3. Physical Layer – Cell Transport Methods

The Cell Transport Method• Early 60’s all switching and transmission systems were analog.

• Experts were watching on PCM to transform analog voice signals into digital bit streams.

• Why? Because too many copper wires in the streets and not enough space for new ones, e.g., using 4 copper wires, a digital stream could transmit many voice signals with better quality than analog systems.

• Around 1965, in Holmdel, NJ, AT&T, the US standards of 24 voice signals multiplexed together to form a 1.544 Mbps DIGITAL SIGNAL called DS-1 was born.

• Each signal needs a 64 kbps stream; this is the product of 8 kHz sampling (due to Nyquist law) and 8 bit per sample coding to tolerate multiple (A/D and D/A) conversions (an important requirement at that time).

• In 1968, Europeans devised a similar standard with 30 voice channels plus a channel for “framing” and a channel for “signaling” for a total of

32*64 kbps = 2.048 Mbps E1 format.

(ETSI -> European Telecommunications Standards Institute)

What is Framing?• It is a method of indicating where to begin counting channels so that the

DEMULTIPLEXER knows which is channel 1,2,3,etc…• A sequence of bits repeated in each frame (8000 frames/sec) forms a

pattern that is difficult for data to initiate.• Thus, by observing the bit stream for a certain period of time, the

framing mechanism can figure out where a channel is.

Frame Frame Frame Frame Frame

Channel 1

Framing Bit (193)8 Bits

125 sec = 1/8000 sec193 Bits each 125 sec = 1.544 Mbps (Aggregate Bit Rate)

8 Bits 8 Bits 8 Bits

8 * 24 = 192 + 1 = 193 Bits8 Bits

• Each voice signal sampled at rate 8000 sample or once every 125 sec.

• Samples quantized 8-bit sequence. Typical PCM requires 64 kbps transmission capacity.

• 24 8-bit voice channels into one time stream operating at 1.544 Mbps.

• Multiplexing means taking a certain number of DS-1 or E-1 signals and putting them together as shown above.

• European: 4 E-1s E-2 at 8 Mbps4 E-2s E-3 at 34 Mbps4 E-3s E-4 at 140 Mbps4 E-3s E-4 at 565 Mbps (not standardized)

• REMARK: DS-1, DS1-C etc… refer to the multiplexing scheme used for carrying information.

• Network providers supply transmission facilities to support these various multiplexed signals referred as CARRIER SYSTEMS designated as “T”.T1 Carrier for DS-1 (in 80’s out; Private Voice, Private Data, Video Teleconf., High Speed Faxing) T3 Carrier for DS-3, etc…

Digital Hierarchy

21

241

41

2

2

1

1

7

…

…

…

DS-0

DS-1

DS1-C

DS-2DS-3

DS-4

64 Kbps/Channel

1.544 Mbps/Channel

3.152 Mbps/Channel6.312 Mbps/Channel

44.376 Mbps/Channel

274.176 Mbps/Channel

Problem : From synchronized network perspective

• Each time it is necessary to pick out or insert a stream, i.e., E-1, from a high-order stream, i.e., 140 Mbps E-4, it is necessary to perform all the operations of the three multiplexers that created the E-4 => Called ADD/DROP.

• These multiplexers create a network in which measuring performance, rerouting signals after network failures and managing rerouted network elements from work centers are all extremely difficult.

What is a Synchronous Network?• The last two decades, digital switching has taken over from analog

switching.

• This means all digital systems can be connected and therefore synchronized with each other.

PDH = Plesiochronous Digital Hierarchy

• At each step, the multiplexer must take into account that each tributary clock has different speeds.

• Each clock is allowed to have certain range of speeds. The multiplexer reads each tributary at the highest allowed clock speed and when there are no bits in the input buffer STUFFING wll be done.

• It also has a mechanism to signal to the demultiplexer that it has performed stuffing and the demultiplexer must know which bit to throw out (this is called positive stuffing).

• “Bit stuffing” used to maintain the clock capacity.

