ASYNCHRONOUS TRANSFER MODE(ATM)
Asynchronous
transfer mode (ATM), also known as cell relay, is similar in
concept
to frame relay. Both frame relay and ATM take advantage of the
reliability
and fidelity of modern digital facilities to provide faster packet
switching
than X.25.
ATM
is even more streamlined than frame relay in its functionality,
and
can support data rates several orders of magnitude greater than frame
relay.
In
addition to their technical similarities, ATM and frame relay have similar
histories.
Frame relay was developed as part of the work of ISDN, but is now finding
wide
application in private networks and other non-ISDN applications, particularly
in
bridges and routers. ATM was developed as part of the work on broadband
ISDN,
but is beginning to find application in non-ISDN environments where very
high
data rates are required.
We
begin with a discussion of the details of the ATM scheme. Then, the
important
concept of the ATM Adaptation Layer (AAL) is examined. Finally, the
key
issue of congestion control in ATM networks is discussed.
PROTOCOL
ARCHITECTURE
Asynchronous
transfer mode (ATM), also known as cell relay, is in some ways similar
to
packet switching using X.25 and frame relay. Like packet switching
and
frame
relay, ATM involves the transfer of data in discrete chunks. Also, like packet
switching
and frame relay, ATM allows multiple logical connections to be multiplexed
over
a single physical interface. In the case of ATM, the information flow on
each
logical connection is organized into fixed-size packets, called cells.
ATM
is a streamlined protocol with minimal error and flow control capabilities;
this
reduces the overhead of processing ATM cells and reduces the number of
overhead
bits required with each cell, thus enabling ATM to operate at high data
rates.
Further, the use of fixed-size cells simplifies the processing required at each
ATM
node, again supporting the use of ATM at high data rates.
The
standards issued for ATM by ITU-T are based on the protocol architecture
shown
in Figure 11.1,
which
illustrates the basic architecture for an interface
between
user and network. The physical layer involves the specification of a
transmission
medium
and a signal encoding scheme. The data rates specified at the physical
layer
include 155.52
Mbps
and 622.08
Mbps.
Other data rates, both higher and
lower,
are possible.
Two
layers of the protocol architecture relate to ATM functions. There is an
ATM
layer common to all services that provides packet transfer capabilities, and an
ATM
adaptation layer (AAL) that is service dependent. The ATM layer defines
the
transmission of data in fixed-size cells and also defines the use of logical
connections.
The
use of ATM creates the need for an adaptation layer to support information
transfer
protocols not based on ATM. The AAL maps higher-layer information
into
ATM cells to be transported over an ATM network, then collects
information
from ATM cells for delivery to higher layers.
The
protocol reference model makes reference to three separate planes:
@
User
Plane. Provides
for user information transfer, along with associated controls
(e.g.,
flow control, error control).
Control Plane. Performs call control and
connection control functions.
@
Management
Plane. Includes
plane management, which performs management
functions
related to a system as a whole and provides coordination
between
all the planes, and layer management, which performs management
functions
relating to resources and parameters residing in its protocol entities.
11.2
ATM LOGICAL CONNECTIONS
Logical
connections in ATM are referred to as virtual channel connections (VCC).
A
VCC is analogous to a virtual circuit in X.25 or a data link connection in
frame
relay;
it is the basic unit of switching in an ATM network. A VCC is set up between
two
end users through the network and a variable-rate, full-duplex flow of fixed-size
cells
is exchanged over the connection. VCCs are also used for user-network
exchange
(control signaling) and network-network exchange (network management
and
routing).
For
ATM, a second sublayer of processing has been introduced that deals with
the
concept of virtual path (Figure 11.2). A virtual path connection (VPC) is a
bundle
of
VCCs that have the same endpoints. Thus, all of the cells flowing over all of
the
VCCs in a single VPC are switched together.
The
virtual path concept was developed in response to a trend in high-speed
networking
in which the control cost of the network is becoming an increasingly
higher
proportion of the overall network cost. The virtual-path technique helps
contain
the
control cost by grouping connections that share common paths through the
network
into a single unit. Network management actions can then be applied to a
small
number of groups of connections instead of to a large number of individual
connections.
Several
advantages can be listed for the use of virtual paths:
a
Simplified
network architecture. Network transport functions can be separated
into
those related to an individual logical connection (virtual channel)
and
those related to a group of logical connections (virtual path).
a
Increased
network performance and reliability. The network deals with fewer,
aggregated
entities.
Reduced
processing and short connection setup time. Much of the work is
done
when the virtual path is set up. By reserving capacity on a virtual path
connection
in anticipation of later call arrivals, new virtual channel connections
can
be established by executing simple control functions at the endpoints
of
the virtual path connection; no call processing is required at transit
nodes.
Thus, the addition of new virtual channels to an existing virtual path
involves
minimal processing.
e
Enhanced
network services. The
virtual path is used internal to the network
but
is also visible to the end user. As a result, the user may define closed user
groups
or closed networks of virtual-channel bundles.
Figure
11.3 suggests in a general way the call-establishment process using virtual
channels
and virtual paths. The process of setting up a virtual path connection
is
decoupled from the process of setting up an individual virtual channel
connection:
e
The
virtual path control mechanisms include calculating routes, allocating
capacity,
and storing connection state information.
