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Port Sound
Bandwidth
Configuring SANs: Dos and Don ts
Chapter 4
60
users connecting at 1.544 Mbps. This may be fine for a local carrier,
but the major network access points around the United States will
have trouble keeping up if DSL and other broadband services are as
widespread as 56 kbps modems are today. The largest access points
MAE-East (Washington, DC) and MAE-West (San Jose, CA) have
combined traffic that has more than quadrupled in each of the 2
years since 1995. Combined traffic reached about 1.5 Gbps on aver-
age in mid-1997. This is an equivalent of about 12,000 phone calls,
and this figure indicates that the Internet is insufferably slow
because there had to be more Internet traffic than the equivalent
12,000 phone calls around the United States in 1997.
Possible Solutions to the Bandwidth Crisis
There are various solutions to ease the bandwidth crisis. Carriers
can install more fiber-optic cable into their infrastructures. For long-
distance carriers whose networks exceed 10,000 miles of cable
across the United States, installing more fiber-optic cable is not
logistically feasible. Even for small-distance carriers (300 km or
less), laying extra cable in a metropolitan area may take months to
achieve, requiring government paperwork and clearances prior to
beginning construction. In either case, carriers are turning to
DWDM as a SAN solution. By combining multiple wavelengths, each
representing separate data channels, the same fiber-optic cable sud-
denly has the bandwidth capacity of multiple cables. An additional
benefit of DWDM is that repeaters commonly used in Time Division
Multiplexing (TDM) networks, such as SONET, are replaced by opti-
cal amplifiers. Unlike repeaters, optical amplifiers can amplify mul-
tiplexed signals without demultiplexing first. This reduces
intermittent bottlenecking. Optical amplifiers also can be placed far-
ther apart than repeaters.
With TDM, bandwidth is inversely proportional to pulse width
twice. In other words, a higher bandwidth in a TDM system will cre-
ate a shorter pulse width, equaling a higher frequency and making
it more susceptible to fiber dispersion. As the bandwidth require-
ment increases in the future, researchers are faced with the dis-
persion issue if TDM is to be competitive. DWDM increases its
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Copyright � 2004 The McGraw-Hill Companies. All rights reserved.
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Configuring SANs: Dos and Don ts
Configuring SANs: Dos and Don'ts
61
bandwidth by adding extra wavelengths and has a theoretical band-
width of 5,000 GHz in the 1,550 nm region.
Notice that in applications where recoverability is essential,
SONET (TDM) will still be the first choice because of SONET s
50-ms recoverability. In the case of a path interruption, this is some-
thing DWDM cannot achieve today by itself.
What Is DWDM?
DWDM is a technology that transmits multiple data signals using dif-
ferent wavelengths of light through a single fiber. As illustrated in
Figure 4-12, incoming optical signals are assigned to specific frequen-
cies within a designated frequency band. The capacity of the fiber is
increased when these signals are multiplexed out onto one fiber.
The best way to describe DWDM technology is by readdressing
the earlier analogy of a car traveling on a highway. The analogy
compared the SAN environment and movement of data to one fiber
traveling to a multilane highway using one lane. It stated that tra-
ditional systems use one lane of the highway. Moving the cars, or sig-
nals, faster within the one lane increases the capacity of the
highway. DWDM technology accesses the unused lanes, increasing
the number of wavelengths on the embedded fiber base and fully
using the untapped capacity within the fiber.
Figure 4-12
DWDM
Independence
of Bit Rates
and Formats
" Merges optical traffic onto one common fiber
" Allows high flexibility in expanding bandwidth
" Reduces costly mux/demux function, reuses existing optical signals
" Individual channels use original OAM&P
DWTM = Dense WDM
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Copyright � 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
Configuring SANs: Dos and Don ts
Chapter 4
62
DWDM is able to reach transmission capabilities four to eight
times faster than traditional systems. This high-speed, high-volume
transmission is made possible by the technology within the optical
amplifier. An optical amplifier is a section of fiberoptic cable that has
been doped with erbium to amplify the optical signal. Erbium-doped
fiber amplifiers (EDFAs) have two advantages: they increase optical
signals, and they do not have to regenerate the signals to boost
strength. A typical optical signal must be regenerated every 100 km.
This is accomplished by converting an optical signal to an electrical [ Pobierz całość w formacie PDF ]
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Any use is subject to the Terms of Use as given at the website.
