Title: ASYNCHRONOUS TRANSFER MODE
1 ASYNCHRONOUS TRANSFER MODE
2ATM definition
- A transfer mode in which information is organized
into cells it is asynchronous in the sense that
the recurrence of cells containing information
from an individual user is not necessarily
periodic". - High-speed transfer technology for voice, video,
and data over public networks.
3ATM VS X.25
- As the speed and number of local area networks
(LANs) continue their relentless growth,
increasing demand is place on wide area
packet-switching networks to support the
tremendous throughput generated by these LANs. - X.25 was designed to support direct connection
of terminals and computers over long distances. - X.25 packets may be of varying length,whereas ATM
packets are of fixed size. - X.25, with its substantial overhead, is being
recognized as an inadequate tool for wide area
networking.
4ATM VS FRAME RELAY
- ATM is normally utilized for high bandwidths of
34 Mbps and upwards. - At speeds of 2 Mbps and below, Frame Relay is
more bandwidth efficient than ATM. - ATM transmits only fixed-size frames, called
cells, not variable-sized frames as frame relay
do. -
- There are two main drivers that caused
businesses migrate from frame relay service to
ATM - 1. The need for greater capacity than frame
relay can handle. - 2. The need to support mixed-media traffic,
especially voice and video.
5- 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. - Frame relay was developed as part of the work of
ISDN - ATM was developed as part of the work on
broadband ISDN -
6ATM VS SONET
- The industry offers two solutions for achieving a
large integrated network SONET/SDH and ATM . - The SONET is a globally accepted, non-proprietary
standard for broadband transmission through
fiber-optic cables. - It handles transmissions from 51 Mbps to 10 Gbps.
- SONET/SDH is a physical transport medium that
occupies the two bottom layers of OSI model. - ATM is a high-speed packet switching technique
suitable for LAN, wide-area network and broadband
ISDN. - SONET is a physical structure, while ATM is a
transmission protocol. - If the ATM protocol is used, a transport medium
is still needed to carry traffic over the network
7ARCHITECTUREATM Devices
- An ATM network is made up of an ATM switch and
ATM endpoints. - ATM switch accepts the incoming cell from an ATM
endpoint or another ATM switch. It then reads and
updates the cell header information and quickly
switches the cell to an output interface toward
its destination. - An ATM endpoint contains an ATM network
interface adapter. - Examples of ATM endpoints are workstations,
routers, LAN switches, and video coder-decoders
(CODECs).
ATM Network Comprises ATM
Switches and Endpoints
8ATM virtual connections
- A Transmission path (TP) is the physical
connection (wire,cable,satellite,and so on)
between an end point and a switch or between two
switches - A Virtual Path (VP) transports ATM cells
belonging to virtual channels which share a
common identifier, called the Virtual Path
Identifier VPI. Connects two switches. - A Virtual Channel (VC) provides the transport of
ATM cells which have the same unique identifier,
called the Virtual Channel Identifier (VCI).
ATM virtual connections
9ATM protocol reference model
- The Physical Layer
- This layer describes the physical transmission
of information through an ATM network. - The original design of ATM was based on SONET
because high data rate of SONETs carrier , the
boundaries of cells can be clearly defined . - SONET specifies the use of pointer to define the
beginning of a payload. - Types of physical media specified for ATM include
shielded and unshielded twisted-pair, coaxial
cable, and fiber-optic cable, which provide cell
transport capabilities ranging from a T1 rate of
1.544Mbps to a SONET range of 622Mbps.
10The ATM Layer
- The ATM layer represents the physical interface
between the ATM Adaptation layer (AAL) and the
Physical layer. Thus, the ATM layer is
responsible for relaying cells from the AAL to
the Physical layer for transmission, and in the
opposite direction from the Physical layer to the
AAL for use in an endpoint. - When transporting cells to the Physical layer,
the ATM layer is responsible for generating the
five-byte cell header for each cell. When
receiving cells from the Physical layer, the ATM
layer performs a reverse operation, extracting
the five-byte header from each cell.
11ATM cell header format
- ATM transfer information in fixed-size units
called cells. - An ATM cell header can be one of two formats UNI
or NNI. The UNI header is used for communication
between ATM endpoints and ATM switches in private
ATM networks. The NNI header is used for
communication between ATM switches. - Each cell consists of 53 octets, or bytes. The
first 5 bytes contain cell-header information,
and the remaining 48 contain the payload (user
information).
