Thread,
There are some truly excellent reads in this month's (6/99) issue of
Lightwave Magazine.
light-wave.com
See the full lineup at:
pennwell.shore.net
The following passive optical network (PON) article is one that I thought
was especially appropriate for the LMT, and I've copied it below for thread
posterity. For the graphics, however, you will need to go to the site, itself:
pennwell.shore.net
Other articles cover DWDM in cable TV nets; solving the reverse problem
with digital; fiber-broadband digital TV; and several treatments on "all-optical"
components and architectures.
Enjoy, and Regards, Frank Coluccio
==================
From: The June 1999 issue of Lightwave Magazine:
Special Reports - June, 1999
ATM PON maximizes bandwidth to homes and businesses
Ray Hogg
Fujitsu Network Communications Inc.
The ever-increasing number of end users accessing the Internet and
taking advantage of today's data-transport capabilities has ignited an
explosive demand for bandwidth. The growing popularity of such
applications has also resulted in longer wait times. Many types of
next-generation access systems are helping to improve this situation by
pushing optical fiber closer to the home or business premises.
Fiber-to-the-home/business (FTTH/B) network architectures offer
major bandwidth improvements compared with alternative optical
access network architectures that rely on copper or coaxial cable for
the final distribution segment to the customer premises. In addition,
small businesses, home offices, and residential users in new
development areas can, and should, begin reaping the benefits of
FTTH/B, with a new optical access technology currently being
standardized within the ITU-T (G.983), generally referred to as ATM
PON (Asynchronous Transfer Mode-based passive optical network).
Opportunity on the horizon
Today's increasing demand for bandwidth has created the need for
FTTH/B. While asymmetric digital subscriber line (ADSL) technology
can provide 1 to 8 Mbits/sec of downstream capacity directly to a
customer premises, this is currently the limit for most existing copper
loops. A hybrid fiber/coaxial-cable (HFC) network, meanwhile, can
provide 30- to 40-Mbit/sec total capacity downstream using a single
6-MHz analog channel spectrum (typically shared by 100 to 250
homes), but HFC has extremely limited upstream capacity.
Based on today's user needs, these data rates should be sufficient until
about 2004. By then, increased multimedia and video services will drive
the need for a new access network architecture such as
fiber-to-the-cabinet (FTTCab) or FTTH/B.
FTTCab can support very high-speed DSL (VDSL) access, which will
increase the data rate to about 13 Mbits/sec at 4500 ft from the
customer premises. Moving the optoelectronics to within 3000 ft of the
customer premises could increase the bandwidth to up to 26 Mbits/sec,
a data rate that should last until about 2010 at today's rapid growth rate.
FTTCab is simply an interim step to FTTH/B, the eventual solution for
the growing bandwidth needs. As a result, telephone companies are
considering running fiber all the way to the home or business as soon as
possible.
Immediately deploying ATM PON may not always be the answer,
however. For example, ATM PON may not be practical in major
metropolitan and commercial areas where excess copper and/or
Synchronous Optical Network (SONET) rings are readily available.
Yet, an FTTH/B architecture based on ATM PON does make sense in
lower-density serving areas, such as outlying residential/suburban areas,
where dynamic and unpredictable demand for voice/data services from
small offices/home offices (SOHOs) and small independent or shared
tenant business premises (such as strip malls) present significant
challenges to local-exchange carrier network planners.
ATM PON allows both service providers and end users to gain the
benefits of FTTH/B. It provides the access piece for the all-fiber
network.
The basics
ATM PON is a point-to-multipoint, cell-based, optical-access
architecture that facilitates broadband communications between an
optical line terminal (OLT) at the central office and multiple remote
optical network units (ONUs) over a purely passive optical-distribution
network with a reach of up to 20 km.
The first operational prestandard ATM PON system was developed by
Fujitsu Ltd. in cooperation with Nippon Telegraph and Telephone
(NTT) of Japan. This system began commercial operation in
September 1997 with leased-line business services in Japan. Currently,
there are hundreds of systems deployed and actively providing service.
