Transcript
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Chapter 12Asynchronous Transfer Mode
Learning Objectives
Understand the background information on ATM technology and protocols
used for broadband networking.
Understand the BISDN UserNetwork Interface concept.
Understand the BISDN Protocol Reference Model.
Understand the functions performed by physical layer.
Understand the ATM cell structure and ATM layer functions.
Understand the ATM Adaptation Layer concept.
Understand how ATM works ? Understand Why ATM service categories are defined ?
Understand ATM Service Architecture and Applications.
IntroductionIn the emerging field of highspeed virtual networking, Asynchronous Transfer Mode
(ATM) is a key component. ATM is a telecommunications concept defined by ANSI
and ITU (formally CCITT) standards for carriage of a complete range of user traffic,
including voice, data, and video signals, on any UsertoNetwork Interface (UNI). As
such, ATM is extremely well suited to highspeed networking. ATM technology canbe used to aggregate user traffic from existing applications onto a single UNI (e.g.
PBX tie trunks, hosttohost private lines, video conference circuits), and to facilitate
multimedia networking between high speed devices (e.g. workstations,
supercomputers, routers or bridges) at multimegabit speeds (e.g. 100s of Mbit/s) and
higher.
On the basis of its numerous strengths, ATM has been chosen by standards
committees (e.g. ANSI T1, ITUT SG13) as an underlying transport technology
within Broadband Integrated Service Digital Network (BISDN) protocol stacks. In
this context, transport refers to the use of ATM switching and multiplexing
techniques at the data link layer (i.e., OSI Layer 2) to convey enduser traffic fromsource to destination within a network. While BISDN is a definition for public
networks. ATM can also be used within private networking products, in recognition
of this fact, and for clarity, here we first define two distinct forms of ATM UNI :
Public UNIwhich will typically be used to interconnect an ATM user with an ATM
switch deployed in a public service providers network.
Private UNI which will typically be used to interconnect an ATM user with an
ATM switch that is managed as part of the same corporate network.
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The primary distinction between these two classes of UNI is physical reach. Both
UNIs share an ATM layer specification, but may utilize different physical media.
Facilities that connect users to switches in public central offices must be capable of
spanning long distances. In contrast, private switching equipment can often be located
in the same room as the user device (e.g. computer, PBX), and hence can use limiteddistance technologies.
The term ATM user represents any device that makes use of an ATM network, via
an ATM UNI, as illustrated in Fig.1.
Fig.1
Implementations of the ATM UNI
For example, an ATM user device may be either of the following :
An Intermediate System (IS), such as an IP router, that encapsulates data into ATM
cells, and then forwards the cells across an ATM UNI to a switch (either privately
owned, or within a public network),
A private network ATM switch, which uses a public network ATM service for the
transfer of ATM cells (between public network UNIs) to connect to other ATM userdevices.
The carriage of user information within ATM format cells is defined in standards as
the ATM Bearer Service.
What is ATM Bearer Service ?
The ATM bearer service as defined by ANSI and ITU standards, provides a
sequencepreserving, connectionoriented cell transfer service between source and
destination with an agreed Quality of Service (QoS) and throughput. The ATM bearer
service involves at a minimum the two lower protocol layers (ATM, Physical) of the
BISDN protocol stack. These two layers are serviceindependent and contain
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ATM
User
ATM
User
ATM
User
Private
ATMSwitch
Public ATM
Network
Private
UNI
Public
UNI
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functions applicable to all upper layer protocols (i.e. they are independent of user
applications). Additionally, the ATM bearer service may involve the CPlane
adaptation layer and signaling protocol for SVC service, UPlane adaptation layers,
which reside above the ATM layer, have been defined in standards to adapt the ATM
bearer service to provide several networking classes of service including ConstantBitRate (CBR) and Variable BitRate (VBR) services.
An ATM bearer service at a Public UNI offers pointtopoint, bidirectional or
pointtomultipoint unidirectional virtual connections at either a virtual path (VP)
level and/or a virtual channel (VC) level. Networks can provide either a VP or VC (or
combined VP and VC) level service. For ATM users that desire only a VP service
from the network, the user will be able to allocate individual VCs (which are not
reserved or allocated for ILMI) within the VP connection (VPC) as long as none of
the VCs is required to have a higher QoS than the VP connection. QoS of a VPC can
be either explicitly specified at subscription time or implicitly specified (through a
variety of mechanisms) and is selected to accommodate the most demanding QoS of
any VC to be carried within that VPC. For VC level service at the UNI, the QoS andthroughput are configured for each virtual channel connection (VCC) individually.
The virtual connection (VPC or VCC) will be established or released via the signaling
protocol or on a subscription basis.
User Network Interface Configuration
Figure 2 illustrates how equipment at both the Private UNI and Public UNI, map into
the BISDN access reference configuration shown in standards. The Public UNI is
modeled after the BISDN UserNetwork Interface defined in ITU Recommendations
and ANSI Standards. It embraces the physical characteristics corresponding to
reference points.
Two elements can be used to describe a reference configuration of the UserNetwork
access of BISDN.
They are
Functional groups
Reference points.
BNT1 functions are similar to Layer 1 o f the OSI Reference model and some of the
functions are
Line Transmission Termination Transmission Interface handling
OAM functions.
BNT2 functions are similar to layer 1 and higher layers of the OSI model. Some
functions of BNT2 are
Adaptation functions for different interface media and topology
Multiplexing and demultiplexing and concentration of traffic
Buffering of TM cells
Resource allocation and usage parameter control
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Signaling protocol handling
Interface handling
Switching of Internal connections.
SB and TB indicate reference points between the terminal (BTE) and the BNT2 and
between BNT2 and BNT1, respectively.Fig. 2
UserNetwork Interfaces Configuration
BISDN : ATM Protocol Reference Model
The BISDN protocol reference model defined in ITUT Recommendation 1.121 is
shown in Fig.3. The reference model is divided into multiple planes as follows :
User plane (Uplane)
with its layered structure, provides for user information flow transfer, along with
associated controls ranging from flow control to error recovery, etc. It contains
Physical Layer, ATM layer and multiple ATM Adaptation Layers required fordifferent service users (e.g. CBR service, VBR service, etc.).