125 s (=1/8000 s)

Framingbit

24 or 30 voice channels

Framingbit

24 or 30 voice channels

……

Structure of a DS-1 or E-1 stream

Imagine four tributary streams

1 bit from into higher-order stream…

Plus a higher order framing bit or byte.

PDH Multiplexing

PDHDS1 Input = 1,544,000 Bps

DS1 Input = 1,545,796 Bps

DS1 Input = 1,540,429 Bps

DS1 Input = 1,544,500 Bps

1,545,796 Bps

1,545,796 Bps

Stuffing = 1296 Bps

Stuffing = 5367 Bps

Stuffing = 0 Bps

Stuffing = 1796 Bps

1,545,796 Bps

1,545,796 Bps

Synchronized the DS1

DS2 Output = 6,312,000 Bps

DS3

1,545,796 Bps(intermediate DS1 rate)-obtained by adding a given DS1 input rate toits associated stuffing rate

6,312,000 Bps(DS2 output rate)-obtained by adding the 4 intermediate DS1 rates and the DS2 overhead rate

Asy

nch

ron

ou

s In

pu

t

DS2 Overhead = 128, 816 Bps

SDH = SYNCHRONOUS DIGITAL HIERARCHY

SONET = SYNCHRONOUS OPTICAL NETWORK

Takes advantage of the totally synchronized network.

Unifies the North-American & European standards.

Can be used on both fiber and radio.

Put some intelligence in the multiplexers for solving operations and maintenance problems, especially protection switching.

Make multi-vendor networks manageable.

Be compatible with existing PDH streams.

What is SDH?• The basic time constant of 8000 frames per second is preserved in SDH.

• What can be transmitted in 125 µsec?

• The “lowest” level of the synchronous hierarchy.Synchronous Transport Module 1 (STM-1) at 155.520 Mbit/s.

The 19,440 bits in a 125 μs frame are represented by this rectangle of 9 rows with 270 bytes/row for a total of 2430 bytes.

270 bytes total

261 bytes for information9 bytes

Pointers

FramingSection overhead

Section overhead

155.520 Mbit/s = (270 9 8) bits/frame 8000 frames / s

0 µsec

125 µsec

Time

Figure. SDH Structure

• All information is collected in bytes and no longer in bits.• The bytes are transmitted one row at a time starting from

the point labeled “0 μsec”.

POINTERS – KEYS TO SUCCESS !!!

• The tributaries to a multiplexer each have a frame that is not aligned in time with the other tributaries, nor with the frame of the output stream.

• In PDH, the multiplexer does not even need to know where this frame is in time, i.e., the task of the demultiplexer in the lower hierarchical level.

• This is why ADD/DROP operations are so expensive.• To solve this problem, the SDH multiplexer finds where the

frame starts in each tributary. It calculates a pointer that tells where in the synchronous transport module level-1 (STM-1) frame it has placed the tributary frame.

……

Framing

Pointers

Framing

Pointers

Beginning of frame of 140Mbps carried in STM-1

End of frame

Time

A 140 Mbps E-4 Signal in an STM-1 Frame

• It begins midway through the STM-1 frame and ends midway through the next one.• A pointer indicates its position.

Remark : The world is not synchronous. If the tributary frame slips with respect to the STM-1 frame, the system just changes the pointer.

VIRTUAL CONTAINERS (VCs) AND ADMINISTRATIVE UNITS (AUs)

• The PDH signal is not just copied into the STM-1 frame as it arrives.• For example, it cannot use the space reserved for overhead and it cannot fill up the space available in the 261x9 bytes . So all PDH signals are packaged in appropriate “VIRTUAL CONTAINERS”. • This repackaging is called “ADAPTATION”.• There are many different VCs, one for each type of PDH signal to be carried. We show VC-4. The VC-4, together with the pointer is called an “ADMINISTRATIVE UNIT 4” or AV-4.

140 Mbps PHD signal

270 bytes

Administrative Unit 4 = data plus pointers

Stuffin

gO

A &

M

info261 bytes

Virtual Container 4

Virtual Container and AdministrativeUnit

HIGHER ORDER MULTIPLEXING : STM-4

• How to construct the next level, called the Synchronous Transport (STM-4) module 4 at 622 Mbps?

