A SEMINAR ON

ATM NETWORKS

 
 
 
 
 
 
 
 
 

                                              BY

                           YALAMANDU.NALLURI 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

                            CONTENTS 
 
 
 

1. Introduction to ATM Networks

 

2. ATM cells

 

3. ATM Architecture/ATM Reference Model

 

4. Services Provided by ATM

 

5. ATM Switching Technology

 

6. Traffic Control Management in ATM

 

7. Security in ATM Networks

 

8. ATM Networks Applications

 

9. ATM Networks Performance

 

10.ATM Networks Standards 

    11 .Bibliography 
 
 
 
 
 
 
 

                            Introduction to ATM Networks 

 

                                   ATM was developed because of developing trends in the networking field. The most important parameter is the emergence of a large number of communication  services with different, sometimes yet unknown requirements. In this information  age, customers are requesting an ever increasing number of new services. The most famous communication services to appear in the future are HDTV(High Definition TV), video conferencing, high speed data transfer, videophone, video library, home education and video on demand.  

                           This large span of requirements introduces the need for one universal network which is flexible enough to provide all of these services in the same way. Two other parameters are the fast evolution of the semi - conductor and optical technology and the evolution in system concept ideas - the shift of superfluous transport functions to the edge of the network. Both the need for a flexible network and the progress in technology and system concepts led to the definition of the Asynchronous Transfer Mode (ATM) principle.

                              ATM is a high-speed, packet-switching technique that uses short fixed length packets called cells. Fixed length cells simplify the design of an ATM switch at the high switching speeds involved. The selection of a short fixed length cell reduces the delay and most significantly the jitter (variance of delay) for delay-sensitive services such as voice and video. ATM is capable of supporting a wide range of traffic types such as voice, video, image and various data traffic.

The reasons for the popularity of the ATM 

      1. ATM is a world wide recognized standard, with which a universal information exchange can be realized  for the first time,  independent of the type of end system and service type of end system and service system(data, video, audio).

      2. ATM is suitable for Local Area Networks, as well as Wide Area Networks.

      3.Today,different services, such as voice, data and video, are transmitted via separate networks, because of their different traffic characteristics. ATM is able to handle all existing information services simultaneously and efficiency; it will therefore improve significantly the performance and efficiency of today’s communication infrastructure.

      4.Since ATM is scalable, and therefore available ion all speed classes, starting from a few Mbit/s up to several Gbits/s, this technology will be able to fulfill the requirements for data communications far into the next millennium.

        

                                    

                                           ATM CELLS 

                                               ATM transfers information in fixed-size units called cells. 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). Small, fixed-length cells are well suited to transferring voice and video traffic because such traffic is intolerant of delays that result from having to wait for a large data packet to download, among other things. Above illustrates the basic format of an ATM cell.

ATM Cell Header Format

                                               The ATM header contains information about destination, type and priority of the cell.  

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. Below figure  depicts the basic ATM cell format, the ATM UNI cell header format, and the ATM NNI cell header format.

 
Figure: An ATM Cell, ATM UNI Cell, and ATM NNI Cell Header Each Contain 48 Bytes of Payload 

Unlike the UNI, the NNI header does not include the Generic Flow Control (GFC) field. Additionally, the NNI header has a Virtual Path Identifier (VPI) field that occupies the first 12 bits, allowing for larger trunks between public ATM switches.

                                    ATM cells can be further allocated to one of four categories: Idle cells, Unassigned cells, Physical layer OAM cells and  VP/VC cells.

Idle cells:

                   Idle cells allow the cell rate to be adjusted to the transfer medium bandwidth. If there are not enough cells to fill the bandwidth provided, idle cells are transmitted. This achieves synchronization with the transmission speed of physical medium.

Unassigned cells:

                         Unassigned cells are cells that have a VPI or VCI value but a blank data field.

Physical layer OAM cells:

                  For direct transfer on the cell-based physical layer, every 27^th cell is used to transfer OAM information concerning the physical layer. After receipt by the physical layer, these cells are not passed on to the ATM layer.

VP/VC cells:

                         The cells used for communication within virtual channels or virtual paths can be subdivided into six functional groups:

             · cells for the transmission of user data,

             · cells for meta-signalling,

             · cells for broadband signaling ,

             · VC OAM cells,

             · SMDS/CBDS cells,

            · ILMI (Interim Local Management Interface                                                                                                                                                    

                                             Specification) cells.

VP/VC OAM cells:

                              The VP/VC OAM cells the performance and availability of virtual paths and channels to be monitored and tested.

The Functions of the ATM cells:

            ATM cells performs the following 6 functions

1. Routing of cells must occur in a connectionless manner.

2. Housekeeping tasks must be done to establish and maintain ATM connection and handle the different service requirements of voice, video, and data.

