数据通信与网络答案17

SONET/SDH

Solutions to Odd-Numbered Review Questions and Exercises

Review Questions

1.The ANSI standard is called SONET and the ITU-T standard is called SDH. The

standards are nearly identical.

3.STS multiplexers/demultiplexers mark the beginning points and endpoints of a

SONET link. An STS multiplexer multiplexes signals from multiple electrical

sources and creates the corresponding optical signal. An STS demultiplexer

demultiplexes an optical signal into corresponding electric signals. Add/drop mul-

tiplexers allow insertion and extraction of signals in an STS. An add/drop multi-

plexer can add an electrical signals into a given path or can remove a desired signal

from a path.

5.Pointers are used to show the offset of the SPE in the frame or for justification.

SONET uses two pointers show the position of an SPE with respect to an STS.

SONET use the third pointer for rate adjustment between SPE and STS.

7.A regenerator takes a received optical signal and regenerates it. The SONET

regenerator also replaces some of the existing overhead information with new

information.

9.The path layer is responsible for the movement of a signal from its source to its

destination. The line layer is responsible for the movement of a signal across a

physical line. The section layer is responsible for the movement of a signal across

a physical section. The photonic layer corresponds to the physical layer of the OSI

model. It includes physical specifications for the optical fiber channel. SONET

uses NRZ encoding with the presence of light representing 1 and the absence of

light representing 0.

Exercises

11.Each STS-n frame carries (9 ×n× 86) bytes of bytes. SONET sends 8000 frames

in each second. We can then calculate the user data rate as follows:

STS-3→ 8000 × (9 ×3× 86) × 8 = 148.608 Mbps

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STS-9→ 8000 × (9 ×9× 86) ×8=445.824 Mbps

STS-12→8000 × (9 ×12× 86) × 8 = 594.432 Mbps

13.The user data rate of STS-1 is (8000 × 9 × 86 × 8) = 49.536 Mbps. To carry a load

with a data rate 49.540, we need another 4 kbps. This means that we need to insert

4000 / 8 = 500 bytes into every 8000 frames. In other words, 500 out of every 8000

frames need to allow the H3 byte to carry data. For example, we can have

sequences of 16 frames in which the first frame is an overloaded frame and then 15

frames are normal.

15.In answering this question, we need to think about the three upper layers in

SONET. The path layer is responsible for end-to-end communication. The line

layer is responsible between multiplexers. The section layer is responsible between

devices.

a.A1 and A2 are used as aligners (synchronizers). They perform the same job as a

preamble or flag field in other networks. We can call them framing bytes. These

bytes are set and renewed at each device to synchronize the two adjacent

devices. There is no need for these bytes at the line or path layer.

b.C1 is used at the section layer to identify multiplexed STSs. This idea can be

compared to statistical TDM in which each slot needs an address. In other

words, C1 is the address of each STS-1 in an STS-n. C2 is like the port numbers

in other protocols. When different processes need to communicate over the

same network, we need port addresses to distinguish between them. There is no

need for C byte at the line layer.

c.D bytes are used for SONET administration. SONET requires two separate

channels at the section (device-to-device) and line (multiplexer-to-multiplexer)

layers.No administration is provided at the line layer.

d.E byte creates a voice communication channel between two devices at the ends

of a section.

e.F bytes also create a voice communication. F1 is used between two devices at

the end of a section; F2 is used between two ends. No bytes are assigned at the

line layer.

f.The only G bytes are used for status reportin

g. A device at the end of the path

reports its status to a device at the beginning of the path. No other layer needs

this byte.

g.H bytes are the pointers. H1 and H2 are used to show the offsetting of the SPE

with respect to STS-1. H3 is used to compensate for a faster or slower user data.

All three are used in the line layer because add/drop multiplexing is done at this

layer. H4 is used at the path layer to show a multiframe payload. Obviously we

do not need an H byte in the section layer because no multiplexing or demulti-

plexing happens at this layer.

h.The only J byte is at the path layer to show the continuous stream of data at the

path layer (end-to-end). The user uses a pattern that must be repeated to show

the stream is going at the right destination. There is no need for this byte at the

other layers.3 i.As we discussed, K bytes are used for automatic protection switching, which

happens at the line layer (multiplexing). Other layers do not need these bytes. j.Z bytes are unused bytes. All of the bytes in SOH are assigned, but in LOH and POH some bytes are still unused.

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