ATM technology is a telecommunications concept defined by international standards for carrying the full range of user traffic, including voice, data and video signals. It was developed to meet the needs of a digital network of broadband services and was originally designed for the integration of telecommunication networks. The ATM abbreviation stands for Asynchonous Transfer Mode and is translated into Russian as "asynchronous data transfer".
The technology was created for networks that need to handle both traditional high-performance data traffic (such as file transfer) and low-latency real-time content (such as voice and video). The reference model for ATM maps roughly to the three lower layers of ISO-OSI: network, data link, and physical. ATM is the primary protocol used over the SONET/SDH (public switched telephone network) and Integrated Services Digital Network (ISDN) circuits.
What is this?
What does ATM mean for a network connection? She providesfunctionality similar to circuit switching and packet switched networks: the technology uses asynchronous time division multiplexing and encodes data into small fixed size packets (ISO-OSI frames) called cells. This is different from approaches such as the Internet Protocol or Ethernet, which use variable-sized packets and frames.
The basic principles of ATM technology are as follows. It uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual communication can begin. These virtual circuits can be "permanent", that is, dedicated connections that are usually pre-configured by the service provider, or "switchable", that is, configurable for each call.
Asynchonous Transfer Mode (ATM stands for English) is known as the communication method used in ATMs and payment terminals. However, this use is gradually declining. The use of technology in ATMs has largely been superseded by the Internet Protocol (IP). In the ISO-OSI reference link (Layer 2), the underlying transmission devices are commonly referred to as frames. In ATM, they have a fixed length (53 octets or bytes) and are specifically called "cells".
Cell size
As noted above, ATM decryption is an asynchronous data transfer carried out by dividing them into cells of a certain size.
If the speech signal is reduced to packets, and theyare forced to be sent on a link with heavy data traffic, no matter what their sizes are, they will encounter large full-blown packets. Under normal idle conditions, they may experience maximum delays. To avoid this problem, all ATM packets or cells have the same small size. In addition, the fixed cell structure means that data can be easily transferred by hardware without the inherent delays introduced by software switched and routed frames.
Thus, ATM designers used small data cells to reduce jitter (in this case, delay dispersion) in the multiplexing of data streams. This is especially important when carrying voice traffic, since the conversion of digitized voice to analog audio is an integral part of the real-time process. This helps the operation of the decoder (codec), which requires a uniformly distributed (in time) stream of data elements. If the next in line is not available when needed, the codec has no choice but to pause. Later, the information is lost because the period of time when it should have been converted into a signal has already passed.
How did ATM develop?
During the development of ATM, the 155 Mbps Synchronous Digital Hierarchy (SDH) with 135 Mbps payload was considered a fast optical network, and many of the Plesiochronous Digital Hierarchy (PDH) links in the network were significantly slower (no more than 45 Mbps /With). Atat this rate, a typical full-size 1500-byte (12,000-bit) data packet should download at 77.42 microseconds. On a low-speed link such as a T1 1.544 Mbps line, it took up to 7.8 milliseconds to transmit such a packet.
The download delay caused by several such packets in the queue can exceed the number of 7.8 ms several times. This is unacceptable for voice traffic, which must have low jitter in the data stream fed into the codec to produce good quality audio.
The packet voice system can do this in several ways, such as using a playback buffer between the network and the codec. This smooths out jitter, but the delay that occurs when passing through the buffer requires an echo canceller, even on local networks. At the time it was considered too expensive. In addition, it increased the delay on the channel and made communication difficult.
ATM network technology inherently provides low jitter (and lowest overall latency) for traffic.
How does this help with network connection?
ATM design is for low jitter network interface. However, "cells" were introduced into the design to allow short queue delays while still supporting datagram traffic. ATM technology broke all packets, data, and voice streams into 48-byte fragments, adding a 5-byte routing header to each so that they could be reassembled later.
This choice of sizewas political, not technical. When CCITT (currently ITU-T) standardized ATM, the US representatives wanted a 64-byte payload as it was considered a good compromise between large amounts of information optimized for data transmission and shorter payloads designed for real-time applications.. In turn, developers in Europe wanted 32-byte packets because the small size (and therefore short transmission time) makes it easier for voice applications in terms of echo cancellation.
The size of 48 bytes (plus header size=53) was chosen as a compromise between the two parties. 5-byte headers were chosen because 10% of the payload was considered to be the maximum price to pay for routing information. ATM technology multiplexed 53-byte cells that reduced data corruption and latency by up to 30 times, reducing the need for echo cancellers.
ATM cell structure
ATM defines two different cell formats: user network interface (UNI) and network interface (NNI). Most ATM network links use UNIs. The structure of each such package consists of the following elements:
- The Generic Flow Control (GFC) field is a 4-bit field that was originally added to support ATM interconnection in the public network. Topologically, it is represented as a Distributed Queue Dual Bus (DQDB) ring. The GFC field has been designed so thatto provide 4 bits of User-Network Interface (UNI) to negotiate multiplexing and flow control among cells of different ATM connections. However, its usage and exact values have not been standardized and the field is always set to 0000.
- VPI - virtual path identifier (8 bit UNI or 12 bit NNI).