3. Segmenting and reassembly must be done to break down frames or packets or other data units into cells at the sender and build them up again at receiver.

4. Cell handling, ATM switch must be done hop by hop through the network in an exact analogy of what our router/switches did before.

5. Convergence must be provided so that different types of physical transport media for cells can be supported.

6.The sending of the physical bits as 1s and 0s over the physical media transport must be done. 
 
 
 
 
 
 
 
 
 

            ATM Architecture/ATM Reference Model

 
 

The ATM architecture uses a logical model to describe the functionality that it supports. ATM functionality corresponds to the physical layer and part of the data link layer of the OSI reference model.

The ATM reference model is composed of the following planes, which span all layers:

• Control—This plane is responsible for generating and managing signaling requests.
• User—This plane is responsible for managing the transfer of data.
• Management—This plane contains two components:
  • Layer management manages layer-specific functions, such as the detection of failures and protocol problems.
  • Plane management manages and coordinates functions related to the complete system.

The ATM reference model is composed of the following ATM layers:

• Physical layer: Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the medium-dependent transmission.

 

• ATM layer: Combined with the ATM adaptation layer, the ATM layer is roughly analogous to the data link layer of the OSI reference model. The ATM layer is responsible for the simultaneous sharing of virtual circuits over a physical link (cell multiplexing) and passing cells through the ATM network (cell relay). To do this, it uses the VPI and VCI information in the header of each ATM cell.

 

• ATM adaptation layer (AAL):Combined with the ATM layer, the AAL is roughly analogous to the data link layer of the OSI model. The AAL is responsible for isolating higher-layer protocols from the details of the ATM processes. The adaptation layer prepares user data for conversion into cells and segments the data into 48-byte cell payloads.

 
 
 
 

                        Services Provided by ATM

                                 Three types of ATM services exist: permanent virtual circuits (PVC), switched virtual circuits (SVC), and connectionless service (which is similar to SMDS).

PVC allows direct connectivity between sites. In this way, a PVC is similar to a leased line. Among its advantages, PVC guarantees availability of a connection and does not require call setup procedures between switches. Disadvantages of PVCs include static connectivity and manual setup. Each piece of equipment between the source and the destination must be manually provisioned for the PVC. Furthermore, no network resiliency is available with PVC.

An SVC is created and released dynamically and remains in use only as long as data is being transferred. In this sense, it is similar to a telephone call. Dynamic call control requires a signaling protocol between the ATM endpoint and the ATM switch. The advantages of SVCs include connection flexibility and call setup that can be handled automatically by a networking device. Disadvantages include the extra time and overhead required to set up the connection.

ATM Virtual Connections

ATM networks are fundamentally connection-oriented, which means that a virtual channel (VC) must be set up across the ATM network prior to any data transfer. (A virtual channel is roughly equivalent to a virtual circuit.)

Two types of ATM connections exist: virtual paths, which are identified by virtual path identifiers, and virtual channels, which are identified by the combination of a VPI and a virtual channel identifier (VCI).

A virtual path is a bundle of virtual channels, all of which are switched transparently across the ATM network based on the common VPI. All VPIs and VCIs, however, have only local significance across a particular link and are remapped, as appropriate, at each switch.

A transmission path is the physical media that transports virtual channels and virtual paths. Figure 27-6 illustrates how VCs concatenate to create VPs, which, in turn, traverse the media or transmission path.

 
Figure : VCs Concatenate to Create VPs 

 
 
 
 
 
 
 
 

                  
 
 
 
 

                       ATM Switching Technology 

                           Various switching architectures were developed in the past for different application such as voice and data, based on modes like STM (Synchronous Transfer Mode) and packet switching.

                        Three major factors have a large impact on the implementation of the ATM switching architecture:

• The high speed at which the switch has to operate (from 150Mbit/sec up to 600Mbit/sec).
• The statistical behavior of the ATM stream passing though the ATM switching system.
• ATM is connection oriented. Therefore, the switching elements have pre-defined routing tables to minimize the complexity of single switch routing.

Structure of an ATM switch:

 

The ATM switch has to handle a minimum of several hundred thousand cells in a second at every switch port. A switch has to connect from a few ports to thousands of ports. In principle, a switch fabric can be implemented by a single switching element. But from practical reasons the switch fabric has to be built of basic switching building blocks – switching elements.

A switching element is the basic unit of the switch fabric. It can be implemented in a single integrated circuit element. At the input port (inlet) the routing information of the incoming cell is analyzed and the cell is then directed to the correct output port (outlet). In general the switching element consists of an interconnection network , and IC (input controller) for each incoming line and an OC (output controller) for each outgoing line. Arriving cells will be synchronized to the internal clock by the IC. The OC transport cells which have been received from the interconnection network toward the destination. The IC and OC are coupled by the interconnection network.

