Asynchronous Transfer Mode (ATM) is a technology designed for the high-speed transfer of voice, video, and data through public and private networks using cell relay technology. ATM is an International Telecommunication Union Telecommunication Standardization Sector (ITU-T) standard. Ongoing work on ATM standards is being done primarily by the ATM Forum, which was jointly founded by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint in 1991.
A cell switching and multiplexing technology, ATM combines the benefits of circuit switching (constant transmission delay, guaranteed capacity) with those of packet switching (flexibility, efficiency for intermittent traffic). To achieve these benefits, ATM uses the following features:
• Fixed-size cells, permitting more efficient switching in hardware than is possible with variable-length packets
• Connection-oriented service, permitting routing of cells through the ATM network over virtual connections, sometimes called virtual circuits, using simple connection identifiers
• Asynchronous multiplexing, permitting efficient use of bandwidth and interleaving of data of varying priority and size
The combination of these features allows ATM to provide different categories of service for different data requirements and to establish a service contract at the time a connection is set up. This means that a virtual connection of a given service category can be guaranteed a certain bandwidth, as well as other traffic parameters, for the life of the connection.
ATM Basics
To understand how ATM can be used, it is important to have a knowledge of how ATM packages and transfers information. The following sections provide brief descriptions of the format of ATM information transfer and the mechanisms on which ATM networking is based.
ATM Cell Basic Format
The basic unit of information used by ATM is a fixed-size cell consisting of 53 octets, or bytes. The first 5 bytes contain header information, such as the connection identifier, while the remaining
48 bytes contain the data, or payload (see in below figure). Because the ATM switch does not have to detect the size of a unit of data, switching can be performed efficiently. The small size of the cell also makes it well suited for the transfer of real-time data, such as voice and video. Such traffic is intolerant of delays resulting from having to wait for large data packets to be loaded and forwarded.
Figure: ATM Cell Basic Format
ATM Device Types
An ATM network is made up of one or more ATM switches and ATM endpoints. An ATM endpoint (or end system) contains an ATM network interface adapter. Workstations, routers, data service units (DSUs), LAN switches, and video coder-decoders (CODECs) are examples of ATM end systems that can have an ATM interface. Figure illustrates several types of ATM end systems—router, LAN switch, workstation, and DSU/CSU, all with ATM network interfaces—connected to an ATM switch through an ATM network to another ATM switch on the other side.
Figure: ATM Network Devices
ATM Network Interface Types
There are two types of interfaces that interconnect ATM devices over point-to-point links: the User-Network Interface (UNI) and the Network-Network Interface (NNI), sometimes called Network-Node Interface. A UNI link connects an ATM end-system (the user side) with an ATM switch (the network side). An NNI link connects two ATM switches; in this case, both sides are network.
UNI and NNI are further subdivided into public and private UNIs and NNIs, depending upon the location and ownership of the ATM switch. As shown in below figure, a private UNI connects an ATM endpoint and private ATM switch; a public UNI connects an ATM endpoint or private switch to a public switch. A private NNI connects two ATM switches within the same private network; a public NNI connects two ATM switches within the same public network. A third type of interface, the Broadband Inter-Carrier Interface (BICI) connects two public switches from different public networks.
Your ATM switch router supports interface types UNI and NNI, including the PNNI routing protocol. For examples of UNI and NNI.
Figure: ATM Network Interfaces
Above figure also illustrates some further examples of ATM end systems that can be connected to ATM switches. A router with an ATM interface processor (AIP) can be connected directly to the ATM switch, while the router without the ATM interface must connect to an ATM data service unit (ADSU) and from there to the ATM switch.
ATM Cell Header Formats
The ATM cell includes a 5-byte header. Depending upon the interface, this header can be in either UNI or NNI format. The UNI cell header, as depicted in below figure, has the following fields:
• Generic flow control (GFC)—provides local functions, such as flow control from endpoint equipment to the ATM switch. This field is presently not used.
• Virtual path identifier (VPI) and virtual channel identifier (VCI)—VPI identifies a virtual path leg on an ATM interface. VPI and VCI together identify a virtual channel leg on an ATM interface. Concatenating such legs through switches forms a virtual connection across a network.
• Payload type (PT)—indicates in the first bit whether the cell contains user data or control data. If the cell contains user data, the second bit indicates whether congestion is experienced or not, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame. If the cell contains control data, the second and third bits indicate maintenance or management flow information.
• Cell loss priority (CLP)—indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network.
• Header error control (HEC)—contains a cyclic redundancy check on the cell header.
Figure: ATM Cell Header—UNI Format
The NNI cell header format, depicted in below figure, includes the same fields except that the GFC space is displaced by a larger VPI space, occupying 12 bits and making more VPIs available for NNIs.
Figure: ATM Cell Header—NNI Format
ATM Services
There are three general types of ATM services:
• Permanent virtual connection (PVC) service—connection between points is direct and permanent. In this way, PVC is similar to a leased line.
• Switched virtual connection (SVC) service—connection is created and released dynamically. Because the connection stays up only as long as it is in use (data is being transferred), an SVC is similar to a telephone call.
• Connectionless service—similar to Switched Multimegabit Data Service (SMDS)
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Note Your ATM switch router supports permanent and switched virtual connection services. It does not support connectionless service.
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Advantages of PVCs are the guaranteed availability of a connection and that no call setup procedures are required between switches. Disadvantages include static connectivity and that they require manual administration to set up.
Advantages of SVCs include connection flexibility and call setup that can be automatically handled by a networking device. Disadvantages include the extra time and overhead required to set up the connection.