- VCI - virtual channel identifier (16 bits).
- PT - payload type (3 bits).
- MSB - network control cell. If its value is 0, a user data packet is used, and in its structure, 2 bits is Explicit Congestion Indication (EFCI) and 1 is Network Congestion Experience. In addition, 1 more bit is allocated for the user (AAU). It is used by AAL5 to indicate packet boundaries.
- CLP - cell loss priority (1 bit).
- HEC - header error control (8-bit CRC).
The ATM network uses the PT field to designate various special cells for operations, administration and management (OAM) purposes, and to define packet boundaries in some adaptation layers (AALs). If the MSB value of the PT field is 0, this is a user data cell and the remaining two bits are used to indicate network congestion and as a general purpose header bit available to adaptation layers. If the MSB is 1, it is a control packet and the remaining two bits indicate its type.
Some ATM (Asynchronous Data Transfer Method) protocols use the HEC field to control a CRC-based framing algorithm that can findcells at no additional cost. The 8-bit CRC is used to correct single-bit header errors and detect multi-bit ones. When the latter are found, the current and subsequent cells are discarded until a cell is found without header errors.
The UNI package reserves the GFC field for local flow control or sub-multiplexing between users. This was intended to allow multiple terminals to share a single network connection. This technology was also used to enable two integrated service digital network (ISDN) phones to share the same basic ISDN connection at a certain rate. All four GFC bits must be zero by default.
The NNI cell format replicates the UNI format in much the same way, except that the 4-bit GFC field is reallocated to the VPI field, expanding it to 12 bits. So one NNI ATM connection can handle almost 216 VCs each time.
Cells and transmission in practice
What does ATM mean in practice? It supports various types of services through AAL. Standardized AALs include AAL1, AAL2, and AAL5, as well as the less commonly used AAC3 and AAL4. The first type is used for constant bit rate (CBR) services and circuit emulation. Synchronization is also supported in AAL1.
The second and fourth types are used for variable bit rate (VBR) services, AAL5 for data. The information about which AAL is used for a given cell is not encoded in it. Instead, it is coordinated or adjusted toendpoints for each virtual connection.
After the initial design of this technology, networks have become much faster. A 1500-byte (12000 bit) full-length Ethernet frame takes only 1.2 µs to transmit on a 10 Gbps network, reducing the need for small cells to reduce latency.
What are the strengths and weaknesses of such a relationship?
The advantages and disadvantages of ATM network technology are as follows. Some believe that increasing the speed of communication will allow it to be replaced by Ethernet in the backbone network. However, it should be noted that increasing the speed by itself does not reduce jitter due to queuing. In addition, the hardware to implement service adaptation for IP packets is expensive.
At the same time, due to a fixed payload of 48 bytes, ATM is not suitable as a data link directly under IP, since the OSI layer on which IP operates must provide a maximum transmission unit (MTU) of at least 576 bytes.
On slower or congested connections (622 Mbps and below), ATM makes sense, and for this reason most asymmetric digital subscriber line (ADSL) systems use this technology as an intermediate layer between the physical link layer and Layer 2 protocol such as PPP or Ethernet.
At these lower speeds, ATM provides the useful ability to carry multiple logics on a single physical or virtual media, although there are other methods such as multi-channelPPP and Ethernet VLANs, which are optional in VDSL implementations.
DSL can be used as a way to access the ATM network, allowing you to connect to many ISPs through a broadband ATM network.
Thus, the disadvantages of the technology are that it loses its effectiveness in modern high-speed connections. The advantage of such a network is that it significantly increases the bandwidth, since it provides a direct connection between various peripheral devices.
In addition, with one physical connection using ATM, several different virtual channels with different characteristics can operate simultaneously.
This technology uses quite powerful traffic management tools that continue to develop at the present time. This makes it possible to transmit data of different types at the same time, even if they have completely different requirements for sending and receiving them. For example, you can create traffic using different protocols on the same channel.
Fundamentals of virtual circuits
Asynchonous Transfer Mode (abbreviation for ATM) operates as a link-based transport layer using virtual circuits (VCs). This is related to the concept of virtual paths (VP) and channels. Each ATM cell has an 8-bit or 12-bit Virtual Path Identifier (VPI) and a 16-bit Virtual Circuit Identifier (VCI),defined in its header.
VCI, together with VPI, is used to identify the next destination of a packet as it passes through a series of ATM switches on its way to its destination. The length of the VPI varies depending on whether the cell is sent over the user interface or over the network interface.
As these packets pass through the ATM network, the switch occurs by changing the VPI/VCI values (replacing tags). Although they do not necessarily match the ends of the connection, the concept of the scheme is sequential (unlike IP, where any packet can reach its destination by a different route). ATM switches use the VPI/VCI fields to identify the virtual circuit (VCL) of the next network that a cell must 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 the frame relay and the logical channel group number in X.25.
Another advantage of using virtual circuits is that they can be used as a multiplexing layer, allowing different services (such as voice and frame relay) to be used. VPI is useful for reducing the switching table of some virtual circuits that share paths.