ATM Switching elements:

                   ATM support the following switching elements

1. A matrix switching elements
2. Bus switching elements
3. Ring switching elements
4. Central memory switching elements

ATM Switch Functions:

                                               ATM is connection oriented. All cells belong to a virtual connection pre-established by the transport network. All traffic is segmented into cells for transmission across the network. The ATM switch has several main tasks:

VCI translation.

The established connection on the ATM network defines the virtual path through different switches across the network. The VCI is local to each switch port. As each cell travels across an ATM switch, the VCI is translated into a new value. The switch has to built the new cell header containing the new VCI (and possibly new VPI- virtual path identifier ) and calculate the new HEC value.

Switching - Cell transport from its input to its output.

The transportation of the information (cell) from an incoming logical ATM channel (inlet) to an outgoing logical ATM channel (outlet), is also the responsibility of the ATM switch. The logical ATM channel is characterized by two identifiers:

1. The physical inlet/outlet which is characterized by a physical port number.
2. The logical channel on the physical port which is identified by the VCI and/or the VPI.

 
 
 
 

                     Traffic Control Management in ATM Networks 

In order to deliver the required performance the ITU has defined a set of capabilities for ATM Networks. Traffic control refers to the necessity for ATM Networks to monitor traffic entering the network to ensure that the network is still capable of delivering the promised connection performance parameters to the users.  

            The traffic control mechanism is much harder to implement in ATM networks. Traffic control is very easy in data networks, where elaborate flow control and congestion control mechanism have evolved in the protocols used there over the years.

           ATM traffic control must perform the following functions 
 

1.Connection admission control: ATM networks must set aside the proper amount resources to service a connection. This is done at connection time, whether a connection is set up at service provision time on a semi permanent basis or by means of a signaling protocol and a dynamic basis if the connection can’t be given these resources, the ATM network will not accept it. 
 

 2.Usage parameter control: ATM networks must “Police” the user network interface (UNI) to make sure cell traffic volumes and so forth don’t effect overall network performance.  
 

3.Priority control: ATM networks must adequately service buffers in the network nodes under all kinds of conditions. Under congestion conditions, when there are just too many cells in the network, a priority mechanism can be used to remedy the congestion situation.

4.Congestion control: ATM networks must prevent congestion control spreading through out the network congestion is widely misunderstood concept in networking circles. Flow control refers to the idea that a sender should never able to send faster than a receiver can receive. Congestion, on the other hand is a global  property of the network itself, not a property of any individual user. No sender may be overwhelming any receiver, but the network may still be hopelessly congestion; there is just too much traffic in it.     

   

                           Security in ATM Networks

                              

                                                For most of the experiments, a UNI compliant ATM interface would be required, with accessible Control and Management Planes of the Protocol Reference Model [8] on the intermediate switches.   

This ATM security consists the following phases:

1. Security requirements of communication networks include:

• Availability
• Secure communication channel
• Accurate auditing information

We consider that aspects like user authentication and non-repudiation of contents (of user messages) should not be expected from the network as an entity, although they might be supported by other means.

2. Threats Analysis

Three classical attacks and their consequences on each ATM flow were studied, to deduce flow's vulnerability:

• data or traffic flow confidentiality loss due to an intruder eavesdropping the network and deducing user data content or user traffic features
• data integrity loss caused by accidental or malicious injection/removal/modification of cells/signalling messages in transfer
• overloading problems following a mass-injection of cells/signalling messages.

Overloading consists in disrupting network entities (e.g. ATM switch) or end-entities (e.g. end-station) by sending a large number of cells/signalling messages whose processing prevents other useful cells/messages processing or at least slows it down. This attack is particularly serious when done with SET UP messages and is also known as Denial of Service (DoS).

3. Security services requirements for ATM

Considering the results of the preceding points, summarised in table 1, security services need to be introduced within ATM planes to protect ATM flows exchanges (see table 2).

Table 1 
 
 

 

Table 2

3.1 Signalling plane

Protecting signalling flows against integrity and overloading attacks requires the introduction of authentication, integrity and replay detection services, naturally complemented by access control mechanisms. Note that not only end-entities (end-stations) but also network entities (switches) need to handle these security services for detecting bogus RELEASE or SET UP messages.

3.2 User plane

User data flows are vulnerable to data confidentiality, traffic flow confidentiality and integrity/overloading attacks so that respectively confidentiality, padding and authentication/integrity/replay detection services must be introduced within user plane.

3.3 Management plane

As shown in table 2, management flows need the introduction of confidentiality, integrity, access control and replay detection services. Note that, in case management cells' content is encrypted, the integrity service is naturally performed thanks to the management cells' CRC field ([10],[11]) being encrypted along with management information. On the other hand, given the fixed management cells structure with only 6 bits being free (the "reserved" field), replay detection seems impossible to realise.

