Asynchronous Transfer Mode
This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages)
|
Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and ITU-T (formerly CCITT) for digital transmission of multiple types of traffic. ATM was developed to meet the needs of the Broadband Integrated Services Digital Network as defined in the late 1980s,[1] and designed to integrate telecommunication networks. It can handle both traditional high-throughput data traffic and real-time, low-latency content such as telephony (voice) and video.[2][3] ATM provides functionality that uses features of circuit switching and packet switching networks by using asynchronous time-division multiplexing.[4][5] ATM was seen in the 1990s as a competitor to Ethernet and networks carrying IP traffic as it was faster and was designed with quality-of-service in mind, but it fell out of favor once Ethernet reached speeds of 1 gigabits per second.[6]
In the
The ATM network reference model approximately maps to the three lowest layers of the OSI model:
Protocol architecture
To minimize
At the time of the design of ATM, 155 Mbit/s
At 155 Mbit/s, a typical full-length 1,500 byte
The design of ATM aimed for a low-jitter network interface. Cells were introduced to provide short queuing delays while continuing to support datagram traffic. ATM broke up all packets, data, and voice streams into 48-byte chunks, adding a 5-byte routing header to each one so that they could be reassembled later. The choice of 48 bytes was political rather than technical.[8] When the CCITT (now ITU-T) was standardizing ATM, parties from the United States wanted a 64-byte payload because this was felt to be a good compromise between larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice. Parties from Europe wanted 32-byte payloads because the small size (and therefore short transmission times) improve performance for voice applications. Most of the European parties eventually came around to the arguments made by the Americans, but France and a few others held out for a shorter cell length. With 32 bytes, France would have been able to implement an ATM-based voice network with calls from one end of France to the other requiring no echo cancellation. 48 bytes (plus 5 header bytes = 53) was chosen as a compromise between the two sides. 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information.[1] ATM multiplexed these 53-byte cells instead of packets which reduced worst-case cell contention jitter by a factor of almost 30, reducing the need for echo cancellers.
Cell structure
An ATM cell consists of a 5-byte header and a 48-byte payload. ATM defines two different cell formats: user–network interface (UNI) and network–network interface (NNI). Most ATM links use UNI cell format.
Diagram of a UNI ATM cell
|
Diagram of an NNI ATM cell
|
- GFC
- The generic flow control (GFC) field is a 4-bit field that was originally added to support the connection of ATM networks to shared access networks such as a distributed queue dual bus (DQDB) ring. The GFC field was designed to give the User-Network Interface (UNI) 4 bits in which to negotiate multiplexing and flow control among the cells of various ATM connections. However, the use and exact values of the GFC field have not been standardized, and the field is always set to 0000.[9]
- VPI
- Virtual path identifier(8 bits UNI, or 12 bits NNI)
- VCI
- Virtual channel identifier(16 bits)
- PT
- Payload type (3 bits)
- Bit 3 (msbit): Network management cell. If 0, user data cell and the following apply:
- Bit 2: Explicit forward congestion indication (EFCI); 1 = network congestion experienced
- Bit 1 (lsbit): ATM user-to-user (AAU) bit. Used by AAL5 to indicate packet boundaries.
- CLP
- Cell loss priority (1-bit)
- HEC
- Header error control (8-bit CRC, polynomial = X8 + X2 + X + 1)
ATM uses the PT field to designate various special kinds of cells for
Several ATM link protocols use the HEC field to drive a CRC-based framing algorithm, which allows locating the ATM cells with no overhead beyond what is otherwise needed for header protection. The 8-bit CRC is used to correct single-bit header errors and detect multi-bit header errors. When multi-bit header errors are detected, the current and subsequent cells are dropped until a cell with no header errors is found.
A UNI cell reserves the GFC field for a local flow control and sub-multiplexing system between users. This was intended to allow several terminals to share a single network connection in the same way that two ISDN phones can share a single basic rate ISDN connection. All four GFC bits must be zero by default.
The NNI cell format replicates the UNI format almost exactly, except that the 4-bit GFC field is re-allocated to the VPI field, extending the VPI to 12 bits. Thus, a single NNI ATM interconnection is capable of addressing almost 212 VPs of up to almost 216 VCs each.[a]
Service types
ATM supports different types of services via AALs. Standardized AALs include AAL1, AAL2, and AAL5, and the rarely used[10] AAL3 and AAL4. AAL1 is used for constant bit rate (CBR) services and circuit emulation. Synchronization is also maintained at AAL1. AAL2 through AAL4 are used for variable bitrate (VBR) services, and AAL5 for data. Which AAL is in use for a given cell is not encoded in the cell. Instead, it is negotiated by or configured at the endpoints on a per-virtual-connection basis.
