Asynchronous Transfer Mode
What is ATM
Asynchronous Transfer Mode (ATM), a dedicated connection switching technology, uses digital signal technology that divides upper-level data (digital data) into 53 byte cell units and to transmit on a physical medium.
A cell is individually asynchronously processed relative to other cells .
It operates independently of the type of transmission being generated at the upper layers and of the type of the physical layer medium below it.
This allows ATM technology to transport all kinds of transmissions(data, voice, video etc.) in a single integrated data stream over any medium at speed of 155 Mbps and beyond.
ATM is a vital part of ISDN (BISDN) along with SONET (Synchronous Optical Network) and other technologies`
What is ATM
ATM transforms information in fixed size units called cells
Each cell consists of 53 bytes . The first 5 bytes contains cell-header information, and the remaining 48 contains the payload(user information)
Why a small cell instead of large packet?
Small cells are well suited for voice and video traffic because traffic is intolerant of delays that result from having to wait for a large data packet to download.
No need for in-route fragmentation.
Why a fixed cell size instead of variable packet size?
Switch architecture can be optimized to the fixed size, so switching can be done in hardware.
Scalable parallel switch designs.
Why 53 bytes?
US wanted 64 payload bytes , Europe wanted 32
Compromised on 48
+5 header = 53
An ATM network consists of ATM switch and ATM endpoints.
ATM switch is responsible for cell transmit through an ATM network.
It accepts the incoming cell from an ATM end point or another ATM switch. Then reads and updates the cell header information & quickly switches the cell to an output interface towards destination.
An ATM endpoint contains an ATM network interface adapter.
Ex: Workstations, routers, Digital Service Units(DSUs), LAN switches, and video coder and decoders.
ATM network comprises ATM switches and endpoints
ATM Network Interfaces
ATM Network Interfaces
An ATM network consists of a set of ATM switches interconnected by point-to-point ATM links or interfaces.
ATM switches primarily uses two types of interfaces:
UNI (User to Network Interface)
NNI (Network to Network Interface)
UNI connects ATM end systems to an ATM switch.
NNI connects two ATM switch.
UNI & NNI is further divided into public & private UNI and NNIs.
B-ICI(Broadband Intercarrier Interface) connects two public switches from different service providers
ATM Cell Header
ATM Cell Header Format
An ATM cell header can be of two formats:
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.
ATM Cell Header Fields
GFC(Generic Flow Control) â€œ Provides local functions , such as identifying multiple stations that share a single ATM interface.
Virtual Path Identifier (VPI) - 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) - In conjunction with the VPI, identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination.
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) - 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.
Three types of ATM services exist:
Permanent virtual circuits (PVC)
Switched virtual circuits (SVC),
Connectionless service (which is similar to SMDS).
PVC: Allows direct connectivity between sites(similar to a leased line).
SVC: Created and released dynamically and remains in use only as long as data is being transferred(similar to a telephone call).
ATM Reference Model
ATM Reference Model
The ATM reference model is composed of the following planes, which span all layers:
Controlâ€This plane is responsible for generating and managing signaling requests.
Userâ€This plane is responsible for managing the transfer of data.
Managementâ€This plane contains two components:
â€œ Layer management manages layer-specific functions, such as the detection of failures and protocol problems.
â€œ Plane management manages and coordinates functions related to the complete system.
ATM Reference Model
The ATM reference model is composed of the following ATM layers:
Physical layerâ€Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the medium-dependent transmission.
ATM layerâ€ 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).
ATM adaptation layer (AAL)â€ 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.
ATM Advantages Quality based
It is scalable and flexible. It can support megabit-to-gigabit transfer speeds and is not tied to a specific physical medium.
It efficiently transmits video, audio, and data through the implementation of several adaptation layers.
Bandwidth can be allocated as needed, lessening the impact on and by high-bandwidth users.
It transmits data in fixed-length packets, called cells, each of which is 53 bytes long, containing 48 bytes of payload and 5 bytes of header.
It is asynchronous in the sense that although cells are relayed synchronously, particular users need not send data at regular intervals.
It is connection oriented, using a virtual circuit to transmit cells that share the same source and destination over the same route.
Large geographical distribution
LAN arena dominated by huge installed Ethernet base
Ethernet growing toward MAN, WAN
Switching and routing performed together.
Only best effort service.
Living up to the hype of early 90s.
ATM Applications ATM technologies, standards, and services are being applied in a wide range of networking environments, as described briefly below.
ATM services:Service providers globally are introducing or already offering ATM services to their business users.
ATM workgroup and campus networksâ€Enterprise users are deploying ATM campus networks based on the ATM LANE standards. Workgroup ATM is more of a niche market with the wide acceptance of switched-Ethernet desktop technologies.
ATM enterprise network consolidation :A full-featured ATM ENS offers a broad range of in-building (e.g., voice, video, LAN, and ATM) and wide-area interfaces (e.g., leased line, circuit switched, frame relay, and ATM at narrowband and broadband speeds).
Multimedia virtual private networks and managed servicesâ€Service providers are building on their ATM networks to offer a broad range of services. Examples include managed ATM, LAN, voice and video services and full-service virtual private-networking capabilities.
Frame-relay backbonesâ€Frame-relay service providers are deploying ATM backbones to meet the rapid growth of their frame-relay services to use as a networking infrastructure for a range of data services and to enable frame relay to ATM service interworking services.
Internet backbonesâ€Internet service providers are likewise deploying ATM backbones to meet the rapid growth of their frame-relay services, to use as a networking infrastructure for a range of data services, and to enable Internet class-of-service offerings and virtual private intranet services.
Residential broadband networksâ€ATM is the networking infrastructure of choice for carriers establishing residential broadband services, driven by the need for highly scalable solutions.
Carrier infrastructures for the telephone and private-line networksâ€Some carriers have identified opportunities to make more-effective use of their SONET/SDH fiber infrastructures by building an ATM infrastructure to carry their telephony and private-line traffic.