Packet Network Foundations 1 PART 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: Packet Broadband Network Handbook 2 Part 1: Packet Network Foundations P art I of this book introduces four widely deployed packet network technologies: X.25, frame relay, asynchronous transfer mode (ATM), and Internet protocol (IP). Before packet networks, communications technology used circuit- switched telephone networks with dedicated, analog circuits that func- tioned on a “always on once activated” basis. A dedicated circuit cannot be used for other purposes even if no communications are taking place at the moment. In regard to telephone conversations, it is estimated that on the average a dedicated circuit carried active traffic only 20 to 25 percent of the time and is idle the other 75 to 80 percent. Moreover, other services such as video data streams cannot be efficiently carried on circuit- switched networks. Packet networks based on packet switching technologies represent a radical departure. The key idea behind packet switching is that a mes- sage or a conversation is broken into independent, small pieces of information called packets that are either equal or variable in size. These packets are sent individually to a destination and are reassembled there. No physical resource is dedicated to a connection, and connections become virtual, thus allowing many users to share the same physical network resource. The concept of packet switching is attributed to Paul Baran who first outlined its principles in an essay published in 1964 in the journal On Dis- tributed Communications. The term packet switching itself was coined by Donald Davies, a physicist at the British National Physical Lab, who came up with the same packet switching idea independently. It is inter- esting to note that a few decades earlier, a similar discovery in physics by Albert Einstein—that waves of light can be broken into a stream of individual photons—led to the development of quantum mechanics. Packet networks allow more efficient use of network resources. Each packet occupies a transmission facility only for the duration of the transmission, leaving the facility available for other users when no trans- mission is taking place. Packet-switched networks are highly fault-tolerant. From the very start of their development, network survivability was a major design goal. Because packet networks do not rely on dedicated physical connec- tions, packets can be routed via alternative routes in case of an outage in the original communications link. Packet networks can support bandwidth on-demand and flexible bandwidth allocation. Bandwidth is allocated at the time of communi- cation, and the amount of bandwidth allocated is based on need. In Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Packet Network Foundations of ARTS Part 1: Packet Network Foundations contrast, a bandwidth of 64 Kbps is built into the infrastructure of cir- cuit-switched telephone networks. Since the very first packet-switched network ARPNET was built in 1969, many packet switching technologies have been developed. Among them, four have endured and achieved large-scale deployment: X.25, frame relay, ATM, and IP. The packet network technologies can be gener- ally divided into the two categories shown in Fig. P1-1: connection-ori- ented and connectionless. A connection-oriented packet network provides a virtual connection for a communications session between a source and a destination either on a permanent or a temporary basis. Packet networks of this category include X.25, frame relay, and ATM. Connectionless packet networks are represented by IP. In a classic IP network, packets of the same message may travel different routes and arrive at the destination out of order. The distinction between connec- tion-oriented and connectionless technologies is not absolute: Connec- tion-oriented packet networks such as ATM and X.25 can also provide connectionless service. In addition, the ubiquitous connectionless IP net- work is moving toward being connection-oriented via new IP network infrastructures such as multiprotocol label switching (MPLS), as will be seen in Part 4 of this book. 3 Figure P1-1 Packet network foundations. Packet-switched network Connection-oriented packet network Connectionless packet network X.25 Frame relay AT M IP Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Packet Network Foundations of ARTS Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Packet Network Foundations of ARTS X.25 Networks 1 CHAPTER 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: Packet Broadband Network Handbook 6 Part 1: Packet Network Foundations 1.1 Introduction X.25 is one of the very first standards to elevate packet networking tech- nology to the global level and lay the foundation for later comers like frame relay and ATM. This section, after providing some background information, introduces the X.25 network model and its components. 1.1.1 A Brief History X.25 is the first generation of public data network standards to serve as a successor to message switching networks and the first public switched data network (PSDN) in parallel to public switched telephone networks (PSTNs). X.25 standards were first defined in 1976 by the CCITT (since renamed the ITU-T). Two major revisions were made subsequently in 1980 and 1984. The X.25 has become synonymous with a set of standards that together define packet network technology: X.32, X.75, X.3, X.28, and X.29, although the X.25 specification itself merely defines an inter- face between user applications and an X.25 network edge switch. As used throughout this chapter, the term X.25 will be used to refer to the over- all X.25 network rather than a particular specification, unless explicitly noted otherwise. X.25 is still one of the most widely used connection-oriented packet networks with guaranteed quality of service (QoS). Its users are mostly business customers with widely dispersed and communications-inten- sive operations in sectors such as utilities, finance, insurance, retail, and transportation. An X.25 packet network can be either public or private. Many corpo- rations have determined that it is more economical to establish and use their own telecommunications facilities. In these cases, packet switches are obtained from network equipment providers, and private X.25 net- works are set up for the exclusive use of and administrated by specific organizations. 