To
set up a virtual channel, there must first be a virtual path connection to the
required
destination node with sufficient available capacity to support the virtual
channel,
with the appropriate quality of service. A virtual channel is set
up
by storing the required state information (virtual channel/virtual path
mapping).
The
terminology of virtual paths and virtual channels used in the standard is
a
bit confusing, and is summarized in Table 11.1. Whereas most of the
networklayer
protocols
that we survey in this lesson relate only to the user-network interface,
the
concepts of virtual path and virtual channel are defined in the ITU-T Recom
Virtual Channel Connection Uses
The
endpoints of a VCC may be end users, network entities, or an end user and a
network
entity. In all cases, cell sequence integrity is preserved within a VCC; that
is,
cells are delivered in the same order in which they are sent. Let us consider
examples
of
the three uses of a VCC.
Between end users. Can be used to
carry end-to-end user data; can also be
used
to carry control signaling between end users, as explained below. A VPC
between
end users provides them with an overall capacity; the VCC organization
of
the VPC is up to the two end users, provided the set of VCCs does
not
exceed the VPC capacity.
* Between an end user and a network entity. Used for
user-to-network control
signaling,
as discussed below. A user-to-network VPC can be used to aggregate
traffic
from an end user to a network exchange or network server.
* Between two network entities. Used for network
traffic management and
routing
functions. A network-to-network VPC can be used to define a common
route
for the exchange of network management information.
Virtual
Path/Virtual Channel Characteristics
ITU-T
Recommendation 1.150 lists the following as characteristics of virtual channel
connections:
Quality
of service. A user
of a VCC is provided with a Quality of Service specified
by
parameters such as cell loss ratio (ratio of cells lost to cells transmitted)
and
cell delay variation.
a
Switched
and semi-pemanent virtual channel connections. Both are
switched
connections,
which require call-control signaling, and dedicated channels can
be
provided.
a
Cell
sequence integrity. The
sequence of transmitted cells within a VCC is
preserved.
a
Traffic
parameter negotiation and usage monitoring. Traffic parameters can
be
negotiated between a user and the network for each VCC. The input of
cells
to the VCC is monitored by the network to ensure that the negotiated
parameters
are not violated.
The
types of traffic parameters that can be negotiated include average rate,
peak
rate, burstiness, and peak duration. The network may need a number of
strategies
to handle congestion and to manage existing and requested VCCs. At the
crudest
level, the network may simply deny new requests for VCCs to prevent congestion.
Additionally,
cells may be discarded if negotiated parameters are violated
or
if congestion becomes severe. In an extreme situation, existing connections
might
be
terminated.
1.150
also lists characteristics of VPCs. The first four characteristics listed are
identical
to those for VCCs. That is, quality of service, switched and semipermanent
VPCs,
cell sequence integrity, and traffic parameter negotiation and
usage
monitoring are all also characteristics of a VPC. There are a number of reasons
for
this duplication. First, redundancy provides some flexibility in how the
network
service
manages its requirements. Second, the network must be concerned
with
the overall requirements for a VPC, and, within a VPC, it may negotiate the
establishment
of virtual channels with given characteristics. Finally, once a WC is
set
up, it is possible for the end users to negotiate the creation of new VCCs. The
WC
characteristics impose a discipline on the choices that the end users may make.
In
addition, a fifth characteristic is listed for VPCs:
"
Virtual
channel identifier restriction within a VPC. One or more virtual channel
identifiers,
or numbers, may not be available to the user of the WC, but
may
be reserved for network use. Examples include VCCs used for network
management.
In
ATM, a mechanism is needed for the establishment and release of VPCs and
VCCs.
The exchange of information involved in this process is referred to as control
signaling,
and takes place on separate connections from those that are being
managed.
For
VCCs, 1.150 specifies four methods for providing an establishment/release
facility.
One or a combination of these methods will be used in any particular
network:
1.
Semi-permanent
VCCs may be used for user-to-user exchange. In this case, no
control
signaling is required.
2.
If there is no pre-established call control signaling channel, then one must be
set
up. For that purpose, a control signaling exchange must take place
between
the user and the network on some channel. Hence, we need a permanent
channel,
probably of low data rate, that can be used to set up VCCs
that
can be used for call control. Such a channel is called a meta-signaling
channel,
as the channel is used to set up signaling channels.
3.
The
meta-signaling channel can be used to set up a VCC between the user and
the
network for call control signaling. This user-to-network signaling virtual
channel
can then be used to set up VCCs to carry user data.
4.
The meta-signaling channel can also be used to set up a user-to-user
signaling
virtual
channel. Such
a channel must be set up within a pre-established WC.
It
can then be used to allow the two end users, without network intervention,
to
establish and release user-to-user VCCs to carry user data.
For
VPCs, three methods are defined in 1.150:
1. A VPC can be established on a semi-permanent
basis by prior agreement. In
this
case, no control signaling is required.
2.
VPC
establishmentlrelease may be customer controlled. In this case, the
customer
uses
a signaling VCC to request the VPC from the network.
3.
VPC
establishmentlrelease may be network controlled. In this case, the
network
establishes
a VPC for its own convenience. The path may be networkto-
network,
user-to-network, or user-to-user.
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