Control plane (Cplane)
has a layered structure and performs the call control and connection control functions;
it deals with the signaling necessary to set up, supervise and release calls and
connections. Thus, Control plane protocols deal with call establishment and release
and other connection control functions necessary for providing switched services. The
Cplane structure shares the Physical and ATM layers with the Uplane as shown in
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Fig.3. It also includes ATM adaptation layer (AAL) procedures and higher layer
signaling protocols.
Management plane (Mplane)
provides two different types of functions : Plane management functions
o not layered, that are related to a system as a whole and provide co
ordination between all the planes.
Layer management functions
o which are related to resources and parameters residing in its protocol
entities; layer management handles the operation and maintenance
(OAM) information flows specific to the layer concerned.
Thus, Management plane provides management functions and the capability to
exchange information between Uplane and Cplane. The Layer Management
performs layerspecific management functions while the Plane Management performs
management and coordination functions related to the complete system.
Fig. 3
BISDN : ATM Protocol Reference Model
The UNI specification involves those protocols, which are either terminated ormanipulated at the usernetwork interfaces. Based on the ATM bearer service
capabilities defined earlier, the protocol layers involved at both UNIs are limited to
the physical and ATM layers, Cplane higher protocol layers for SVC support and
other protocols required for UNI management. Many physical layers (e.g. SDH :
STM1/SONET : DS3) can be specified at both the private or public UserNetwork
Interfaces
Additional physical layers (e.g. blockcoded) are specified for the private UNI. The
applicability of any physical layer at a given interface will depend on technology
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limitations (e.g. maximum reach) or cost effectiveness (e.g. complexity). The UNIs
may also contain Physical Layer Management functions (e.g. SDH/SONET OAM)
and ATM Layer Management functions.
In terms of ATM protocol stack, above the Physical layer there is the ATM Layer that
provides cell transfer for all services and the ATM Adaptation Layer (AAL) providingservicedependent functions to the layers above (indicated as higher layers). The layer
above AAL in the control plane provides call control and connection control; the
management plane provides network supervision functions. The functions of each
layer are detailed in Table 1, which also shows sublayers : Convergence Sublayer
(CS) and Segmentation and Reassembly Sublayer (SAR) for the AAL, Transmission
Convergence (TC) and Physical Medium (PM) for the Physical Layer.
ATM Physical Layer
Physical layer controls transmission and receipt of bits on the physical medium. It
also keeps track of ATM cell boundaries and packages cells into the appropriate type
of frame for the physical medium being used.
The ATM physical layer is divided into two parts :
Physical medium sublayer
Transmission convergence sublayer.
Table 1
Functions of the BISDN : ATM in relation to the Protocol Reference Model
Layer
Management
Higher layer Higher layers
Convergence CSAAL
Segmentation and Reassembly SAR Generic flow control
ATMCell header generation/extraction
Cell VIP/VCI translation
Cell multiplex and demultiplex
Cell rate decoupling
TCPhysical
layer
HEC head. sequ. generation/verification
Cell delineation
Transmission frame adaptation
Transmission frame generation/recovery
Bit timing PM PMPhysical medium
The physical medium sublayer is responsible for sending and receiving a continuous
flow of bits with associated timing information to synchronize transmission and
reception. Because it includes only physicalmediumdependent functions, its
specification depends on the physical medium used. It provides bit transmission
capability including bit alignment. It performs Line coding and also electrical/optical
conversion, if necessary.
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ATM can use any physical medium capable of carrying ATM cells. Some existing
standards that can carry ATM cells are SONET (Synchronous Optical
Network)/SDH STM 1, DS3/E3, 100Mbps local fibre [Fibre Distributed Data
Interface (FDDI) physical layer), and 155Mbps local fibre (Fiber Channel Physical
Layer). Various proposals for use over twistedpair cables were under considerationand Physical layer specification for UTP Cat 5 and Cat 3 have already been
developed.
The transmission convergence sublayer is responsible for the following five functions
Cell rate decoupling
Inserts or suppresses idle (unassigned) ATM cells to adapt the rate of valid ATM cells
to the payload capacity of the transmission system, i.e. in cell rate decoupling it
inserts the idle cells in transmitting direction in order to adapt the rate of the ATM
cells to the payload capacity of the transmission system. It suppresses all idle cells in
the receiving direction. Only assigned and unassigned cells are passed to the ATM
layer.
Header Error Control (HEC) sequence generation and verification
Generates and checks the header error control code to ensure valid data. HEC
sequence generation is done in the transmit direction and its value is recalculated and
compared with the received value and thus used in correcting the header errors. If the
header errors cannot be corrected, the cell will be discarded.
Cell delineation
Maintains ATM cell boundaries. This function enables the receiver to recover the cell
boundaries. Scrambling and Descrambling are to be done in the information field of a
cell before the transmission and reception respectively to protect the cell delineation
mechanism.
Transmission frame adaptation
Packages ATM cells into frames acceptable to the particular physicallayer
implementation, i.e. transmission frame adaptation takes care of all actions to adapt
the cell flow according to the used payload structure of the transmission system in the
sending direction. It extracts the cell flow from the transmission frame in the
receiving direction. The frame can be a synchronous digital hierarchy (SDH) envelope
or an envelope according to ITUT Recommendation G.703.
Transmission frame generation and recovery
Generates and maintains the appropriate physicallayer frame structure. This is the
lowest functions of TC sublayer.
Service expected from the Physical Layer
The ATM layer expects the Physical layer to provide for the transport of ATM cells
between communicating ATMentities. The information exchanged between the
ATM layer and the Physical layer across the PHYSAP includes the following
primitives :
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Primitive Request Indicate Confirm Respond1
PHYUNITDATAX X
1 : The ATMentity passes one cell per PHYUNITDATA request and accepts one
cell per PHYUNITDATA indicate.
