Asynchronous Transfer Mode

By Gilbert Held

Chapter 14 from Understanding Data Communications, 6^th Edition, published by New Riders Publishing

This chapter is focused on a rapidly evolving networking technology that provides the capability to transport voice, data, video, and images. Commonly referred to by the acronym ATM, Asynchronous Transfer Mode provides the opportunity for both end users and communications carriers to transport virtually any type of information using a common format. In this chapter, you will examine the rationale for the development of ATM and its underlying technology.

On This Page

Evolution
The Rationale and Underlying Technology
Architecture
Advantages of the Technology
Cell Routing
The ATM Protocol Reference Model
Service Definitions
LAN Emulation
What You Have Learned
Quiz for Chapter 14
About the Author

Evolution

Asynchronous Transfer Mode (ATM) represents a relatively recently developed communications technology designed to overcome the constraints associated with traditional, and for the most part separate, voice and data networks. ATM has its roots in the work of a CCITT (now known as ITU-T) study group formed to develop broadband ISDN standards during the mid-1980s. In 1988, a cell switching technology was chosen as the foundation for broadband ISDN, and in 1991, the ATM Forum was founded.

The ATM Forum represents an international consortium of public and private equipment vendors, data communications and telecommunications service providers, consultants, and end users established to promote the implementation of ATM. _To accomplish this goal, the ATM Forum develops standards with the ITU and other standards organizations.

The first ATM Forum standard was released in 1992. Various ATM Forum working groups are busy defining additional standards required to enable ATM to provide a communications capability for the wide range of LAN and WAN transmission schemes it is designed to support. This standardization effort will probably remain in effect for a considerable period due to the comprehensive design goal of the technol-ogy, which was developed to support voice, data, and video on both local and wide area networks.

The Rationale and Underlying Technology

ATM can be considered to represent a unifying technology because it was designed to transport voice, data, and video (including graphics images) on both local and wide area networks. Until the development of ATM, networks were normally developed based on the type of data to be transported. Thus, circuit-switched networks, which included the public switched telephone network and high-speed digital transmission facilities, were primarily used to transport delay-sensitive information, such as voice and video. In comparison, on packet-based networks, such as X.25 and Frame Relay, information can tolerate a degree of delay. Network users can select a networking technology to satisfy a specific communications application, but most organizations support a mixture of applications. Thus, most organizations are forced to operate multiple networks, resulting in a degree of inefficiency and escalating communications costs. By combining the features from both technologies, ATM enables a single network to support voice, data, and video.

ATM is designed to be scalable, enabling its 53-byte cell to be transported from LAN to LAN via WAN, as well as for use on public and private wide area networks at a range of operating rates. On LANs, ATM support is currently offered at 25 and 155Mbps, whereas access to WAN-based ATM carrier networks can occur at T1 (1.544Mbps), at T3 (45Mbps), or via different SONET facilities at data rates up to 622Gbps, all based on the transportation of 53-byte cells. A key to ATM's ubiquitous transmission capability is its fixed 53-byte cell length, which remains static regardless of changes in media, operating rates, or framing.

The use of a fixed-length cell enables low-cost hardware to be developed to perform required cell switching based on the contents of the cell header, without requiring more complex and costly software. Thus, ATM can be considered to represent a unifying technology that will eventually become very economical to implement when its development expenses are amortized over the growing production cycle of ATM communications equipment.

Although many organizations merged voice and data through the use of multiplexers onto a common circuit, this type of merger is typically not end-to-end. For example, traffic from a router connected to a LAN might be fed into a port on a high-speed multiplexer with another connection to the multiplexer from the company PBX. Although this type of multiplexing enables a common WAN circuit to be used for voice and data, it represents an interim and partial solution to the expense associated with operating separate voice and data networks. In addition, the emergence of multimedia applications requiring the transmission of video can wreak havoc with existing LANs and WANs due to their requirement for high bandwidth for short periods. ATM represents an emerging technology designed to provide support for bandwidth-on-demand applications, such as video, as well as voice and data. A comparison of the key features associated with each technology can give you an appreciation for ATM technology in comparison to conventional data communications- and telecommunications-based technology. Table 14.1 compares nine features of data communications and telecommunications networks with those of an ATM network.