Port Sound
Bandwidth
Configuring SANs: Dos and Don ts
Chapter 4
60
users connecting at 1.544 Mbps. This may be fine for a local carrier,
but the major network access points around the United States will
have trouble keeping up if DSL and other broadband services are as
widespread as 56 kbps modems are today. The largest access points
MAE-East (Washington, DC) and MAE-West (San Jose, CA) have
combined traffic that has more than quadrupled in each of the 2
years since 1995. Combined traffic reached about 1.5 Gbps on aver-
age in mid-1997. This is an equivalent of about 12,000 phone calls,
and this figure indicates that the Internet is insufferably slow
because there had to be more Internet traffic than the equivalent
12,000 phone calls around the United States in 1997.
Possible Solutions to the Bandwidth Crisis
There are various solutions to ease the bandwidth crisis. Carriers
can install more fiber-optic cable into their infrastructures. For long-
distance carriers whose networks exceed 10,000 miles of cable
across the United States, installing more fiber-optic cable is not
logistically feasible. Even for small-distance carriers (300 km or
less), laying extra cable in a metropolitan area may take months to
achieve, requiring government paperwork and clearances prior to
beginning construction. In either case, carriers are turning to
DWDM as a SAN solution. By combining multiple wavelengths, each
representing separate data channels, the same fiber-optic cable sud-
denly has the bandwidth capacity of multiple cables. An additional
benefit of DWDM is that repeaters commonly used in Time Division
Multiplexing (TDM) networks, such as SONET, are replaced by opti-
cal amplifiers. Unlike repeaters, optical amplifiers can amplify mul-
tiplexed signals without demultiplexing first. This reduces
intermittent bottlenecking. Optical amplifiers also can be placed far-
ther apart than repeaters.
With TDM, bandwidth is inversely proportional to pulse width
twice. In other words, a higher bandwidth in a TDM system will cre-
ate a shorter pulse width, equaling a higher frequency and making
it more susceptible to fiber dispersion. As the bandwidth require-
ment increases in the future, researchers are faced with the dis-
persion issue if TDM is to be competitive. DWDM increases its
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright � 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
Configuring SANs: Dos and Don ts
Configuring SANs: Dos and Don'ts
61
bandwidth by adding extra wavelengths and has a theoretical band-
width of 5,000 GHz in the 1,550 nm region.
Notice that in applications where recoverability is essential,
SONET (TDM) will still be the first choice because of SONET s
50-ms recoverability. In the case of a path interruption, this is some-
thing DWDM cannot achieve today by itself.
What Is DWDM?
DWDM is a technology that transmits multiple data signals using dif-
ferent wavelengths of light through a single fiber. As illustrated in
Figure 4-12, incoming optical signals are assigned to specific frequen-
cies within a designated frequency band. The capacity of the fiber is
increased when these signals are multiplexed out onto one fiber.
The best way to describe DWDM technology is by readdressing
the earlier analogy of a car traveling on a highway. The analogy
compared the SAN environment and movement of data to one fiber
traveling to a multilane highway using one lane. It stated that tra-
ditional systems use one lane of the highway. Moving the cars, or sig-
nals, faster within the one lane increases the capacity of the
highway. DWDM technology accesses the unused lanes, increasing
the number of wavelengths on the embedded fiber base and fully
using the untapped capacity within the fiber.
Figure 4-12
DWDM
Independence
of Bit Rates
and Formats
" Merges optical traffic onto one common fiber
" Allows high flexibility in expanding bandwidth
" Reduces costly mux/demux function, reuses existing optical signals
" Individual channels use original OAM&P
DWTM = Dense WDM
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright � 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
Configuring SANs: Dos and Don ts
Chapter 4
62
DWDM is able to reach transmission capabilities four to eight
times faster than traditional systems. This high-speed, high-volume
transmission is made possible by the technology within the optical
amplifier. An optical amplifier is a section of fiberoptic cable that has
been doped with erbium to amplify the optical signal. Erbium-doped
fiber amplifiers (EDFAs) have two advantages: they increase optical
signals, and they do not have to regenerate the signals to boost
strength. A typical optical signal must be regenerated every 100 km.
This is accomplished by converting an optical signal to an electrical [ Pobierz całość w formacie PDF ]