12ATM Cell Header Fields
- Generic Flow Control (GFC)The 4-bit GFC field
provides flow control at the UNI level.The ITU-T
has determined that this level of flow control is
not necessary at the NNI. - Virtual Path Identifier (VPI)The VPI is an 8-bit
field in a UNI cell a 12-bit field in an NNI
cell.In conjunction with the VCI, identifies the
next destination of a cell as it passes through a
series of ATM switches on the way to its
destination. - Virtual Channel Identifier (VCI)VCI is 16-bit
field in both. - Payload Type (PT)Indicates in the first bit
whether the cell contains user data or control
data. If the cell contains user data, the bit is
set to 0. If it contains control data, it is set
to 1. The second bit indicates congestion (0 no
congestion, 1 congestion), and the third bit
indicates whether the cell is the last in a
series of cells that represent a single AAL5
frame (1 last cell for the frame). - Cell Loss Priority (CLP)Indicates whether the
cell should be discarded if it encounters extreme
congestion as it moves through the network. If
the CLP bit equals 1, the cell should be
discarded in preference to cells with the CLP bit
equal to 0. - Header Error Control (HEC)Calculates checksum
only on the first 4 bytes of the header. HEC can
correct a single bit error in these bytes,
thereby preserving the cell rather than
discarding it. -
13The ATM Adaptation Layer
- Was developed to enable two ATM concepts.
- This layer is responsible for providing an
interface between higher-layer protocols and the
ATM layer. - AAL maps the data stream originated by the
higher-layer protocol into the 48-byte payload of
ATM cells, with the header placement being
assigned by the ATM layer. - In the reverse direction, the AAL receives the
payload of ATM cells in 48-byte increments from
the ATM layer and maps those increments into the
format recognized by the higher-layer protocol
14- Class A services are data streams with a constant
bit rate, running over established connections. - Class B services are similar, but instead of
being locked to a regular data rate they send
'peaks' of data at some times, and little or none
at others. Examples include compressed video. - Class C services are those carrying data
messages on established connections. These are
inherently variable bit-rate, as Class B.
Examples include X.25 and Frame Relay. - Class D services are the so called
connectionless datagrams, where a packet of data
is sent into the network and contains its own
destination address. Examples include many
traditional local-area networks such as
Ethernet, wide area networks and the new
switched multimegabit
Class Timing Relationship Bit Rate Type of Connection
A Yes Constant Connection-oriented
B Yes Variable Connection-oriented
C No Variable Connection-oriented
D No Variable Connectionless
The ATM Application Classes
15- In AAL1 cell payload the Sequence Number
Protection (SNP) field protects the Sequence
Number (SN) field from the effect of bit errors
occurring during transmission, in effect
providing a forward error detection and
correction capability. - AAL1 is designated for transporting constant bit
rate (CBR) data, such as real-time voice and
video traffic. - First byte in the normal 48-byte cell payload is
used for cell sequencing and protection of the
sequence number, limiting the actual payload to
47 bytes per AAL1-generated cell.
16- The AAL2 was intended to support a VBR .
- But now used for low-bit-rate traffic short
frame traffic such as audio,video or fax .ex
mobile telephony. - AAL3 is designed to transport delay-insensitive
user data, such as Frame Relay, X.25, or IP
traffic. - AAL3/4 uses four additional bytes beyond the cell
header. The use of those bytes makes 44 bytes in
the cell available for transporting the actual
payload. - In comparison, AAL5 uses all 48 bytes beyond the
cell header to transport the payload, providing a
minimum 10 enhanced throughput in comparison to
AAL3/4.
17THE ATM CONNECTION-ESTABLISHMENT PROCESS
- ATM signalling uses the one-pass method of
connection setup that is used in all modern
telecommunication networks, such as the telephone
network. - 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 can then begin.
18ATM LAN EMULATION
- LAN Emulation (LANE) is a standard defined by
the ATM Forum that gives to stations attached via
ATM the same capabilities that they normally
obtain from legacy LANs. - The LANE protocol defines mechanisms for
emulating either an IEEE 802.3 Ethernet or an
802.5 Token Ring LAN.
- The LANE protocols make an ATM network look and
behave like an Ethernet or Token Ring - LANE requires no modifications to higher-layer
protocols to enable their operation over an ATM
network.
19LANE Components
- LAN Emulation client (LEC)- It is an entity in an
end system that performs data forwarding, address
resolution, and registration of MAC addresses
with the LAN Emulation Server (LES). An ATM end
system that connects to multiple ELANs has one
LEC per ELAN. - LESThe LES provides a central control point for
LECs to forward registration and control
information.When a station receives a frame to be
sent to another station using a physical address
, LEC sends a special frame to the LES. - The server creates a virtual circuit between the
source the destination station.The source
station can now use this virtual circuit( the
corresponding identifier) to send the frame or
frames to the destination.