Fig. 4
PHYSAP Services required by the ATM Layer
Physical Layer UNI Interfaces
Because of the different kinds of details in the coupling between the fibre or other
physical medium, the transmission convergence sublayer is different, depending on
the physical layer.
155 Mbps SONET STS3c/SDH STM 1
Lets start with SONET/SDH, which is probably the physical layer most often
associated with ATM.
The essential feature of SONET/SDH is to keep track of boundaries of streams that
dont really depend on the particular medium. So, although we typically think about it
as fibre, it will in fact operate over other media. Some of the work going on currently
in the ATM Forum on a physical specification for using (copper) unshielded-twisted
pair will be using the SONET type framing.
155 Mbps, SONET STS3c/SDH STM1
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1 Synchronous
Payload Envelope
(1 column of overhead)
Maintenance
and
Operations
9
R
O
WS
125 sec
270 columns
9 X 260 X (8/125) msec = 149.76 Mbps payload9 bytes
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This is the SONET frame at 155 Mbps. To read this chart, let us start in the upperlefthand corner. The bytes are transmitted across the medium a row at a time
wrapping to the next row. By the time we go through all nine rows, the elapsed time is
nominally 125 microseconds.
The first 9 bytes of each row have various overhead functions. For example, the first
two bytes here are used to identify where the beginning of this frame is so the receiver
can lock on to the frame.
In addition, although not shown here, there is another column of bytes, which are
included in the Synchronous Payload Envelope that is additional overhead, with the
result that each row has 260 bytes of information. Consequently, 260 bytes per rowtimes 9 rows times 8 bits divided by 125 microseconds, we get 149.76 Mbps of
payload.
This is called the STS3C. It is also known as the STM1 because in the international
carrier networks, this will be the smallest package that we see available in terms of the
Synchronous Digital Hierarchy (SDH), the international flavor of SONET. The bit
rates for SDH STMn are three times the bit rates for SONET STSn for the same n
example.
HEC Cell Delineation
The cells within the SONET/SDH STM1 payload are delineated by using the Header
Error Check (HEC) in the ATM cell.
The receiver, when its trying to find the cell boundaries, takes five bytes and says, I
wonder if this five bytes is a header. It does the HEC calculation on the first four
bytes and matches that calculation against the fifth byte. If it matches, the receiver
then counts 48 bytes and tries the calculation again. And if it finds that calculation
correct several times in a row, one can probably safely assume that in fact its found
the cell boundaries. If it tries the calculation and it fails, you just slide the window and
try the calculation again.
This kind of process must be used because, of course, we dont really know whats inthe 48 bytes of payload, but the chances that the user data would contain these
patterns separated by 48 bytes is essentially zero for any length of time.
Consider for a moment what happens if you come across a series of empty cells. Then
how do you determine the cell boundaries ? This is especially important since the
CRC for an all zero (empty cell) header would be all zeros. Consequently, the HEC
must be based on something other than a simple CRC.
The answer is that the HEC is calculated by first calculating the CRC value, then
performing an exclusive or operation of the CRC value with a bit pattern called the
coset, resulting in a nonzero HEC. Thus, the HEC is unique from the zeros in the
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empty cells, and the HEC may still be used for cell delineation. At the receiving end,
another exclusive or operation is performed, resulting in the original CRC for
comparison.
2.048 Mbps E1The 2.048 Mbps interface is particularly important in Europe, where this speed (E1),
is the functional equivalent of North American DS1 interfaces.
2.048 Mbps E1
The diagram here shows the basic E1 framing format. The 2.048 Mbps rate is an exact
multiple of 64 kbps.
The basic E1 frame consists of a collection of 32 bytes, recurring every 125microseconds. Instead of using framing bits, this format uses the first (Byte 0) and
seventeenth (Byte 16) for framing and other control information. The receiver uses
the information within the framing bytes to detect the boundaries of the physical layer
block, or frames. The remaining 30 bytes are used to carry ATM cells.
Consequently, the physical layer payload capacity for the E1 interface is 1.920 Mbps.
The HEC is used to find the cell boundaries.
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31 Bytes15 16 171 2 3
32 Bytes/125 s
Cell Carrying Bytes
Framing and Overhead Bytes
32 x 8/125 s = 2.048 Mbps HEC cell delineation used
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34 Mbps E3
The diagram shows a single 125 microsecond frame, so this pattern time each second.
It consists of 9 rows of 59 bytes each, plus 6 extra framing and overhead bytes. The
result, once we do the arithmetic, is 34.368 Mbps of physical layer capacity. Theactual capacity available for carrying cells is 33.92 Mbps, once the overhead bytes are
subtracted.
The HEC is used to find the cell boundaries.
25.6 Mbps UTP3
Turning to the private UNI, the lowest speed interface is the 25 Mbps interface over
UTP3. This is designed as a physical layer that can use the typical existing wiring
within the office environment, such as between the wiring closet and the desktop.
Thus, this is targeted at desktop ATM.
Use IEEE 802.5 physical layer with 4B/5B coding.
32 Mbaud x 4/5 = 25.6 Mbps
Cells delineated by special symbol pairs
25.6 Mbps UTP3
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59 Columns
125 s Frame
9 Rows
Framing and Overhead Bytes
[(59x9) + 6] x 8/125 s = 34.368 MbpsHEC Cell Delineation used
CellX X
Reset Scramble
CellX 4
No Scramble Reset
or
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In fact, this actually just takes the Token Ring physical layer and does a couple of
interesting tricks with it. In particular, what it does is it uses whats called a 4B/5B
block code. Every five bits in the physical layer are considered a fivebit block and
this actually represents a fourbit pattern. Thus, we have 32 possible fivebit
symbols. Sixteen of the symbols will be for data and 16 of the symbols can be usedfor other things such as control. The reason of doing this is it effectively takes the 16
Mbps Token Ring rate and makes it 25 megabits.
Defining, or declining, cells is very easy here. We define a brandnew symbol called
the X symbol, which will never show up in the cell because the cell is all data and
always uses symbols from the 16 data symbols. So, whenever a receiver sees an X
symbol actually sees two X symbols it knows that what follows is the symbols for
the rest of the cell.
