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4. Adding traffic
.1 Executive summary of chapter
This chapter describes the various external systems that use the optical WDM netor! to transport information
and the transponders and muxponders acting as interfaces beteen the external systems and the WDM netor!.
This chapter also explains ho the information is framed hen it is forarded beteen the netor! nodes over
the WDM light path. A closer loo! at ho the Transmode uni"ue internal framing #$ntelligent WDM % iWDM& can
be leveraged in mobile bac!haul and 'igabit Ethernet access netor!s. The chapter explains( )D*+ ),-ET and ,T-.
ast Ethernet+ 'igabit Ethernet+ 1/ 'bit Ethernet and higher speed Ethernet.
)torage Area -etor!s #iber 0hannel and E)0,-&.
ine side data rates ith iWDM or ,T- framing.
iWDM applications.
.2 $ntroduction
The preceding to chapters describe ho continuous connections of avelengths 3 light paths 3 are established
and managed in a WDM optical netor!. The light paths are the circuits5 of the optical netor!+ circuits hich
are put to or! and used by traffic units 3 the transponders and the muxponders. The light paths ma!e up the
physical5 transmission lin!s beteen the client systems attached to the optical netor!. #igure 62&
igure 62.Example of a light path using avelength 7 beteen to transponders.
A light path in a WDM netor! relies upon analog technology and therefore re"uires a strictly controlled optical
environment for error free transmission. The transponders and muxponders serve as buffers beteen the servedtraffic layer and the optical #WDM& layer. As a result+ client e"uipment of different types #e.g. )D* multiplexers
and $8 routers& and from different vendors can be connected to the TM%)eries netor! ithout impacting the
parameters of the transmission path through the optical domain.
When designing the optical layer of the netor!+ i.e. the grid of light paths beteen endpoints+ to main
parameters are fundamental( The number of avelengths needed to establish the desired connections ithout collisions on the
ay5+ i.e. each light path must have its on avelength#s& assigned for the entirety of its route. The distances involved+ hich affects the type of transceivers #)89 :8& to be used and if the WDM
netor! must include amplifiers.
$n the traffic layer#s&+ i.e. the end to end logical connection beteen the client systems+ digital signals are
processed in the electrical domain+ ith transponders9muxponders performing functions such as fixed time
division multiplexing or statistical time division multiplexing #pac!et sitching&. Transponders9muxponders
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aggregate and bring a variety of loer%speed voice+ data and leased line services into the WDM netor!. Each of
these client netor!s is important in its on right and can typically operate over point%to%point fiber lin!s as ell
as over a more sophisticated optical layer+ using the light paths of WDM. inally+ the transponders9 muxponders
also add management and control information for the lin! and netor! to the bits sent over the light path.
When designing the traffic layers of the netor!+ an additional set of parameters become important( The data rate #Mbit9s& of the signal to be transported. $n time division multiplexing+ the outgoing line rate
of the transponder9muxponder must be sufficient to accommodate all the incoming multiplexed data streams.The line rates currently available ith the TM%)eries are 2.; 'bit9s+ 4 'bit9s+ 1/ 'bit9s 14+ 4/ 'bit9s and 1//'bit9s.
The format #framing& of the client signal and ho signaling beteen the end points is done+ i.e. the
protocol employed. $f data can be manipulated5+ e.g. compressed by the transponder9 muxponder or if the connection
must be fully transparent. *o and if exact time synchronising the terminology of optical netor!ing+ a client system is any digital communications e"uipment that is
connected to and ma!es use of the optical netor!. A client system can be an )D*9),-ET multiplexer+ an $8
router or an Ethernet sitch+ all capable of sending and receiving digital signals. The connection beteen the
client system and the netor! #i.e. the interface beteen the client system and the transponder9muxponder& can
be optical over a multimode or single mode fiber+ or electrical+ e.g. a coaxial cable.
The client system exchanges data ith its peer client systems according to a given set of rules for ho data shall
be formatted hen being sent and received 3 it follos a communications protocol. )ince each communications
protocol has its on characteristics+ it becomes an important parameter hen selecting a suitable transponder or
muxponder. $t is therefore necessary to understand the basics of the most common telecommunications
protocols+ hen designing an optical netor! ith the TM%)eries.