20-
- Broadcast and Unknown Server (BUS)Multicasting
Broadcasting require the use of another server
called the broadcast/unknown server(BUS).If a
station needs to send a frame to a group of
stations or to every station,the frame first goes
to the busthis server has permanent virtual
connnection to every station.The server creates
copies of the received frame sends a copy to a
group of stations or to all stations,simulating a
multicasting or broadcasting process.The server
can also deliver a unicast frame by sending the
frame to every station.In this case the
destination address is unknown.This is sometimes
more efficient then getting the connection
identifier from the LES - LAN Emulation Configuration Server (LECS)This is
used for the initial connection between the
client LANE. - This server is always waiting to receive the
initial contact.It has well known ATM address
that is known to every client in the system. - The LECS maintains a database of LECs and the
ELANs to which they belong.
21ATM QUALITY OF SERVICE
- Quality-of-service (QoS) that guarantees
traffic contract, traffic shaping, and traffic
policing is based on the service class , user
related attributes, network-related attributes. - Traffic contract specifies an envelope that
describes the intended data flow. 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 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 for
the multimedia applications and provide overall
optimization of network resources
22Service Class Quality of Service Parameter
constant bit rate (CBR) It is designed for customers who need real time audio or video services. The cell rate is constant with time. CBR applications are quite sensitive to cell-delay variation. Examples of applications that can use CBR are telephone traffic , videoconferencing, and television.
variable bit ratenon-real time (VBRNRT) This class allows users to send traffic at a rate that varies with time depending on the availability of user information.It is designed for those users who do not need real time services but use compression techniques to create a variable bit rate. Multimedia e-mail is an example of VBRNRT.
variable bit ratereal time (VBRRT) This class is similar to VBRNRT but is designed for applications that are sensitive to cell-delay variation. It is designed for those users who need real time services use compression techniques to create a variable bit rate. Examples for real-time VBR is interactive compressed video.
available bit rate (ABR) This class of ATM services provides rate-based flow control and is aimed at data traffic such as file transfer and e-mail. Although the standard does not require the cell transfer delay and cell-loss ratio to be guaranteed or minimized, it is desirable for switches to minimize delay and loss as much as possible. Depending upon the state of congestion in the network, the source is required to control its rate. The users are allowed to declare a minimum cell rate.If more network capacity is available, this minimum rate can be exceeded.ABR is particularly suitable for applications that are bursty.
unspecified bit rate (UBR) This class is a best effort delivery service that does not guarantee anything and is widely used today for TCP/IP.
23User related attributes
- SCR The Sustained cell rate is the average cell
rate over a long time interval.The actual cell
rate may be lower or higher than this value, but
the average should be equal to or less than the
SCR. - PCR The peak cell rate defines the senders
maximum cell rate. The users cell rate can
sometimes reach this peak,as long as the SCR is
maintained. - MCR The minimum cell rate defines the minimum
cell rate acceptable to the sender.For example,if
the MCR is 50,000, the network must guarantee
that the sender can send atleast 50,000 cells per
second. - CVDT The cell variation delay tolerance is a
measure of the variation in cell transmission
times.For example,if the CVDT is 5 ns ,this means
that the difference between the minimum the
maximum delays in delivering the cells should not
exceed 5 ns.
24Network related attributes
- The network related attributes are those that
define characteristics of the network.The
following are some network related attributes - CLR The cell loss ratio defines the fraction of
cells lost(or delivered so late that they are
considered lost) during transmission.For example,
if the sender sends 100 cells one of them is
lost,the CLR is - CLR
1/100 10-2 - CTD The cell transfer delay is the average time
needed for a cell to travel from source to
destination. The maximum CTD the minimum CTD
also considered attributes. - CDV The cell delay variation is the difference
between CTD maximum the CTD minimum. - CER The cell error ratio defines the fraction
of cells delivered in error.
25IP over ATM
- When IP works with ATM , the IP packets are
segmented into fixed length cells of ATM,
transmitted through the ATM network, then
reassembled into IP packets at the receiving end. - Each entry/exit point is a router. An ATM
backbone can span an entire continent and may
have tens or even hundreds of ATM switches. - Most ATM backbones have a permanent virtual
channel (VC) between each pair of entry/exit
points. -
26- For n entry points, n(n - 1) permanent VCs are
needed to directly connect n entry/exit points.
