As in the phone system, ATM needs to
have a way to establish a connection between two points and, after
completing the session, a way to terminate the connection. The
mechanism that the originating computer uses to inform the network
that a connection needs to be established is called signaling.
When an ATM device wants to establish
a connection with another ATM device, it sends a signaling-request
packet to its directly connected ATM switch. This request contains
the ATM address of the desired ATM endpoint, as well as any QoS
parameters required for the connection.
ATM signaling protocols vary by the type of ATM link can be either
User-Network Interface (UNI) signals or Network Node Interface
(NNI) signals. UNI is used between an ATM end-system and ATM switch
across ATM UNI, while NNI is used across NNI links.
The ATM Forum UNI 3.1 specification is
the current standard for ATM UNI signaling. The UNI 3.1 specification
is based on the Q.2931 public network signaling protocol developed
by the ITU-T. UNI signaling requests are carried in a well-known
default connection: VPI = 0, VPI = 5.
Standards currently exist only for ATM UNI signaling, but standardization
work is continuing on NNI signaling.
ATM Connection-Establishment Process
ATM signaling uses a mechanism to do
connection setup that is similar to the telephone network. An
ATM connection setup proceeds in the following manner. First,
the source end system sends a connection request which is propagated
through the network and connections are set up through the network.
The connection request reaches the final destination, which either
accepts or rejects the connection request.
ATM Connection-Management Messages
A number of connection management message
types, including Setup, Call Proceeding, Connect, and Release,
are used to establish and tear down an ATM connection. The source
end-system sends a Setup message (including the destination end-system
address and any traffic QoS parameters) when it wants to set up
a connection. The ingress switch sends a Call Proceeding message
back to the source in response to the Setup message. The destination
end system next sends a Connect message if the connection is accepted.
The destination end system sends a Release message back to the
source end system if the connection is rejected, thereby clearing
Connection-management messages are used to establish an ATM connection
in the following manner. First, a source end system sends a Setup
message, which is forwarded to the first ATM switch (ingress switch)
in the network. This switch sends a Call Proceeding message and
invokes an ATM routing protocol. The signaling request is propagated
across the network. The exit switch (called the egress switch)
that is attached to the destination end system receives the Setup
message. The egress switch forwards the Setup message to the end
system across its UNI, and the ATM end system sends a Connect
message if the connection is accepted. The Connect message traverses
back through the network along the same path to the source end
system, which sends a Connect Acknowledge message back to the
destination to acknowledge the connection. Data transfer then
Figure 1. Connection Establishment
Source PC uses Q.2931 procedures (VPI=0/VCI=5)
to setup an ATM connection. The local ATM switch receives
the signaling message and passes it to the Connection Manager.
The Connection Manager the level of services for that machine
calculates the bandwidth required to setup this connection
for the forward and reverse paths. If the bandwidth on either
the reverse channel or the forward channel is not available,
alternate paths are examined.
The setup message is then sent back to the Network
Switch to allocate the necessary links.
The Network switch forwards the connection setup
request to the destination machine.
The Network switch receives the connection-accept
message from the destination machine.
Once the Network switch receives the connection-accept
message from the called PC, it passes the information to the
The Connection Manager allocates the reverse
channel capacity for the called party.
Connection Manager informs the Local switch
that the connection has been accepted.
Connection Manager informs the Local switch
that the connection has been accepted.
Connection Manager confirms the connection to
the destination PC.
Connection Manager informs the source PC and
data transmission can begin.
Types of Connections
Unlike a phone connection, data connections
usually have several different types of connections that area
established. Voice traffic is continuous, and in the network,
samples of speech are transmitted every 125 micro-seconds (8,000
times a second).
ATM supports Quality of Service (QoS)
which allows voice and video to be transmitted smoothly. Constant
Bit Rate (CBR) guarantees bandwidth for real-time voice and video.
Variable Bit Rate (VBR) allows time delivery guarantees but the
amount of data at each interval may vary. Available Bit Rate (ABR)
adjusts bandwidth according to congestion levels for LAN traffic.
Unspecified Bit Rate (UBR) provides a best effort for remote users.
In order to support QoS delivery of cells through a network, network
devices must implement algorithms that control when cells are
transmitted. These mechanisms are comprised of traffic contract,
traffic shaping, and traffic policing.
A traffic contract specifies an envelope
that describes the intended data flow. This envelope specifies
values for peak bandwidth, average sustained bandwidth, and burst
size, among others. When an ATM end-system connects to an ATM
network, it enters a "contract" with the network based
on QoS parameters.