It turns out theres another technical detail here. Theres a scrambling technique
which helps make the spectrum of frequency a littler smoother. There are two ways todo this scrambling.
One is to not reset the scrambling. In this case, the transmitter will use two Xs. If the
receiver is to reset the scrambler, we put an X followed by a 4 data symbol. But again,
well never find the X within the cell because its not a data symbol, so its very easy
to lock onto the cell boundaries.
100 Mbps, 4B/5B Coding
Theres also a 100 Mbps physical layer. One of the reasons this exists is that it is
basically reusing the FDDI technology. FDDI uses a block coding technique using 5
bit (baud) blocks to encode 4 bits of information. There are 16 symbols used for data,
and there are 16 remaining symbols used for control.
5 bit symbols are used to encode 4 bits of data and certain control information
16 used for data.
16 symbol pairs defined in the FDDI standard
Operates at 125 Mbaud or 100 Mbps data rate over multimode fibre.
Cells delimited with TT symbol pair.
108 symbols needed per cell and 25 Msymbols per second implies 85.89 Mbps of cell
payload.
Cells can be separated by JK idle symbols.
25.6 Mbps UTP3
The technique for finding the cells is to define a symbol called the TT symbol and
thats inserted in front of every cell. This becomes very easy. Since this symbol pair
cannot appear in any data, it will not appear anyplace else in a stream. The receiver
needs only to scan for the first TT symbol. It then has locked into the cell
boundaries immediately. If we go through the calculations of cell payload, this is
effectively 6 bytes of overhead for every 48 bytes of payload. Thus, this yields a little
less than 89 Mbps for the cell payloads.
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TTTTCell Cell Cell
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ATM LayerThe ATM layer is responsible for establishing connections and passing cells through
the ATM network. To do this, it uses the information contained in the header of each
ATM cell.
ATM Layer Functions
ATM layer is the layer above the physical layer. As shown in the Fig.3, it does the 4
functions, which can be explained as follows :
Cell header generation/extraction
This function adds the appropriate ATM cell header (except for the HEC value) to the
received cell information field from the AAL in the transmit direction. VPI/VCI
values are obtained by translation from the SAP identifier. It does opposite, i.e.
removes cell header in the receive direction. Only cell information field is passed to
the AAL.
Cell multiplex and demultiplex
This function multiplexes cells from individual VPs and VCs into one resulting cell
stream in the transmit direction. It divides the arriving cell stream into individual cell
flows with respect to VC or VP in the receive direction.
VPI and VCI Translation
This function is performed at the ATM switching and/or crossconnect nodes. At the
VP switch, the value of the VPI field of each incoming cell is translated into a new
VPI value of the outgoing cell. The values of VPI and VCI are translated into new
values at a VC switch.
Generic Flow Control (GFC)
This function supports control of the ATM traffic flow in a customer network. This is
defined at the BISDN Usertonetwork interface (UNI).
The ATM layer provides for the transparent transfer of fixed size ATM layer Service
Data Units (ATMSDUs) between communicating upper layer entities (e.g., AAL
entities). This transfer occurs on a preestablished ATM connection according to a
traffic contract. A traffic contract is comprised of a QoS class, a vector of traffic
parameters, a conformance definition and other items. Each ATM endpoint is
expected to generate traffic, which conforms to these parameters. Enforcement of thetraffic contract is optional at the Private UNI. The Public Network is expected to
monitor the offered load and enforce the traffic contract.
Two levels of virtual connections can be supported at the UNI
A pointtopoint or pointtomultipoint Virtual Channel Connection (VCC)
which consists of a single connection established between two ATM VCC
endpoints.
A pointtopoint or pointtomultipoint Virtual Path Connection (VPC)
which consists of a bundle of VCCs carried transparently between two ATM
VPC endpoints.
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ATM layer performs no retransmission of lost or corrupted information. The ATM
layer also provides its users with the capability to indicate the loss priority of the data
carried in each cell. The information exchanged between the ATM layer and the upper
layer (e.g., the AAL) across the ATMSAP includes the following primitives :
Primitive Request Indicate Confirm Respond
ATMDATA X X
Fig. 5
ATM Service Access Point (SAP) Primitives
ATM Cell StructureEquipment supporting the UNI shall encode and transmit cells according to the
structure (see Fig.6).
GFC VPI
VPI VCI
VCI
VCI PT CLP
HEC
Cell Payload
(48 octets)
GFC : General Flow Control VPI : Virtual Path Identifier
VCI : Virtual Channel Identifier PT : Payload Type
CLP : Cell Loss Priority HEC : Header Error Check
Fig. 6
ATM Cell Structure at the UNIThe structure of the ATM cell shown in Fig.6 contains the following fields :
Generic Flow Control (GFC)
This field has local significance only and can be used to provide standardized local
functions (e.g. flow control) only the customer site. The value encoded in the GFC is
not carried endtoend and will be overwritten by the ATM switches. The GFC is
envisaged to provide contention resolution and simple flow control for shared
medium access arrangement at the customer premises equipment (CPE). Thus, the
GFC field is present at the cells between the users and the network that can be used to
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BIT
8 7 6 5 4 3 2 1
1
2
3
4
5
6
.
.
53
OC
T
ET
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provide local functions, such as identifying multiple station that share a single ATM
interface.
Two modes of operation have been defined for operation of the GFC field. These are
uncontrolled access and controlled access. The uncontrolled access mode ofoperation has been used in early ATM environment. This mode has no impact on the
traffic that a host generates. CPE at the UNI shall encode the GFC value to all zeros
(0000).
Public network equipment at the public UNI shall encode the GFC value to all zeros
(0000).
Virtual Channel Identifier
The VCI is used to establish connection using translation tables at switching nodes
that map an incoming VCI to an outgoing VCI. Circuits established using VCIs
connections are referred to as virtual circuits, and VCIs endtoend connection is
called a virtual connection. In this sense, that bandwidth is not utilized unless userinformation is actually transmitted. The VCI field in the header of the ATM has 16
bits.
