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The Situation in the Telecommunication World Before ATM
Performance Reqirements from ATM
THE SITUATION IN THE TELECOMMUNICATION WORLD BEFORE ATM
Today's telecommunication networks are characterized by specialization. This
means that for every individual telecommunication service at least one network
exists that transports this service.
Each of these networks was specially
designed for that specific service and is often not at all applicable to
transporting another service.
When designing the network of the future, one
must take into account all possible existing and future services.
The networks of today are very specialized and suffer from a large number of disadvantages:
* Service Dependence
Each network is only capable of transporting one
specific service .
*Inflexibility
Advances in audio, video and speech coding and compression
algorithms and progress in VLSI technology influence the bit rate generated by a
certain service and thus change the service requirements for the network. In the
future new services with unknown requirements will appear. A specialized network
has great difficulties in adapting to new services requirements.
*Inefficiency
The internal available resources are used
inefficiently.
Resources which are available in one network cannot be made
available to other networks.
It is very important that in the future only a single network will exist and
that this network is service independent.
This implies a single network
capable of transporting all services, sharing all its available resources
between the different services.
It will have the following advantages:
*
Flexible and future safe
A network capable of transporting all types of
services that will be able to adapt itself to new needs.
* Efficient in the use of its available resources All available resources can be shared between all services, such that an optimal statistical sharing of the resources can be obtained.
* Less expensive
Since only one network needs to be designed, manufactured
and maintained the overall costs of the design, manufacturing, operations and
maintenance will be lower.
PROGRESS IN TECHNOLOGY - ATM IS POSSIBLE
The definition of a service independent network has been influenced by an
evolution in technology and system concepts.
System Concept Progress
The ideal network in the future must be flexible.
The most flexible network in terms of bandwidth requirements and the most
efficient in terms of resource usage, is a network based on the concept of
packet switching.
Any bandwidth can be transported over a packet switching
network and the resources are only used when useful information has to be
transported.
The basic idea behind the concept changes is the fact that
functions must not be repeated in the network several times if the required
service can still be guaranteed when these functions are only implemented at the
boundary of the network.
Progress In Technology In recent years large progress has occurred both in
field of electronics and in the field of optics.
Broadband communication
systems can be developed based on different technologies, the most promising
being CMOS. (Complementary Metal Oxide Semiconductor)
Cmos allows high
complexity and reasonably high speed (up to 200 to 300 Mbits/s). The low power
dissipation of Cmos is particularly important, and allows the realization of
high complexity, high speed systems on a very small chip surface.
With the
increased complexity per chip, the system cost can easily be reduced since the
large integration will continuously allow the volume of the system to shrink or
to increase the functionality at a constant cost.
Optical technology is also evolving quite rapidly.
Optical fiber has been
installed for transmission services for several years.
Performance Requirements from ATM
In the future broadband network a large number of services have to be
supported. These services are :
* low speed like telemetry, low speed data
,telefax,..
* medium speed like hifi sounds, video telephony, high speed
data,..
* very high speed like high quality video, video library ...
A single typical service description does not exist. All services have
different characteristics both for their average bit rate and burstiness.
To
anticipate future unknown services we must try to characterize as general a
service as possible.
The optimal transfer mode should support the
communication of various types of information via an integrated access. Ideally
the transfer mode must provide the capability to transport information, whatever
type of information is given at the network, very much like the electricity
network, which provides power to it's customers without regarding the way the
customer uses his electricity.
Two other important factors are:
*Semantic transparency - determines the
possibility of network to transport the information error free.
The number of
end to end errors introduced by the network is acceptable for the service
No
system is perfect. Most of the imperfections of telecommunication systems are
caused by noise. Other factors contribute to a reduced quality: limited
resources causing blocking; any system errors. One of the most important
parameters used to characterize imperfections is the BER (bit error rate) - the
ratio between erroneous bits and transmitted bits.
*Time transparency -
determines the capability of the network to transport the information through
the network from source to destination in a minimal time acceptable for the
service.
Time transparency can be defined as the absence of delay and delay
jitter(different part of the information arrive at the destination with
different delay). The value of end to end delay is an important parameter for
real time services, such as voice and video. If the delay becomes too large echo
problems may arise in a voice connection.
SERVICE BER DELAY Telephony 10^(-7) 25 - 500 ms Data transmission 10^(-7) 1000 ms Broadcast Video 10^(-6) 1000 ms Hifi Sound 10 ^(-5) 1000 ms
INFORMATION TRANSFER
ATM is considered a packet
oriented transfer mode based on:
* asynchronous time division
multiplexing
* the use of fixed length cells
An ATM cell structure is displayed in the following figure:
Each cell consist of an information field and a header.