.=.1 )D* and ),-ET
)ynchronous Digital *ierarchy #)D*& and )ynchronous ,ptical -etor!ing #),-ET& are standardi
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into all its individual components. 0onse"uently+ less e"uipment is needed and the management of multiple+ real
time+ bit streams become less costly than ith earlier multiplexing schemes. urthermore+ the )D*9),-ET
multiplexing is fully compatible ith earlier standards such as 8D*16+ alloing the encapsulated data to have its
on frame rate and be able to float around5 relative to the )D*9),-ET frame structure and rate. Another
important feature of )D*9),-ET is that it has been designed for use in public netor!s and includes extensivenetor! management capabilities to provide a carrier grade service of high availability. When WDM is used to
upgrade the capacity of a public metro or long haul netor!+ it is very li!ely that some of the client systems are
)D*9),-ET multiplexers+ due to the fre"uent use of this technology.
The basic unit of framing in )D* is the )TM%1 #)ynchronous Transport Module+ level 1&+ hich operates at 1;;.;2
Mbit9s. ),-ET refers to this basic unit as an )T)%=c #)ynchronous Transport )ignal =+ concatenated& or ,0%=c+
depending on hether the signal is carried electrically #)T)& or optically #,0&+ but its high%level functionality+
frame si
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igure 6 .The ),-ET and )D* multiplexing hierarchies.
)D* and ),-ET netor!s have been in existence for several decades and often encompass complex structures
of the public telecom netor!. The folloing figure shos a typical example of such a ),-ET netor!+
comprising fiber lin!s beteen add9drop multiplexers #ADM&+ digital cross connects #D0)& and optical l ine
terminals #TM&. When designing a WDM%system for this netor!+ light paths carrying signals ith sufficient data
rates have to be created beteen all the individual ),-ET netor! elements.
1;The 'eneric raming 8rocedure #'8& is an )D*9),-ET and ,T- adaptation method that or!s for a variety of
data netor!ing protocols+ including $8+ Ethernet and ibre 0hannel.
168lesiochronous Digital *ierarchy. An $T>%T standard from the mid%1C6/s for the multiplexing of digital voice
circuits ithout re"uirement on full signal synchroni
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igure 6;.)D*9),-ET client interfaces of TM%)eries Transponders and Muxponders.
.=.2 ,ptical Transport -etor! #,T-&
The ,ptical Transport -etor! #,T-& is a more recent addition to the standards for public telecommunications
netor!s and is sometimes referred to by its $T>%T name './C. The standard as designed to transport both
pac!et mode traffic such as $8 and Ethernet+ and legacy )D*9 ),-ET traffic over fiber optics. $t supports forard
error correction #E0& and management functions for monitoring a connection end%to%end over multiple transport
segments.
The amount of installed ,T- e"uipment is currently still limited in many regions but ,T- is becoming more
idely deployed as ),-ET9)D* solutions reach end of life. Today+ ,T- has its main application in the long haul
netor! here error correction and interoperability beteen several operators e"uipment are important.
,T- is a digital rapper technology that raps any client signal in overhead information for operations+
administration and management. The basic unit of information transport in the protocol is the ,ptical 0hannel
Data >nit #,D>& hich is carried ithin an ,ptical 0hannel Transport >nit #,T>& defining the line rate of the
connection. The line rates of ,T- are referred to as ,T>1+ ,T>2 and ,T>= and are different from the )D*9
),-ET line rates. #igure 66&
igure 66.,T- line rates compared ith )D*9),-ET line rates.
The $T>%T './C standard defines the format and data rates for the ,D> signals. #igure 6&
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igure 6A.$T>%T './C ,D> definitions and application examples. )ource( Wi!ipedia
The TM%)eries includes the folloing traffic units for transparent transport of ,T-9,D> signals. #igure 6&
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sitched Ethernet. $n sitched Ethernet all Ethernet stations have their on+ individual+ full duplex+ connection to
a central sitch #sometimes called multiport bridge&. The sitch has a forarding table hich matches Ethernet
addresses ith a corresponding port+ and sends the frame to the correct destination. )itches are then
interconnected and form more or less complex mesh structures. #igure /&
igure A/.A meshed and sitched Ethernet netor!.
