138 Fiber Optic Essentials
Government supported networks like these are often used as examples in
the technical literature to illustrate network traffic management models.
6.3 Latency
Dense wavelength division multiplexing devices also function as channel
extenders, allowing many datacom protocols to reach previously impos-
sible distances (50–100 km or more). Combined with optical amplifier
1
E
W
2
E
W
3
E
W
ATM IP TDM
Terminal site
1
E
W
2
E
W
3
E
W
ATM IP TDM
Terminal site
(a)
3
E
W
2
E
W
ATMIP
ATMIP
1
E
W
ATM IP
OADM site OADM site
OADM site
1
E
W
2
E
W
3
E
W
ATM IP TDM
Terminal Site
(b)
Figure 6.3 WDM network topologies (a) point-to-point (b) hubbed ring.
6. Wavelength Multiplexing 139
1
E
W
IP
OADM
2
E
W
ATM
OADM
1
E
W
2
E
W
ATM IP
Terminal
1
E
W
2
E
W
ATM
IP
Terminal
(c)
1
E
W
2
E
W
OADM or
Terminal site
2
E
W
OADM or
Terminal site D
1
E
W
OADM or
Terminal site A
1
E
W
OADM or
Terminal site C
2
E
W
OADM or
Terminal site B
(d)
1
E
W
2
E
W
ATM IP
OADM site
Terminal site
2
E
W
IP
Terminal site
1
E
W
ATM
(e)
Figure 6.3 (continued) (c) dual hubbed ring (d) meshed ring (e) linear optical
add/drop multiplexer (OADM).
Abilene International Network Peers
Pacific Wave
AARNET,
CA*net, GEMNet,
KOREN/KREONET2,
SingAREN, TANet2
New York
SINET,
SURFnet
/IEEAF-TYC
StarLight
ASNet, CA*NET, CERN/DataTAG, GÉANT*/EuroLink,
HARNET, JGN2 (APAN), KOREN/KREONET2, GLORIAD**,
SURFnet, TANet2, CERNET
MANLAN
CA*Net, GÉANT*,
HEANET, SINET,
SURFnet
/IEEAF-TYC
Qatar FN
Los Angeles
APAN/TransPAC
UNINET
Pacific Wave
Seattle
Sunnyvale
Houston
San Diego
(via CALREN2)
CUDI
EI Paso
(via UTEP, UT)
CUDI
Miami (via AMPATH)
ANSP, RETINA,
RNP2, REACCIUN-2
Denver
Chicago
Kansas City
Atlanta
Washington
GÉANT*
Indianapolis
*vis GEANT: ACOnet, ARNES, BELNET, CARNet, CERN, CESNET, CYNET, EENet, Forskningsneltet, Funet,
G-WIN, GARR, GRNET, HEAnet, HUNGARNET, IUCC, JANET, LANET, LITNET, Univ, Malta, POL34, RBnet,
RCTS2, RedIRIS, Renater, RESTENA, REUNA2, Rhnet, RoEduNet, SANET, SUNET, SURFnet, SWITCH,
ULAKBYM, UNINETT
*via APAN/TransPAC: WIDE/JGN, IMna1, CERNet
/CSTnet /NSFCNET, KOREN/KREONET2, PREGINET,
SingAREN, TANET2, ThaiSARN, WIDE (v6)
**vis GLORIAD: CSTNET, RBnet
Figure 6.4 Peer nodes for the initial implementation of the next generation optical Internet.
6. Wavelength Multiplexing 141
technology, this has led some industry analysts to proclaim “the death
of distance,” that is connection distances should no longer pose a seri-
ous limitation in optical network design. However, in many real world
applications, it is not sufficient to simply extend a physical connection;
performance of the attached datacom equipment must also be consid-
ered. Latency, or propagation delay due to extended distances, remains
a formidable problem for optical data communication. The effects of
latency are often protocol specific or device specific. For example, using
DWDM technology it is possible to extend an ESCON channel to well
over 50 km. However, many ESCON control units and DASD are syn-
chronous, and exhibit timing problems at distances beyond about 43 km.
Some types of asynchronous DASD overcome this limitation; however,
performance of the ESCON protocol also degrades with distance. Due
to factors such as the buffer size on an ESCON channel interface card
and the relatively large number of acknowledgments (ACKs) or hand-
shakes required to complete a data block transfer (up to six or more),
ESCON begins to exhibit performance droop at around 9 km on a typical
channel, which grows progressively worse at longer distances. For exam-
ple, at 23 km a typical ESCON channel has degraded from a maximum
throughput of 17.5 MB/s to about 10 MB/s; if the application is a SAN
trying to back up a petabyte database, the time required to complete a full
backup operation increases significantly. This problem can be addressed
to some degree by using more channels (possibly driving the need for a
multiplexer to avoid fiber exhaust) or larger data block sizes, and is also
somewhat application dependent. Other protocols, such as FICON, can
be designed to perform much better at extended distances.
A further consideration is the performance of the attached computer
equipment. For example, consider the effect of a 10 km duplex channel
with 100 ms round trip latency. A fast PC running at 500 MHz clock
rate would expend 50,000 clock cycles waiting for the attached device
to respond, while a mainframe executing 1000 MIPS could expend over
100,000 instruction cycles in the same amount of time. The effect this
may have on the end user depends on factors such as the application
software. As another example, there is tremendous activity in burst mode
routing and control traffic for next generation Internet (NGI) applications.
In many cases, it is desirable for data to be transported from one point
in the network to one or more other points in the least possible time. For
some applications the time sensitivity is so important that minimum delay
is the overriding factor for all protocol and equipment design decisions.
A number of schemes have been proposed to meet this requirement,

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