www.ece.gatech.eduresearchlabsbwnee6609notes by dffhrtcv3


									  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
• Why? à 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 àE1 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      8 Bits    8 Bits               8 Bits

         8 Bits                  8 * 24 = 192 + 1 = 193 Bits

                                    125 msec = 1/8000 sec
                          193 Bits each 125 msec = 1.544 Mbps (Aggregate Bit Rate)
• Each voice signal sampled at rate 8000 sample
  or once every 125 msec.
• 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.
                           Digital Hierarchy

                                                               1               DS-4

                                                 1              DS-3       274.176 Mbps/Channel
                                 1               DS-2       44.376 Mbps/Channel

                                   DS1-C    6.312 Mbps/Channel

    2               DS-1     3.152 Mbps/Channel
      DS-0    1.544 Mbps/Channel
  64 Kbps/Channel
• Multiplexing means taking a certain number of DS-1 or E-1 signals and putting them
  together as shown above.
• European:           4 E-1s à E-2 at 8 Mbps
                      4 E-2s à E-3 at 34 Mbps
                      4 E-3s à E-4 at 140 Mbps
                      4 E-3s à E-4 at 565 Mbps (not standardized)
• REMARK: DS-1, DS1-C etc… refer to the multiplexing scheme used for carrying
• 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…
What is a Synchronous Network?
• The last two decades, digital switching has taken over from analog
• This means all digital systems can be connected and therefore
  synchronized with each other.

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.
   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 ms (=1/8000 s)
              Framing                    Framing
                bit                         bit
                    24 or 30 voice channels     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
                     DS1 Input = 1,544,000 Bps
                                                    1,545,796 Bps
                            Stuffing = 1796 Bps
                                                                            Synchronized the DS1

                     DS1 Input = 1,545,796 Bps
Asynchronous Input

                                                   1,545,796 Bps
                               Stuffing = 0 Bps
                                                                                   DS2 Output = 6,312,000 Bps
                     DS1 Input = 1,540,429 Bps

                            Stuffing = 5367 Bps    1,545,796 Bps
                     DS1 Input = 1,544,500 Bps

                           Stuffing = 1296 Bps     1,545,796 Bps
                                                                    1,545,796 Bps             6,312,000 Bps
                                                                    (intermediate DS1         (DS2 output rate)
                                                                    rate)                     -obtained by adding
                                                                    -obtained by adding a     the
                     DS2 Overhead = 128, 816 Bps                    given DS1 input rate      4 intermediate DS1
                                                                    to                        rates and the DS2
                                                                    its associated stuffing   overhead rate

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
                 9 bytes                               261 bytes for information    0 µsec



                       155.520 Mbit/s = (270 ´ 9 ´ 8) bits/frame ´8000 frames / s   125 µsec
                                         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”.


• 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.
    A 140 Mbps E-4 Signal in an STM-1 Frame
           Pointers                                                               Time

                                 Beginning of frame of 140Mbps carried in STM-1


                      Pointers       End of 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.
                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
• 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.                          270 bytes

                                                                                      261 bytes
                                         Administrative Unit 4 = data plus
                                         pointersOA & M info
  Virtual Container and Administrative
  Unit                                                         140 Mbps PHD signal

                                                                Virtual Container 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 msec 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 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.
                                   4 x 270 bytes
               9 bytes
             Pointers                  ...

                            4 x 270 bytes
   9 bytes


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


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  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.
      SDH-BASED INTERFACE at 622.080 Mbps
                                   270 x 4 bytes

                           9x4                 261 x 4

                 9 rows

                                                                   125 usec
                          SOH       STM-4 payload

                          SOH     Section overhead
                          STM-4   Synchronous transport module 4
• 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.
                            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       50.112
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
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
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 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
•   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
  Service   DS1, DS3, cells

       Path layer                                      Envelope

      Line Layer                  STS-N blocks
     Section Layer      Frame
     Photonic layer

      Terminal                       Regenerator                   STS multiplexer                Terminal
(a) Logical hierarchy
            SONET                                  Add-Drop                                 SONET
            multiplexer                            multiplexer                              multiplexer
            (PLE + LTE)                            (LTE)                                    (PLE + LTE)

                                  Repeater                                 Repeater
                                  (STE)                                    (STE)

Terminals             Section            Section                 Section          Section                 Terminals
                                  Line                                     Line
(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.
  TABLE. STS-1 Overhead Bits
                                                             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.

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