Following the initial design of ATM, networks have become much faster. A 1500 byte (12000-bit) full-size Ethernet frame takes only 1.2 μs to transmit on a 10 Gbit/s network, reducing the motivation for small cells to reduce jitter due to contention. The increased link speeds by themselves do not eliminate jitter due to queuing.
ATM provides a useful ability to carry multiple logical circuits on a single physical or virtual medium, although other techniques exist, such as
Virtual circuits
An ATM network must establish a connection before two parties can send cells to each other. This is called a
Motivation
ATM operates as a channel-based transport layer, using VCs. This is encompassed in the concept of the virtual paths (VP) and virtual channels. Every ATM cell has an 8- or 12-bit virtual path identifier (VPI) and 16-bit virtual channel identifier (VCI) pair defined in its header.[11] The VCI, together with the VPI, is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination. The length of the VPI varies according to whether the cell is sent on a user-network interface (at the edge of the network), or if it is sent on a network-network interface (inside the network).
As these cells traverse an ATM network, switching takes place by changing the VPI/VCI values (label swapping). Although the VPI/VCI values are not necessarily consistent from one end of the connection to the other, the concept of a circuit is consistent (unlike IP, where any given packet could get to its destination by a different route than the others).[12] ATM switches use the VPI/VCI fields to identify the virtual channel link (VCL) of the next network that a cell needs to transit on its way to its final destination. The function of the VCI is similar to that of the data link connection identifier (DLCI) in Frame Relay and the logical channel number and logical channel group number in X.25.
Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, IP). The VPI is useful for reducing the switching table of some virtual circuits which have common paths.[13]
Types
ATM can build virtual circuits and virtual paths either statically or dynamically. Static circuits (permanent virtual circuits or PVCs) or paths (permanent virtual paths or PVPs) require that the circuit is composed of a series of segments, one for each pair of interfaces through which it passes.
PVPs and PVCs, though conceptually simple, require significant effort in large networks. They also do not support the re-routing of service in the event of a failure. Dynamically built PVPs (soft PVPs or SPVPs) and PVCs (soft PVCs or SPVCs), in contrast, are built by specifying the characteristics of the circuit (the service contract) and the two endpoints.
ATM networks create and remove switched virtual circuits (SVCs) on demand when requested by an
Routing
Most ATM networks supporting SPVPs, SPVCs, and SVCs use the
Traffic engineering
Another key ATM concept involves the traffic contract. When an ATM circuit is set up each switch on the circuit is informed of the traffic class of the connection. ATM traffic contracts form part of the mechanism by which quality of service (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.
- CBR – Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.
- VBR – Variable bit rate: an average or Sustainable Cell Rate (SCR) is specified, which can peak at a certain level, a PCR, for a maximum interval before being problematic.
- ABR – Available bit rate: a minimum guaranteed rate is specified.
- UBR – Unspecified bit rate: traffic is allocated to all remaining transmission capacity.
VBR has real-time and non-real-time variants, and serves for bursty traffic. Non-real-time is sometimes abbreviated to vbr-nrt. Most traffic classes also introduce the concept of cell-delay variation tolerance (CDVT), which defines the clumping of cells in time.
Traffic policing
To maintain network performance, networks may apply
If the traffic on a virtual circuit exceeds its traffic contract, as determined by the GCRA, the network can either drop the cells or set the Cell Loss Priority (CLP) bit, allowing the cells to be dropped at a congestion point. Basic policing works on a cell-by-cell basis, but this is sub-optimal for encapsulated packet traffic as discarding a single cell will invalidate a packet's worth of cells. As a result, schemes such as partial packet discard (PPD) and early packet discard (EPD) have been developed to discard a whole packet's cells. This reduces the number of useless cells in the network, saving bandwidth for full packets. EPD and PPD work with AAL5 connections as they use the end of packet marker: the ATM user-to-ATM user (AUU) indication bit in the payload-type field of the header, which is set in the last cell of a SAR-SDU.
Traffic shaping
Traffic shaping usually takes place in the network interface controller (NIC) in user equipment, and attempts to ensure that the cell flow on a VC will meet its traffic contract, i.e. cells will not be dropped or reduced in priority at the UNI. Since the reference model given for traffic policing in the network is the GCRA, this algorithm is normally used for shaping as well, and single and dual leaky bucket implementations may be used as appropriate.
Reference model
The ATM network reference model approximately maps to the three lowest layers of the
- At the physical network level, ATM specifies a layer that is equivalent to the OSI physical layer.
- The ATM layer 2 roughly corresponds to the OSI data link layer.
- The OSI network layer is implemented as the ATM adaptation layer (AAL).
Deployment
ATM became popular with telephone companies and many computer makers in the 1990s. However, even by the end of the decade, the better
Wireless or mobile ATM
Wireless ATM,
See also
Notes
- ^ In practice some of the VP and VC numbers are reserved.