Fujitsu is still the sole provider of fully functional OLT equipment to
NTT.
ATM PON was first proposed as a standardized FTTH solution in the
early 1990s by the Full-Service Access Network (FSAN) initiative,
which comprised 14 telephone companies from around the world.
FSAN is an effort to set common requirements for full-service
optical-access networks among all operators globally. The requirements
generated within this group were forwarded to the ITU-T Study Group
15 as recommendation G.983.1, which was formally approved last
October.
Fig. 1. The ATM PON architecture supports up to 64 ONUs
or ONTs via a single fiber. The system supports both
155-Mbit/sec upstream and either 155- or 622-Mbit/sec
downstream traffic.
In an ATM PON system, a maximum of 64 ONUs can share the
capacity of a single fiber using ATM transport and passive optical
splitter/combiner technology (see Fig. 1). Full-duplex (simultaneous
upstream and downstream) transmission via a single fiber facility can
also be achieved using independent wavelengths (1310/1550 nm) for
each direction. In FTTH/B network architectures, the functions of an
ONU and a network termination unit are often integrated into a single
unit referred to as an "optical network termination" (ONT).
The OLT broadcasts a downstream signal consisting of a continuous
time-division multiplexed stream of fixed-length (53-byte) time slots
carrying physical-layer operations, administration, and maintenance
(PLOAM) cells, ATM cells, and idle cells for cell-rate decoupling. The
nominal bit rate of the downstream signal can be either 155.52 or
622.08 Mbits/sec. The maximum downstream cell-transfer capacity on
a single fiber (excluding a fixed reserved capacity for PLOAM cells) is
149.97 or 599.86 Mbits/sec, depending on the nominal line rate chosen
for the downstream facility.
A time-division multiple-access (TDMA) technique is used for
upstream communications using 56-byte time slots with burst-mode
synchronization performed by the OLT receiver. Upstream
media-access coordination and control (MAC) is provided by the OLT,
which issues explicit messages via downstream PLOAM cells that
grant access to each ONU for transmission within specified upstream
time slots.
When an ONU is granted access to an upstream time slot, it transmits
a 3-byte header followed by a 53-byte cell at a nominal line rate of
155.52 Mbits/sec. Due to the overhead bytes, the upstream
cell-transfer capacity is limited to 147.2 Mbits/sec and is shared among
the ONUs based on their allotted upstream bandwidth. Some of this
upstream capacity is needed by the OLT for physical-layer overhead
and MAC control (the actual amount of upstream overhead bandwidth
required is implementation-dependent).
Fig. 2. The 155-Mbit/sec frame format shown in (a)
includes a pair of PLOAM cells and 54 ATM cells. The
622-Mbit/sec format is shown in (b) for comparison.
A framing structure is applied to both the downstream and upstream
signals to support the TDMA operation. The downstream frame format
for the 155.52-Mbit/sec line-rate option consists of 56 consecutive
53-byte time slots containing two evenly spaced PLOAM cells 28 time
slots apart (see Fig. 2, which also illustrates the 622-Mbit/sec frame
format). The upstream frame format consists of 53 consecutively
numbered 56-byte time slots. The two downstream PLOAM cells
collectively contain 53 upstream grants (each associated with a specific
time slot in the next upstream frame) along with additional
physical-layer OAM information broadcast to the ONUs.
Multiple consecutive mini-slots
Fig. 3. Slots in the upstream frame can be divided into
"mini-slots." It is likely that these mini-slots will be used
by the ONUs to submit on-demand bandwidth requests to
the OLT's MAC arbitration controller.
In addition to upstream bandwidth allocation via fixed-length time slots,
a "divided-slot" mechanism is defined to allow a single upstream time
slot to be subdivided into multiple consecutive mini-slots (see Fig. 3).