$n a meshed Ethernet+ there are several paths beteen nodes+ and frames could be forarded in infinite loops
ithin the netor!+ if no countermeasures ere ta!en 3 there must be one and only one open route beteeneach node of the netor!+ and all other interconnecting ports of the sitches must be bloc!ed. )uch a netor!
topology is called a spanning tree. #igure 1&
igure A1.A spanning tree #blue lines& in a mesh grid
The )panning Tree 8rotocol #)T8& in the Ethernet standard is a distributed algorithm run by the sitches to form a
spanning tree. The sitches identify alternative routes+ use eights5 assigned to the lin!s to select the
shortest9fastest lin!+ and bloc! ports not to be used. $n case of lin! failure+ a recalculation is done and an
alternative route for the frames is selected. The original protocol has been further improved #@)T8+ @apid
)panning Tree&+ to reduce the convergence time to compute a ne spanning tree hen there is a topological
change. $t could ta!e the )T8 protocol =/ 3 ;/ s to recalculate the spanning tree after a lin! outage+ but @)T8
and further refinements have ta!en the recovery time don to some milliseconds.
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Another important Ethernet feature is the possibility to define a virtual A- #HA-&. A HA- allos bandidth to
be divided up beteen groups of nodes+ and in a ay so that users in each group can only communicate ithin its
on HA-. All Ethernet frames in a HA- have a distinct identifier called a HA- tag. HA-s can be used to
implement virtual private netor!s #H8-& and HA- frames include priority fields that can be used to support a
differentiation of "uality of service. #igure 2&
igure A2.To virtual A-s sharing a common ide area connection
.=.=.2 Ethernet physical media #8*I&
The Ethernet standard has a data lin! layer and a physical media #8*I& part+ the latter being specific for the
media and data rate employed. When Ethernet is transported over a WDM ide area netor!+ the 'igabit
Ethernet and the 1/%'igabit Ethernet 8*I standards are of most interest since these are the types of Ethernet
deployed in metropolitan and other ide area netor!s. The TM%)eries includes transponders and muxponders
that can be e"uipped ith transceivers supporting fast Ethernet+ 'igabit Ethernet and 1/94/91//%'igabit
Ethernet on the client system side.
A)T ET*E@-ET #E& 8*I)$0A AIE@
ast Ethernet+ i.e. Ethernet at 1// Mbit9s+ has to predominant physical formats( 1//?A)E%T: that runs over to
ire unshielded tisted copper pairs #>T8& and 1//?A)E%: that runs over optical fiber. 1//?A)E%: uses 1=//
nm light transmitted via to strands of optical fiber+ one for receive #@:& and the other for transmit #T:&. Maximum
length is 4// m for halfduplex connections and 2 !m for full%duplex over multi%mode optical fiber.
The 1// in the media type designation refers to the transmission speed of 1// Mbit9s. The ?A)E5 refers to
baseband signaling+ hich means that only Ethernet signals are carried on the medium. The T: and : refer to
the physical medium that carries the signal.
The TM%)eries traffic units support both the electrical 1//?A)E%T: and the optical 1//?A)E%: client interfaces.
'$'A?$T ET*E@-ET #'?E& 8*I)$0A AIE@
'igabit Ethernet #'bE& can be transmitted over shielded fiber cables and over shielded copper cables. $t can also
be transmitted over unshielded tisted pairs of copper. The transmission is set up to operate in full duplex #mostcommon& or half duplex mode. The standard defines a physical media dependent #8MD& sub layer hich specifies
the transceiver for the physical medium in use. There are three types of 8MDs for 'bE( )hort range+ long range
and shielded copper. The short range uses ;/ nm light ith a reach of 22/ 3 2;/ m on multimode fiber. The
long range 8MD uses 1=1/ nm light ith a reach of ;;/ m on multimode fiber and ; !m on single mode fiber. The
8MD for shielded copper reaches only 2; m.
or unshielded copper+ hich is common in many office installations+ multiple tisted pairs are used to send
multilevel signals in a ay that extends the reach to 1// m. #igure =&
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A= .'igabit Ethernet physical media. )ource( Wi!ipedia
The TM%)eries traffic units support both the electrical #1///?A)E%T& and the optical #1///?A)E%& variants for
single and multimode fiber in the 'bE physical layer hen interfacing to client systems.