In a data communications environment, the network can range in scope from a token-ring LAN to an X.25 or Frame Relay WAN. Thus, although some features are common to both LAN and WAN environments, there is also some variability. In general, a data communications network transports data by using variable-length packets. Although many WAN protocols are connection-oriented, some are connectionless. Similarly, many LAN protocols are connectionless, whereas others are connection-oriented. Because data communications networks were designed to transport files, records, and screens of data, transmission delay or latency, if small, does not adversely affect users. In comparison, in a telecommunications network, a similar amount of latency that is acceptable on a data network could wreak havoc with a telephone conversation. Recognizing the differences among voice, video, and data transportation, ATM was designed to adapt to the time sensitivity of different applications. It includes different classes of service that enable the technology to match delivery to the time sensitivity of the information it transports.

Table 14.1 Comparing Network Features

Thus, ATM provides a mechanism for merging voice, data, and video onto LANs and WANs. You can gain an appreciation for how ATM accomplishes this by learning about its architecture.

Architecture

ATM is based on the switching of 53-byte cells, in which each cell consists of a 5-byte header and a payload of 48 bytes of information. Figure 14.1 illustrates the format of the ATM cell, including the explosion of its 5-byte header to indicate the fields carried in the header.

Figure 14.1: The 53-byte ATM cell.
See full-sized image.

The 4-bit Generic Flow Control (GFC) field is used as a mechanism to regulate the flow of traffic in an ATM network between the network and the user. The use of this field is currently under development. As we will shortly note, ATM supports two major types of interfaces: Network-to-User (UNI) and Network-to-Network (NNI). When a cell flows from the user to the network or from the network to the user, it will carry a GFC bit value. However, when it flows within a network or between networks, the GFC field is not used. Instead of being wasted, its space can be used to expand the length of the Virtual Path Identifier field.

The 8-bit Virtual Path Identifier (VPI) field represents one half of a two-part connection identifier used by ATM. This field identifies a virtual path that can represent a group of virtual circuits transported along the same route. Although the VPI is eight bits long in a UNI cell, the field expands to 12-bit positions to fill the Generic Flow Control field in an NNI cell. It is described in more detail later in this chapter.

The Virtual Channel Identifier (VCI) is the second half of the two-part connection identifier carried in the ATM header. The 16-bit VCI field identifies a connection between two ATM stations communicating with one another for a specific type of application. Multiple virtual channels (VCs) can be transported within one virtual path. For example, one VC could be used to transport a disk backup operation, while a second VC is used to transport a TCP/IP-based application. The virtual channel represents a one-way cell transport facility. Thus, for each of the previously described operations, another series of VCIs is established from the opposite direction. You can view a virtual channel as an individual one-way end-to-end circuit, whereas a virtual path that can represent a collection of virtual channels can be viewed as a network trunk line. After data is within an ATM network, the VPI is used to route a common group of virtual channels between switches by enabling ATM switches to simply examine the value of the VPI. Later in this chapter, you will examine the use of the VCI.

The Payload Type Identifier (PTI) field indicates the type of information carried in the 48-byte data portion of the ATM cell. Currently, this 3-bit field indicates whether payload data represents management information or user data. Additional PTI field designators have been reserved for future use.

The 1-bit Cell Loss Priority (CLP) field indicates the relative importance of the cell. If this field bit is set to 1, the cell can be discarded by a switch experiencing congestion. If the cell cannot be discarded, the CLP field bit is set to 0.

The last field in the ATM cell header is the 8-bit Header Error Control field. This field represents the result of an 8-bit Cyclic Redundancy Check (CRC) code, computed only over the ATM cell header. This field provides the capability for detecting all single-bit errors and certain multiple-bit errors that occur in the 40-bit ATM cell header.

Advantages of the Technology

The use of cell-switching technology in a LAN environment provides some distinct advantages over the shared-medium technology employed by Ethernet, token-ring, and FDDI networks. Two of those advantages are obtaining full bandwidth access to ATM switches for individual workstations and enabling attaching devices to operate at different operating rates. Those advantages are illustrated in Figure 14.2, which shows an ATM switch that could be used to support three distinct operating rates. Workstations could be connected to the switch at 25Mbps, and a local server could be connected at 155Mbps to other switches either to form a larger local LAN or to connect to a communications carrier's network via a different operating rate.

