ATM NETWORKS

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12 Δεκ 2013 (πριν από 3 χρόνια και 3 μήνες)

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3.

Physical Layer




Cell Transport Methods

SONET


(
S
YNCHRONOUS
O
PTICAL

NET
WORK)





* 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 “signalin
g”

for a total of



32*64 kbps = 2.048 Mbps

E1 format.

(ETSI
-
> European Telecommunications Standards Institute)

The Cell Transport Method


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 sec

193 Bits each 125

sec = 1.544 Mbps (Aggregate Bit Rate)

8 Bits

8 Bits

8 Bits

8 * 24 = 192 + 1 = 193 Bits

8 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.

What is Framing?


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 Mbps



4 E
-
2s


E
-
3 at 34 Mbps





4 E
-
3s


E
-
4 at 140 Mbps



4 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

2

1

24

1

4

1

2

2

1

1

7







DS
-
0

DS
-
1

DS1
-
C

DS
-
2

DS
-
3

DS
-
4

64 Kbps/Channel

1.544 Mbps/Channel

3.152 Mbps/Channel

6.312 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.


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
.

What is a Synchronous Network?


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)

Framing

bit

24 or 30 voice channels


Framing

bit

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

PDH = Plesiochronous Digital Hierarchy


PDH

DS1 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 to

its associated stuffing rate

6,312,000 Bps

(DS2 output rate)

-
obtained by adding the

4 intermediate DS1 rates and
the DS2 overhead rate

Asynchronous Input

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
μ
s 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 information

9 bytes

Pointers

Framing

Section
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.

SDH

……

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

Stuffing

OA & M info

261 bytes

Virtual Container 4

Virtual Container and Administrative Unit




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.

HIGHER ORDER MULTIPLEXING: STM
-
4



STM





The STM cell stream is mapped into the C
-
4 frame which is 9 row x 260

column 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

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









STM

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 payload

SOH

9 x 4

261 x 4

SOH

Section overhead

STM
-
4

Synchronous transport module 4

125 usec

9 rows





OC Level

SONET Designation

CCITT Designation

Data Rate

Payload Rate




(STS Level)



(SDH Level)

(MBPS)


OC
-
1

STS
-
1




51.84

50.112

OC
-
3

STS
-
3


STM
-
1


155.52

150.336

OC
-
9

STS
-
9


STM
-
3


466.54

451.008

OC
-
12

STS
-
12


STM
-
4


622.08

601.344

OC
-
18

STS
-
18


STM
-
6


933.12

902.016

OC
-
24

STS
-
24


STM
-
8


1244.16

1202.688

OC
-
36

STS
-
36


STM
-
12


1866.24

1804.032

OC
-
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

SONET/SDH SIGNAL HIERARCHY

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


1.544





VT2.0


VC
-
12




2.048


VT3.0




3.152





VT6.0


VC
-
2


6.312


6.312




VC
-
3


44.736


34.368




VC
-
4




139.264


STS
-
1




51.84





STS
-
3


STM
-
1


155.52


155.52


STS
-
12


STM
-
4


622.08


622.08


SONET/SDH SIGNAL HIERARCHY


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



SONET/SDH SIGNAL HIERARCHY

Table. North American Digital Hierarchy

Signal 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 Hierarchy

STS
-
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/SDH SIGNAL 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).

SONET SYSTEM HIERARCHY

Path layer

Line Layer

Section Layer

Photonic layer

Service

DS1, DS3, cells

Frame

Light

STS
-
N blocks

Envelope

Terminal

Terminal

Regenerator

STS multiplexer

a) Logical Hierarchy

SONET

multiplexer

(PLE + LTE)

Add
-
Drop

multiplexer

(LTE)

SONET

multiplexer

(PLE + LTE)

Repeater

(STE)

Repeater

(STE)

Terminals

Terminals

Section

Section

Section

Section

Line

Line

Path

b) Physical Hierarchy

SONET System Hierarchy



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.

SONET System Hierarchy


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 pay
loa
d 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 equipmen
t 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
whe
n packing lower
-
rate channels( virtual
tributaries) into the SPE.

Z3
-
Z5: Reserved for future use.

TABLE. STS
-
1 Overhead Bits