Each router interface that connects to the ATM
network will have two addresses. The router
interface will have an IP address, as usual, and
the router will have an ATM address, which is
essentially a LAN address. - Consider now an IP datagram that is to be moved
across the ATM backbone To four IP routers, the
backbone appears as a single logical linkATM
interconnects these four routers just as Ethernet
can be used to connect four routers. - Let us refer to the router at which the datagram
enters the ATM network as the entry router and
the router at which the datagram leaves the
network as the exit router. - The entry router does the following
- 1. Examines the destination address of the
datagram. - 2. Indexes its routing table and determines
the IP address of the exit router - 3. To move the datagram to the next router,
the physical address of the next-hop router must
be determined. - 4. IP in the entry router then passes down to
the link layer (that is, ATM) the datagram along
with the ATM address of the exit router.
27- ATM must now move the datagram to the ATM
destination address. This task has two subtasks - Determine the VCI for the VC that leads to the
ATM destination address. - Segment the datagram into cells at the sending
side of the VC (that is, at the entry router),
and reassemble the cells into the original
datagram at the receiving side of the VC (that
is, at the exit router). - ATM uses AAL5 to provide a more efficient way to
segment and reassemble a datagram. Recall that IP
in the entry router passes the datagram down to
ATM along with the ATM address of the exit
router. ATM in the entry router indexes an ATM
table to determine the VCI for the VC that leads
to the ATM destination address. AAL5 then creates
ATM cells out of the IP datagram - The datagram is encapsulated in a CPCS-PDU using
the format in fig. - The CPCS-PDU is chopped up into 48-byte chunks.
Each chunk is placed in the payload field of an
ATM cell. - All of the cells except for the last cell have
the third bit of the PT field set to 0.
The AAL_indicate bit is used to reassemble IP
datagrams from ATM cells.
28- The last cell has the bit set to 1. AAL5 then
passes the cells to the ATM layer. ATM sets the
VCI and CLP fields and passes each cell to the TC
sublayer. For each cell, the TC sublayer
calculates the HEC and inserts it in the HEC
field. The TC sublayer then inserts the bits of
the cells into the PMD sublayer. - The ATM network then moves each cell across the
network to the ATM destination address. At each
ATM switch between the ATM source and the ATM
destination, the ATM cell is processed by the ATM
physical and ATM layers, but not by the AAL
layer. At each switch the VCI is typically
translated and the HEC is recalculated. - When the cells arrive at the ATM destination
address, they are directed to an AAL buffer that
has been put aside for the particular VC. The
CPCS-PDU is reconstructed using the AAL_indicate
bit to determine which cell is the last cell of
the CPCS-PDU. Finally, the IP datagram is
extracted out of the CPCSPDU and is passed up the
protocol stack to the IP layer
29ATM Advantages
- Provides scalable bandwidth from a few megabits
per second (Mbps) to many gigabits per second
(Gbps). - Fixed-length cells enables low-cost hardware to
be developed to perform required cell switching
based on the contents of the cell header, without
requiring more complex and costly software - Fixed-size cells allow ATM to support
quantifiable QoS - Because of its asynchronous nature, ATM is more
efficient than synchronous technologies, such as
TDM. - Simplified Network Management.
- ATM is a cell-switching and multiplexing
technology that combines the benefits of circuit
switching (guaranteed capacity and constant
transmission delay) with those of packet
switching (flexibility and efficiency ).
30ATM disadvantages
- Overhead of cell header (5 bytes per cell)
- Complex mechanisms for achieveing QoS
- Congestion may cause cell losses
- ATM handles data traffic smoothly, but runs into
delay problems with voice transmissions.
31CONCLUSION
- ATM is a high-speed packet switching technique
suitable for LAN, wide-area network and broadband
ISDN (integrated services digital network)
transmissions. - The decision of when to use ATM and when to use
frame relay largely depends on the applications
businesses want to run over their enterprise
networks, the amount of bandwidth they need and
their performance requirements. - ATM is ideally suited for converged voice,data
and video networks because it assures quality of
service. - It also provides the high amounts of bandwidth
that businesses are increasingly demanding for
data and other applications. - Frame relay, on the other hand, continues to be a
highly economical and reliable choice, especially
for medium-speed, data only applications. - It provides scalable bandwidth from a few
megabits per second (Mbps) to many gigabits per
second (Gbps). ATM provides no retransmissions on
a link-by-link basis. - Combining the ATM SONET offers scalability and
flexibility. While Asynchronous Transfer Mode
(ATM) and Synchronous Optical Network (SONET)
technologies are still emerging technologies, the
combination of the two will drastically alter
future corporate LAN design -
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