The selection of a 53-byte cell length results in a minimum of latency in comparison to the packet length of traditional LANs, such as Ethernet, which can have a maximum 1526-byte frame length. Because the ATM cell is always 53 bytes in length, cells transporting voice, data, and video can be intermixed without the latency of one cell adversely affecting other cells. Because the length of each cell is fixed and the position of information in each header is known, ATM switching can be accomplished via the use of hardware. In comparison, on traditional LANs, bridging and routing functions are normally performed by software or firmware, which executes more slowly than hardware-based switching.

Figure 14.2: ATM is based on the switching of 53-byte cells.
See full-sized image.

Two additional features of ATM that warrant discussion are its asynchronous operation and its connection-oriented operation. ATM cells are intermixed via multiplexing, and cells from individual connections are forwarded from switch to switch via a single-cell flow. However, the multiplexing of ATM cells occurs via asynchronous transfer, in which cells are transmitted only when data is present to send. In comparison, in conventional time division multiplexing, keep-alive or synchronization bytes are transmitted when there is no data to be sent. Concerning the connection-oriented technology used by ATM, this means that a connection between the ATM stations must be established before data transfer occurs. The connection process results in the specification of a transmission path between ATM switches and end stations, enabling the header in ATM cells to be used to route the cells on the required path through an ATM network.

Cell Routing

The actual routing of ATM cells depends on whether a connection was pre-established or set up as needed on a demand basis. The pre-established type of connection is referred to as a Permanent Virtual Connection (PVC), and the other type is referred to as a Switched Virtual Connection (SVC). Examine the 5-byte ATM cell header shown in Figure 14.1 and note the VCI and VPI fields. The VPI is 8 bits in length, whereas the VCI is 16 bits in length, enabling 256 virtual paths of which each path is capable of accommodating up to 65,536 (2¹⁶) virtual connections.

By using VPs and VCs, ATM employs a two-level connection identifier that is used in its routing hierarchy. A VCI value is unique only in a particular VPI value, whereas VPI values are unique only in particular physical links. The VPI/VCI value assignment has only local significance, and those values are translated at every switch a cell traverses between endpoints in an ATM network. The actual establishment of a virtual path is based on ATM's network management and signaling operations. During the establishment of a virtual path routing table, entries in each switch located between endpoints map an incoming physical port and a Virtual Path Identifier pair to an outgoing pair. This initial mapping process is known as network provisioning, and the change of routing table entries is referred to as network reprovisioning.

Figure 14.3 illustrates an example of a few possible table entries for a switch, where a virtual path was established such that VPI=6 on port 1 and VPI=10 on port 8, representing two physical links in the established connection.

Figure 14.3: Switch operations based on routing table entries.
See full-sized image.

Next, examine the entries in the routing table shown in Figure 14.3, and note that the table does not include values for VCIs. This is by design because a VP in an ATM network can support up to 65,536 VC connections. Thus, only one table entry is required to switch up to 65,536 individual connections if those connections all follow the same set of physical links in the same sequence. This method of switching, which is based on the VPI and port number, simplifies the construction and use of routing tables and facilitates the establishment of a connection through a series of switches. Although VCIs are not used in routing tables, they are translated at each switch. To help you understand the rationale for this technique, you must focus on their use. As previously noted, a VCI is unique within a VP and is used at an endpoint to denote a different connection within a virtual path. Thus, the VPI/VCI pair used between an endpoint and a switch has a local meaning and is translated at every switch; however, the VCI is not used for routing between switches.

The establishment of a connection between two end stations is known as a Virtual Channel Connection (VCC). To illustrate the routing of cells in an ATM network based on a VCC, consider Figure 14.4, which represents a small two-switch–based ATM network. The VCC represents a series of virtual channel links between two ATM endpoints. In Figure 14.4, one VCC could be represented by VCI=1, VCI=3, and VCI=5, which collectively form a connection between workstations at the two endpoints shown in the network. A second VCC could be represented by VCI=2, VCI=4, and VCI=6. The second VCC could represent the transportation of a second application between the same pair of endpoints or a new application between different endpoints served by the same pair of ATM switches.

