SONET Turns Data-Friendly
By Charlotte Wolter and Peter Lambert
Optical transport technologies rapidly are becoming more friendly to emerging data
services, and new data-optimized optical equipment emerging this year also is proving
interoperable across various vendor brands, thanks to cooperation among key manufacturers
begun last fall. Happily for carriers, vendors also are finding ways to introduce
data-friendly components to traditional equipment incrementally without forcing any
wholesale change in optical horses midstream.
Formal vendor cooperation began in the fall of 1998 with the founding of Data Aware
Transport Activity (D.A.T.A.), led by charter founders ADC Telecommunications Inc.,
Minneapolis; Atmosphere Networks Inc., Cupertino, Calif.; Fujitsu Network Communications
Inc., Richardson, Texas; and Nortel Networks, Richardson, Texas.
D.A.T.A. has been focused particularly on synchronous optical network (SONET) systems
(also designated synchronous digital hierarchy [SDH] outside of North America), the
standardized fiber optic transport used to pick up, aggregate and drop off carrier traffic
in fiber optic networks worldwide.
SONET originally was designed and optimized for fixed channels of narrowband,
constant-rate, time-division multiplexed (TDM) voice traffic. However, SONET has proved
inefficient for handling the more variable and bursty bandwidth needs of data traffic,
which has exploded for most SONET-based carriers over the past several years.
According to D.A.T.A. members, the primary reason for this data unfriendliness lies
with the fact that SONET standards define fixed increments of bandwidth, growing from
1.5-megabits-per-second (mbps) (DS-1) channels to 51mbps (STS-1) channels, to 155mbps
(OC-3), 622mbps (OC-12), 2.5-gigabits-per-second (gbps) (OC-48) and 10gbps (OC-192)
This hierarchy is well-suited to 64-kilobits-per-second (kbps) voice circuits, hundreds
and thousands of which can be aggregated for transport over metropolitan and long distance
networks. However, squeezing bursty, variable-rate data into such fixed increments of
bandwidth can prove highly inefficient.
For example, to accommodate a standard 10mbps, 10BaseT Ethernet service over its SONET
network, a carrier would have to allocate an entire 51mbps synchronous transport signal
level 1 (STS-1) channel, translating to 41 megabits of wasted capacity.
To solve that inefficiency, most of the D.A.T.A. members now are integrating
asynchronous transfer mode (ATM) data-packet switching into their SONET add/drop
multiplexer (ADM) equipment. In the past, there have been separate network elements,
called service access multiplexers (SAMs), to interwork ATM and SONET, but adding more
layers to a network increases costs. Those costs can be reduced or even eliminated when
those functions are implemented directly in the transport infrastructure.
SONET incorporates enough intelligence to pick up and drop off individual voice or data
sessions at specific sites, or nodes, on a SONET network. ATM adds another layer of
intelligence, which also can enable a SONET channel to be shared dynamically by multiple
types of traffic sessions, says Paul Havala, manager of wideband product development for
Fujitsu, which in June introduced the FLASH600 ADX, the first member of a family of data
transport multiplexers incorporating SONET, ATM and Internet protocol (IP) interfaces.
For example, Havala says, with the new ATM-powered ADMs, over a 155mbps STS-1 channel,
"while one node can add and drop 10BaseT Ethernet, the next node might add and drop a
DS-3 [45-megabit] ATM link while the next node adds and drops frame relay or native IP
traffic, so multiple services are sharing an STS on the same ring.
In other words," he adds, "each service circuit can now be any
Further, Havala says that sharing of an ATM-based SONET channel bandwidth can be
dynamic. Whereas traditional, TDM-based SONET would continue to allocate bandwidth to an
Internet browsing or file transfer or other type of data session even when that session
remains idle for any period of time, ATM-based SONET instead can make that idle data
session’s bandwidth available to other services in the same channel.
In Fujitsu’s case, a carrier can make its SONET network data-aware by inserting a
FLASH600 circuit board, or "blade," into existing SONET ADMs. The FLASH600 ADX
can deliver ATM virtual paths across a SONET ring to accommodate IP over 10mbps or 100mbps
Ethernet; ATM data traffic via DS-3, OC-3 or OC-12 links; traditional telephony traffic
via DS-1, DS-3, OC-3 and OC-12 interfaces; and the SONET network through OC-3, OC-12 and
OC-48 line rates.
"You need to add the blade only at every node where you want this data-aware
visibility," Havala says.
Converging Traffic Streams
Another advantage of data-aware equipment, says Alex Dobrushin, vice president of
marketing, Atmosphere Networks, is that all traffic types can be converted into a single
packet or traffic stream. This is very important for carriers starting to build converged
backbones with packet technology. With this ability to concentrate multiple services in
the SONET access network, it becomes an extension of packet backbones. "You can
extend the power of the packet backbones all the way to the edge of the network,"
For example, Dobrushin explains, a customer today who wants a transparent local area
network (LAN) service and Internet access service would have to buy two different services
and two interfaces to the network because there is no convenient way of providing those
services on the same interface. With data-aware functions in a carrier infrastructure,
those services can be delivered over a single interface.
In Atmosphere Networks’ products, "The transport infrastructure is intelligent
enough to decide if a packet from a subscriber needs to go onto the transparent LAN
service, or if it is destined for the Internet," Dobrushin says. "It can
actually route packets as well. So this gives great flexibility to a carrier to create
differentiated services. With traditional SONET, all you can do is deliver bandwidth to
Until recently, these kinds of solutions were proprietary, so that a SONET ring
equipped with Fujitsu data-aware equipment could not pass its ATM-over-SONET traffic to
another vendor also offering data-aware ATM over SONET.
One reason was that vendors were offering data-aware capability as so-called
"hybrids"; that is, an add-on or upgrade to their existing TDM-based SONET
systems, Dobrushin notes.
There is a group of startups, he says, that is taking a forward-looking approach and
using all-packet transport for all services, but even within that group, some are using
ATM as a transport mechanism for transport and multiplexing, and other companies are
looking to the possibility of using IP as their transport layer.
"When you have this mix of different implementations in a market, the question is:
If you want to supply a particular transport service, how do you deliver it across
different [SONET] infrastructures?" Dobrushin says.
The next activities of the group, according to Tony Farinholt, ATM transport program
manager, ADC Communications, will be to define interoperability for packet-over-SONET or
IP-over-SONET, and to define the area the group will be taking up in the next few months.
Also, cost-saving is a significant factor in the development of data-aware systems.
Dobrushin says Atmo-sphere’s data-aware solution is capable of delivering 100baseT
Ethernet to a subscriber at the same cost that a T1 circuit can be delivered over
conventional infrastructure today. "It has actually driven the cost points down to a
level where implementing packet-based networks is as economical as traditional
Now, D.A.T.A. members have reached interoperability agreements among themselves and
will demonstrate multiple devices working together in Chicago at the National Fiber Optic
Electronics Conference (NFOEC) later this month.
Charlotte Wolter is infrastructure editor and Peter Lambert is features editor for