The OLT can allocate one or more divided slots to a group of ONUs,
each of which can transmit one or more mini-slots in a prescribed
sequence. Each mini-slot consists of a 3-byte header followed by a
fixed-length payload (configurable to any integer number of bytes that
can be encapsulated within the available divided-slot payload).
Configuration of the divided-slot time slot and its associated mini-slots is
accomplished via PLOAM messages exchanged between the OLT and
associated ONUs.
It has been agreed that among other potential applications, mini-slots
will be used by ONUs to submit on-demand bandwidth requests to the
MAC arbitration controller within the OLT. However, the format and
semantics for these on-demand bandwidth requests have not been
defined. Furthermore, it is unclear as to whether an on-demand MAC
protocol will ever be subject to standardization.
The OLT establishes an upstream physical-layer OAM channel with
each individual ONU by issuing specific grants requesting the
transmission of a PLOAM cell. The rate of this upstream PLOAM
channel is directly controlled by the OLT; but an upstream PLOAM
cell will be requested at least every 100 msec. Upstream/downstream
PLOAM cell flows facilitate direct OAM message exchange between
the OLT and each individual ONU for physical-layer management and
control operations.
Due to the physical convergence of the upstream burst transmissions
from each ONU via one or more passive optical splitter/combiner
elements, the timing of each ONU transmission must be precisely
synchronized with delay compensation to account for unequal distances
between the OLT and each individual ONU. To accomplish
transmission-delay equalization among all ONUs, the OLT must
perform a ranging procedure (to measure the logical reach distance to
and from each ONU) and assign a specific equalization delay
adjustment to each ONU. When granted access (under normal
operation), an ONU synchronizes its upstream transmission based on
timing recovered from the downstream signal timing and applies its
assigned equalization delay adjustment.
Associating unique cell flows
The virtual-path identifier (VPI) field of each ATM cell is used as the
multiplexing identifier for associating unique cell flows between the
OLT and each specific ONU. That is, an ONU only processes
downstream ATM cells containing VPI values that have been explicitly
assigned to them (ignoring all other ATM cells broadcast on the shared
downstream channel). Point-to-point virtual-path (VP) connections can
be established between the OLT and any ONU by assigning a unique
VPI value to a single ONU.
A point-to-multipoint VP connection (with the OLT serving as the root
node and one or more ONUs as the leaf nodes) can be established by
assigning the same VPI value to all associated leaf-node ONUs. For an
ONU/ONT to support a standard ATM UNI as a subscriber interface,
it must perform VPI translation to maintain local significance of the
VPI space.
Due to the multicast nature of the PON facility, downstream cells are
broadcast to all ONUs. An optional "churning" function can be
individually enabled for point-to-point VP connections to provide
additional security against potential eavesdropping. This is a
byte-oriented encoding scheme based on a private key (churn key)
exchanged between a given ONU and the OLT. The churn key is
generated by the ONU and provided to the OLT on request. As an
added security measure, the OLT requests the churn key be updated
with a new value on a periodic basis (with at least one update per
second).
The physical characteristics of the optical splitter/combiner technology
induce severe attenuation on the signal transmission from the upstream
transmitter of one ONU to the receiver of another. Therefore, there is
no need to churn upstream transmissions and it is not likely that one
ONU could intercept a private churn key exchange between another
ONU and the OLT. If churning is not considered sufficient security for
a given service, a more suitable encryption mechanism must be
employed at a higher layer.
ATM PON benefits
The most obvious benefit that ATM PON offers to end users is a vast
increase in the amount of bandwidth delivered to a home or business
premises. It opens the door to more bandwidth-intensive applications
such as video-on-demand (VOD) and will eliminate any access
network-induced wait time experienced when surfing the Web.
Initial G.983 systems will likely only support the symmetrical line-rate
configuration (155.52 Mbits/sec upstream and downstream) with
statically provisioned upstream bandwidth allocation and limited split
ratios. Future releases will add support for an asymmetric line-rate
configuration (622.08 Mbits/sec downstream/155.52 Mbits/sec
upstream), MAC enhancements such as supported dynamic
session-oriented bandwidth reservation, and arbitrated on-demand
bandwidth allocation and support for up to 64 ONUs per PON facility.
