Introduction of ATM

 
Today, there’s much talk about information highways and Asynchronous Transfer Mode (ATM). Just as the Internet revolutionized worldwide communication, ATM brings new meaning to high-speed networking. However ATM is emerging as the primary networking technology for next-generation, multimedia communication. Besides, the ATM protocol designed to handle isochronous (time critical) data such as video and telephony (audio), in addition to more conventional data communications between computers.
 
 
 

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What is ATM?

ATM can define as the transport protocol. It’s a way of transmitting data, voice and video from one place to another. However, ATM gives you more than a basic transmission capability such as
 

1) It support both private and public networks

2) Uses the same technology for Local and Wide Area Network

3) Transports voice, video, and data traffic on a common circuit

4) Delivers bandwidth on demand
 

However, it comes at a low price. Contrary to common misconceptions, ATM is a very complex technology, perhaps the most complex ever developed by the networking industry. While the structure of ATM cells and cell switching do facilitate the development of hardware intensive, high performance ATM switches, the development of ATM network requires the overlay of a highly complex, software intensive, protocol infrastructure. This infrastructure is required to both allow individual ATM switches to be linked into a network, and for such networks to inter-networking with the vast installed base of existing local and wide area network.
 

ATM is the networking technology that satisfies these requirements. For LAN users, you’ll get more bandwidth with ATM than with many of today’s LANs. You can begin to use new applications and improve the performance of existing applications. ATM also provides more flexible bandwidth for your Wide Area Network (WAN). WAN users can expect enchanted bandwidth management and bandwidth on demand, which can reduce the cost.
 

Furthermore, the advantages of the ATM network are ATM is a very flexible bandwidth allocation and It simple routing due to connection oriented technology. Besides, It is the high bandwidth utilization due to statistical multiplexing.
 

However the disadvantages of the ATM are overhead of cell header and complex mechanisms for achieving Quality of Service. Finally, it very congestion and it may cause cell losses.

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Basic Concept in ATM

Nowadays, there are many reasons why ATM will be the networking technology for the near future and beyond. The success of this technology has not been left to chance, because ATM technology is been engineered in a manner to proactively address the problems that will be encountered with next-generation networks. One of the reason it successes is that ATM technology is designed to handle all aspects of multimedia (video, audio and data) for the LAN, MAN, and WAN. However there are few basic concept for the ATM how it be the next generation networking.

• Information Transfer: ATM is a fast packet oriented transfer mode based on asynchronous time division multiplexing and its uses fixed length (53 bytes) cells. Each ATM cell consists of 48 bytes for information field and 5 bytes for header (see figure 1). The header is used to identify cells belonging to the same virtual channel. ATM Adaptation layers (AAL) are used to support


                         (Figure 1)

Various services and provide service specific functions. This AAL specific information is contained in the information field of the ATM cell.

 Routing: ATM is a connection-oriented mode. The header values (i.e. VCI and VPI etc.) are assigned during the connection set up phase and translated when switched from one section to other. Signaling information is carried on a separate virtual channel than user information. In routing, there are two types of connection i.e. Virtual channel connection (VCC) and Virtual path connection (VPC). A VPC is an aggregate of VCCs. Switching on cells is first done on the VPX and then on the VCC. We will discuss these two connections later.

 

• ATM Resources: ATM is connection oriented and the establishment of the connections includes the allocation of a VCI i.e. virtual channel identifier and VPI i.e. Virtual path identifier and also includes the allocation of the required resources on the user access and inside the network. These resources, expressed in terms of throughput and quality of service, can be negotiated between user and network either before the call setup or during the call.

 

• ATM Cell Identifiers: ATM cell identifiers, i.e. Virtual Path Identifier, Virtual Channel Identifier and Payload Type Identifier (PTI) are used to recognise an ATM cell on a physical transmission medium. VPI and VCI are same for cells belonging to the same virtual connection on a shared transmission medium.

 

• Throughput: Peak Cell Rate (PCR) can be defined as a Throughput parameter which in turn is defined as the inverse of the minimum interarival time T between two consecutive basic events and T is the peak emission interval of the ATM connection. PCR applies to both constant bit rate (CBR) and variable bit rate (VBR) services for ATM connections. It is an upper bound of the cell rate of an ATM connection and there is another parameter sustainable cell rate (SCR) allows the ATM network to allocate resources more efficiently.

 

• Quality Of Service: Quality of Service (QOS) parameters include cell loss, the delay and the delay variation incurred by the cells belonging to the connection in an ATM network. QOS parameters can be either specified explicitly by the user or implicitly associated with specific service requests. A limited number of specific QOS classes will be standard in practice.

 

• Usage Parameter Control: In ATM, excessive reservation of resources by one user affects (traffic for other users. So the throughput must be policed at the user-network interface by a Usage Parameter Control function in the network to ensure that the negotiated connection parameters per VCC or VPC between network and subscriber is maintained by each other user. Traffic parameters describe the desired throughput and QOS in the contract. The traffic parameters are to be monitored in real time at the arrival of each cell. CCITT recommends a check of the peak cell rate (PCR) of the high priority cell flow (CLP = 0) and a check of the aggregate cell flow (CLP =1), per virtual connection.

 

• Flow Control In order to control the flow of traffic on ATM connections from a terminal to the Cletwork, a General Flow Control)(GFC) mechanism is proposed by CCITT at the User to Network Interface (UNI). This function is-supported b~ GFC field in the ATM cell header. Two sets of procedures are associated with the GFC field i.e. Uncontrolled Transmission which is for use in point-to-point configurations and Controlled Transmission which can be used in bothpoint-to-point and shared medium configurations.