Virtual Paths and Virtual Channels
ATM networks are fundamentally connection oriented. This means that a virtual connection needs to be established across the ATM network prior to any data transfer. ATM virtual connections are of two general types:
• Virtual path connections (VPCs), identified by a VPI.
• Virtual channel connections (VCCs), identified by the combination of a VPI and a VCI.
A virtual path is a bundle of virtual channels, all of which are switched transparently across the ATM network on the basis of the common VPI. A VPC can be thought of as a bundle of VCCs with the same VPI value (see in figure ).
Figure : ATM Virtual Path And Virtual Channel Connections
• VPIs and VCIs are not addresses, such as MAC addresses used in LAN switching.
• VPIs and VCIs are explicitly assigned at each segment of a connection and, as such, have only local significance across a particular link. They are remapped, as appropriate, at each switching point.
Using the VCI/VPI identifier, the ATM layer can multiplex (interleave), de multiplex, and switch cells from multiple connections.
Point-to-Point and Point-to-Multipoint Connections
Point-to-point connections connect two ATM systems and can be unidirectional or bidirectional. By contrast, point-to-multipoint connections (see in below figure) join a single source end system (known as the root node) to multiple destination end-systems (known as leaves). Such connections can be unidirectional only, in which only the root transmits to the leaves, or bidirectional, in which both root and leaves can transmit.
Figure: Point-to-Point and Point-to-Multipoint Connections
AAL 5 does not have any provision within its cell format for the interleaving of cells from different AAL5 packets on a single connection. This means that all AAL5 packets sent to a particular destination across a particular connection must be received in sequence, with no interleaving between the cells of different packets on the same connection, or the destination reassembly process would not be able to reconstruct the packets.
This is why ATM AAL 5 point-to-multipoint connections can only be unidirectional; if a leaf node were to transmit an AAL 5 packet onto the connection, it would be received by both the root node and all other leaf nodes. However, at these nodes, the packet sent by the leaf could well be interleaved with packets sent by the root, and possibly other leaf nodes; this would preclude the reassembly of any of the interleaved packets.
Solutions
For ATM to interoperate with LAN technology, it needs some form of multicast capability. Among the methods that have been proposed or tried, two approaches are considered feasible (see in below Figure).
• Multicast server. In this mechanism, all nodes wishing to transmit onto a multicast group set up a point-to-point connection with an external device known as a multicast server. The multicast server, in turn, is connected to all nodes wishing to receive the multicast packets through a point-to-multipoint connection. The multicast server receives packets across the point-to-point connections, serializes them (that is, ensures that one packet is fully transmitted prior to the next being sent), and retransmits them across the point-to-multipoint connection. In this way, cell interleaving is precluded.
• Overlaid point-to-multipoint connections. In this mechanism, all nodes in the multicast group establish a point-to-multipoint connection with each other node in the group and, in turn, become a leaf in the equivalent connections of all other nodes. Hence, all nodes can both transmit to and receive from all other nodes. This solution requires each node to maintain a connection for each transmitting member of the group, while the multicast server mechanism requires only two connections. The overlaid connection model also requires a registration process for telling nodes that join a group what the other nodes in the group are, so that it can form its own point-to-multipoint connection. The other nodes also need to know about the new node so they can add the new node to their own point-to-multipoint connections.
Of these two solutions, the multicast server mechanism is more scalable in terms of connection resources, but has the problem of requiring a centralized resequencer, which is both a potential bottleneck and a single point of failure.
Figure: Approaches to ATM Multicasting
Applications
Two applications that require some mechanism for point-to-multipoint connections are:
• LAN emulation—in this application, the broadcast and unknown server (BUS) provides the functionality to emulate LAN broadcasts.
• Video broadcast—in this application, typically over a CBR connection, a video server needs to simultaneously broadcast to any number of end stations.
Operation of an ATM Switch
An ATM switch has a straightforward job:
1. Determine whether an incoming cell is eligible to be admitted to the switch (a function of Usage Parameter Control [UPC]), and whether it can be queued.
2. Possibly perform a replication step for point-to-multipoint connections.
3. Schedule the cell for transmission on a destination interface. By the time it is transmitted, a number of modifications might be made to the cell, including the following:
– VPI/VCI translation
– setting the Early Forward Congestion Indicator (EFCI) bit
– setting the CLP bit
The functions of UPC, EFCI, and CLP are discussed in "."
Because the two types of ATM virtual connections differ in how they are identified, as described in the “Virtual Paths and Virtual Channels”, they also differ in how they are switched. ATM switches therefore fall into two categories—those that do virtual path switching only and those that do switching based on virtual path and virtual channel values.
The basic operation of an ATM switch is the same for both types of switches: Based on the incoming cell's VPI or VPI/VCI pair, the switch must identify which output port to forward a cell received on a given input port. It must also determine the new VPI/VCI values on the outgoing link, substituting these new values in the cell before forwarding it. The ATM switch derives these values from its internal tables, which are set up either manually for PVCs, or through signaling for SVCs.
In below figure shows an example of virtual path (VP) switching, in which cells are switched based only on the value of the VPI; the VCI values do not change between the ingress and the egress of the connection. This is analogous to central office trunk switching.
Figure: Virtual Path Switching
In below figure shows an example of switching based on both VPI and VCI values. Because all VCIs and VPIs have only local significance across a particular link, these values get remapped, as necessary, at each switch. Within a private ATM network switching is typically based on both VPI and VCI values.
Figure: Virtual Path/Virtual Channel Switching
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Note Your Cisco ATM switch router performs both virtual path and virtual channel switching.
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