Virtual Path Identifier
The VPI is used like VCI to establish a virtual path connection for one or more
logically equivalent VCIs in terms of route and service characteristics. The VPI
allows simplified network routing functionality and management. The VPI field has 8
bits in case of UNI, or 12 bits in case of NNI, i.e. depending on the location of the
ATM cell. The VPI is used in setting up the endtoend virtual path connection of
multiple virtual path segments. A virtual path contains multiple virtual channels.
The bits within the VPI and VCI fields used for routing are allocated using the
following rules :
The allocated bits of the VPI subfield shall be contiguous;
The allocated bits of the VPI subfield shall be the least significant bits of the
VPI subfield, beginning at bit 5 of octet 2;
The allocated bits of the VCI subfield shall be contiguous;
The allocated bits of the VCI subfield shall be the least significant bits of the
VCI subfield, beginning at bit 5 of octet 4;
Any bits of the VPI subfield that are not allocated are set to 0. For a given VP,
any bits of the VCI subfield that are not allocated are set to 0.
Payload Type (PT)
This is a 3bit field used to indicate whether the cell contains user information or
Connection Associated Layer Management Information (F5 flow). It is also used to
indicate a network congestion state or for network resource management.
The first bit is used to discriminate cells of data from cells of maintenance and
operation. Assuming that the cell is a data cell, the second bit is called the Explicit
Forward Congestion Indication (EFCI) bit. If a cell passes through a point in the
network that is experiencing congestion, this bit is set. At this point, this bit is used in
congestion control for Available Bit Rate (ABR). Again, assuming data cells, the third
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bit is carried transparently by the network. Currently, its only defined use is in one of
the ATM Adaptation Layers AAL5.
Cell Loss Priority (CLP)
This is a 1bit field which allows the user or the network to optionally indicate theexplicit loss priority of the cell, i.e. it indicates whether the cell should be discarded if
it encounters extreme congestion as it moves through the network.
Header Error Control (HEC)
The HEC field is used by the physical layer for detection/correction of bit errors in the
cell header. It may also be used for cell delineation.
The last eight bits in the header are the header error check (HEC). HEC is needed
because if a cell is going through a network and the VPI/VCI values get errored, it
will get delivered to the wrong place. As a security issue, it was deemed useful to put
some error checking on the header. Of course, the HEC also is used, depending on the
physical medium, e.g. in SONET, to delineate the cell boundaries.HEC actually has two modes. One is a detection mode where if there is an error with
the CRC calculation, the cell is discarded. The other mode allows the correction of
onebit errors. Whether one or the other mode is used depends on the actual medium
in use. If fibre optics is used, the onebit error correction may make a lot of sense
because typically the errors are isolated. It may not be the right thing to do in a copper
medium because errors tend to come in bursts. When the onebit error correction is
used, you increase the risk of a multiplebit error being interpreted as a singlebit
error, mistakenly corrected, and sent someplace. So the error detection capabilities
drop when the correction mode is used.
Notice that the HEC is recalculated link by link because it covers the VPI/VCI value,
and the VPI/VCI values changes as cells go through the network.
ATM SwitchingATM uses Virtual Paths (VPs) and Virtual Channels (VCs) to accomplish the endto
end routing. The ATM process has no internal method of cell sequencing, and so
unlike X.25, the cells must be sent and received in the correct order. This is achieved
using the Virtual Circuit principle. A virtual circuit can be thought of as a dedicated
pipe between communicating devices and down this pipe all data between those
devices will be sent. This connection is achieved using a virtual circuit, and because
all data between these two specific points uses the same route, the problem of cell
sequencing is solved.The concept virtual circuits, which are known as Virtual Channel Connections
(VCCs), can be described in the following way :
A VCC is set up between any source and any destination in the ATM network,
regardless of the way it is being routed across the network. Fundamentally, ATM is a
connectionoriented technology. The way the network sets up the connection is,
therefore, by signaling, i.e. by transmitting a setup request that passes across the
network to the destination. If the destination agrees to form a connection, the VCC is
set up between the two endsystems. A mapping is defined between the Virtual
Channel Identifiers (VCIs)/Virtual Path Identifiers (VPIs) of both UNIs, and between
the appropriate input link and the corresponding output link of all intermediate
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switches. A VCC is a connection between two communicating ATM endentities. It
may consist of a concatenation of several ATM VC links. All communication
proceeds along this same VCC which preserves cell sequence and provides a certain
quality of service. Note that the Virtual Channel Identifier (VCI) in the ATM cell
header is assigned per network entitytoentity link, i.e. it may change across thenetwork within the same VCC. A Virtual Path (VP) group VCs carried between two
ATM entities and may also involve many ATM VP links. The VCs associated with a
VP are globally switched without unbundling or processing the individual VC in any
way or changing their VCI numbers. Thus, the cell sequence of each VC is still
preserved and the quality of service of the VP depends on that of its most demanding
VC. As the cell address mechanism uses both the VCI and the VPI, different VPs may
also use the same VCI without conflict. A cell may also not be associated with any
VP. In this case, it would have a null VPI and only a unique VCI. By means of VCs
and VPs, virtual circuits can be set up either permanently (by using socalled
Permanent Virtual Channels, PVCs) or on demand (Switched Virtual Channels,
SVCs). It is likely that VPs will be used mostly between switches (i.e., across NNIs)to carry across large number of virtual circuits. In any case, all the ATM switch has to
do is to identify, on the basis of the cells VPI, VCI or both, which output a received
cell needs to be routed to and what the new VPI/VCI on this output link is. The
operation of an ATM network is, therefore, very simple and inherently can scale to
very high speeds. Fig. 7A and 7B illustrate the concept of virtual path (VP) and
virtual channel (VC).
Fig. 7A
Virtual Path & Virtual Channel
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Fig. 7B
Virtual Path & Virtual Channel
Implementation of VP and VC simplifies the switching process. VP becomes aconvenient way to bundle traffic, and, therefore, the ATM switching equipment has
only to check the VPI of each cell before it can be relayed to the next network node.