• 4 AV-4 are combined into an “ADMINISTRATIVE UNIT GROUP”, (AUG) and placed in an STM-4 frame which is still 125 sec long but has four times as many bytes as an STM-1.

Generation of the SDH-based User-Network Interface Signal

• The STM cell stream is mapped into the C-4 frame which is 9 row x 260column container corresponding to the transfer capability of 149.760 Mbps.

• C-4 is packed in the virtual container VC-4 along with the VC-4 POH.

• The C-4 is then mapped into the 9 x 270 byte frame called STM-1.

• The AU-4 pointer of the STM-1 frame is used to find the first VC-4 byte. The POH bytes J1, B3, C2, G1 and H4 are activated.

• The H4 pointer will be set at the sending side to indicate the next occurrence of a cell boundary.

Framing

Pointers . . .

4 x 270 bytes

9 bytes

Section overhead

Section overhead

. . .

Framing

9 bytes

4 x 270 bytes

(a)

(b)

Figure. STM-4

STM-16 is created in the same way as the STM-4, by interleaving 4 STM-4 signals.

This is the 2.4 Gbps rate, the highest are defined so far.

Every byte in every VC of all 4 tributaries is easily found using the pointers.

Framing

Pointers

☆ 7

• • • ▼ ☎

¶ ♠ O ♥

SDH-BASED INTERFACE at 622.080 Mbps

• 622.080 Mbps frame (STM-4) can be created straightforwardly from four STM-1s.

• The STM-4 payload can be structured either simply as 4 x VC-4 or as one block.

• The available ATM cell transfer capability would be 4 x 149.760 mbps = 599.040

Mbps for the first case.

• In the second case [ 9 x 261 x 4 byte - 9 bytes POH ] x 8 kHz = 600.768 Mbps.

270 x 4 bytes

STM-4 payloadSOH

9 x 4 261 x 4

SOH Section overheadSTM-4 Synchronous transport module 4

125 usec

9 ro

ws

SONET/SDH SIGNAL HIERARCHY

SONET/SDH Signal Hierarchy

OC Level SONET Designation CCITT Designation Data Rate Payload Rate (STS Level) (SDH Level) (MBPS)

OC-1 STS-1 51.84 50.112OC-3 STS-3 STM-1 155.52 150.336OC-9 STS-9 STM-3 466.54 451.008OC-12 STS-12 STM-4 622.08 601.344OC-18 STS-18 STM-6 933.12 902.016OC-24 STS-24 STM-8 1244.16 1202.688OC-36 STS-36 STM-12 1866.24 1804.032OC-48 STS-48 STM-16 2488.32 2405.376 . . . . . . . . . .OC-192 STS-192 STM-64 9953.28 .

OC : Optical Carrier STS : Synchronous Transport Signal STM : Synchronous Transport Module

General Formula N*51.84 STM *n OC-N STS-N

Table. SONET Equivalent to Plesiochronous Digital Hierarchy

North American SONET CCITT/ITU SDH SONET Rate SDH Rate VT VC (Mbps) (Mbps)

VT1.5 VC-11 1.544VT2.0 VC-12 2.048VT3.0 3.152VT6.0 VC-2 6.312 6.312

VC-3 44.736 34.368VC-4 139.264

STS-1 51.84STS-3 STM-1 155.52 155.52STS-12 STM-4 622.08 622.08

Table. Summary of International Plesiochronous Digital Hierarchy

Digital Bit Rate (Mbps) Multiplexing Number of Level Voice Channels North America Europe Japan 0 1 0.064 0.064 0.064 1 24 1.544 1.544

30 2.048

48 3.152 3.152 2 96 6.312 6.312

120 8.448

3 480 34.368 32.064 672 44.376

1344 91.0531

1440 97.728 4 1920 139.264

4032 274.176

5760 397.200 5 7680 565.148

Table. North American Digital HierarchySignal Name Rate Structure Number of DS0s

DS0 64k bps Time Slot 1

DS1 1.544 Mbps 24xDS0 24

DS1c 2xDS1 48

DS2 2xDS1c 96

DS3 44.736 Mbps 7xDS2 672

Table. North American Digital HierarchySTS-N or OC-N level Bit Rate (Mbps) Number of DS0s Number of DS1s Number of DS3s

