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Asynchronous Transfer Mode
(ATM)
DEFINITION:
"ATM is a technology that allows transmission and switching (circuit/packet) of fixed length packets; encoded by TDM (Time Division Multiplexing) known as cells, through an ATM switching network asynchronously."
EXPALNATION:
ATM is basically a switching technique used by telecommunication networks that is defined by fixed cell structure of cells. Fixed cell structure of ATM means it can be easily switched by hardware without delays introduced by routed frames and software switching. This is the reason people believe that ATM is the key to Internet Bandwidth problem. ATM creates a fixed route between two points before data transfer begins, which differs from TCP/IP; where data is divided into packets, each packet take different routes to get to its destination. This make it easier to bill data usage. ATM is used for real-time services such as video. However, an ATM network is less adaptable to a sudden network traffic surge.
With TDM, each user is assigned to a time slot, and no other station can send in that time slot. If a station has much data to send, it can send only when its time slot comes up, even if all other time slots are empty. However, if a station has nothing to transmit when its time slot comes up, the time slot is sent empty and is wasted. Because ATM is asynchronous, time slots are available on demand with information identifying the source of the transmission contained in the header of each ATM cell.
About ATM we can say that:
"A virtually circuit or virtually connection must be established before the two end points can actually exchange data."
ATM is connection oriented low overhead concepts of virtual channels which has no flow control or error recovery. ATM guarantees the cell sequence. Following two hierarchical levels are used to do so:
- VCI- logical connections It describes the uni-directional transport of ATM cells associated logical connections by a common unique identifier value, that identifier is called Virtual Channel Identifier.
- (^) VP- bundle of channels having same end point It describes the uni-directional transport of ATM cells belonging to Virtual Channels that are associated by a common identifier value, that identifier is called Virtual Path Identifier.
SWITCHES:
ATM provide functionality that is similar to both circuit switching & packet switching.
Circuit Switching is a method of implementing telecommunications network in which two network nodes establish a dedicated communication channel (circuits) through the network before nodes may communicate.
Packet Switching is digital networking communications method that group all transmitted data into suitably sized blocks; called packets, which are transmitted via a medium that may be shared by multiple simultaneous communication sessions. Efficiency & robustness are its main advantage.
Typically used to trunk multiple service providers.
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.
ATM SERVICES:
A telecommunication service classifications defined on the basis of following parameters:
- (^) Timing relationship between source & destination.
- Bit-rate
- Connection mode
There are five basic types (and several variants) which each have a set of parameters describing the connection.
- Constant Bit Rate (CBR) SERVICE CATEGORY: CBR SC roughly provides the service given by a circuit switched network. CBR is intended to support real-time applications requiring tightly constrained cell delay, cell delay variation and cell loss ratio. It is used by connections demanding a static bit rate to be available during the duration of connection.
- Variable Bit Rate (VBR) SERVICE CATEGORY: The VBR SC is intended for those applications having a burst traffic, i.e. having a transmission rate which fluctuates between different rate transmission levels. If CBR SC was used for this kind of traffic, PCR (Peak Cell Rate) should allocate the maximum required transmission rate, and the network resources would be used inefficiently when source transmitting at lower rates.
In VBR SC categorize their traffic in way that network operator doesn't need to reserve bandwidth for maximum transmission rate. VBR have two sub categories: rt-VBR (real time-VBR)
- Applications requiring tightly delay constrained.
- Quality of Service (QoS) parameters is specified in it.
&
nrt-VBR (not real time-VBR)
- Applications having no delay constrained.
- Quality of Service (QoS) parameters is specified in it.
- Unspecified Bit Rate (UBR) SERVICE CATEGORY: UBR SC is intended for non-real time traffic that do not have delays nor bandwidth constrains. The goal of UBR is to offer an economical SC which employs the unused network resources. The network doesn't guarantee any QoS.
- Available Bit Rate (ABR) SERVICE CATEGORY: ABR SC has been introduced to support traffic form sources which are able to adapt their cell rate to changing network conditions and available bandwidth left by guaranteed rate traffic. Information about cell rate adjustments is sent to the sources as feedback information through special control cells, called Resource Management cells (RM-cells). There is no guarantee of delay variation.
- Guaranteed Bit Rate (GBR) SERVICE CATEGORY: Unlike other SC's, the concept of frame is introduced in GFR. The term frame refers to a consecutive group of cells conveying a single data unit (packet) that has been fragmented at a source end system to be accommodated into ATM cell stream. Frames are introduced because in case of congestion the network will discard cells in a frame basis in order to avoid delivering the "corrupted packets".
Reference
https://books.google.com.pk/books?id=8SvUX0YVDyEC&pg=PP7&dq=atm+in +telecommunication&hl=en&sa=X&ved=0ahUKEwiy2J6Vut7SAhWNDBoKHS29BV 8Q6AEIHDAB#v=onepage&q=atm%20in%20telecommunication&f=false
https://www.techopedia.com/definition/5339/asynchronous-transfer-mode-atm
https://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode
http://docwiki.cisco.com/wiki/ Asynchronous_Transfer_Mode_Switching#ATM_Network_Interfaces
http://www.tesisenred.net/bitstream/handle/10803/5979/01Lca01de01.pdf? sequence=