Fig.7C shows virtual paths and virtual channels switching concept.
Fig. 7CVirtual Path & Virtual Channel Switching
A user can get ATM services in two ways by setting up either a Permanent Virtual
Circuit (PVC) or a Switched Virtual Circuit (SVC).
PVC
In setting up a PVC, usually following procedure is followed.
User calls the service provider with a request for PVC.
User provides the destination address, average bandwidth requirements or committed
information rate, and duration of PVC circuit.
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Service provider enters the information on the control terminal to setup circuit path at
the subscription time and circuit is established for desired duration on permanent
basis.
User pays a monthly fee for the circuit and pays only for usage of that circuit. If that
circuit is not used, the user pays only the monthly circuit fee like rental. This is justlike monthly telephone bills.
SVC
SVC operation is similar to making a directdialed telephone call. the connection
across the network using a virtual path and virtual circuit is established using
signaling and network switching.
ATM NetworksA simplified example for the structure of an ATM network is shown in Fig.8. It is
important to understand that the various UNI and NNI connections could be carried
via different physical media, such as the existing Plesiochronous Digital Hierarchy
(PDH) layers or the new Synchronous Digital Hierarchy (SDH). Several standards
have been defined on how to interface the physical layers and work is continuing to
specify additional physical layers to be used to transport ATM cells.
Fig. 8
ATM Networks
Adaptation Layer ConceptATM is a packet technology that directs traffic using a label contained in the packets
header. Unlike other packet technologies, such as X.25 or frame relay, ATM uses
short fixedlength packets called cells. As we already know the ATM cell structure
consisting of 53 bytes long : 48 bytes for the information field and 5 bytes for the
preceding header. The header field contains information about the virtual channel
(VCI : Virtual Channel Identifier) and Virtual Path (VPI : Virtual Path Identifier) in
use, Payload type (PT) and cell loss priority (CLP). Inserting payload data into the
48byte information field of the ATM cell is accomplished by the ATM Adaptation
Layer (AAL). The AAL is what gives ATM the flexibility to carry entirely different
types of services within the same format. It is important to understand that the AAL is
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not a network process but instead is performed by the network terminating equipment,
i.e. end systems/stations. Thus, the networks task is only to route the cell from one
point to another, depending on its header information. It should be noted that up to
four bytes might be used by the adaptation process itself with some AAL types,
leaving 44 bytes for payload information. Several adaptation layers have already beenstandardized. And these are :
Type 1
Constant Bit Rate (CBR) services. AAL1 handles traffic where there is a strong
timing relation between the source and the destination. Examples include PCM
encoded voice traffic, constant bit rate video and the emulation of public network
circuits (e.g. the transport for E1 links).
Type 2
Variable Bit rate (VBR) timing sensitive services. AAL2 handles traffic where a
strong timing relation between the source and the destination is required, but the bitrate may vary. Examples include variable bit rate voice and compressed, for instance
MPEGcoded, video.
Type 3/4
Connectionoriented and connectionless VBR data transfer. AAL3/4 is a fairly
complex layer that can handle VBR (i.e. bursty) data both with and without pre
establishing an ATM link. Examples for the connectionoriented type include large
file transfers like CAD files or data back up. The connectionless type is intended for
short, highly bursty transfers as might be generated by LANs.
Type 5
Simple and Efficient Adaptation layer (SEAL). AAL5 may be looked upon as a
simplified version of AAL3/4 that is designed to meet the requirements of local, high
speed LAN implementations. AAL5 is intended for connectionless or connection
oriented VBR services.
ATM Service CategoriesThe introduction of new ATM service categories increased the benefits of ATM,
making the technology suitable for a virtually unlimited range of applications. An
ATM network can provide Virtual Path (VP) or Virtual Channel (VC) connections
with different levels of service. The concept of negotiating the behaviour expectedfrom the ATM layer in terms of traffic and performance for each connection allows
users to optimise network capabilities to suit the applications requirements.
The first ATM implementation offered limited options. A typical network behaviour,
common to most of the first generation ATM Networks, is to reserve a fixed amount
of bandwidth for each connection for the duration of the call on the basis of the
maximum emission rate of the source (i.e., the peak cell rate, PCR) and to provide a
single level of quality of service. The ATM service categories represent new service
building blocks that make it possible for users to select specific combinations of
traffic and performance parameters.
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Why New ATM Service Categories ?
ATM is a multiservice technology. Actually, most of the requirements that are
specific to a given application may be resolved at the edges of an ATM network by
choosing an appropriate ATM Adaptation Layer (AAL). However, in accordance with
the standards definitions the ATMlayer behaviour should not rely on the AALprotocols since these are service specific (and are in many cases supported by the user
terminal, i.e. outside the core network visibility), nor on higher layer protocols which
are application specific.
Given the presence of a heterogeneous traffic mix and the need to adequately control
the allocation of network resources for each traffic component, a much greater degree
of flexibility, fairness and utilisation of the network can be achieved by providing a
selectable set of capabilities within the ATMlayer itself. The Service Categories
have been defined with this goal in mind. Both users and network operators can
benefit from the availability of a selectable set of ATMlayer services. These services
are, in effect, the tools, which will allow the promise of ATM to be fully met :
Customer perspective
ATM customers (e.g. endusers, IT and telecommunications managers) aim to save
on network usage costs, provided that their substantial efficiency and quality
requirements are matched. Requirements vary in nature depending on what
application (e.g. data, voice, video, multimedia) is running. As a matter of fact, users
that produce variable traffic patterns would like to be able to get bandwidth just when
actually needed and, in case of elastic sources, to have fast access to as much
available bandwidth as possible, achieving a satisfactory compromise between
performance and cost.
Network and service operators perspective
All types of operators investing in ATM infrastructures and services aim to achieve
maximum use of the deployed resources, avoiding congestion while being able to
share network resources among a large number of customers and fulfilling the
different user needs in a costeffective way. This allows for appropriate tariff
strategies to be deployed. The ability to offer a range of network services, with
selectable cost/performance levels, is a key issue for network operators, particularly in
a competitive market.