Figure 14.4: Connections in an ATM network.
See full-sized image.

As indicated by the previous examples, each VC link consists of one or more physical links between the location where a VCI is assigned and the location where it is either translated or removed. The assignment of VCs is the responsibility of switches during the call setup process.

The ATM Protocol Reference Model

Three layers in the ATM architecture form the basis for the ATM Protocol Reference model, illustrated in Figure 14.5. Those layers are the Physical layer, the ATM layer, and the ATM Adaptation layer.

Figure 14.5: The ATM protocol suite.
See full-sized image.

The Physical Layer

As indicated in Figure 14.5, the lowest layer in the ATM protocol is the Physical layer. This layer describes the physical transmission of information through an ATM network. It is not actually defined with respect to this new technology. The absence of a Physical layer definition results from the design goal of ATM to operate on various physical interfaces or media types. Thus, instead of defining a specific Physical layer, ATM depends on the Physical layers defined in other networking protocols. Types of physical media specified for ATM include shielded and unshielded twisted-pair, coaxial cable, and fiber-optic cable, which provide cell transport capabilities ranging from a T1 rate of 1.544Mbps to a SONET range of 622Mbps.

The ATM Layer

The ATM layer represents the physical interface between the ATM Adaptation layer (AAL) and the Physical layer. Thus, the ATM layer is responsible for relaying cells from the AAL to the Physical layer for transmission, and in the opposite direction from the Physical layer to the AAL for use in an endpoint. When transporting cells to the Physical layer, the ATM layer is responsible for generating the five-byte cell header for each cell. When receiving cells from the Physical layer, the ATM layer performs a reverse operation, extracting the five-byte header from each cell.

The actual manner by which the ATM layer performs its relaying function depends on the location of the layer at a switch or at an endpoint. If the ATM layer is located in an endpoint, it receives a stream of cells from the Physical layer and transmits either cells with new data or empty cells if there is no data to send to the AAL. When located in a switch, the ATM layer is responsible for determining where incoming cells are routed and for multiplexing cells by placing cells from individual connections into a single-cell stream.

The ATM Adaptation Layer

The ATM Adaptation layer (AAL) represents the top layer in the ATM Protocol model. This layer is responsible for providing an interface between higher-layer protocols and the ATM layer. Because this interface normally occurs based on a voice, data, or video application accessing an ATM network, the operations performed by the AAL occur at endpoints and not at ATM switches. Thus, the AAL is shown in Figure 14.5 to reside at ATM endpoints.

The primary function of the ATM Adaptation layer is format conversion. That is, the AAL maps the data stream originated by the higher-layer protocol into the 48-byte payload of ATM cells, with the header placement being assigned by the ATM layer. In the reverse direction, the AAL receives the payload of ATM cells in 48-byte increments from the ATM layer and maps those increments into the format recognized by the higher-layer protocol.

Because it is not possible to address the requirements of the diverse set of applications designed to use ATM within a single AAL, the ITU-T classified the functions required by different applications based on their traffic and service requirements. This classification scheme defined four classes of applications based on whether a timing relationship is required between end stations, the type of bit rate (variable or constant), and the type of connection (connection-oriented or connectionless) required. Table 14.2 summarizes the four classes of applications with respect to their timing relationship, bit rate, and type of connection.

In Table 14.2, note that the timing relationship references whether one is required between end stations. Real-time services, such as the transportation of voice or video, represent two examples of applications that require a timing relationship. In comparison, a file transfer represents an application that does not require a timing relationship. When a timing relationship is required, clocking between two end stations must be aligned.

A constant bit-rate application represents an application that requires an unvarying amount of bandwidth, such as voice or real-time video. In comparison, a variable bit-rate application represents "bursty" traffic, such as LAN data or transmission via a packet network.

The capability to support connection-oriented or connectionless applications enables ATM to support various existing higher-layer protocols. For example, Frame Relay is a connection-oriented protocol, whereas IP is a connectionless protocol. Through they use of different AALs, both can be transported by ATM.

