Home > Articles > Networking

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

2.4 Equal-Sized Packets Model: ATM

A networking model in which packets are of equal size can be constructed. Equalsized packets, or cells, bring a tremendous amount of simplicity in the networking hardware, since buffering, multiplexing, and switching of cells become extremely simple. However, a disadvantage of this kind of networking is the typically high overall ratio of header to data. This issue normally arises when the message size is large and the standard size of packets is small. As discussed in Section 1.3, the dominance of headers in a network can cause delay and congestion. Here, we describe Asynchronous Transfer Mode technology as an example of this model.

The objective of Asynchronous Transfer Mode (ATM) technology is to provide a homogeneous backbone network in which all types of traffic are transported with the same small fixed-sized cells. One of the key advantages of ATM systems is flexible multiplexing to support multiple forms of data. ATM typically supports such bursty sources as FAX, coded video, and bulk data. Regardless of traffic types and the speed of sources, the traffic is converted into 53-byte ATM cells. Each cell has a 48-byte data payload and a 5-byte header. The header identifies the virtual channel to which the cell belongs.

Similar to a telephone network, ATM is a set of connection-oriented protocols, which means that a connection must be preestablished between two systems in a network before any data can be transmitted. ATM is capable of supporting and integrating data, voice, and video over one transmission medium with high bit data rate delivery services into a single network. ATM is bandwidth-on-demand networking and is scalable in bandwidth with the ability to support real multimedia applications.

The use of fixed-size cells can greatly reduce the overhead of processing ATM cells at the buffering and switching stages and hence increase the speed of routing, switching, and multiplexing functions. However, the great ratio of header to data makes the technology unsuitable for wide area networks and limits its applications in small networks. Like IPv6, ATM supports QoS mainly to reserve resources that guarantee specified maximum delay, minimum throughput, and maximum data loss. The QoS support allows ATM to concurrently handle all kinds of traffic.

ATM connections are identified by a virtual channel identifier (VCI) and a virtual path identifier (VPI). VCI and VPI are combined to be used in a switch to route a cell. As shown in Figure 2.11, the identity of a "physical" link is identified by two "logical" links: virtual channel (VC) and virtual path (VP). When a connection is set up, the values of these identifiers remain unchanged for the lifetime of the ATM connection.

Figure 2.11

Figure 2.11 Overview of a typical ATM transmission medium

Example. Figure 2.12 shows a routing table in an ATM switch, with routing information for all active connections passing through the switch. The routing information consists of the new VPI/VCI and new outgoing link for every incoming VC. Link 5, with VPIs 1, 3, and 5, can be switched on link 10 with VPIs 3, 7, and 8 through the ATM switch. A routing table provides the detail of the switching function. For example, a cell with VPI 3 and VCI 9 on link 5 is set to be forwarded with VPI 7 and VCI 2 on link 10.

Figure 2.12

Figure 2.12 A routing table in an ATM switch

2.4.1 ATM Protocol Structure

The ATM protocol structure is shown in Figure 2.13. The three-dimensional model includes four layers in the vertical dimension. The tightly linked layers consist of the physical layer, the ATM layer, the ATM adaptation layer (AAL), and higher layers. The physical layer includes two sublayers: the physical medium and transmission convergence. The physical medium sublayer defines the physical and electrical/optical interfaces with the transmission media on both the transmitter and the receiver. This layer also provides timing information and line coding. The transmission convergence sublayer provides frame adaptation and frame generation/recovery.

Figure 2.13

Figure 2.13 ATM protocol reference model

The ATM layer provides services, including cell multiplexing and demultiplexing, generic flow control, header cell check generation and extraction, and most important, remapping of VPIs and VCIs. The AAL layer maps higher-layer service data units, which are fragmented into fixed-size cells to be delivered over the ATM interface. In addition, this layer collects and reassembles ATM cells into service data units for transporting to higher layers. The four types of AALs support different classes of services.