Using VOD as an example, the benefit of these enhancements can be
better understood. With the increased downstream bandwidth capacity,
several subscribers would each be able to order a unique movie
requiring, as an example, 6 Mbits/sec of bandwidth. A bandwidth
reservation would be granted to one or more ONUs for the duration of
the movie, thus reducing the total bandwidth-contention pool remaining
for on-demand access by all ONUs on the same PON facility. At the
completion of a movie session, the bandwidth reserved for this session
would then be returned to the pool of total bandwidth available for
on-demand access.
Although ATM PON will enable higher bandwidths to be delivered
directly to the customer premises without an increase in cost to the
subscribers, it probably will not lower their overall cost, either. In some
cases, such as for SOHOs in residential areas, ATM PON may provide
the opportunity for a lower-tariffed service. Even though the cost for
access may not be reduced, the cost per bit transported will improve
significantly with the introduction of ATM PON.
While cable modem service can provide more bandwidth for end users'
money compared with Integrated Services Digital Network and ADSL
services, it does not offer the secure and deterministic environment that
ATM PON provides. FTTH/B provides a dedicated optical connection
that is secure all the way to the central office. This type of security is
particularly important to SOHO users with a home-office connection to
the corporate local area network.
If ATM PON and coaxial cable (used for cable modem service) are
both broadcast media, why is ATM PON more secure? The passive
optical splitter/combiner technology used by ATM PON is highly
directional, so attenuation is relatively low upstream (from every ONU
to the OLT) but very high from one ONU to another. Coaxial cable
uses a multi-tap (RF splitter/combiner) element to provide service to an
individual customer premises. This element is not as highly directional
as an optical splitter/combiner and therefore creates a much less secure
environment.
The maximum length of the coaxial drop cable in an HFC system is a
few hundred feet, resulting in distance constraints on the assignment of
individual drops to one coaxial segment (RF splitter/combiner group). In
comparison, aside from an overall optical-loss budget, ATM PON has
no constraints on the length of a drop segment or the placement of the
splitter/combiner elements (only the maximum fiber distance any single
ONU may be located from the OLT). This freedom allows a
splitter/combiner group to easily be re-engineered to meet any specific
demands in terms of absolute reserved bandwidth or privacy
requirements for a single home or business premises.
With the OLT at the central office and an ONT in the subscriber's
home or business premises, there are no active electronics (such as
digital loop carrier or FTTCab electronics) in the outside plant—just
optical fiber to the splitter and then to the home. Because the optical
splitter is passive, it has no power or electronics associated with it,
greatly increasing the life and reliability of the loop plant and thus
reducing the chances of a problem occurring to a subscriber's
connection or service.
ATM PON also offers subscribers quicker service delivery and no
delay in the implementation of new services. With ATM PON, new
subscribers can be easily added to an existing PON facility by
interconnection at an existing splitter or installing a second-tier splitter
for increased drop capacity.
The future choice
In new development areas, where the ground is already being
excavated and laying fiber is relatively simple, the cost of laying fiber is
economically justifiable. Here, access-network providers can deploy
fiber as an overlay to copper, so subscribers can still receive traditional
lifeline "plain old telephone service."
In existing serving areas where the loop infrastructure is already in
place, the need for more bandwidth will eventually require rehabilitation
of the existing copper loop plant. At that time, telephone companies can
consider FTTH/B as a viable, higher-speed alternative to an FTTCab
deployment, particularly in areas served by an aerial plant or where
dark fiber or conduit already exists deep within the buried plant.
Advancements in ATM PON technology will allow increased split
ratios, permit higher downstream capacity, and offer reserved and
on-demand dynamic bandwidth allocation. These capabilities will enable
end users to request additional bandwidth as needed.
Ray Hogg is principal product planner at Fujitsu Network
Communications Inc.
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