ModemDigital Carrier Systems
Digital carrier standard
Europe, Mexico, South America
Digital carrier is a digital signaling represent a telecommunications service. In North America and Japan Digial service in North America definds a four level transmission hierachy called T-carrier, range from T1, T2, T3 and T4.
In Europe and South America, there is five-level transmission hierachy called E-carrier, range from E1, E2, E3, E4 and E5.
Both systems use PCM to encode an analog signal in digital from. The signal is sampled 5000 times per second, and each sample valve is encoded in an 8-bit valve. The signal transmission uses TDM.
4/11/40 page *
T1 carrier system
DS-1 frame
In North America and Japan, 24 voice channels are grouped together. The system use time division multiplexing to combine 24 voice channels into one frame.
The designations T1 and DS-1 refer to the circuit and signal respectively.
4/11/40 page *
DS-1 frame
8 bit
125 ms
A DS-1 frame contains 193 bits divieded into 24 slots (one for each channel) of 8 bits each. This leaves one extra bit called a framing bit, which is used for synchronizing.
Eight-bit voice samples are taken from each of 24 channels at a rate of 8000 times per second (64 kpbs rate). To support this speed, T-1 must transmit a DS-1 frame every 1/8000 of a second (125 ms). This yields bit rate : (24x8+1) bits/125 ms = 1.544 Mbps.
4/11/40 page *
T - carrier
DSI
DSIC
DS2
DS3
DS4
DSIC
DS2
DS2
DS2
DS2
DS2
DS2
DS3
DS3
DS3
DS3
DS3
DSI
Two 1.544 Mbps DS1 channels are multiplexed into a single 3.152 Mbps DS1C channel
Two DS1C channels are multiplexed into a single 6.312 Mbps DS2 channel
Seven DS2 channels are multiplexed into a single 44.736 Mbps DS3 channel
Six DS3 channels are multiplexed into a single 274.176 Mbps DS4 channel
To achieve higher aggregate link bit rates, several DS-1 channels are multiplexed together.
carrier frame format no. of channels Data rate in Mbps
T-1 DS-1 24 1.544
T-1C DS-1C 48 3.152
T-2 DS-2 96 6.312
T-3 DS-3 672 44.376
T-4 DS-4 4032 274.176
In both T1 and E1 systems, the lower bit rates are known as fractional T1/E1
4/11/40 page *
E1-frame
frame synchronization
signaling channel
0
1
2
16
31
E1 is a carrier channel configuration defined by ITU-T. The E1 carrier channel is built up of 64 kbps voice channels and two 64 kbps signaling channel.
This yields bit rate : 32x8 bits/125 ms = 2.048 Mbps
4/11/40 page *
E - carrier
E1
E1
E1
E1
E2
E2
E2
E2
E3
E3
E3
E3
E4
E4
E4
E4
E5
Thirty 64 kbps channels are multiplexed to create one 2.048 Mbps E1 channel
Four E1 channels are multiplexed into a single 8.448 Mbps E2 channel
Four E2 channels are multiplexed into a single 34.368 Mbps E3 channel
Four E3 channels are multiplexed into a single 139.264 Mbps E4 channel
Four E4 channels are multiplexed into a single 565.148 Mbps E5 channel
To achive higher bit rate, several E-1 channels are multiplex together :
circuit no. of channels Data rate (Mbps)
E1 30 data +2 control 2.048
E2 120 8.448
E3 480 34.368
E4 1920 139.264
E5 7680 565.148
Digital carrier comparison
PDH
Different streams have small differences in clock signals.
Solve by adding justification bit
PDH = Plesiochronous Digital Hierachy
almost synchronous
All telecommunication traffic has until recently been carried over equipment based on PDH. The term Plesiochronous cames from Greek language, and means-Almost operating to gether.
An examples would be two alarm clocks, both of which keep good time. over a period, the two clocks would start to drift part.
In the same way, digital signals from different places typically have their own clocks. all operating at almost (but not quite) 2.048 Mbps (E1). When combining four E1 to achieve E2, these four lines gradually drift apart, extra bits must occasionally be added to one or other to keep up.
4/11/40 page *
PDH Network
ADM = Add-drop multiplexer
The demultiplexing/multiplexing operation is performed by a device known as a rop-and-insert?or add-drop multiplexer (ADM)
4/11/40 page *
PDH deficiencies (I)
Lack of flexibility
impossible to identify a lower bit rate channel from the higher-order bit stream.
demux the high bit
remux back into higher
level for onward transmission
Extraction of 2 Mbps channel from 140 Mbps channel
It difficult, using PDH based equipment, to drop and insert a low rate channel from a high rate channel, without the use of a ultiplexer mountain? The normal example given is that of dropping out a 2 Mbps channel from a passing 140 Mbps stram. The cause of this partucular problem lies in the amount of information processing necessary to locate and extrace the resuired traffic channel. The basic PDH multilexer process involves a bit interleaving operation which obscures knowledge of the individual byte boubdaries, and thus leads to an inflexible system.