Based on the four application classes, four different types of AALs were defined: AAL1, 2, 3/4, and 5. At one time, AAL3 and AAL4 were separate types; however, they had a sufficient degree of commonality to be merged. Figure 14.6 illustrates the relationship between application classes and ATM Adaptation layers with respect to the different parameters used to classify the application classes.

Table 14.2 The ATM Application Classes

Figure 14.6: Application classification and associated AALs.
See full-sized image.

ATM Adaptation layers are distinguished from one another based on the method by which the 48-byte cell payload constructed as a data stream generated by a higher-level protocol is passed to the ATM layer. For example, consider a Class A application represented by a voice conversation. Because misordering cells can be viewed as being worse than losing cells, the payload is constructed to include a sequence number when Class A traffic is transported. Figure 14.7 illustrates the format of an AAL1 cell payload. Note that the Sequence Number Protection (SNP) field protects the Sequence Number (SN) field from the effect of bit errors occurring during transmission, in effect providing a forward error detection and correction capability.

AAL1 is designated for transporting continuous bitrate (CRR) data, such as real-time voice and video traffic. The AAL1 specification defines the manner by which a continuous signal is transported in a sequence of individual ATM cells. As indicated in Figure 14.7, the first byte in the normal 48-byte cell payload is used for cell sequencing and protection of the sequence number, limiting the actual payload to 47 bytes per AAL1-generated cell. The AAL2 cell will eventually be used to transport packet video services and should be defined in the near future.

Figure 14.7: AAL 1 cell payload format.
See full-sized image.

AAL3 is designed to transport delay-insensitive user data, such as Frame Relay, X.25, or IP traffic. There is a high degree of probability that such data will have to be fragmented because the maximum payload of an ATM cell is 48 bytes. AAL3/4 uses four additional bytes beyond the cell header. The use of those bytes makes 44 bytes in the cell available for transporting the actual payload. In comparison, AAL5 uses all 48 bytes beyond the cell header to transport the payload, providing a minimum 10% enhanced throughput in comparison to AAL3/4.

Although several aspects of different AAL operations remain to be specified, the use of different AALs provides the mechanism for the cell-based switching technology on which ATM is based to transport different types of information using a common cell structure.

Service Definitions

Perhaps the major benefit of ATM is that it enables users to obtain a Quality of Service (QoS) for each class of service. The QoS represents a guaranteed level of service that can be based upon such parameters as peak cell rate (PCR), sustained cell rate (SCR), cell delay variation tolerance (CDVT), minimum cell rate (MCR), and burst tolerance (BT). Each of these parameters is used with other parameters to define one of the five classes of service for which a carrier may offer cell loss, cell delay, and bandwidth guarantees. Those classes of service include Continuous Bit Rate (CBR), Variable Bit Rate–Real Time (VBR–RT), Variable Bit Rate–Non-Real Time (VBR-NRT), Unspecified Bit Rate (UBR), and Available Bit Rate (ABR).

Continuous Bit Rate and Variable Bit Rate–Real Time services generally correspond to Class A and Class B services, respectively. Variable Bit Rate–Non-Real Time is a less time-stringent version of VBR–RT.

Both UBR and ABR services are for transporting delay-insensitive traffic, corresponding to Classes C and D. UBR represents a best-effort delivery mechanism for which cells can be discarded during periods of network congestion. In comparison, an ABR service is allocated all the bandwidth required by the application that is available on a connection, with a feedback mechanism employed to control the rate the originator transmits cells to minimize cell loss when available bandwidth contracts. Table 14.3 provides a summary of the five types of ATM services.

Table 14.3 ATM Services

Legend:

PCR = Peak Cell Rate
CDVT = Cell Delay Variation Tolerance
SCR = Sustained Cell Rate
BT = Burst Tolerance
MCR = Minimum Cell Rate

LAN Emulation

Although numerous advantages are associated with the use of ATM, its use in corporate and government offices causes a degree of interoperability problems when it's used to support legacy LANs, such as Ethernet and token-ring networks. Figure 14.8 illustrates the interoperability problems associated with using ATM as a backbone to interconnect legacy LAN switches. In Figure 14.8, note that ATM uses virtual path and virtual channel identifiers for addressing. In comparison, legacy LANs that include Ethernet use MAC addressing. Another difference between the two is that ATM is a connection-oriented protocol, whereas Ethernet is connectionless. This means there is no direct equivalent to a legacy broadcast transmission capability.