4. Availability

We consider the availability of some of these services (namely to the Control and Management Planes) is essential for the robustness of the network itself. In fact, we find that the integrity of the network depends on the existence of means to avoid some forms of attack (Denial of Service, Masquerade, Spoofing and Repudiation), on signalling and management protocols, even if user security services could be performed at higher layers.

5. Standardisation

Standardisation work at the ATM Forum is under way regarding the future shape of ATM Security infrastructure [5]. This infrastructure considers the use of special signalling procedures to allow for negotiation of security parameters between communicating parties.

                             ATM Applications

There are several practical applications using ATM Technology. ATM is going to be the Backbone Network for many broadband applications including Information SuperHighway. Some of the key applications can be mentioned as follows.

Video Conferencing

Desktop Conferencing

Multimedia Communications

ATM Over Satellite Communications

Mobile Computing over ATM for Wire-less Networks  
 
 
 
 
 

                       ATM Performance 
 

                      An ATM network built for simultaneous transport of voice, audio, video and  data must have acceptable performance characteristics for each kind of service. Unfortunately, they all have different requirements with regard to a number of crucial parameters. Some of there parameters are shown in the below fig: 
 

•  

If compression is used 

                                            Therefore, it will not be easy. It is not a bandwidth problem;  it is many problems. In terms of the effects of delay, error  tolerance, and the “burstiness” of data, each service has its own distinctive requirements. And compressed voice, audio, and video now become “bursty” themselves. This does not help the problem; it just moves it. ATM has defined a number of performance parameters that will make the performance of the ATM layer acceptable for all these different services.       
 
 

1.ATM Network layer performance:

                      In an ATM network, the performance ATM delivers is separate from the performance of the underlying transport. Bit errors may occur, and delays may vary from physical medium to physical medium.

           A cell with an bit error(e.g. a 0 bit is send instead of 1) in it is not the only possible bad outcome on and ATM network, Cell

Errors that are not detected in the header may result in misdirected cells turning up on the wrong conditions.

         ITU-TSS(CCITT) Recommendation I.35B defines four possible things that may happen when a cell enters an ATM network:

• Successfully delivered cell.
• Errored cell.
• Lost cell
• Inserted cell.

ATM cell transfer and delay variation:

               

                                  We know that the main purpose of  ATM networks is to give acceptable traffic delay patterns for voice, video, and data over the same physical network, here is what is meant by acceptable.

                        A cell is sent from a source A  and arrives at a destination B.

In between, there are links and network nodes.  The links have a characteristic propagation delay, and network nodes have a widely varying processing delay. If the effects of all these are combined, the from the first bit sent from source A until the last bit arrives at destination B is the cell transfer delay. If this exceeds the time T after which a cell is supposed to be delivered, the receiver discards the arriving cell, whether in error or not. This is illustrated in the below Fig.

SOURCE A                                              DESTNATION  B    

CELL

SENT                                                                           T

                                                                               CELLARRIVE

                The cell delay variation(CDV) refers to the fact that some cells will be switched very rapidly through the ATM network, but other cells may longer, due to such effect on constant –bit-rate(CBR). 

ATM Performance parameters:

              The following parameters fully characterize the performance of any ATM network. 

1. Cello loss ratio: The cell loss ratio is the ration of the number of lost cells to the sum of the number of lost and successfully delivered cells.

2. Cell insertion rate: The cell insertion ratio is the number of cells inserted into an ATM network with in a specified time period.

• Severely errored cell ratio: The severely errored cell ratio is the ratio of severely errored  cells to the number of successfully delivered cells.
• Cell transfer capacity: The cell transfer capacity is the maximum number of successfully delivered cells occurring over a specified ATM connection during a unit of time.
• Cell transfer delay: The time from the first bit sent from source to the last bit reach at destination.

                    Standards for ATM

 
 

                                      ATM is based on the efforts of the ITU-T Broadband Integrated Services Digital Network (B-ISDN) standard. It was originally conceived as a high-speed transfer technology for voice, video, and data over public networks. The ATM Forum extended the ITU-T's vision of ATM for use over public and private networks. The ATM Forum has released work on the following specifications:

· User-to-Network Interface (UNI) 2.0

           · UNI 3.0

           · UNI 3.1

     ·  UNI 4.0

          · Public-Network Node Interface (P-NNI)

     ·  LAN Emulation (LANE)

    ·  Multiprotocol over ATM  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

                                                        BIBLIOGRAPHY 

Books: 

• Introduction to ATM Networking

                       -Walter J.Goralski. 

• ATM Networks

                   -Othmar Kyas. 
 

WEB SITES:  

1.   www.atmfourm.com

2. Coffey@cis.ohio-state.edu