabit
fiber link by increasing the launch power and receive sensitivity of
the fiber optic transceivers used. These long haul options are
currently not available from major switch manufacturers. There are
several applications for "Long Haul" Gigabit devices.
MANs -
Metropolitan Area Networks
Large Campus
LANs
There is a growing
popularity of MANs (metropolitan area networks) which allow native
LAN protocols to be transmitted between remote facilities over leased
or privately owned single mode fiber. Entities that have remote
facilities can effectively make a LAN backbone connection between
buildings that are as far as 65 kilometers away from each other. In
recent years, companies such as telcos, public utilities, etc., have
been installing vast amounts of single mode fiber optic cable in
their right-of-ways. Much of this fiber (as much as 90%) is currently
"dark" (not currently being used). Companies with excess fiber are,
in many cases, offering access to their fiber through a lease
arrangement.
There are
significant benefits to making a native Gigabit backbone connection
between facilities compared to other options. Connecting LANs
together using more conventional methods, such as T1 or leased lines,
are expensive and require additional active hardware (routers, etc.)
to facilitate the translation of transmission protocols. Also, most
commonly used WAN protocols cannot transmit at speeds anywhere near
Gigabit (i.e. T1 transmits at only 1.544 Mbps). Lastly, a leased or
private fiber connection offers you security (you are the only one
who has access to the fiber), and the ability to manage the
connection remotely via your existing SNMP management software
(converters as well as Gigabit switches are capable of management via
SNMP). Transmitting native Gigabit is more efficient, easier to
maintain, and less expensive than other options. There may also be
applications for long haul Gigabit devices in larger campus
environments where single mode connections must be made over
distances greater than 5 kilometers (theoretical maximum distance for
Gigabit over standard single mode connection).
There may also be
instances in campus environments where there is excess attenuation
resulting from loss at multiple connection points (i.e. patch
panels). A larger number of termination points may be used in
campuses as the single mode fiber moves from building to building. To
overcome attenuation that falls outside the norm, a high powered
single mode device can be employed.
Media Converters
offer the option of long haul connections over single mode fiber that
range from 5 kilometers (standard) to 15 kilometers (long haul) all
the way up to 65 kilometers (long wavelength 1550nm long haul).
Whether you need distance extension on an existing multimode fiber or
1000BASE-T Gigabit link, media converters can meet most requirements
efficiently and economically.
Figure 2: Media Converter Application
Note: Keep in mind
that distances are estimates based on industry averages for
attenuation. To calculate the actual distance you can transmit across
a given fiber link you should always properly calculate the link
budget.
The IEEE 802.3z and 802.3ab task forces have developed standards that
have enabled network hardware manufacturers to produce products with
transmission speeds of 1 Gigabit per second. Most recently, they have
developed standards that allow Gigabit speed over UTP copper cabling.
Although some issues do exist with regard to cost (cabling
infrastructure) and throughput (half-duplex), 1000BASE-T is well on its
way to becoming a viable standard. Fiber-based solutions for Gigabit,
although relatively expensive, offer network managers the additional
reach to connect virtually any two network devices together. In most
cases, a need will exist for both the cost benefit of copper and the
distance benefit of fiber in a Gigabit-enabled network. Various options
exist to facilitate the co-existance of copper and fiber in a Gigabit
network. Conversion technology offers an easy to implement, low-cost
option for connecting disparate interfaces with long haul options not
available from major switch manufacturers.