A unified approach to the definition of ATMlayer services in the ATM Forum and in
ITUT is presented in Table 2.An ATM Service Category (ATM Forum name) or ATMlayer Transfer Capability
(ITUT name) is intended to represent a class of ATM connections that have
homogeneous characteristics in terms of traffic pattern, QoS requirements and
possible use of control mechanisms, making it suitable for a given type of resource
allocation.
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Table 2
Correlation of ATM Forum and ITUT ATM Services
ATM Forum TM 4.0
ATM Service
Category
ITUT 1.371
ATM Transfer
Capability
Typical use
Constant Bit Rate (CBR)Deterministic Bit Rate
(DBR)
Realtime,
QoS guarantees
RealTime Variable Bit
Rate (rtVBR)For further study
Statistical mux,
realtime
NonRealTime Variable
Bit Rate (nrtVBR)Statistical Bit Rate (SBR) Statistical mux
Available Bit Rate (ABR) Available Bit Rate (ABR)Resource feedback
control
Unspecified Bit Rate
(UBR)(No equivalent)
Best effort,
no guarantees
Guaranteed Frame Rate
(GFR)
Non real time
application minimum
rate guaranteed.
(no equivalent) ATM Block Transfer (ABT)Burst level feedback
control.
A first classification of these services/capabilities may be seen from a network
resource allocation viewpoint. We can identify :
A category based on a constant (maximum) bandwidth allocation. This is called
Constant Bit Rate (CBR) in the ATM Forum and Deterministic Bit Rate (DBR) in
ITUT;A category based on a statistical (average) bandwidth allocation. This corresponds to
the ATM Forum Variable Bit Rate (VBR) and ITUT Statistical Bit Rate (SBR). The
ATM Forum further divides VBR into realtime (rtVBR) and nonrealtime (nrt
VBR), depending on the QoS requirements. A further partitioning, commonly
adopted, defines three VBR subclasses depending on the conformance criteria
adopted;
A category based on elastic bandwidth allocation, where the amount of reserved
resources varies with time, depending on network availability. This is the Available
Bit Rate (ABR). The same name is used both in the ATM Forum and ITUT;
A category considered only in the ATM Forum is the Unspecified Bit Rate (UBR).
No explicit resource allocation is performed; neither bandwidth nor QoS objectivesare specified;
A further category is considered in ITUT only, and is based on block (or burst)
allocation. This is called ATM Block Transfer (ABT). The feature of this class is the
idea that network resources can be negotiated and allocated on a per block basis rather
than on a per connection basis.
The GFR Service category is intended to support nonreal time applications. It is
designed for applications that may require a minimum rate guarantee and can benefit
from accessing additional bandwidth dynamically available in the network. The
service guarantee is based on AAL 5 PDUs (Frames) and under congestion
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conditions, the network attempts to discard complete PDU instead of discarding cells
without reference to frame boundaries.
The ATM Service Architecture
The ATM Service Architecture makes use of procedures and parameters for trafficcontrol and congestion control whose primary role is to protect the network and the
endsystem in order to achieve network performance objectives. An additional role is
to optimize the use of network resources. The design of these functions is also aimed
at reducing network and endsystem complexity while maximizing network
utilization. To meet these objectives, the set of functions forming the framework for
managing and controlling traffic and congestion can be used in appropriate
combinations.
ATM Service Category (or Transfer Capability) relates quality requirements and
traffic characteristics to network behaviour (procedures and parameters). It is intended
to specify a combination of Quality of Service (QoS) commitment and traffic
parameters that is suitable for a given set of applications (user interpretation) and that
allows for specific multiplexing schemes at the ATM layer (network interpretation).
A Service Category used on a given ATM connection, among those that are made
available by the network, has to be implicitly or explicitly declared at connection
setup. All service categories apply to both Virtual Channel Connections (VCCs) and
Virtual Path Connections (VPCs).
Functions such as Connection Admission Control (CAC), Usage Parameter Control
(UPC), Feedback Controls, Resource Allocation, etc. are made available within the
ATM node equipment and are, in general, structured differently for each ServiceCategory.
Generic Network Functions
Connection Admission Control (CAC0 is defined as the set of actions taken by the
network during the call (virtual connection) setup phase, or during call re
negotiation phase, to determine whether a connection request can be accepted or
rejected. Network resources (port bandwidth and buffer space) are reserved to the
incoming connection at each switching element traversed, if so required, by the
service category.
Usage Parameter Control (UPC) or Policing is defined as the set of actions taken bythe network to monitor and control the traffic offered and the validity of the ATM
connection at the User to Network Interface (UNI). It is an essential requirement for
any network supporting multiple services. The main purpose of UPC is to protect
network resources from malicious and unintentional misbehaviour, which can affect
the QoS of other already established connections. Procedures based on a Generic Cell
Rate Algorithm (GCRA) may be applied to each cell arrival to assess conformance
with respect to the traffic contract for the connection. Violations of negotiated
parameters are detected and appropriate actions can be taken (e.g. cell tagging,
discard).
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Feedback controls are defined as the set of actions taken by the network and by the
endsystems (possibly cooperating) to regulate the traffic submitted on ATM
connections according to the state of network elements. Specific Feedback Control
procedures may be associated with a service category.
Traffic Parameters
A source traffic parameter describes an inherent characteristic of a source. A set of
these parameters constitute a Source Traffic Descriptor which, along with Cell Delay
Variation Tolerance (CDVT) and a Conformance Definition, characterize an ATM
Connection.
The following parameters are considered for the purpose of defining the Service
Categories :
Table 3
Traffic Parameters
Peak Cell Rate (PCR)
Sustainable Cell Rate (SCR)Maximum Burst Size (MBS)
Minimum Cell Rate (MCR)
QoS Parameters
Throughput
Peak Cell Rate (PCR) can be defined as a Throughput parameter which in turn is
defined as the inverse of the minimum interarrival time T between two consecutive
basic events and T is the peak emission interval of the ATM connection. PCR applies
to both constant bit rate (CBR) and variable bit rate (VBR) services for ATM
connections. It is an upper bound of the cell rate of an ATM connection and there is
another parameter sustainable cell rate (SCR) allows the ATM network to allocate
resources more efficiently.