With all the different levels of plesichronous traffic, it is impossible to find and directly demultiplex a higher order bit rate, This is need to demultiplex the high bit rate down to the lower level. This stream must then be remultiplexed back into higher level for onward transmission
4/11/40 page *
PDH deficiencies (II)
Lack of performance
No management channel
The PDH does not currently have any internationsl standardised ways of monitoring the performance of traffic channels. The PDH does not have any aggreed management channels or protocol satcks
4/11/40 page *
PDH deficiencies (III)
Lack of id-Fibre meet
undefined interface specification on the line side of a line transmission
LTE
G.703
interface
PDH
SDH
Although the PDH specifies the exact format of the bit stream at the aggregate port of any PSH multiplexer, it puts no such constraints on the bit stream on the line side of a line termination. Consequently, every macufacture has used his own proprietary line code and optical interface specification, making it impossible for a PTO to interconnect line terminals from two different manufactures.
4/11/40 page *
SDH & SONET
multiplexing standard for optical fiber transmission
SONET = Synchronous Optical Network
refers to the system used within the U.S. and Canada
SDH = Synchronous Digital Hierarchy
international community term (ITU-T recommendtions)
SDH (Sychronous Digital Hierarchy) was developed initially by Bellcore in the USA under the title of Synchronous Optical Network (SONET). ITU-T sets a SDH standard with series of recommendations G.707, G.708 and G.709 as well as other relevant recommendations
SONET is a new transmission system designed for use over new fibre optic links It is a more intelligent frmaing ans transmission system which has many advantages over the older PDH systems. SONET consists of a hierarchy of physical data transmission rates.
4/11/40 page *
SDH goals
Provide a way to multiplex multiple digital signal togethers
provide support for operations, administration, and maintenace
Characteristics
Bit stream can be a added or extracted directly
Basic transmission rate = 155.52 Mbps
Design Goals :
(1). Defines a common signaling standard with respect to wavelength, timeming, framing structure and others.
(2) Digital sytem in U.S., European and Japan were based on 64 Kbps PCM channels, but all of which combined them in different and imcompatibility ways.
(3) Part of SDH/SONET mission was to continue the hierarchy to gigabits/sec and beyond
(4) Previous PDH system did not do this OAM (Operation, Administration, Management) very well
4/11/40 page *
Signaling rates
STM = Synchronous Transport Module
SONET is a signaling hierarchy based on a basic signaling structure called Synchonous Transport Signal (STS). The STS signaling system provides a basic signaling structure that built upon to implement higher line rates.
STS is also called Optical Carrier (OC) signaling. Optical carrier is used to cannote light signaling over fiber rather than electrical signaling over copper.
The STS/OC signaling hierarchy is based on a basic signaling rates of 51.84 Mbps. The higher signaling rate is amultiple of the STS/OC number times the basic rate of 51.84 Mb/s. The STS-3 line rate is three times the line rate of STS-1. There are rates for STS/OC-2,4,5,7,8 etc. within this hierarchy. However, these rates not used.
Note that the basic rate of SDH starts with a signaling rate of 155.52 Mbps called STM-1 (Synchronous Transport Module) which is equivalent to STS-3/OC-3
4/11/40 page *
Synchronous Container
1.5 Mbps
2 Mbps
6 Mbps
in an appropiate container
monitoring purpose
The Generl way that any of PDH stream are transported by SDH is to map such a stream into a synchronous container. A synchronous container can be viewd as a subdivision of the basic SDH frame structure, and it consists of a predefined number of 64kbps channels. The entire family of synchronous containers comprises only a few different types, each of which has been sized to accommodate one or more of the commom plesiochronous tranmission rates, without wasting too much bandwidth.
The operation of mapping a plesiochronous stream in to such a container is very similar to the normal stuffing operations performed in a conventions PDH multiplexer. However, in this case, the plesiochonous channel is being synchronized not to the master oscillator in a PDH multiplexer, but to the frequency of synchronous container, which in turn, synchronous to the basic SDH frame structure.
Once a PDH signal has been loaded to the container, a path overhead (POH) is appended to it. The combination of container and its POH is known as virtual container (VC). The VC POH bytes allow a PTO to monitor several parameters, the most important of which is the error rate of the VC between the points at which it was loaded and unloaded PDH signal.
The process of loading containers and then attaching POHs is repeated at several levels in SDH, resulting in the nesting of smaller Vcs in the largers ones. the nesting hierarchy stops when the largest level of VC is loaded into the data area of a STM.