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Why using ATM Network?

Following we will discuss about why we using ATM Network as our next generation network? The following is the benefit of using ATM network:

• One Network-ATM will provide a single network for all traffic types-voice, data, and video. ATM allows for the integration of networks improving efficiency and manageability.
• Enables new applications-Due to its high speed and the integration of traffic types, ATM will enable the creation and expansion of new applications such as multimedia to the desktop.
• Compatibility-Because ATM is not based on a specific type of physical transport, it is compatible with currently deployed physical networks. ATM can be transported over twisted pair, coax and fiber optics.

• Incremental Migration-Efforts within the standards organisations and the ATM Forum continue to assure that embedded networks will be able to gain the benefits of ATM incrementally-upgrading portions of the network based on new application requirements and business needs.
• Simplified Network Management-ATM is evolving into a standard technology for local, campus/backbone and public and private wide area services. This uniformity is intended to simplify network management by using the same technology for all levels of the network.
• Long Architectural Lifetime-The information systems and telecommunications industries are focusing and standardising on ATM. ATM has been designed from the onset to be scalable and flexible in:
• Geographic distance
• Number of users
• Access and trunk bandwidths (As of today, the speeds range from Megabits to Gigabits)
• This flexibility and scalability assures that ATM will be around for the long time.

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ATM Network Operation

 
In this part, I will discuss the architecture of ATM networks, ATM connection and ATM connection type.
 

An ATM network, it consist of a set of ATM switches interconnected by point-to-point ATM links or interfaces. ATM switch support two kinds of interfaces, there are User-Network Interface (UNI) and network-node interfaces (NNI). Before I go in detail I will differential what is UNI and NNI. Actually, the UNI is the network interface, but the different is subtle and unimportant. However, the NNI is a cell. It unlike the UNI cells, because there is no Generic Flow Control (GFC) field, and the first four bits of the cell are used by an expanded (12-bit) VPI field. Since the GFC is rarely used, so that there are no difference functional between UNI and NNI cells, other than in the fact that the latter can support a larger VPI space.
 

From the figure 2 below the UNI connect ATM end-system which included hosts, router and so on to an ATM switch, while an NNI may be imprecisely defined as an interface connection two ATM switches together; slightly different cell formats are defined across UNI and NNI. More precisely, however, an NNI is any physical or logical link across which two ATM switches exchange the NNI protocol. So that, for this reason the connection between a private ATM switch and public ATM switch is a UNI – known as a Public UNI – since these switches do not typically exchange NNI information.

                         (Figure 2 ATM Network Interface)

ATM networks are fundamentally connection oriented. This means that a virtual circuit needs to be set up across the ATM network prior to any data transfer. ATM circuits are of two types: virtual paths, identified by virtual path identifiers (VPI); and virtual channels identified by the combination of VPI and of virtual channel identifier (VCI). A virtual path is a bundle of virtual channels, all of which are switched transparently across the ATM network on the basis of the common VPI. All VCI and VPI, however, have only local significance across a particular link, and are remapped, as appropriate, at each switch. In normal operation, switches allocate all UNI connections within VPI=0; the use of other virtual paths is discussed later in this paper.

The basic operation of an ATM switch is very simple: to receive a cell across a link on a know VCI or VPI value; to look up the connection value in a local translation table to determine the outgoing port (or ports) of the connection and the new VPI/VCI value of the connection link with the appropriate connection identifiers such as the figure 3.

                                                                (Figure 3)

The switch operation is simple such as the following diagram (Figure 4) because external mechanisms set up the local translation tables prior to the transmittal of any data. The manner in which in which these tables are set up determine the two fundamental types of ATM connection :

 
                    ( Figure 4 Virtual Circuit and Vritual Path Switching)
 

• Permanent Virtual Connections (PVC): A PVC is a connection setup by some external mechanism, typically network management, in which a set of switches between an ATM source and destination ATM system are programmed with the appropriate VPI/VCI values. As is discussed later, ATM signaling can facilitate the setup of PVCs, but by definition, PVCs always require some manual configuration. As such, their use can often be cumbersome.
• Switched Virtual Connection (SVC): A SVC is a connection that is set up automatically through a signaling protocol. SVCs do not require the manual interaction needed to set up PVCs and, as such are likely to be much more widely used. All higher layer protocols operating over ATM primarily use SVCs, and it is these that are primarily considered in this paper.

On the other hand, the ATM routing protocols is route ATM connection requests across the ATM network such as the figure 5. There are two fundamental types of ATM connections:
 

                                           (Figure 5 Types of ATM Connection)
 

    What is notably missing from these types of ATM connections is an analog to the multicasting or broadcasting capability common in many shared medium LAN technologies such as Ethernet or Token Ring. In such technology multicasting allows multiple end systems to both receive data from other multiple systems, and transmit data to these multiple systems. Such capabilities are easy to implement in shared media technologies such as LANs, where all nodes on a single LAN segment must necessarily process all packets sent on that segment. The obvious analog in ATM to a multicast LAN group would be a (bi-directional) multipoint-to-multipoint connection. Unfortunately, this obvious solution cannot be implemented when using AAL5; the most common ATM Adaptation Layer (AAL) used to transmit data across ATM network.
 

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Summary

As a conclusion ATM will offer significantly greater bandwidth, flexibility and quality of service support than is possible on today's networks built with shared media hubs and routed internetworks. The most important reason for impending success of ATM is that this technology is standards-based. Standards and interpretability are the most important factors behind Internet worked communications. Networking standard that is technically sound and rigidly followed are more important than much technical limitation within a system.

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Prepare by Wang Fook Weng
28th September 1997