1 51.84 672 28 1

3 155.52 2,016 84 3

6 311.04 4,032 168 6

9 466.56 6,048 252 9

12 622.08 8,064 336 12

18 933.12 12,096 504 18

24 1,244.16 16,128 672 24

36 1,866.24 24,192 1008 36

48 2,488.32 32,256 1344 48

96 4,976.00 64,512 2688 96

192 9,952.00 129,.024 5376 192

SONET SYSTEM HIERARCHY

• PHOTONIC. (Type of fiber; dispersion characteristics: lasers).

• SECTION. (Basic SONET Frames are created. Electronic signals are converted to photonic ones).

• LINE. (For synchronization, multiplexing of data into the SONET frames protection and maintenance functions and switch).

• PATH. (End-to-end transport of data at an appropriate signaling speed).

Path layer

Line Layer

Section Layer

Photonic layer

Service DS1, DS3, cells

Frame

Light

STS-N blocks

Envelope

Terminal TerminalRegenerator STS multiplexer(a) Logical hierarchy

SONETmultiplexer(PLE + LTE)

Add-Drop multiplexer(LTE)

SONETmultiplexer(PLE + LTE)

Repeater(STE)

Repeater(STE)

Terminals TerminalsSectionSectionSectionSection

Line Line

Path(b) Physical hierarchy

Figure. SONET System Hierachy

• Figure shows the physical realization of the logical layers.

• A section is the basic physical building and represents a single run of optical cable between two optical fiber transmitter/receivers.

• For shorter runs, the cable may run directly between two end units. For longer distances, regenerating repeaters needed.

• The repeater is a simple device that accepts a digital stream of data on one side and regenerates and repeats each out the other side

• Issues of synchronization and timing need to be addressed.

• A line is a sequence of one or more sections such that the internal signal or channel structure of the signal remains constant.

• Endpoints and intermediate switches/multiplexers that may add or drop channels terminate a line.

• Finally, a path connects to end terminals; it corresponds to an end-to-end circuits. • Data are assembled at the beginning of a path and are not accessed or modified until they are disassembled at the other end of the path.

Section Overhead

A1,A2: Framing bytes = F6, 28 hex

C1: STS-1 1D identifies the STS-1 number ( 1 to N) for each STS-1 within an STS-N multiplex

B1: Bit-interleaved parity type providing even parity previous STS-N frame after scrambling

E1: Section-level 64-kbps PCM orderwire (local orderwire)

F1: 64-kbps channel set aside for user purposes

D1-D3: 192-kbps data communications channel for alarms, maintenance, control, and administration between sections

Line Overhead

H1-H3: Pointer bytes used in frame alignment and frequency adjustment of payload data

B2: bit-interleaved parity for line-level error monitoring

K1,K2: Two bytes allocated for signaling between line-level automatic protection switching equipment

D4-D12: 576-kbps data communications channel for alarms,maintenance, control, monitoring, and administration at the line level

Z1-Z2: Reserved for future use

E2: 64-kbps PCM voice channel for line-level orderwire

Path Overhead

J1: 64-kbps channel used to repetitively send a 64-byte fixed-length string so a receiving terminal can continuously verify the integrity of a path; the contents of the message are user-programmable.

B3: Bit-interleaved parity at the path level

C2: STS path signal label to designate equipped versus unequipped STS signals and, for equipped signals, the specific STS payload mapping that might be needed in receiving terminals to interpret the payloads

G1: Status byte sent from path-terminating equipment back to path-originating equipment to convey status of terminating equipment and path error performance

F2: 64-kbps channel for path user

H4: Multiframe indicator for payloads needing frames that are longer than a single STS frame; multiframe indicators are used when packing lower-rate channels( virtual tributaries) into the SPE.

Z3-Z5: Reserved for future use.

TABLE. STS-1 Overhead Bits