  1. AAL1 supports class A traffic, the required timing between a transmitter and a receiver, and the constant bit rate (CBR) traffic.
  2. AAL2 supports class B traffic and time-sensitive—between source and sink—but variable bit rate (VBR) data traffic.
  3. AAL3/4 supports class C or class D traffic and VBR data traffic.
  4. AAL5 supports class D traffic in which VBR traffic can be transported and no timing relationship between source and sink is required.

The higher layers incorporate some of the functionality of layers 3 through 5 of the TCP/IP model. The control plane at the top of the cube shown in Figure 2.13 involves all kinds of network signaling and control. The user plane involves the transfer of user information, such as the flow-control and error-control mechanisms. The management plane provides management function and an information-exchange function between the user plane and the control plane. The management plane includes (1) plane management that performs management and coordination functions related to a system as a whole, and (2) the layer management that monitors bit error rates on a physical communications medium.

An ATM network can support both a user-network interface (UNI) and a network-node interface (NNI). A UNI is an interface connection between a terminal and an ATM switch, whereas an NNI connects two ATM switches. A summary of these two interfaces is shown in Figure 2.14. If privately owned switches are in the network, the interface between the public and private parts of the network is called the public NNI, and the connection between two private switches is known as the private NNI (P-NNI).

Figure 2.14

Figure 2.14 Overview of signaling: (a) UNI format; (b) NNI format

2.4.2 ATM Cell Structure

An ATM cell has two parts: a 48-byte payload and a 5-byte header, as shown in Figure 2.15. The choice of a 48-byte payload was a compromise among various design teams considering two important factors: packetization delay and transmission efficiency. (Refer back to Section 1.3 on packet size optimization.) The header consists of several fields. However, the ATM cell header has two different formats: UNI and NNI. The details of the UNI 5-byte header are as follows.

  • The 4-bit generic flow control (GFC) field is used in UNI only for controlling local flow control. This field enables the participating equipment to regulate the flow of traffic for different grades of service. Two modes are defined for this field: the controlled GFC, to provide flow control between a user and a network, and the uncontrolled GFC, to indicate that the GFC function is not used.
  • Together, the virtual path identifier and the virtual channel identifier represent an ATM address. A VPI identifies a group of virtual channels with the same end point. A VCI identifies a virtual channel within a virtual path.
  • The 3-bit payload type field is used to indicate the type of data located in the payload portion of the cell: for example, 000, the current cell is a data cell and no congestion is reported; 010, this is a user data cell, and congestion is experienced. The payload could be congestion information, network management message, signaling information, or other forms of data.
  • The 1-bit cell-loss priority (CLP) field is used to prioritize cells. When congestion occurs, cells with CLP set to 1 (considered low priority) are discarded first. If the bit is set to 0, the cell gets higher priority and should be discarded only if it could not be delivered.
  • The 8-bit header error control (HEC) field is used for error checking. HEC functions include correcting single-bit errors and detecting multiple-bit errors.
Figure 2.15

Figure 2.15 An ATM cell and its header structure

The main difference between NNI and UNI formats is that the 4 bits used for the GFC field in the UNI cell header are added to the VPI field in the NNI cell header. Thus, for NNI, VPI is 12 bits, allowing for more VPs to be supported within the network. VCI is 16 bits for both cases of UNI and NNI. The values of VPI and VCI have local significance only with a transmission link. Each switching node maps an incoming VPI/VCI to an outgoing VPI/VCI, based on the connection setup or routing table, as shown in Figure 2.12.

HEC works like other checking methods. First, the transmitter calculates the HEC field value, and the receiver side runs an algorithm consisting of two modes of operation. At initialization step, this field starts with an error-correction mode. If a single-bit error is detected in the header, the error-correction algorithm identifies the error bit and then corrects it. If a multibit error is detected, the mode moves to detection mode; errors are discarded but not corrected. Error-detection mode remains whenever cells are received in error, moving back to correction mode only when cells are received without error.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020