To obtain compatibility between ATM and legacy LANs, the ATM Forum developed a protocol called LAN Emulation (LANE). The goal of LANE is to provide a mechanism that enables ATM to interoperate with legacy LANs while hiding the ATM network from the legacy network. To accomplish this, the ATM LANE protocol emulates the characteristics of the legacy network.

LANE functions are performed on switches at the edge of an ATM network. As you might surmise, such switches are referred to as ATM edge devices. Four components are needed to provide LAN Emulation: a LAN Emulation Configuration Server (LECS), a LAN Emulation Server (LES), and a Broadcast and Unknown Server (BUS).

MAC addressing; connectionless Workstations

Figure 14.8: Interoperability problems associated with using ATM as a backbone to connect legacy LANs.
See full-sized image.

The Client

The functionality of an LEC is typically located in an ATM adapter card installed in a legacy switch. That card is configured with two addresses: an IEEE 48-bit MAC address and a 20-byte ATM address. The LEC is responsible for address resolution, data forwarding, and registration of MAC addresses with the LANE server (LES). It also communicates with other LECS via ATM virtual channel connections established across the ATM network.

The LECS

The LANE Configuration Server maintains a database of emulated LANs (ELANs) and the ATM address of LAN Emulation Servers (LESs) that control the ELANs. When a LANE client needs an ATM address, it first searches its connections, called Virtual Channel Connections (VCCs), it previously opened. The LEC maintains a translation table of destination MAC addresses mapped to VCCs. If the destination address is in the table, the LEC can use the existing VCC to send the message. If not, the LEC must perform an address resolution procedure using the LAN Emulation Address Resolution Protocol (LE-ARP). To do so, it queries the LECS, which returns the ATM address that serves the appropriate emulated LAN. The LEC then uses that address to query the LES. The LECS database is defined and maintained by the network manager or LAN administrator and represents the only manual process in the entire emulation process.

The LES

The LES represents a central control point for a predefined group of LECs. The LES maintains a point-to-multipoint Virtual Control Channel to all the LECs it controls. When the LEC queries the LES, the LES verifies that the LEC can joint the ELAN. Assuming it can, it examines the request of the LEC to resolve a MAC to ATM address by searching its tables for the appropriate ATM address that provides a path to the desired MAC address. Those tables are formed by LECs registering their ATM-to-MAC address translations with the LES. If the address is in the LES' cache memory, the LES returns the ATM address to the LEC that uses that address to establish an ATM connection. If the LES does not have that address in cache memory, it uses the services of the BUS.

The BUS

The Broadcast and Unknown Server (BUS) functions as a central point for transmitting broadcasts and multicast messages. It is required because ATM is a point-to-point connection-oriented technology that lacks a broadcast or one-to-many transmission capability. If the LES does not have the address required by the LEC, it uses the services of the BUS. That is, the BUS transmits an address resolution request to all stations that make up the ELAN, and the station that recognizes its own MAC address returns its ATM address. The LES updates its cache memory and returns the ATM address to the LEC. The LEC can then establish a connection across the ATM _network.

Although communications carriers have expended a significant amount of effort to develop an ATM infrastructure for transporting information between carrier offices, the expansion of this evolving technology to customer premises—as well as its common use on LANs—will probably take several years. This is because, as with any new technology, the cost of ATM equipment is relatively high in comparison to the cost of older technology. Over the next few years, you can expect several important standards to be promulgated, and you can also expect to see the cost of ATM equipment become more reasonable as development costs are amortized over a larger base of products. As this occurs, the use of ATM will expand considerably.

What You Have Learned

• ATM represents cell-switching technology designed to transport voice, data, and video by using a common cell format on both local and wide area networks.

• ATM represents a scalable technology for which 53-byte cells can be transported at a range of operating rates from 25Mbps on LANs to 622Gbps on SONET.

• Recognizing the differences between voice and data transportation requirements, ATM is designed to adapt to the time sensitivity of different applications.