When an ATM end station connects to the ATM network, it is essentially making a
contract with the network based on quality of service (QoS) parameters. This contract
specifies an envelope that describes the intended traffic flow. This envelope specifies
values for peak bandwidth, average sustained bandwidth, and burst size.
It is the responsibility of the ATM device to adhere to the contract by means of traffic
shaping. Traffic shaping is the use of queues to constrain data bursts, limit peak data
rate, and smooth jitter so that the traffic will fit within the promised envelope.
ATM switches have the option of using traffic policing to enforce the contract. Theswitch can measure the actual traffic flow and compare it against the agreed upon
traffic envelope. If it finds that traffic is outside of the agreed upon parameters, the
switch can set the CLP bit of the offending cells. Setting the CLP bit makes the cell
discard eligible, which means that the switch, or any other switch handling the cell, is
allowed to drop the cell during periods of congestion.
Congestion control is a primary concern of ATM designers. For example, dropping
just one cell that is part of a FDDI frame can result in the retransmission of 93 cells.
Retransmission can lead to an exponential increase in congestion as ATM switches
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drop individual cells from different packets, resulting in retransmission of more
packets, which causes even more cells to be dropped.
Quality of Service (QoS) parameters include cell loss, the delay and the delay
variation incurred by the cells belonging to the connection in an ATM network. QoSparameters can be either specified explicitly by the user or implicitly associated with
specific service requests. The QoS parameters selected to correspond to a network
performance objective may be negotiated between the endsystems and the network,
e.g., via signaling procedures, or can be taken as default. One or more values of the
QoS parameters may be offered on a per connection basis.
Table 4
QoS Parameters
Cell Delay Variation (CDV)
Maximum Cell Transfer Delay (Max CTD)
Cell Loss Ratio (CLR)
A number of additional QoS parameters have been identified e.g. Cell Error Ratio
(CER), Severely Errored Cell Block Ratio (SECBR), Cell Misinsertion Rate (CMR).
Traffic Contract and Negotiation
A traffic contract specifies the negotiated characteristics of a VP/VC connection at an
ATM User Network Interface (either Private or Public UNI). The traffic contract at
the Public UNI shall consist of a connection traffic descriptor and a set of QoS
parameters for each direction of the ATM layer connection and shall include the
definition of a compliant connection. The values of the traffic contract parameters can
be specified either explicitly or implicitly. A parameter value is explicitly specified in
this initial call establishment message. This can be accomplished via signaling for
SVCs (Switched Virtual Connections) or via the Network Management System
(NMS) for PVCs (Permanent Virtual Connections) or at subscription time. A
parameter value is implicitly specified when its value is assigned by the network
using default rules.
Table 4
ATM Service Category Attributes and Guarantees
Service
Category
Traffic Description
GuaranteesUse of
feedback
control
Min.
Loss(CLR)
Cell Delay/
Variance
Band
width
CBR PCR X X X NO
rtVBR PCR, SCR, MBS X X X NO
nrtVBR PRC, SCR, MBS X NO X NO
ABRPCR, MCR+
behaviour parametersX NO X X
GFRPCR, MCR, MBS,
MFSX NO X NO
UBR PCR NO NO NO NO
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Fig. 9
Link Usage by different traffic types (CBR, VBR, ABR)
Applications SummaryFollowing tabulation is an attempt to sum up the indications outlined above. The
association and the score assignment are based on a subjective perception.
Table 6
Application Areas for ATM Service Categories
ApplicationCBR rt
VBR
nrt
VRB
ABR GFR UBR
Critical Data XX X XXX X X N/S
LAN interconnection
LAN emulationX X XX XXX XXX XX
Data Transport/interworking
(IPFRSMDS) X X XX XXX XXX XXCircuit Emulation PABX XXX XX N/S N/S N/S N/S
POTS/ISDN
Video ConferenceXXX N/S N/S N/S
Compressed Audio X XXX XX XX N/S X
Video Distribution XXX XX X N/S N/S N/S
Interactive Multimedia XXX XXX XX XX N/S X
Score to indicate the advantage :
Optimum : xxx Good : xx Fair : x N/S : Not suitable
Not quoted : Presently considered not applicable with advantage
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T i m e
Bandwidth
C o n s t a n t B i t R a t e t r a f f ic
V a r i a b l e B i t R a t e t r a f f i c
U n s p e c i f i e d B i t R a t e o r A v a i l a b l e B i t R a t e T r a f f i c
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Comparison between ATM and Frame Relay Service
Cell Relay Service (ATM) Frame Relay Service
Connection Type
Virtual Path & Virtual
Connection Virtual Connections
Local Address VPI/VCI DLCI
PDU Length Fixed (48+5 = 53 Octets) Variable
Delineation Method HEC Cell Delineation Flag Delineation
Traffic DescriptorMultiparameter (PCR, SCR,
Bt)
Multiparameter (CIR, Bc,
Be, T)
Priority Indication Cell Loss Priority (CLP) Discard Eligibility (DE)
Error ProtectionHeader only
(+AAL Functions)
CRC 16 over entire
Frame
Congestion Ind.EFCI (Explicit Forward
Congestion Indication)
FECN, BECN
(NNotification)
Physical Interfaces
Following physical interfaces have been standardized for ATM :
Long Distance Media Payload (Mbps)
2.048/1.544 Mbit/s (E1/T1)
34/45 Mbit/s (E3/T3)155 Mbit/s (STM1/OC3C)
622 Mbit/s (STM4/OC12C)
2488 Mbit/s (STM16/OC48C)
UTP/coax.
Coax.SMF
SMF
SMF
1.920/1.536
33.920/40.6149.760
Campus or LAN
25.6 Mbit/s
155 Mbit/s (STM1/OC3C)
UTP3
UTP5, MMF
25.1
149.760
*******************
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