The header is used to identify cells belonging to the same virtual channel
and to perform the appropriate routing.
To guarantee a fast processing in the
network, the ATM header has very limited function. Its main function is the
identification of the virtual connection by an identifier which is selected at
call set up and guarantees a proper routing of each packet. In addition it
allows an easy multiplexing of different virtual connections over a single link.
The information field length is relatively small, in order to reduce the
internal buffers in the switching node, and to limit the queuing delays in those
buffers - small buffers guarantee a small delay and a small delay jitter as
required in real time systems.
The information field of ATM cells is carried
transparently through the network. No processing is performed on it inside the
network.
All services (voice, video, data) can be transported via ATM ,
including connectionless services.
ROUTING
ATM is connection oriented. Before information
is transferred from the terminal to the network, a logical/virtual connection is
set.
The header values are assigned to each section of a connection for the
complete duration of the connection, and translated when switched from one
section to another.
Signalling and user information are carried on separate
virtual channels Two sorts of connections are possible:
* Virtual Channel
Connections VCC
* Virtual Path Connections VPC
When switching or
multiplexing on cells is to be performed, it must first be done on VPC ,then on
the VCC.
Virtual Channels
This function is performed by a header sub field - VCI.
Since the ATM network is connection oriented each connection is characterized by
a VCI which is assigned at call set up. A VCI has only a local significance on
the link between ATM node and will be translated in the ATM nodes. When the
connection is released , the VCI values on the involved links will be released
and can be reused by other connections.
An advantage of this VCI principle is
the use of multiple VCI values for multicomponent services. For instance video
telephony can be composed of 3 components: voice , video and data each of which
will be transported over a separate VCI. This allows the network to add or
remove components during the connection. For instance, the video telephony
service can start with voice only and the video can be added later.
Virtual Path
The network has to support semi-permanent connections, which
have to transport a large number of simultaneous connections. This concept is
known as virtual path.
All ATM switches can be schematically described as follows.
A number of
incoming links ( I1,I2,..In) transport ATM information to the switch, where
depending on the value of the header this information is switched to outgoing
link (O1,O2,..On). The incoming header and the incoming link number are used to
access a translation table. The result of the access to the table is an outgoing
link and a new header value.
RESOURCES
As ATM is connection oriented, connections
are established either semi-permanently, or for the duration of a call, in case
of switched services.
This establishment includes the allocation of a VCI
(Virtual Channel Identifier)and/or VPI (Virtual Path Identifier), and also the
allocation of the required resources on the user access and inside the network.
These resources are expressed in terms of throughput and Quality of
Service.
They may be negotiated between user and network for switched
connection during the call set up phase
Lets look at the following topics
ATM Cell Identifiers
ATM cell identifiers
are:
* Virtual Path Identifier
* Virtual Channel Identifiers
* Payload
Type Identifiers
They support recognition of an ATM cell on a physical
transmission medium. Recognition of a cell is a basis for all further
operations.
VPI and VCI are unique for cells
belonging to the same virtual connection on a shared transmission medium. As
such they are limited resources. Within a particular virtual circuit, cells may
be further distinguished by their PTI, which cannot be allocated freely but
depends on the type of payload carried by the cell. This field indicates whether
the cell is carrying user information to be delivered transparently through the
network or special network information.
In case the field indicates network
information, part of the information field indicates the type of network control
whereas the remaining part of information field may be processed inside the
network.
Throughput
Bandwidth has to be reserved in the
network for each virtual connection. ATM offers the possibility to realize
resources saving in the total bandwidth needed when multiplexing traffic of many
variable Bit Rate connections.
The amount which can be saved depends heavily
on the number of multiplexed connections, on the burstiness of the traffic they
carry, on the correlation between them and on the quality of service they
require.
Quality of Service
The quality of service of a connection
relates to the cell loss, the delay and the delay variation incurred by the
cells belonging to that connection in an ATM network. For ATM, the quality of
service of a connection is closely linked to the bandwidth it uses. When
providing limited physical resources using more bandwidth increases the cell
loss, the delay, and the delay variation incurred, i. e. decreases the QOS for
cells of all connections which share those resources.
Usage Parameter Control
In ATM there is no physical
limitation on the user access rate to the physical transmission medium, apart
from the physical cell rate on the medium itself. Multiplexing equipment will do
its utmost to avoid cell loss to offer the highest possible throughput whatever
the user chooses to send.