• A two-part identifier consisting of a Virtual Path Identifier and Virtual Channel Identifier enables multiple connections to be carried on the same path.

• The ATM Protocol Reference model has three layers: the Physical layer, the ATM layer, and the ATM Adaptation layer.

• The routing of ATM cells between switches is based on Virtual Path Identifiers and port number routing table entries in the two switches.

• A Virtual Path Connection (VPC) represents a concatenation of virtual paths between switches; a Virtual Channel Connection (VCC) represents a connection between two end stations via a VPC.

• ATM users can obtain a guaranteed level of service referred to as a Quality of Service (QoS).

• LAN Emulation provides the mechanism for overcoming incompatibilities between ATM and legacy LAN addressing and their use of connection-oriented and connectionless operations.

Quiz for Chapter 14

1. What is The ATM forum?

   1. A government agency that regulates the use of ATM

   2. An international consortium that promotes the implementation of ATM

   3. A branch of the ITU-T involved in the ATM standardization effort

   4. A debating society involving ATM operations

2. ATM scalability references the capability of the technology to support what?

   1. Different cell lengths

   2. Different wide area networks

   3. Different local area networks

   4. A range of operating rates using the same old format

3. Latency is a measure of:

   1. LAN operations.

   2. Cell length.

   3. Transmission delay.

   4. WAN operations.

4. The Cell Loss Priority field indicates:

   1. The loss of a cell.

   2. The priority of a cell.

   3. Whether a cell can be discarded.

   4. The capability of a cell to be prioritized.

5. What is the function of the Header Error Control field in an ATM cell?

   1. To protect user data

   2. To correct user data

   3. To indicate the occurrence of any possible error in the cell header

   4. To indicate the occurrence of certain types of errors in the cell header

6. Which of the following is an advantage of using ATM over using conventional LANs?

   1. Shared medium access

   2. Full bandwidth access

   3. Support of fixed operating rates

   4. Direct sharing of the medium

7. What is the lowest layer in the ATM protocol?

   1. ATM Adaptation layer

   2. Convergence layer

   3. ATM layer

   4. Physical layer

8. Which layer generates the five-byte ATM cell header?

   1. ATM layer

   2. Physical layer

   3. ATM Adaptation layer

   4. Convergence layer

9. The routing of ATM cells between switches is based on the:

   1. Virtual Path Identifier.

   2. Port number.

   3. VPI and port number.

   4. Virtual Channel Identifier.

10. Which of the following is defined as a connection between two end stations?

    1. Virtual path

    2. Virtual Channel Connection

    3. Virtual link

    4. Virtual LAN

11. Which ATM layer is responsible for providing an interface to higher-layer _protocols?

    1. Physical layer

    2. Convergence layer

    3. ATM layer

    4. ATM Adaptation layer

12. Which ATM application class requires a timing relationship, a variable bit rate, and a connection-oriented type of connection?

    1. Class A

    2. Class B

    3. Class C

    4. Class D

13. A constant bit rate application represents an application that requires:

    1. An unvarying amount of bandwidth.

    2. File transfer capability.

    3. A varying amount of bandwidth.

    4. A connectionless type of connection.

14. Which type of ATM Adaptation layer is designed to transport delay-insensitive traffic?

    1. AAL1

    2. AAL2

    3. AAL3

    4. AAL4

15. The Quality of Service represents:

    1. A guaranteed cell rate.

    2. A guaranteed cell delay.

    3. A guaranteed burst tolerance.

    4. A guaranteed level of service.

16. Which LAN Emulation component has to be manually configured?

    1. The LAN Emulation Client

    2. The LAN Emulation Configuration Server

    3. The LAN Emulation Server

    4. The Broadcast and Unknown Server

17. Which of the following is the central control point for a group of LAN Emulation Clients?

    1. The LES

    2. The LECS

    3. The BUS

    4. The high-order MAC address

18. Which component of ATM LAN Emulation transmits broadcasts and multicast messages?

    1. The LES

    2. The LECS

    3. The BUS

    4. The low-order MAC address

About the Author

Gilbert Held is an internationally known award-winning lecturer and author. He is the author of more than 40 technical books and 300 articles covering the fields of personal computing and computer communications.

Copyright © 1999 by New Riders Publishing

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