3. physical layer – cell transport methods
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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
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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
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