4/11/40 page *
Syncronization techniques
data #1
data #1
data #1
data #1
data #2
data #2
POH #1
POH #2
sample frame
frame #1
frame #2
It very difficult to maintain a complete tranmission network in rigid synchronisation for all time. Even if we could tolerate the delays introduced by the addition of numerous ander buffers?necessary to accommodate the slow changes in transmission medium delay, there is no guarantee that different PTOs?networks would all be synchronised to the same master clock. For a network based on the SDH, this problem trasnlates to that of how to synchronously multiplex and demultiplex many individual VCs, which theu have been created in disparate parts of the same, oe even different SDH networks, may have slighly different short term bit rates.
The solution adoprd by SDH is to associate a pointer with each VC so that when it is multiplexd, along with others, into a larger VC, its phase offset in bytes can be identified relative to some reference point in this larger VC.Furthur more, there is also a mechanism for allowing the value of this pointer to change if, for some capacity VC is running either slighly slower, or slightly faster then the larger VC. In fact, each of the smller capacity VCs has its own pointer, which can change independently of any of the others.
The pointer mechanism is at the very heart of the SDH standard. It is this mechansim that enables us to construct networks that are nearly, but not completely synchronous, and yet still allow us to easily locate each traffic channel(VC), tigether with its associated management and control information. It could be argued that SDH networks are not really synchronous at all, but are actually very tightly controlled asynchronous networks.
4/11/40 page *
SONET System
section
section
section
section
line
line
path
mux
repeaters
repeaters
mux
mux
In SONET/SDH terminology, a fiber going directly from any device to any other device, with nothing in between, is called a section. A run between two multiplexers (possibly with one or more repeaters in the middle) is called a line. Finally, the connection between the source and destinations is called a path.
The SONET/SDH topology can be a mesh, but often a dual ring.
4/11/40 page *
SONET Basic Frame structure
1 2 3 4 5 6 7 8 9
STS-1/OC1
row/column mapping
A SONET frame can be viewd logocally as 9 rows of 90 bytes each. Therefore, an STS-1 frame is a unit of 810 bytes of information. SONET frame is transmitted serially bit-by-bit beginning with the first bit of the first byte. Each frame
The 810 bytes frame are best described as a rectangular of bytes, 90 column wide by 9 rows high. Thus 810x8 =6480 bits are transmitted 8000 times per second, for a gross data rate of 51.84 Mbps.
The first thress column of each row are reserved for system management information. the first three rows contain section overhead. The nex six contain the line overhead. Section overhead and line overhead are called transport overhead.
The remaining 87 column hold 87x9x8x8000 = 50.112 Mbps of user data called Synchronous Payload Envelope or Information Payload or Payload.
4/11/40 page *
SDH Basic Frame structure
1 2 3 4 5 6 7 8 9
STM-1
row/column mapping
A STS-3 frame has 9 cloumn of overhead and 261 bytes of payload, results 270x9= 2430 bytes. The 2430x8 bites are transmitted 8000 times per second, for a gross data rate of 155.52 Mbps.
4/11/40 page *
STM-1 Frame
9 bytes
261 bytes
frame #1
frame #2
Payload #1
Payload #1
Payload #1
Payload #1
Payload #2
Payload #2
POH #1
POH #2
This user data do not always begin at row1 col 4. The SPE can begin everywhere with the frame. The first row of line overhead contain a pointer to the first byte of user data. The first column of the SPE is called the path overhead.
The ability to allow the SPE to begin anywhere with in SDH frame,and even to span two frames, as shown above, gives added flexibility to the system. For example, if a payload arrives at the source while a dummy frmae is being constructed. It can be inserted into the current frame, insteadof being held until the start of the next one. This feature is also useful when the payload does not fit exactly in one frame,a s in the case of a sequence of 53-byte ATM cells. The first row of line overhead can then point to the start of the first full cell, to provide synchronization.
