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Chapter 3. SONET Overview > SONET Advantages

SONET Advantages

The big advantage of SONET is that it was designed to provide the following functions needed in networking at that time:

  • Single-step multiplexing

  • Access to low-level signals directly

  • Carry existing DS1, DS3, ATM, and packet traffic

  • Synchronous timing to eliminate bit stuffing

  • Overhead room for acceptable network management information

  • Allow transmission of data at higher speeds (50 Mbps+)

The following sections describe these functions in more detail.

Single-Step Multiplexing

In single-step multiplexing, lower-rate traffic is combined into higher-rate traffic without having to go through stages. So if you have an STS1 and want to put it in an STS3 for transmission, you can do so in one step. If you want something to be transported inside an STS-48, again one step is needed. The process is provided through byte interleaving like that used in creating a DS1.

Access to Low-level Signal Directly

Direct access to low-level signals allows the receiving system to know exactly where an STS-1 is located within a STS-n and extract it for use at the local site. A new STS-1 can also be inserted into the traffic for delivery to another node. This can be done without having to fully demultiplex the signal, buffer the contents, extract the desired content, and then multiplex a new signal for transport across the wire.

Carry Existing DS1, DS3, ATM, and Packet Traffic

A single STS-1 is able to carry a payload of 50.112 Mbps. Placing a single DS1, which is 1.544 Mbps, into a single STS-1 frame would be a waste of space. Because backward- compatibility was always important in SONET, a system was devised as part of the standard to subdivide an STS-1 when necessary.

An STS-1 frame can be subdivided into seven virtual tributary groups (VTG) of 108 bytes each. Each VTG can carry one of four different types of traffic organized into virtual tributaries (VTs). VTs can differ in size based on the traffic they carry. Figure 3-3 shows a representation of virtual tributaries. VTs can be one of the following types:

  • VT 1.5(1.728 Mbps)— A VT1.5 is 27 bytes and carries a full DS1. A DS1 contains 24 bytes plus 1 bit for framing. A VTG is 108 bytes, so one VTG can hold 4 VT1.5s, which carries 4 DS1s. The extra unused bytes are used for VT overhead.

  • VT 2(2.304 Mbps)— A VT2 is used to transport the European E1 frame. A VT 2.0 can carry three E1s with 12 bytes left for overhead.

  • VT 3(3.456 Mbps)— A VT3 is designed to carry a DS1C signal, which is 2 DS1s pasted together. Once again, the fit is such that 12 bytes are left over for VT overhead.

  • VT 6(6.912 Mbps)— A VT6 can carry a DS2, which is 4 DS1s. Again, the leftover bytes are 12 bytes.

Figure 3-3. Virtual Tributaries

A SONET frame can carry any combination of VT groups, but a VT group can carry only one type of traffic. Because a VTG can carry 4 VT1.5s and there are 7 VTGs in an STS1, an STS1 can carry 28 DS1s, like a DS3. If a SONET frame carries VT groups, it cannot carry non-VT group traffic. If the traffic is not in the form of a DS1, E1, DS1C or DS2, the traffic can be mapped directly into a SONET frame. This includes DS3, ATM, High-Level Data Link Control (HDLC, which includes Dynamic Packet Transport [DPT] and packet over SONET [PoS]), and Ethernet. Figure 3-4 compares payloads.

Figure 3-4. Comparison of Payloads

Synchronous Timing to Eliminate Bit Stuffing

SONET is synchronous. This requires a constant timing source for both the receiving and transmitting of traffic. The advantage of this common timing source is that it is possible to have byte interleaving rather than bit interleaving at these higher speeds. This results in being able to access the underlying traffic directly instead of having to demultiplex the entire signal.

In a plesiochronous system such as that used with DS1 and DS3 traffic, the clock of the sender is run independently of the receiver's clock. In a system running at 1.544 Mbps, an individual bit is sent every 648 nanoseconds. Electrically the voltage of a 1 bit only exists for half that time, so accuracy has to be down to 324 nanoseconds. Variations in the clocking rate of the sender and the receiver does not greatly impact the reception of the traffic at slower speeds because the receiver can adjust its clock by utilizing the sender's clock extracted from the signal. Issues develop when multiple senders are delivering traffic, such as DS1s, to be aggregated for passage across the network inside a larger multiplexed signal, such as a DS3. When the aggregating switch that is using its own clock source accumulates bits from each of the independently timed sources, it needs them there at precise intervals to successfully create the larger frame. The timing variances of each source can cause problems. If a bit is not ready to be processed because one of the originating systems was delayed, the switch still needs to send something to preserve its timing. The solution is for the switch to stuff a filler bit in the frame. Bit stuffing maintains the timing but prevents the demultiplexing device from accessing the underlying traffic directly because the position of any given bit cannot be guaranteed. In this case, the entire DS3 must be demultiplexed to get access to any of the DS1s. (See Figure 3-5.)

Figure 3-5. Timing Source

SONET uses a pointer system to account for these timing variances, which allows it to keep its synchronous nature. This avoids the use of bit stuffing, by allowing the frame to be created using the more efficient byte interleaving process. The pointer is called the SPE pointer located in the line overhead (LOH), which points to the location of the first byte of the STS-1 SPE. (See Figure 3-6.)

Figure 3-6. Synchronous Payload Envelope (SPE)

Overhead Room for Network Management Information

A SONET frame carries multiple types of overhead. A single STS-1 has 9 bytes of section overhead, 18 bytes of line overhead, 9 bytes of path overhead, and if the payload is packaged into VT groups, another 12 bytes of VT overhead for each of the 7 VT groups. All this overhead carries a wealth of management and other information between devices. Although the amount of space dedicated to overhead in SONET is considered excessive by some, as a percentage of the traffic, it is fixed at 3.45 percent. This is true at all speeds of SONET. SONET overhead is discussed in detail in the section “SONET Framing” later in this chapter. (See Figure 3-7.)

Figure 3-7. STS-1 Frame format

Allow Transmission of Data at Higher Speeds

At the time of the development of SONET, standards in the United States were maxed out at T3, which was approximately 45 Mbps. Other higher rates were available, but they were proprietary, which meant vendor interoperability was not an option. SONET basically picked up from this rate and moved upward. Refer to Table 3-1 for a recap of the speeds. Although not included in the table, work is being done on even higher speeds of OC-768 or approximately 40 Gbps across optical fiber.

Now that you have reviewed the benefits of SONET, the next section covers SONET's architecture.

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