1. 3. Physical Layer – Cell Transport Methods

2. 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 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 = Mbps E1 format.
(ETSI -> European Telecommunications Standards Institute)

3. 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
Channel 1
Framing Bit (193)
8 Bits
8 Bits
8 Bits
8 Bits
8 Bits
8 * 24 = = 193 Bits
125 sec = 1/8000 sec
193 Bits each 125 sec = Mbps (Aggregate Bit Rate)

4. 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 Mbps.

5. Digital Hierarchy 2 1 24 4 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
Mbps/Channel
Mbps/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  E-2 at 8 Mbps 4 E-2s  E-3 at 34 Mbps 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…

6. 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.
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.

7. 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.

8. Structure of a DS-1 or E-1 stream
125 s (=1/8000 s)
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

9. PDH DS1 Input = 1,544,000 Bps 1,545,796 Bps Stuffing = 1796 Bps
Synchronized the DS1
DS1 Input = 1,545,796 Bps
1,545,796 Bps
Stuffing = 0 Bps
DS2 Output = 6,312,000 Bps
Asynchronous Input
DS1 Input = 1,540,429 Bps
Stuffing = 5367 Bps
1,545,796 Bps
DS3
DS1 Input = 1,544,500 Bps
Stuffing = 1296 Bps
1,545,796 Bps
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
DS2 Overhead = 128, 816 Bps

10. 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.

11. 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 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 information
9 bytes
Framing
Section overhead
Mbit/s = (270  9  8) bits/frame 8000 frames / s
0 µsec
125 µsec
Time
Pointers
Figure. SDH Structure

12. 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.

13. A 140 Mbps E-4 Signal in an STM-1 Frame
Framing ……
Pointers
Time
Beginning of frame of 140Mbps carried in STM-1
Framing
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.

14. 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.
270 bytes
261 bytes
Administrative Unit 4 = data plus
pointers
OA & M info
140 Mbps PHD signal
Stuffing
Virtual Container and Administrative
Unit
Virtual Container 4

15. 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.

16. 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 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.

17. . . . . . . Figure. STM-4 Framing Section overhead Section overhead
4 x 270 bytes
9 bytes
Framing
Section overhead
. . .
Pointers
Section overhead
(a)
4 x 270 bytes
9 bytes
Framing
. . .
(b)
Figure. STM-4

18. 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-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.

19. SDH-BASED INTERFACE at 622.080 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
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 mbps =
Mbps for the first case.
In the second case [ 9 x 261 x 4 byte - 9 bytes POH ] x 8 kHz = Mbps.

20. SONET/SDH SIGNAL HIERARCHY
OC Level SONET Designation CCITT Designation Data Rate Payload Rate
(STS Level) (SDH Level) (MBPS)
OC-1 STS
OC-3 STS-3 STM
OC-9 STS-9 STM
OC-12 STS-12 STM
OC-18 STS-18 STM
OC-24 STS-24 STM
OC-36 STS-36 STM
OC-48 STS-48 STM
OC-192 STS-192 STM
OC : Optical Carrier STS : Synchronous Transport Signal STM : Synchronous Transport Module
General Formula N* STM *n OC-N STS-N

21. Table. SONET Equivalent to Plesiochronous Digital Hierarchy
North American SONET CCITT/ITU SDH SONET Rate SDH Rate
VT VC (Mbps) (Mbps)
VT1.5 VC
VT2.0 VC VT
VT6.0 VC VC VC
STS
STS-3 STM
STS-12 STM

22. Table. Summary of International Plesiochronous Digital Hierarchy
Digital Bit Rate (Mbps)
Multiplexing Number of
Level Voice Channels North America Europe Japan

23. 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
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 3
155.52
2,016 84 6
311.04
4,032
168 9
466.56
6,048
252 12
622.08
8,064
336 18
933.12
12,096
504 24
1,244.16
16,128 36
1,866.24
24,192
1008 48
2,488.32
32,256
1344 96
4,976.00
64,512
2688
192
9,952.00
129,.024
5376

24. 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).

25. Figure. SONET System Hierachy
Service
DS1, DS3, cells
Envelope
Path layer
Line Layer
Section Layer
Photonic layer
STS-N blocks
Frame
Light
Terminal
Regenerator
STS multiplexer
Terminal
(a) Logical hierarchy
SONET
multiplexer
(PLE + LTE)
Add-Drop
multiplexer
(LTE)
SONET
multiplexer
(PLE + LTE)
Repeater
(STE)
Repeater
(STE)
Terminals
Section
Section
Section
Section
Terminals
Line
Line
Path
(b) Physical hierarchy
Figure. SONET System Hierachy

26. 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.

27. 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.