Using cells and virtual circuits to organize traffic
ATM technology includes additional traffic movement. When the circuit is configured, each switch in the circuit is informed of the connection class.
ATM traffic contracts are part of the mechanismproviding "quality of service" (QoS). There are four main types (and several variants), each of which has a set of parameters that describe the connection:
- CBR - constant data rate. Specified Peak Rate (PCR) which is fixed.
- VBR - variable data rate. Specified average or steady state value (SCR), which can peak at a certain level, for the maximum interval before problems occur.
- ABR - available data rate. Minimum guaranteed value specified.
- UBR - undefined data rate. Traffic is distributed across the remaining bandwidth.
VBR has real-time options, and in other modes is used for "situational" traffic. Incorrect time is sometimes shortened to vbr-nrt.
Most traffic classes also use the concept of Cell Tolerance Variation (CDVT), which defines their "aggregation" over time.
Data transmission control
What does ATM mean given the above? To maintain network performance, traffic rules for virtual networks can be applied to limit the amount of data transferred at connection entry points.
The reference model validated for UPC and NPC is the Generic Cell Rate Algorithm (GCRA). As a rule, VBR traffic is usually controlled using a controller, unlike other types.
If the amount of data exceeds the traffic defined by GCRA, the network can either resetcells, or flag the Cell Loss Priority (CLP) bit (to identify the packet as potentially redundant). The main security work is based on sequential monitoring, but this is not optimal for encapsulated packet traffic (because dropping one unit will invalidate the entire packet). As a result, schemes such as Partial Packet Discard (PPD) and Early Packet Discard (EPD) have been created that are capable of discarding a whole series of cells until the next packet begins. This reduces the number of useless pieces of information on the network and saves bandwidth for complete packets.
EPD and PPD work with AAL5 connections because they use the end of the packet marker: the ATM User Interface Indication (AUU) bit in the Payload Type field of the header, which is set in the last cell of the SAR-SDU.
Traffic Shaping
The basics of ATM technology in this part can be represented as follows. Traffic shaping typically occurs at a network interface card (NIC) in the user equipment. This attempts to ensure that the cell flow on the VC will match its traffic contract, i.e. the units will not be dropped or reduced in priority at the UNI. Since the reference model given for traffic management in the network is GCRA, this algorithm is commonly used for shaping and routing data as well.
Types of virtual circuits and paths
ATM technology can create virtual circuits and paths asstatically as well as dynamically. Static circuits (STS) or paths (PVP) require the circuit to consist of a series of segments, one for each pair of interfaces it passes through.
PVP and PVC, although conceptually simple, require considerable effort in large networks. They also don't support service rerouting in case of failure. In contrast, dynamically built SPVPs and SPVCs are built by specifying the characteristics of a schema (a service "contract") and two endpoints.
Finally, ATM networks create and delete switched virtual circuits (SVCs) as required by the end piece of equipment. One application for SVCs is to carry individual telephone calls when a network of switches is interconnected via ATM. SVCs were also used in an attempt to replace ATM LANs.
Virtual routing scheme
Most ATM networks that support SPVP, SPVC, and SVC use the Private Network Node interface or the Private Network-to-Network Interface (PNNI) protocol. PNNI uses the same shortest path algorithm used by OSPF and IS-IS to route IP packets for exchange of topology information between switches and route selection through the network. PNNI also includes a powerful summarization mechanism that allows for the creation of very large networks, as well as a Call Access Control (CAC) algorithm that determines the availability of sufficient bandwidth along a proposed route through the network to meet the service requirements of a VC or VP.
Receiving and connecting tocalls
The network must establish a connection before both sides can send cells to each other. In ATM, this is called a virtual circuit (VC). This can be a permanent virtual circuit (PVC) that is administratively created at the endpoints, or a switched virtual circuit (SVC) that is created as needed by the transmitting parties. The creation of an SVC is controlled by signaling, in which the requestor specifies the address of the receiving party, the type of service requested, and any traffic parameters that may be applicable to the selected service. The Network will then confirm that the requested resources are available and that a route exists for the connection.
ATM technology defines the following three levels:
- ATM adaptations (AAL);
- 2 ATM, roughly equivalent to OSI data link layer;
- physical equivalent to the same OSI layer.
Deployment and distribution
ATM technology became popular with telephone companies and many computer manufacturers in the 1990s. However, even by the end of this decade, the best price and performance of Internet Protocol products began to compete with ATM for real-time integration and packet network traffic.
Some companies still focus on ATM products today, while others provide them as an option.
Mobile Technology
Wireless technology consists of an ATM core network with a wireless access network. Cells here are transmitted from base stations to mobile terminals. FunctionsMobilities are performed on an ATM switch in the core network, known as "crossover", which is analogous to the MSC (Mobile Switching Center) of GSM networks. The advantage of ATM wireless communication is its high throughput and high handover rate performed at layer 2.
In the early 1990s, some research laboratories were active in this area. The ATM forum was created to standardize wireless networking technology. It was backed by several telecommunications companies, including NEC, Fujitsu, and AT&T. ATM mobile technology aims to provide high-speed multimedia communications technologies capable of providing mobile broadband beyond GSM and WLAN networks.