1.1.2 X.25 Network Reference Model As shown in Fig. 1-1, the X.25 network includes the functions of the bot- tom three layers of the open systems interconnection (OSI) network ref- erence model (Black 1994): the physical layer, the data link layer, and the Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25 Networks Chapter 1: X.25 Networks network layer. The X.25 standards focus on the network layer, but offer some specifications for the physical layer and the data link layer as well. 1.1.3 X.25 Network Components An X.25 network is made up of four types of network elements, with an analogue transmission line connecting them, as shown in Fig. 1-2. In this logical view of an X.25 network, functional components are speci- fied in the X.25 specification. Multiple functional components are often combined into one network device in an actual implementation of the X.25 network. For example, the data-terminal equipment (DTE) and the packet switching equipment (PSE) can be physically combined inside an X.25 switch. 1.1.3.1 PAD The packet assembler/disassembler (PAD) can be viewed as a special network interface provided for character-mode DTEs, such as terminals. When a user sends data to the network, the PAD interface takes a stream of data from a character-mode DTE and assembles it into packets to be sent to the network. At the receiving end, the PAD disas- sembles packets from the network into streams of data to be sent to a character-mode DTE. The PAD function is often implemented in soft- 7 X.25 protocol stack X.25 frame layer X.25 physical layer X.21, X.21 Bis Application layer Presentation layer Session layer Transport layer Network layer Data link layer Physical layer OSI reference model X.25 network reference model X.25 packet layer Figure 1-1 X.25 network reference model. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25 Networks 8 Part 1: Packet Network Foundations ware that is built into the same device as the DTE (ITU-T 1997a; ITU-T 1997b; ITU-T 2000a; ITU-T 2000b). 1.1.3.2 DTE Data-terminal equipment (DTE) is an interface point between a user equipment and an X.25 network, and it is implemented in a computer or computer-related device (ISO/IEC 1995; ITU-T 2000a). DTE devices such as networked computers are where user applications reside. DTEs are divided into packet-mode DTEs and character-mode DTEs. Packet-mode DTEs are typically computer systems that implement the X.25 protocol in hardware and software and are capable of sending and receiving packets. Character-mode DTEs are asynchronous devices, such as terminals and printers, that send or receive data one character at a time and require a PAD component to interact with other X.25 network components. 1.1.3.3 DCE Data-circuit-terminating equipment (DCE) is a network interface to packet-mode DTEs. The DTE-DCE interface represents the boundary between a user and a network, and a DCE device is often at the edge of a public data network. The DCE function is often built X.25 switch X.25 switch DTE user station PA D X. 25 modem Network host DTE Mainframe computer X.25 switch X.25 switch Figure 1-2 An X.25 network overview. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25 Networks Chapter 1: X.25 Networks into a X.25 switch located at the edge of a public X.25 network (ITU-T 1996). 1.1.3.4 PSE Packet-switching elements (PSEs) are packet switches con- nected over telecommunications facilities (phone lines, for example) in a PSDN. A main function of PSEs is to determine and pass packets to the next switch in a path. 1.2 Physical Layer of X.25 Networks The physical layer of the X.25 network deals with the transmission medium and provides procedural and functional interfaces between a DTE and a DCE. This layer is specified in the CCITT X.21, X.21-bis, and V.24 recommendations (ITU-T 1998): ITU-T Recommendation X.21 specifies the operations of digital circuitry. X.21, initially defined in 1976, specifies the digital signaling interface of how a DTE can set up and clear calls by exchanging signaling messages with DTE (ITU-T 1992). The X.21 interface operates over eight interchange circuits: signal ground, DTE common return, transmit, receive, control, indication, signal element timing, and byte timing. For example, a DTE uses specialized circuits like transmit and control to transmit data and control information. A DCE uses a specialized receiver and indication circuits for data and control information. The functions of the circuits are defined in recommendation X.24, and their electrical characteristics are defined in recommendation X.27. ITU-T Recommendation X.21-bis defines an analogue interface to support the access to digital circuit-switched networks using an analogue access line. X.21-bis provides procedures for sending and receiving addressing information to enable a DTE to establish switched circuits with other DTEs that have access to a digital network (ITU-T 1988). ITU-T Recommendation V.24 provides procedures to enable a DTE to operate over a leased analogue circuit that connects the DTE to a packet switching node or concentrator. 9 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25 Networks 10 Part 1: Packet Network Foundations The physical medium X.25 networks operate on can be either analog or digital transmission lines. One assumption for the X.25 protocols is that transmission facilities like analog lines are inherently unreliable and error-prone. The assumed maximum data rate is up to 64 Kbps. 1.3 Data Link Layer of X.25 Networks The data link layer of X.25 networks takes a bit stream received from the physical layer and presents to the packet layer a view of an error-free link to transmit packets. X.25 networks adopt the most commonly used high-level data link control (HDLC) protocol for data link layer. This sec- tion first provides a brief historical background of data link layer proto- cols, and then moves on to a detailed description of the HDLC frame fo r m at . 