References
- ^ a b Ayanoglu, Ender; Akar, Nail (25 May 2002). "B-ISDN (Broadband Integrated Services Digital Network)". Center for Pervasive Communications and Computing, UC Irvine. Retrieved 3 June 2011.
- ^ Telcordia Technologies, Telcordia Notes on the Network, Publication SR-2275 (October 2000)
- ISBN 0-13-393828-X, Prentice Hall PTR, 1995, page 2.
- ^ "Recommendation I.150, B-ISDN Asynchronous Transfer Mode functional characteristics". ITU.
- ^ a b McDysan (1999), p. 287.
- ^ https://www.google.com.pa/books/edition/_/ghy9BOw6svMC?hl=en&gbpv=1&pg=PP1&dq=atm+network
- ISBN 0-07-060362-6, McGraw-Hill series on computer communications, 1995, page 563.
- ^ D. Stevenson, "Electropolitical Correctness and High-Speed Networking, or, Why ATM is like a Nose", Proceedings of TriCom '93, April 1993.
- ^ "ATM Cell Structure". Retrieved 13 June 2017.
- ^ "A Brief Overview of ATM: Protocol Layers, LAN Emulation, and Traffic Management". www.cse.wustl.edu. Retrieved 21 July 2021.
- ^ Cisco Systems Guide to ATM Technology (2000). Section "Operation of an ATM Switch". Retrieved 2 June 2011.
- ^ Cisco Systems Guide to ATM Technology (2000). Section "ATM Cell Header Formats". Retrieved 2 June 2011.
- ^ "What is VPI and VCI settings of broadband connections?". Tech Line Info. Sujith. Retrieved 1 July 2010.
- ^ ITU-T, Traffic control and congestion control in B ISDN, Recommendation I.371, International Telecommunication Union, 2004, page 17
- ^ ITU-T, Traffic control and congestion control in B ISDN, Recommendation I.371, International Telecommunication Union, 2004, Annex A, page 87.
- ISBN 0-13-393828-X, Prentice Hall PTR, 1995.
- ^ "Guide to ATM Technology for the Catalyst 8540 MSR, Catalyst 8510 MSR, and LightStream 1010 ATM Switch Routers" (PDF). Customer Order Number: DOC-786275. Cisco Systems. 2000. Archived (PDF) from the original on 9 October 2022. Retrieved 19 July 2011.
- ^ Steve Steinberg (October 1996). "Netheads vs Bellheads". Wired. Vol. 4, no. 10. Retrieved 24 September 2011.
- ^ "What's in store for FORE?". Network World. 16 September 1996. p. 12. Retrieved 24 September 2011.
- ^ "Optical Ethernet firms brave stormy industry seas". Network World. 7 May 2001. p. 14. Retrieved 24 September 2011.
- ^ "About the Broadband Forum: Forum History". Archived from the original on 9 October 2011. Retrieved 24 September 2011.
- ^ Wireless ATM
- ^ Book on Wireless ATM Networks - Chai Keong Toh, Kluwer Academic Press 1997
- ^ WATMnet: a prototype wireless ATM system for multimedia personal communication, D. Raychaudhuri, et al.
- ^ "Cambridge Mobile ATM work". Archived from the original on 25 June 2015. Retrieved 10 June 2013.
- Black, Uyless D. (1998). ATM—Volume III: Internetworking with ATM. Toronto: Prentice Hall. ISBN 0-13-784182-5.
- De Prycker, Martin (1993). Asynchronous Transfer Mode. Solutions for Broadband ISDN. Prentice Hall.
- Joel, Amos E. Jr. (1993). Asynchronous Transfer Mode. IEEE Press.
- Golway, Tom (1997). Planning and Managing ATM Network. New York: Manning. ISBN 978-0-13-262189-2.
- McDysan, David E.; Darren L. Spohn (1999). ATM Theory and Applications. Montreal: McGraw-Hill. ISBN 0-07-045346-2.
- Neelakanta, P. S. (2000). A Textbook on ATM Telecommunications, Principles and implementation. CRC Press. ISBN 0-8493-1805-X.
- ATM Cell formats- Cisco Systems
- "Asynchronous Transfer Mode (ATM)". Cisco Systems. Archived from the original on 29 October 2007.
External links
- "ATM forum". Archived from the original on 1 July 2005.
- ATM Info and resources Archived 2 January 2013 at the Wayback Machine
- ATM ChipWeb - Chip and NIC database
- A tutorial from Juniper web site
- ATM Tutorial
- "Asynchronous Transfer Mode Switching". DocuWiki. Cisco Systems. Archived from the originalon 31 January 2018.