4. The term broadband is used to describe a type of data transmission in
which a single medium (wire) can carry several channels at once. Cable
TV, for example, uses broadband transmission. and DSL modems
(which use your existing phone line).
Relationship Between ATM and B-ISDN
ATM evolved from the standardization efforts for B-ISDN.
ATM is the technology upon which B-ISDN is based.
5. WHAT’S ATM?
• ATM is Asynchronous Transfer Mode.
• ATM is a connection-oriented, high-speed, low-delay switching and
transmission technology that uses short and fixed-size packets, called
cells, to transport information.
• ATM is originally the transfer mode for implementing Broadband ISDN
(B-ISDN) but it is also implemented in non-ISDN environments where
very high data rates are required
6. ATM Overview
• Used in both WAN and LAN settings
• Packets are called cells (53 bytes)
• 5-byte header + 48-byte payload
• Commonly transmitted over SONET
• other physical layers possible
• Connections can be switched (SVC), or permanent (PVC).
• ATM operates on a best effort basis.
• ATM guarantees that cells will not be disordered.
5 octets 48 octets
7. Asynchronous TDM
• ATM uses asynchronous time-division
multiplexing-that is why it is called
Asynchronous Transfer Mode-to
multiplex cells corning from different
channels. It uses fixed-size slots (size of
a cell). ATM multiplexers fill a slot with a
cell from any input channel that has a
cell; the slot is empty if none of the
channels has a cell to send
9. WHY ATM ??
International standard-based technology (for interoperability)
Low variance of delay (for voice and video transmission)
Guaranteed quality of service
High capacity switching (multi-giga bits per second)
Bandwidth flexibility (dynamically assigned to users)
Medium not shared for ATM LAN (no degradation in performance as traffic load or
number of users increases)
Supports a wide range of user access speeds
Supports audio, video, imagery, and data traffic (for integrated services)
• Frame Networks
Before ATM, data communications at the data
link layer had been based on frame switching and
frame networks. Different protocols use frames of
varying size and intricacy. As networks become
more complex, the information that must be
carried in the header becomes more extensive.
The result is larger and larger headers relative to
the size of the data unit.
11. Problems (cont.)
• Large Data Fields
In response, some protocols have enlarged
the size of the data unit to make header use
more efficient (sending more data with the
same size header). Unfortunately, large data
fields create waste. If there is not much
information to transmit, much of the field
goes unused. To improve utilization, some
protocols provide variable frame sizes to
• Mixed Network Traffic
As you can imagine, the variety of frame sizes makes traffic
unpredictable. Internetworking among the different frame
networks is slow and expensive at best, and impossible at
Another problem is that of providing consistent data rate
delivery when frame sizes are unpredictable and can vary so
13. ATM provides the solution
• Cell Networks
• Many of the problems associated with frame
internetworking are solved by adopting a concept called
• A cell is a small data unit of fixed size. In a cell network, which uses
the cell as the basic unit of data exchange, all data are loaded into
identical cells that can be transmitted with complete predictability
As frames of different sizes and formats reach
the cell network from a tributary network, they
are split into multiple small data units of equal
length and are loaded into cells. The cells are
then multiplexed with other cells and routed
through the cell network. Because each cell is
the same size and all are small, the problems
associated with multiplexing different-sized
frames are avoided.
19. Transmission path (TP)
• A transmission path (TP) is the physical connection (wire, cable, satellite, and so
on) between an endpoint and a switch or between two switches. Think of two
switches as two cities.
• A transmission path is divided into several virtual paths. A virtual path (VP)
provides a connection or a set of connections between two switches.
• Cell networks are based on virtual circuits (VCs). All cells belonging to a single
message follow the same virtual circuit and remain in their original order until
they reach their destination.
• In a virtual circuit network, to route data
from one endpoint to another, the virtual
connections need to be identified.
• Virtual path identifier (VPI) : defines the
• Virtual-circuit identifier (VCl): defines a
particular VC inside the VP.
• The VPI is the same for all virtual
connections that are bundled (logically)
into one VP.
VPI=Virtual path identifier
VCI=Virtual Circuit Identifier
21. Length of Identifiers
Length of VPI in User Network Interface:
Length of VPI in Network To Network Interface:
Length of VCI will remain same in UNI and NNI:
22. ATM Cell Format
ATM cell is only 53 bytes long with 5 bytes allocated to the header
and 48 bytes carrying the payload.
23. GFC - the Generic Flow Control: field provides local functions such as identifying
multiple stations that share a single ATM interface
VPI - the Virtual Path Identifier: is used together with the Virtual Channel
Identifier (VCI) to identify the virtual circuit.
VCI - the Virtual Channel Identifier: is used together with the Virtual Path
Identifier (VPI) to identify the virtual circuit along which the cell will be directed as it
passes through an ATM network on the way to its destination (values of 0 to 31 are
PT –Payload Type:
• If 1st bit=1, then cell has Control data
• If 1st bit=0, then cell contains User data
• If 2nd bit=1, Congestion occurs
• If 2nd bit=0, No Congestion
• If 3rd bit=1, this cell is the last one
CLP - the Cell Loss Priority: bit field indicates whether the cell should . be discarded
if it encounters extreme congestion as it moves through the network. If set to 1, the cell
should be discarded before cells that have the bit set to 0.