1/%'$'A?$T ET*E@-ET #1/'?E& 8*I)$0A AIE@
1/%'igabit Ethernet #1/'bE& can be transmitted over fiber optics and copper cables+ but copper cables are only
used over very short distances such as interconnections ithin a chassis.
or fiber optic cables+ the physical layer can be implemented in to main variants( A- 8*I and WA- 8*I+
optimi
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over a short range+ long range+ extended range or long reach 8MD. )hort range uses ;/ nm over multimode
fibers up to =// mF long range uses 1=1/ nm and reaches 26/ m on multimode and 1/ !m on single mode fiber.
The extended reach 8MD uses 1;;/ nm and has a maximum reach of 4/ !m on single mode fiber.
The TM%)eries traffic units can be e"uipped ith client system interfaces that support both the A- 8*I and
WA- 8*I over the various 8MDs on single and multimode fiber. .=.=.= 0arrier Ethernet
With the pervasiveness of Ethernet in enterprises+ many operators today offer Ethernet connectivity across
multiple sites as a service. )ervices are often standardi
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$8 is a netor!ing protocol designed to or! over a multitude of loer data lin! layers+ a fact that has contributed
to its idespread success. )everal layering structures are possible to map $8 into the optical layer and WDM+ i.e.
the term $8 over WDM may refer to several alternative approaches. #igure ;&
igure A;.Alternative implementations of $8 over WDM( a& 8ac!et over ),-ET ith *D0 framing. b& 8ac!et
over ),-ET ith '8 framing. c& Ethernet framing.
rom the TM%)eries perspective+ it is important to note that $8 relies upon underlying physical layer protocols and
that these protocols #e.g. )D*9 ),-ET and Ethernet& are the ones that are interfaced to the transponders and
muxponders. The TM%)eries WDM optical netor! is completely transparent to all $8 pac!ets+ hich are
forarded inside5 the selected physical layer protocol.
Multiprotocol abel )itching #M8)& is a technology that can be used in $8 netor!s to simulate more
permanent connections5 beteen end points. M8) simplifies the implementation of routing and pac!et
processing in the $8 routers+ and enables "uality of service mechanisms of value to netor! operators. M8) can
be thought of as a layer sandiched in5 beteen the $8 layer and the physical layer+ hence the TM)eries is
also completely transparent also to M8) traffic.
.=.; )torage Area -etor!s #)A-& ith ibre 0hannel and E)0,-
A storage area netor! #)A-& is a dedicated data netor! that connects storage devices+ such as dis! arrays+ tape
libraries+ and optical Gu!eboxes to mainframe computers and servers+ so that the devices appear li!e locally
attached devices to the operating system. A )A- typically has its on netor! of storage devices that are
generally not accessible through ide or local area netor!s by other devices. $n the early days+ the entire )A-
as located at one data center+ but today a )A- is often distributed over a ider metropolitan area to duplicate
data and provide resilience against disasters. #igure 6&
igure A6.Architecture of a storage area netor! #)A-&.
)A-s often utili
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adapts the signal for transmission over fiber and copper ires #the latter seldom used&. ibre 0hannel transceivers
operating at 1=// nm and 1;;/ nm can reach tens of !ilometers on single mode fiber. ;/ nm transceivers are
used for multimode fiber ith a reach of a fe hundred meters. Just as for )D*9),-ET and ,T-+ ibre 0hannel is
available in a hierarchy of data rates+ from 1// Mbyte9s up to 1 /// Mbyte9s. #igure &
igure AA.ibre 0hannel storage area netor! data rates. )ource Wi!ipedia.