1.3.1 Overview of Link Layer Protocols The responsibilities of the data link layer for X.25 networks (as well as other networks) include the following (ISO/IEC 1997): Interfacing the physical layer to receive or send data in a bit stream Delineating the received bit stream into link layer frames Synchronizing the link to ensure that the receiver is in step with the transmitter Detecting transmission errors and recovering from such errors Identifying and reporting certain protocol errors to higher layers Since the early 1970s, the data link layer protocols have repeatedly evolved, and the industry has settled on a few that have achieved wide deployment. In the course of that evolution, data link layers themselves grew from being character-based to being bit-oriented, “character-based” meaning they handled one character at a time with a minimum unit of 8-bit characters. It was IBM in the early 1970s that developed the first bit- oriented data link layer protocol for data communication, called syn- chronous data link control (SDLC), which allowed the transfer of an arbitrary binary sequence of data without alignment at 8-character Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25 Networks [...]... DCE acknowledges receipt of the packet by sending a receive-ready packet 3 Then the data packet is forwarded through the public X.25 data network and delivered to the remote DCE The remote DCE passes the data packet to the remote DTE 4 The remote DTE acknowledges the receipt of the data packet with a receive-ready packet sent to the remote DCE, which in turns forwards the packet back to the local DCE... teller machine and then sent to the packet layer The packet layer creates packets out of blocks of data from the PAD by adding a packet header which includes , fields such as the virtual circuit number on which the packet should be sent, the packet type, and so on When the packet is complete, it is delivered to the data link layer which builds a frame from each packet after , adding a frame header... as given at the website X.25 Networks 20 Part 1: Packet Network Foundations 3 The clear-indication packet, in a similar fashion, gets forwarded back to the originating DTE, which clears the virtual circuit locally The SVC has been disconnected 1.4.4 Traffic and Congestion Control at Packet Layer In addition to the link layer flow control, the X.25 packet layer provides a packet sequence number-based... a significant performance impact if errors occur often 1.4 X.25 Packet Layer The data link layer of an X.25 network, after performing the data link layer processing, strips the frame header and passes the data units to the network layer also known as the packet- to -packet layer of X.25 networks , This section first introduces a generic packet format and then discusses the important concept of virtual... negotiation and packet retransmission This service allows a DTE to change the packet and window sizes for flow control that is used at the interface between a DTE and a local DCE The service of packet retransmission allows a DTE to initiate a retransmission of an unacknowledged data packet by issuing a DTE reject packet to the network, with a sequence number specified by the reject packet Throughput-class... to the Terms of Use as given at the website X.25 Networks 24 Part 1: Packet Network Foundations 8 In an intermediate X.25 switch, what is the highest layer of X.25 protocol stack that examines an incoming packet to determine how to switch the packet, the data link layer or the packet layer? , 9 Discuss one of the main reasons for X.25 to remain the network of choice for many large corporations in sectors... control mechanism between a source and a destination DTE The packet layer send-sequence number P(S) identifies the current packet with respect to the packet header sequence number modulus A receiver uses P(R ) in the acknowledge packet to indicate the send-sequence number of the next expected packet from the sender Note that this sequence number-based packet layer flow control is on a per call basis while... (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Byte 3 X.25 Networks 16 Part 1: Packet Network Foundations packet type and packet header length, with 01 indicating a data packet with a 3-octet header 1.4.1.2 Logical Channel Identifier The 12-bit logical channel identifier (LCI) identifies a virtual circuit (VC) and... DTE, , generates a clear-request packet and passes the request to the connected DCE 2 The DCE forwards the packet through the X.25 network to the remote DTE, and the packet arrives at the terminating DTE as a clear-indication packet The DTE clears the virtual circuit and returns any local resources to the available resource pool Then the DTE sends back a clear-indication packet Figure 1-5 Three phases... call-request packet and sends it to the local DCE 2 The request is forwarded through a X.25 public data network and eventually delivered to the remote DTE in the form of an X.25 incoming call packet 3 After validating the incoming call request and checking its own parameters, the remote DTE generates a call-accept packet to accept the call 4 The packet is passed through the public X.25 network and arrives . given at the website. Source: Packet Broadband Network Handbook 2 Part 1: Packet Network Foundations P art I of this book introduces four widely deployed packet network technologies: X.25, frame. be seen in Part 4 of this book. 3 Figure P1-1 Packet network foundations. Packet- switched network Connection-oriented packet network Connectionless packet network X.25 Frame relay AT M IP Downloaded. at the website. Source: Packet Broadband Network Handbook 6 Part 1: Packet Network Foundations 1.1 Introduction X.25 is one of the very first standards to elevate packet networking tech- nology