Unit 7

Lesson 1 - Point-to-Point WAN Services

   SONET

Synchronous Optical Network (SONET) is a Physical Layer standard that provides rules for converting electrical signals to pulses of light, moving the light signals over fiber optic cable, and converting the signals back to electrical form at their destination.

Like the T-carrier system, SONET defines a method of multiplexing, as well as the bandwidth required to accomplish the multiplexing. Also, like the T-carrier system, each SONET signal can be multiplexed into a larger signal. Put it all together and we have the Optical Carrier (OC) hierarchy shown in the OC Hierarchy Table.

OC Hierarchy

SONET (OC) Level

Bandwidth

Number of T1 Lines

Number of 64-Kbps Channels

  OC-1

52 Mbps

28

672

  OC-3

155.5 Mbps

84

2,016

  OC-9

466.5 Mbps

252

6,048

  OC-12

622 Mbps

336

8,064

  OC-18

933 Mbps

504

12,096

  OC-24

1.2 Gbps

672

16,128

  OC-36

2 Gbps

1,008

24,192

  OC-48

2.5 Gbps

1,344

32,256

  OC-96

5 Gbps

2,688

64,512

  OC-192

10 Gbps

5,376

129,024

Notice that OC-1 is designed to carry the same number (672) of 64-Kbps channels as a T3 circuit. However, the bandwidth requirement is different for T3 (45 Mbps) and OC-1 (52 Mbps). The difference is because SONET uses more bandwidth for the control information necessary to keep its multiplexed signals in synch.

Notice also that, beginning with OC-24, we are no longer talking about bandwidth in Mbps. At OC-24 and above, SONET carrier lines handle gigabits per second (Gbps), or billions of bits.

SONET Rings

SONET is a ring network technology. It can be built as a true ring, using two parallel rings that carry signals in opposite directions. The same signal is sent simultaneously over both rings, thus both rings carry identical traffic. This redundancy ensures that if one ring fails, a signal will still get through.

A SONET network can also use a point-to-point physical topology. However, signals still flow in a logical ring.

SONET was originally developed to provide high-speed connections between telephone company CO switches across entire regions. However, the SONET standard also extends down to the office desktop. Today, telephone companies offer SONET connections to their large customers that need high-speed optical transmission in their business networks.

However, how can an individual business network connect to a public network that operates at Gbps data rates? The SONET architecture provides what are called "virtual tributaries" of channels, just as a large river is fed by smaller tributaries, and sometimes divides into smaller side streams. A SONET network uses MUXs to add and remove smaller signals from the main data flow. A MUX also cross-connects communication channels, so that one customer location can be connected to a communication channel at another customer location.

   Activities

See the Activities and Extended Activities section in Unit 7 Lesson 1 in your textbook Introduction to Networking to test what you have learned so far.

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