Another common legacy protocol for attaching peripheral e"uipment to mainframe computers is E)0,-+
#Enterprise )ystems 0onnection&+ originally created by $?M. E)0,- is an optical fiber+ half%duplex+ serial
interface. $t originally operated at a rate of 1/ Mbyte9s+ hich as later increased to 1 Mbyte9s. The current
maximum distance specified is 4= !ilometers.
.=.;.1 ,vervie of )torage Area -etor! client interfaces in the TM%)eries
#igure &
igure AE.)A- client interfaces in the TM%)eries.
.=.6 Hideo protocols
Transport of video signals over long haul and metropolitan netor!s is an important application for WDM
netor!s. )tandards for digital broadcasting have been developed by the Digital Hideo ?roadcasting #DH?&
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8roGect+ an industry consortium of broadcasters+ netor! operators and the TH industry. The first phase of DH?s
or! involved establishing standards to enable the delivery of digital TH to the consumer via the traditional5
broadcast netor!s. Thus+ the three !ey standards during this phase ere DH?%) for satellite netor!s+ DH?%0 for
cable netor!s and DH?%T for terrestrial netor!s. $n addition to these+ a hole range of supporting standards
as re"uired covering areas such as service information #DH?%)$&+ subtitling #DH?%)>?&+ interfacing #e.g. DH?%A)$&+etc. The TM%)eries traffic units include interfaces for DH?%A)$9%)D$9*D%)D$.
. 0ombining protocols in one traffic unit
@eal orld metropolitan transport netor!s are characteri
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.; ine side data rates and framing
After having discussed the various traffic layer protocols of the client systems connected to the WDM optical
netor!+ it is time to loo! into the line side of the Transmode traffic units. #igure 1&
igure E1.Definition of the line side of a traffic unit.
As stated earlier+ the transponder9muxponder adapts the client signal to the WDM netor!. $t also encapsulates
the client signal into a digital rapper+ i.e. extra bytes are added to the client signal at the client ingress point and
removed at the client egress point. These ,*%bytes can be used for a number of features+ such as $ntroduction of "uality chec! of the WDM signal enabling 8erformance Management #8M& as ell as
ault Management #M& in the transmission domain. $nsertion of management channels that are used to connect to other nodes in the netor! for
management purposes 7 Embedded or inband management channels. $nsertion of coding that detects and corrects bit errorsF orard Error 0orrection #E0&. There are
different variants of E0 and these are sometimes needed for long%haul transport of 1/94/91// 'bit9s signalsin amplified netor!s.
$nsertion of path information that can be used to validate the connection.
)afety and security features li!e Automatic aser )hut%don #A)& that shuts don the laser upon a fiber
cut.
The TM%)eries traffic units 3 the transponders and muxponders 3 are designed to operate at a specific date rate
on their line side( 2.; 'bit9s+ 4 'bit9s+ 1/ 'bit9s+ 4/ 'bit9s or 1// 'bit9s. This is the data rate of the digital signal
transmitted on the WDM avelength 7+ a avelength that is created by the )89:8 of the traffic unit and that
enters the filters+ multiplexers+ @,ADMs etc+ of the optical netor!. The line side data rate thus defines the
highest client side data rate and the number of client systems that can be time division multiplexed upon one
single avelength forming a light path beteen to traffic units.
Just as client systems interact ith each other according to different protocols+ the to traffic units at the end of a
given light path interact ith each other according to a specific framing format and protocol #signaling&. The
framing and the signaling specify ho data from the clients is multiplexed and allos for the management
information to flo beteen the traffic units. #igure 2&
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igure E2.The traffic units at each end of a light path interact ith each other according to a line side framing
format and a signaling protocol.
.;.1 ),-ET9)D* and ,T- framing
As previously described in this document in the section on client side protocols+ both ),-ET9)D* and ,T-
support the necessary framing for transmission over fiber or WDM netor!s.