Traffic shaping is the use of queues
to constrain data bursts, limit peak data rate, and smooth jitters
so that traffic will fit within the promised envelope. ATM devices
are responsible for adhering to the contract by means of traffic
shaping. ATM switches can use traffic policing to enforce the
contract. The switch can measure the actual traffic flow and compare
it against the agreed-upon traffic envelope. If the switch finds
that traffic is outside of the agreed-upon parameters, it can
set the cell-loss priority (CLP) bit of the offending cells. Setting
the CLP bit makes the cell "discard eligible," which
means that any switch handling the cell is allowed to drop the
cell during periods of congestion.
CBR (Constant Bit Rate)
The CBR service class is intended for
real-time applications, i.e. those requiring tightly constrained
delay and delay variation, as would be appropriate for voice and
video applications. The consistent availability of a fixed quantity
of bandwidth is considered appropriate for CBR service. Cells
that are delayed beyond a value specified are assumed to be significantly
less value to the application.
Figure 2. CBR Connection
VBR (Variable Bit Rate)
Real time VBR
The real time
VBR service class is intended for real-time applications, i.e.,
those requiring tightly constrained delay and delay variation,
as would be appropriate for voice and video applications. Sources
are expected to transmit at a rate which varies with time. Equivalently
the source can be described "bursty". Cells which are
delayed beyond a value are assumed to be of significantly less
value to the application. Real-time VBR service may support statistical
multiplexing of real-time sources, or may provide a consistently
Non-real time VBR
time VBR service class is intended for non-real time applications
which have 'bursty' traffic characteristics and which can be characterized
in terms of a GCRA. For those cells which are transferred, it
expects a bound on the cell transfer delay. Non-real time VBR
service supports statistical multiplexing of connections.
Figure 3. VBR Connection
ABR (Available Bit Rate)
Many applications have the ability to
reduce their information transfer rate if the network requires
them to do so. Likewise, they may wish to increase their information
transfer rate if there is extra bandwidth available within the
network. There may not be deterministic parameters because the
users are willing to live with unreserved bandwidth. To support
traffic from such sources in an ATM network will require facilities
different from those for Peak Cell Rate of Sustainable Cell Rate
traffic. The ABR service is designed to fill this need.
Figure 4. ABR Connection
UBR (Unspecified Bit Rate)
The UBR service class is intended for
delay-tolerant or non-real-time applications, i.e., those which
do not require tightly constrained delay and delay variation,
such as traditional computer communications applications. Sources
are expected to transmit non-continuous bursts of cells. UBR service
supports a high degree of statistical multiplexing among sources.
UBR service includes no notion of a per-VC
allocated bandwidth resource. Transport of cells in UBR service
is not necessarily guaranteed by mechanisms operating at the cell
level. However it is expected that resources will be for UBR service
in such a way as to make it usable for some set of applications.
UBR service may be considered as interpretation of the common
term "best effort service".
A circuit is a path through the network.
For phone service, a circuit was an actual wire connection set
up between two phones. Virtual Circuits embody the same idea of
connecting two machines together having many attributes of having
the security of a private line. It becomes a virtual circuit because
there are many of these individual circuits sharing a common wire.
Figure 5. Virtual Circuits
Two main forms of these Virtual Circuits
(VCs) are Switched Virtual Circuits (SVCs) and Permanent Virtual
PVCs are the simpler of the two types
of Virtual Circuits because the path the network is established
at some point in time and then the stations "turn on"
the link or "turn off" the link. The PVC is defined
only between two specific end points.
SVCs act more like a phone connection.
When you enter the phone number (i.e. the address), the best path
for that type of connection is computed and the requested bandwidth,
delay, and jitter are guaranteed. You may enter any address (i.e.
phone number) and each time you establish a connection, a new
route is computed and the connection is not limited to specific
end stations. While SVCs are more flexible, they require a lot
of network information. For example, public and private networks
need to share information about sources, destinations, capabilities
and maps of network switches.
The ATMForum has specifications available
Additional ATM seminars:
An Introduction to ATM - An overview of ATM, the types
of connections, and a description of the various cell formats.
The ATM Adaptation Layers (AAL) - Large data packets
are required to be segmented into the smaller ATM cells and later
the ATM cells are reassembled back into packets. There are several
different AALs that can be used based on the type of data being
Connection Management requires the involvement
of all elements along the network path.
There are four primary classes of Quality of
Service (QoS): CBR, VBR, ABR, UBR.
There are two types of Virtual Circuits: PVCs