Asynchronous Transfer Mode
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28-08-2009, 02:13 AM
Asynchronous Transfer Mode
The successful deployment of broadband networks and services requires a comprehensive assessment of the capabilities of the impacted network elements. In particular, conformance, performance, and interoperability testing of switching systems and routers is necessary for all aspects of network and service deployment. To ensure timely, high quality deployment of products and services on Multi Protocol Label Switching, Asynchronous Transfer Mode (ATM), Frame Relay, and/or Internet Protocol (IP)-based networks, network service providers need to select network equipment with proven reliability and the interoperability necessary to support the services they plan to offer. Often, service providers will refuse to purchase new products and capabilities from their existing telecommunications suppliers if they have experienced numerous or significant field problems with that supplierâ„¢s products in the past.
In this intensely competitive equipment market, telecommunications suppliers need to ascertain with confidence that their network equipment will meet the applicable specifications and standards expected by the customers and further, that this equipment will integrate successfully with other network equipment from a diversified market of suppliers to deliver the quality of service expected by customers.
Telecommunications suppliers need to determine whether their equipment:
1) Meets industry standards and network requirements for the network service it intends to support
2) Delivers an acceptable quality of service given the wide variety and quantity of traffic that may be experienced in the network
3) Conforms to the necessary standards that enable it to integrate with other network equipment, so that it can deliver reliable end-to-end service.
IP and ATM are major examples for connectionless and connection-oriented services. Connectionless IP is more efficient for browsing, e-mail, and other non-real time services demanding quality and real time delivery. But for services demanding guaranteed quality and real-time delivery, fixed-path ATM is a much better candidate. A standalone ATM or IP network has additional problems. For ATM, despite the efforts, there is still little confidence that statistical multiplexing can be economically implemented in a gigabit network.
After many years of research, it is abundantly clear that future networks need two modes of operation: datagram (connectionless) and virtual circuit (connection-oriented). IP and asynchronous transfer mode (ATM) are the two archetypal examples. Connectionless IP is more efficient for browsing, e-mail, and other non-real-time services, but for services demanding guaranteed quality and real-time delivery, fixed-path ATM is a much better candidate. A standalone ATM or IP network has additional problems. For ATM, despite overwhelming efforts, there is still little confidence that statistical multiplexing can be economically implemented in a gigabit network. It is unrealistic to expect all users know the call descriptors of their connections. Without statistical multiplexing, the potentially large amount of unused bandwidth from the real-time virtual channels may be wasted. For IP, supporting real-time services with guaranteed quality requires bandwidth reservation; but the constant change of paths in an IP network makes bandwidth reservation difficult to implement.
Pinning, however, can cause IP to lose the flexibility of a connectionless network. Even if pinning is used, the network still needs to deal with the issues of call admission control rate policing, pricing, and how to support a variety of charging policies (like 800 services). To successfully tackle these issues requires extensive signaling capability, which is the characteristic of a connection-oriented network. With each additional step added to IP, the network will look more like an ATM network.
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15-02-2012, 01:56 PM
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04-10-2012, 10:52 AM
Asynchronous Transfer Mode
Atm-intro.ppt (Size: 1.78 MB / Downloads: 62)
Need for Network Convergence
PSTN sometimes used as a data network backbone – but since it is circuit switched (voice optimized) not very WAN efficient
Delay sensitive traffic such as voice not possible on data networks since there is no guarantee of QoS
Types of Traffic and demand on a communication channel
Its generation is asynchronous (a speaker may speak anytime)
Its transmission must be synchronous (once the message starts, it must flow continuously as it is spoken)
The bandwidth required for a voice conversation in digital communication is relatively small and constant (64K)
The signals may contain a high degree of error and the information can still be retrieved correctly
The generation is synchronous (continuous)
Its transmission is synchronous. The bandwidth required is variable and it could range from under 64 Kbps to several Mbps in the same session.
Error control should be tight - otherwise the wrong information on the monitor may trigger severe wrongful actions
Its generation could be either asynchronous (text) or synchronous (telemetry)
Its transmission in general can be asynchronous (data typically can wait patiently in buffers)
The information is extremely error-sensitive, so extreme caution must be exercised in transmission and error control must be very tight.
How can we combine voice , data and video on the same link?
Fixed and relatively short packets
Delays associated with each packet are going to be short and fixed – predictable transmission
If Voice and Video can be given priority handling – then mixing is possible without any diminishing in quality
ATM – Asynchronous Transfer Mode
It is a high speed, connection – oriented switching and multiplexing technology capable of transmitting voice, video and data and interconnecting LAN’s
ATM is asynchronous because information streams can be sent independently without the need of a common clock
Features and Benefits of ATM
Convergence of Voice , Video and Data on one network
High speed switching at hardware level
Bandwidth on demand
Predefined and guaranteed QoS and CoS
Superior Management features
Scalability in network size and speed
Ease of integration with other technologies
Important terms relevant to ATM
Quality of Service (QoS) :A broadly used term that refers to the performance attributes of an end-to-end connection. A QoS definition for data would address attributes such as error rates, lost packet rates, throughput, and delay
Class of Service (CoS) :It is a way of managing traffic in a network by grouping similar types of traffic together and treating each type as a class with its own level of service priority
Fast Packet Switching :A packet switching technique that increases the throughput by eliminating overhead. Overhead reduction is accomplished by allocating flow control and error correction functions to either the user applications or the network nodes that interface with the user. Cell relay is an implementation of this.
Cells and Cell relay
A Cell is a formatted packet that uses a fixed length data unit
Cell relay is the process of moving these cells through switching elements
Fixed size cells can be switched at a very high speed and add predictability to data transmissions
Variable length frames produce unpredictable patterns and performances as the buffer time cannot be determined
Cell tax – overhead imposed by ATM cells which can cut into amount of data that can be transferred
Fundamental ATM Operations Concept
A virtual or logical connection is established
ATM forms a packet of fixed length – 53 octets ( 5 octet header and 48 octet information field )
Cells are placed in a queue, on reaching ATM switch
Cells are then multiplexed asynchronously with other cells for transmission
Switch adapts the incoming bit rate to match the transmit channel bit rate
Switch inserts dummy cells to meet the aggregate bit stream rate of 155.52 Mbps