),-ET9)D* and ,T- framing can be particularly useful in long%haul applications here e"uipment from several
vendors has to interor! in the same optical netor!+ other protocols may be re"uired.
The TM%)eries Transponders provide fully transparent transport of ,T- ,T>1 signals at 2.; 'bit9s and for ,T-
,T>2 signals at 1/ 'bit9s. The TM)eries also supports transparent transport of ),-ET9)D* signals+ i.e. it is
possible to use )D* framing on the line side of selected transponders.urthermore muxponder options are available that multiplex loer speed signals onto an ,0%49)TM%16+ an ,0%
1C29)TM%64 line or an ,T>2 ,T- line.
.;.2 -ative Ethernet framing
Transmode also supports a -ative 8ac!et ,ptical architecture that integrates ayer 2 Ethernet sitching into the
TM%)eries optical platform. This solution is outside the scope of this document as it is a ide ranging subGect in
itself. ?ut it is orth noting in this section that the TM%)eries also supports -ative Ethernet as a protocol for line
framing.
.;.= Transmode iWDM
TM
framing
$n addition to the )D*9),-ET+ ,T- and -ative Ethernet framing options outlined above+ Transmode has also
developed $ntelligent WDM framing to provide additional options to address some of the flexibility and cost
efficiency challenges that need to be addressed in metropolitan netor!s+ Transmode provides this additional
optional framing format 3 iWDM framing 3 for traffic unit interaction+ i.e. for point to point avelengths over a
fixed or @,ADM based optical netor!. The iWDM framing as invented by Transmode and provides cost%
optimi
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igure E=.iWDM framing allos intelligent time division multiplexing of multiple types of traffic+ hile providing
management and control capabilities over one or more avelengths. The figure shos an example here
Ethernet and )D*9),-ET traffic from different sources is multiplexed over one avelength.
The ability to support multiple services+ and the "uality of these services+ is vital for transport netor!s. iWDM
framing provides aggregation at the physical transmission layer and at higher layers to the most cost%efficient
transport bit rate that includes 2.; 'bit9s+ 4 'bit9s or 1/ 'bit9s depending on the application. With iWDM
framing+ it is for example possible to multiplex up to 1/ independent 'bE signals into one 1/ 'bit9s stream on a
single avelength+ hile still maintaining full transparency for both data and individual synchroni
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igure E .0omparison of some ,T- and iWDM framing characteristics.
.6 To applications ta!ing advantage of iWDM framing
The versatility of the iWDM framing enables much more than the basic multiplexing and management of several
client signals over a WDM netor!. The iWDM framing can for example be optimi
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igure E;.
8rinciples of a WDM mobile bac!haul netor!.
The figure illustrates ho traffic from a number of cell sites covering a region is aggregated over WDM and
handed over to a long haul netor! for transport to the central nodes of the mobile netor!. The bul! of the
traffic is $8 pac!ets over Ethernet+ but there are also streams of )D*9 ),-ET E19T1 or E=9T= time division
multiplexed circuits for voice calls. $n addition+ the ireless transceivers need to maintain an exactsynchroni
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standards such as sync%E #'.2629I.1=62& and Timing over 8ac!ets #1;v2& for its on+ independent
synchroni
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igure EE.Transport of 1/ full 'bE lin!s over a 1/ 'bit9s channel ith iWDM.
Ethernet aggregation is Gust one of several possible applications of this 1/' muxponder M)%M:891/'. The M)%
M:891/' implements a true multi%service concept+ enabling a traffic unit to be configured and used in multiple
applications. $nstead of having traffic units that are dedicated for a certain service and application+ the TM%)eries
comprises units that can be reconfigured to support different traffic formats as ell as being configured to
provide various functions. This flexible capability in combination ith pluggable optics gives the loest total cost
of onership #T0,&.
When the M)%M:891/' traffic unit is used as a 1/ x Ethernet muxponder all client side Ethernet ports support( ,ptical and electrical 'igabit Ethernet.
Electrical ast Ethernet.
$ndividual )ync%E #'.2629I.1=62& hen using optical )8s.
The transport beteen to M)%M:891/' units is fully synchroni