HEC - the Header Error Control: field contains a checksum calculated on the first 4
bytes of the header. It can be used to correct a single bit error in these byte.
25. ATM uses two types of connections:
• ATM provides permanent virtual
connections and switched virtual
Permanent Virtual Connections (PVC)
Permanent connections set up manually by
Switched Virtual Connections (SVC)
Set up and released on
demand by the end user
via signaling procedures.
26. STEPS Of Establishing ATM Connection:
• 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
27. • The Network switch forwards the connection setup request to the
• The Network switch receives the connection-accept message from the
• Once the Network switch receives the connection-accept message from the
called PC, it passes the information to the Connection Manager.
• The Connection Manager allocates the reverse channel capacity for the
• Connection Manager informs the Local switch that the connection has been
• Connection Manager confirms the connection to the destination PC.
• Connection Manager informs the source PC and data transmission can
29. The ATM routing protocol
• The Private Network Node Interface (PNNI) protocol provides both topology-
discovery and call-establishment services.
• PNNI uses the same shortest-path-first algorithm (OSPF).
• A PNNI routing table within each switch maintains information about network
routes and the bandwidth available on individual links.
• PNNI routing table to determine a path to the intended destination that meets
the specified Quality of Service (QoS).
30. SERVICE CATEGORIES:
• Represents particular combination of traffic and QoS parameters:
Constant Bit Rate (CBR):Supports real-time applications. Supports real-time
applications requiring tightly constrained delay variation.
Real-Time Variable Bit Rate (RT-VBR):Deals with real-time applications with
Non Real-Time Variable Bit Rate (NRT-
VBR):Deals with non-real time
applications wit bursty traffic.
Unspecified Bit Rate (UBR):Represents
“best effort service” for non real-time
Available Bit Rate (ABR):Similar to UBR
but enhanced to handle applications of
33. ATM Layers
• ATM works on three basic layers:
ATM adaption Layer
34. Physical Layer:
• The physical layer provides for the
transmission and reception of ATM cells
across a physical medium between two
ATM devices. This can be a transmission
between an ATM endpoint and an ATM
switch, or it can be between two ATM
• The physical layer is subdivided into a
Physical Medium Dependent sublayer
and Transmission Convergence sublayer.
35. PMD Sublayer
• The Physical Medium
Dependent (PMD) sublayer is
responsible for the transmission
and reception of individual bits
on a physical medium.
• These responsibilities
encompass bit timing, signal
encoding, interacting with the
physical medium, and the cable
or wire itself.
36. Transmission Convergence
• The Transmission Convergence
(TC) sublayer functions as a
converter between the bit stream
of ATM cells and the PMD
• When transmitting, the TC
sublayer maps ATM cells onto the
format of the PMD sublayer
37. ATM Layer
• The ATM layer multiplexes and de-
multiplexes and routes ATM cells,
and ensures their sequence from end
• If a cell is dropped by a switch due to
congestion or corruption, it is not the
responsibility of the ATM layer to
correct the dropped cell by means of
retransmission or to notify other
layers of the dropped cell.
• Layers above the ATM layer must
detect the lost cell and decide
whether to correct it or disregard it.
38. ATM Adaptation Layer
• The ATM Adaptation Layer (AAL)
creates and receives 48-byte
payloads through the lower
layers of ATM on behalf of
different types of applications.
• There are five different types of
AALs, each providing a distinct
class of service.
39. ATM Adaptation Layer (AAL) Types
In order for ATM to support a variety of services with different traffic
characteristics and system requirements:
• It is necessary to adapt the different classes of applications to the ATM layer.
• This function is performed by the AAL, which is service-dependent.
• Four types of AAL were proposed, but two of these (3 and 4) have now been
merged into one, AAL 3/4:
– AAL 1: Supports connection-oriented, constant bit rate, time-dependent services.
– AAL 2: Supports connection-oriented services that do not require constant bit
– AAL 3/4: Intended for both connectionless and connection oriented variable bit
– AAL 5: Supports connection-oriented variable bit rate data services.
• More efficient compared with AAL 3/4 at the expense of error recovery and built in
ATM has received a lot of attention recently because of its exciting promises.
With ATM, the bottlenecks of the current networks will be solved, networked
multimedia capabilities like video conferencing, home shopping, home
education, video library, etc. will be brought to the desktops.
Due to its high speed and the integration of traffic types (voice, data, and video
can be transported on the same network).
ATM can accommodate the upcoming new teleservices easily even the
requirements for these services are unknown. It also improves the efficiency and
simplifies the management of the network by using the same technology for all
levels of the network.
The ATM Forum has sponsored interoperability demonstrations to prove the
technology and continues to meet to discuss the uncertain problems of ATM. At
the moment, ATM coexists with current LAN/WAN Technology. Equipment,
Services and Applications are available today and are being used in live networks.
The telecommunications industry is converging on ATM!