4/11/40 page *
SOH, LOH and POH
A1 A2 C1 J1
B1 E1 F1 B3
D1 D2 D3 C2
H1 H2 H3 G1
B2 K1 K2 F2
D4 D5 D6 H4
D7 D8 D9 Z3
D10 D11 D12 Z4
Z1 Z2 E2 Z5
SOH
A1-A2 framing bytes = F6H, 28H, used to synchronize the beginning of the frame
B1 8-bit even parity check of last SPE
C1 identifies the STS-1 number (1 to N)for each STS-1 within an STS-N
D1-D3 data communications channel for alarms, maintenance, control, and
adminsistrations between sections
F1 64Kbps channel set aside for user purpose
LOH
H1-H3 pointer bytes used in frame alignment and frequency adjustment of payload data
B2 8-bit parity check for line level error monitoring
D4-D12 data communications channel for alarms, maintenance, control, and
adminsistrations between lines
K1-K2 signaling channel for automatic protection switching equipment
Z1-Z2 reserved for national used
POH
J1 64 Kbps channel fro receiving terminal can verifie the integrity of the path
B3 8-bit parity check for path level error monitoring
C2 signaling identification (type of mapping appeared in payload)
G1 path status
H4 indictes whether the payload is part of a multiframe
Z3-Z5 reserved for national used
4/11/40 page *
SDH mux scheme
STM-1
SONET-specific
Europe-specific
Universal
PDH
Tributaries
PDH tributaries streams is cariired in differnet containers. Note that the first digit of the lowest level container is a 1 and the second digit indicates whether it contains 1.5 Mbps (C11) or 2 Mbps (C12)
4/11/40 page *
SDH Elements
Administrative Unit
STM-1
The containner and its path overhead collectively forma virtual container (VC). The Vcs and its pointer are known as a tributaru unit (TU) if the VC carrires a lower-order tributarire or a tributary group if it carries a number of lower-order trubutaries. The largest VC in an STM-1 frame is known as asn admintrative unit (AU).
4/11/40 page *
Containers mapping
fixed stuff
alignment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
17 18 19 20 21 22 23 24 25 26 27 28 29 30
signaling
Containers, VC & TU
plus POH
plus pointer
9 bytes
3 bytes
4 bytes
12 bytes
Having mapping a PDH stream into a container, the next operation is to generate and attach the path overhead (POH), whicg enables the containers to be identified, monitored for errors and routed through the SDH network. The additions of this POH byte to the container creates a virtual container.
The POH stays attached to its container all the way from the point where it was generated, to the point at which the PDH payload exits the SDH network. The VC and its pointer are known as tributary unit (TU).
4/11/40 page *
TU-X format
3 columns
9 rows
4 columns
9 rows
12 columns
9 rows
125 ms
125 ms
125 ms
1728 kbps
2304 kbps
6912 kbps
Several virtual containers such as VC-12 are loaded in to a larger container, which subsequencetly has its own POH addes, thus created a larger higher bit rate.
In CEPT countries, this operatoins results in the creation of a VC4, which is large enough to accommodated up to 63 VC12s. Unfortunately complication arise in this operation, because the network element that is performing this task, may not itself have created all the VC12s. this leads to the possiblity that not all the VC12s are completely synchronous with one another or, more importantly, with the VC4 into which they are being loaded.
Because of the importance of pointers for locating low order VCs within high order VCs, the combination of a low order VC plus its pointer is refered to as a tributary unit (TU).
4/11/40 page *
TUG-2 to VC-3
4xTU-11
3xTU-12
1xTU-2
.The reason for introducing the concept of TUG is that many size of virual containers have been defined (VC-11, VC-12 and VC-2) In the event that a PTO needing to transport various VCs e.g. 2 Mbps PDH (VC-12) and 6.3 Mbps PDH (VC2), they can both be accomodated within the same frame by assigning some TUGs to carry groups of 3xTU12s whilel other TUGs are assigned to each carry a single TU2
Seven of TUG-2 can be multiplex byte-by-byte to be VC-3 with 1 column of POH results a VC-3
4/11/40 page *
AU-3 to AUG
A B C A B C
A B C
H1 H2 H3
H1 H2 H3
H1 H2 H3
AU-3 is used mainly in North America (SONET), where by the payload consists of a group of three VC-3s, together with their associated pointers.
4/11/40 page *
TUG-3 to VC-4
fixed stuff
TUG-3 have two path ways, either from TUG-2 or TU-3. A TUG-3 is slightly larger than 7 TUG-2s, and designed to accommodate a single 45 Mbps signal ( from C-3 to VC-3 to TU-3 to TUG-3). A 34 Mbps signal can also be mapped into a VC-3, in an operation that appears somewhat wasteful on bandwidth, as the C-2 has been sized for a 45 Mbps signal. It is interesting to note that, rather than use a more aptly sized container, this bandwidth sacrifice was aggreed to by European PTOs in order to reduce the potential network control problem posed by a larger variety of VCs.
The combination of VC-4 plus its pointer is known as administrative unit 4 (AU4), rather than a TU-4.
4/11/40 page *
High order mux
stream
The multiplexing of multiple data stream, plays an important role in SONET/SDH. Multiplexing is done byte for byte. For example, when three STM-1 are merged into one STM-3 stream at 466.56 Mbps, the multiplexer first outputs 1 byte from tributary 1, then 1 from tributary 2 , and finally from tributary 3, before going back to 1.
4/11/40 page *
1xOC3 and 3xoC1
a 155.52 Mbps carriers consisting of 3 separated OC3 carriers
OC3c = 1X155.52Mbps carriers
Higher-order frames also exist (e.g. OC12c).
The amount of actual user data in an OC3c stream is slighly higher thanin an OC-3 stream!