In 1984 the central research group at Bell Communications Inc., jointly owned at the time
by the Regional Bell Operating Companies, introduced SONET. SONET is the U.S. standard of
synchronous data transmission. It is the transport service used on fiber optic cabling,
and is deployed in long distance and telecommunications networks. The international
equivalent of SONET is synchronous digital hierarchy (SDH).
Together, these are the standards that ensure digital networks can interconnect throughout
the world. Through tributary attachments, they also ensure that existing conventional
transmission systems can take advantage of optical media. SONET is a standard way to
interconnect traffic from multiple vendors and transport data at high optical carrier
speeds (OC). Data from ATM switches, T-1 and T-3 multiplexers is also transported by
Unlike ATM, which is a switching and multiplexing technique, SONET is a Layer 1 service,
which defines interfaces to physical media such as copper and fiber optic cabling. ITU has
approved standards for OC speeds, defined as follows:
SONET Rings For Greater
Reliability is extremely important due to the higher speeds attainable on fiber. When
conversation is carried from one telephone subscriber by a medium such as copper, a copper
cut only impacts one customer. The SONET speed OC-192, carries 192,000 transmissions, so a
cut in this would affect a very large number of customers. If a SONET ring fails that
serves a major hospital, police department or armed forces unit, the health, safety, and
possibly national defense of the locale would be affected.
If an outage affects more than 30,000 users, telcos are required to notify the FCC
(Federal Communications Commission). To increase reliability, ring topology is often used
in SONET deployment. Two sets of fiber strands are used. One set is used for sending and
receiving, while the other is the spare set. If one set of fiber strands is broken, the
spare set re-routes the traffic in the other direction. This is an important advantage
over fiber in a straight line. If there is a fiber cut on a single straight line, there is
no other route for the traffic to take without the carrier intervening and rerouting
Telephone Company SONET
Local telcos sell spare fiber-based SONET ring capacity to commercial customers. They
offer to interconnect T-1 and (mostly) T-3 services to the SONET rings to give these
services the extra reliability provided by the SONET ring structure. In particular, these
services are often aimed at call centers, which require high reliability as well as high
Reliability is important for call centers, such as airline reservation centers, where the
businesses are highly competitive. Large amounts of money would be lost during an outage
since callers can easily call another airline.
Another example of an industry that requires high reliability is the financial services
industry. Another application for selling SONET reliability is to connect inter-exchange
carriers customers to the local telephone companys SONET rings. The speeds
offered are at the OC-3, OC-12, and OC-48. The local telcos claim 50-millisecond network
restoration in the case of a network failure or degradation. In the case of a failure at
the central office, they also run SONET rings to multiple inter-exchange carrier switches
and local telco facilities.
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To convert the SONET light signals
to electrical signals and vice-versa, matching SONET lightwave multiplexers are required
at the customer premise and at the telephone company office.
SONET technology: High Capacity
Networks at Lower Costs
Utilizing SONET helps reduce the amount of equipment required for data transmission.Fewer
fiber runs are required to carry vast amounts of traffic. This helps lower financial
barriers for entry into the construction of long-distance networks.
New carriers are at an advantage in reaching OC-192 speeds. OC-192 requires zero
dispersion fibers. These are special fibers that are thinner and have fewer impurities
than standard single-mode fibers previously purchased for carrier networks.
New carriers do not have to upgrade older multiplexers and SONET devices, nor do they have
to contend with having lower grade fiber optic cabling in their cabling plants.