As virtual connections share physical resources,
transmission media and buffer space, unforeseen excessive occupation of
resources by one user may impair traffic for other users. Throughput must be
monitored at the user - network interface by a Usage Parameter Control function
in the network to ensure that a negotiated contract per VCC or VPC between
network and subscriber is respected.
It is very important that the traffic
parameters which are selected for this purpose can be monitored in real time at
the arrival of each cell.
FLOW CONTROL
In principle, no flow control will be
applied to information streams at the ATM layer of the network. In some cases it
will be necessary to be able to control the flow of traffic on ATM connections
from a terminal to the network. In order to cope with this a GFC (general flow
control) mechanism may be used. This function is supported by a specific field
in the ATM cell header. Two sets of procedure are used: Uncontrolled
Transmission - for the use of point to point configuration.
Controlled
Transmission - can be used in both point to point and shared medium
configuration.
Another principle is no error protection on link by link basis.
If a link
in the connection, either the user to network link or the internal links between
the network nodes, introduces an error during the transmission or is temporarily
overloaded thereby causing the loss of packets, no special action will be taken
on that link to correct this error (= no requesting for retransmission).This
error protection can be omitted since the links in the network have a very high
quality
SIGNALLING
The negotiation between the user and the
network with respect to the resources is performed over a separate signalling
virtual channel. The signalling protocol to be used over the signalling virtual
channel is an enhancement of those used in ISDN signalling.
AAL 2 - Adaptation for variable bit rate services
This type AAL offers a transfer of information with a variable bit rate. In
addition, timing information is transferred between source and destination.
Since the source is generating a variable bit rate, it is possible that cells
are not completely filled and that the filling level varies from cell to cell.
Therefore more functions are required from the SAR .
* the SN field (Sequence Number) contains the sequence number to allow the
recovery of lost or misrouted cells.
* The IT (Information Type) indicates
the beginning of a message (BOM), continuing of a message (COM),the end of a
message(EOM) or that the cell transports timing or other information
.
BOM,COM or EOM indicate that the cell is the first, middle or last cell of
a message, i.e. an information unit as defined in the CS layer with possibly a
variable length.
* The LI (length indicator) field indicates the number of
useful bytes in partially filled cells.
*The CRC field allows SAR to detect
bit errors in the SAR SDU
In the CS sublayer the following functions have to be performed:
* Clock
recovery by means of insertion and extraction of time information.
* Handling
of lost or incorrectly delivered cells.
* Forward error correction for video
and audio services
AAL 3/4 - Adaptation for data services
This AAL is recommended for transfer of data which is sensitive to loss, but
not to delay. The AAL may be used for connection oriented as well as for
connectionless services, since functions like routing and network addressing are
performed on the network layer.
Two modes of AAL 3/4 are defined: * Message
Mode
The AAL SDU is passed across the AAL interface in exactly one AAL
Interface Data Unit (IDU) . This service is provided for the transport of fixed
or variable length AAL SDU.
* Streaming mode
The AAL SDU is passed in one
or more AAL IDU. Transfer of these IDUs may occur separate in time. The service
provided for long variable length AAL SDUs.
The SAR sublayer
functions:
*Segmentation and reassembly of variable length CS PDUs. The SAR
PDU contains two fields for this purpose:
1. ST Segment Type - indicates
which part of the CS PDU is carried by the SAR PDU: first middle or last
2.
LI Length Indicator
* Error Detection - using CRC field
* Multiplexing of
multiple CS PDUs on a common bearer in the ATM layer. Multiplexing is supported
by a multiplexing identifier.
The CS
functions
* Delineation and transparency of SDUs
* Error detection and
handling - Corrupted SDUs are either discarded or optionally delivered to the
service specific convergence sublayer.
* Buffer allocate size- each PDU
carries up front an indication to the receiving entity of the maximum buffer
required to receive the PDU * Abort - a partially transmitted PDU can be aborted
AAL 5 - Adaptation for data services
This AAL is recommended for high speed connection oriented data service. This
AAL offers a service with less overhead and better error detection.
The SAR
sublayer functions:
The SAR sublayer accepts variable length SAR SDUs which
are multiples of 48 octets from the CS sublayer, and generates SAR PDUs
containing 48 octets of data.
The CS
functions
The functions implemented by the AAL5 are the same as the ones
offered by the AAL 3/4 except
that the AAL 5 does not give a buffer allocation size indication to the
receiving peer entity. Also error protection in the AAL 5 is fully handled at
the CS layer itself, instead of being shared between SAR and CS.
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