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This chapter provides a brief overview of ATM technology. It covers basic principles of ATM, along with the common terminology, and introduces key concepts you need to be familiar with when configuring ATM network equipment. If you already possess this basic knowledge, you can skip this chapter and go on to "ATM Signaling and Addressing."
This chapter includes the following sections:
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:
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.
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 Figure 1-1). 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.
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 1-2 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.
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 Figure 1-3, 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.
Figure 1-3 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.
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 Figure 1-4, has the following fields:
The NNI cell header format, depicted in Figure 1-5, 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.
There are three general types of ATM services:
Advantages of permanent virtual connections 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 switched virtual connections 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.
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 virtual path connection is a bundle of virtual channels (see Figure 1-6).
Using the VCI/VPI identifier, the ATM layer can multiplex (interleave), demultiplex, and switch cells from multiple connections.
Point-to-point connections connect two ATM systems and can be unidirectional or bidirectional. By contrast, point-to-multipoint connections (see Figure 1-7) 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.
Note that there is no mechanism here analogous to the multicasting or broadcasting capability common in many shared medium LAN technologies, such as Ethernet or Token Ring. In such technologies, multicasting allows multiple end systems to both receive data from other multiple systems, and to transmit data to these multiple systems. Such capabilities are easy to implement in shared media technologies such as LANs, where all nodes on a single LAN segment must necessarily process all packets sent on that segment. The obvious analog in ATM to a multicast LAN group would be a bidirectional multipoint-to-multipoint connection. Unfortunately, this obvious solution cannot be implemented when using AAL5, the most common ATM Adaptation Layer (AAL) used to transmit data across ATM networks.
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.
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 Figure 1-8).
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.
Two applications that require some mechanism for point-to-multipoint connections are:
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:
The functions of UPC, EFCI, and CLP are discussed in the chapter "ATM Technology Fundamentals."
ATM switches 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, VCI, or both, the switch must identify which output port to forward a cell received on a given input port, and it must 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 permanent virtual connections, or through signaling for switched virtual connections.
Figure 1-9 shows an example of virtual path 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 1-10 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.
The ATM architecture is based on a logical model, called the ATM reference model, that describes the functionality it supports. In the ATM reference model (see Figure 1-11), the ATM physical layer corresponds approximately to the physical layer of the OSI reference model, and the ATM layer and ATM adaptation layer (AAL) are roughly analogous to the data link layer of the OSI reference model.
The layers of the ATM reference model have the following functions:
Switched virtual connections work much like a telephone call. When you place a telephone call you must have the address (telephone number) of the called party. The calling party signals the called party's address and requests a connection. This is what happens with ATM switched virtual connections; they are set up using signaling and therefore require address information.
The types and formats of ATM addresses, along with their uses, are described in the chapter "ATM Signaling and Addressing."
ATM connections are further characterized by a traffic contract, which specifies a service category along with traffic and quality of service (QoS) parameters. Five service categories are currently defined, each with a purpose and its own interpretation of applicable parameters.
The following sections describe the components of the traffic contract, the characteristics of the service categories, and the service-dependent AAL that supports each of the service categories.
ATM provides five standard service categories that meet these requirements by defining individual performance characteristics, ranging from best effort (Unspecified Bit Rate [UBR]) to highly controlled, full-time bandwidth (Constant Bit Rate [CBR]). Table 1-1 lists each service category defined by the ATM Forum along with its applicable traffic parameters and QoS characteristics.
| Service Category | Traffic Parameters | QoS Characteristics | |
|---|---|---|---|
| Cell Loss | Cell Delay | ||
CBR---constant bit rate | PCR | low | low |
VBR-RT---variable bit rate real-time | PCR, SCR, MBS | low | low |
VBR-NRT---variable bit rate non-real time | PCR, SCR, MBS | low | unspecified |
ABR---available bit rate | PCR, MCR | unspecified | unspecified |
UBR---unspecified bit rate | (no guarantees) | unspecified | unspecified |
The characteristics and uses of each service category are summarized as follows:
The service-dependent AAL provides a set of rules for segmentation and reassembly of packets. The sender segments the packet and builds a set of cells for transmission, while the receiver verifies the integrity of the packet and reassembles the cells back into packets---all according to a set of rules designed to satisfy a particular type of service. Table 1-2 lists the four AAL types recommended by the ITU-T, along with the service categories commonly supported by each and the corresponding connection mode.
| AAL | Service Category | Connection Mode and Characteristics |
|---|---|---|
AAL1 | CBR | Connection-oriented; supports delay-sensitive services that require constant bit rates and have specified timing and delay requirements, such as uncompressed video. |
AAL2 | VBR | Connection-oriented; supports services that do not require constant bit rates, such as video schemes that use variable bit rate applications. AAL2 is presently an incomplete standard. |
AAL3/4 | UBR | Connectionless; mainly used for SMDS applications. |
AAL5 | ABR, UBR, VBR | Connection-oriented and connectionless; supports services with varying bit rate demands; offers low bandwidth overhead and simpler processing requirements in exchange for reduced bandwidth capacity and error-recovery capability. |
Table 1-3 shows the most commonly used physical interface types for ATM.
| Framing/Interface Type | Data Rate (Mbps) | Physical Media |
|---|---|---|
DS-1 T1 E1 |
1.544 2.048 |
twisted pair twisted pair and coaxial cable |
DS-3 T3 E3 |
44.736 34.368 |
coaxial cable coaxial cable |
ATM25 | 25.6 | UTP-3 |
4B/5B (TAXI) | 100 | multimode fiber |
SONET/SDH OC-3 STS-3c/STM-1 OC-12 OC-48 |
155.52 155.52 622.08 2488.32 |
multimode and single-mode fiber UTP-5 single-mode fiber single-mode fiber |
A physical interface on an ATM switch must support all three characteristics---framing type, data rate, and physical medium. As Table 1-3 shows, an OC-3 interface---the most commonly used one for ATM---can run over multimode or single-mode fiber. If you planned to use an OC-3 SM fiber link, you would need a physical interface (port adapter or interface module) that supports the SONET framing at 155.52 Mbps over single-mode fiber.
The choice of physical interface depends upon a number of variables, including bandwidth requirements and link distance. In general, UTP is used for applications to the desktop, multimode fiber between wiring closets or buildings, and SM fiber across long distances.
Posted: Mon Aug 16 14:09:41 PDT 1999
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