X.25 Overview Next The Mnemonic In Network Stack Page 05% 01 Layer7: The application Layer interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer. Layer6: The Presentation layer establishes a context between application layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. Layer5: The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. Layer4: The Transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. Layer3: The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Layer2: The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. Originally, this layer was intended for point-to-point and point-to- multipoint media, characteristic of wide area media in the telephone system. Layer1: The Physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.

X.25 Overview Next What Is Protocol ? In Network Stack Page 10% 02

X.25 Overview Next Layers Protocols In Network Stack Page 15% 03

X.25 Overview Next Introduction to X.25 In Network Stack Page 20% 04 Layer TCP/IP suiteOSI suite #Name 7Application NNTP, SIP, SSI, DNS, FTP, Gopher, HTTP, NFS, NTP, SMPP, SMTP, SNMP, Telnet, FTAM, X.400, X.500, DAP 6PresentationMIME, XDR, SSL, TLS (Not a separate layer)ISO 8823, X.226 5Session Sockets. Session establishment in TCP. SIP. (Not a separate layer with standardized API.) ISO 8327, X.225 4TransportTCP, UDP, IPSec, PPTP, L2TPTP0, TP1, TP2, TP3, TP4 3NetworkIP, ARP, ICMP, DHCP, RIP, OSPF, BGP, IGMP, IS-ISX.25 (PLP), CLNP 2Data LinkPPP, SLIPX.25 (LAPB), Token Bus 1Physical X.25 (X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703) You can find this table with more details in

X.25 Overview Next X.25 Devices In X.25 Overview Page 25% 05 X.25 is an ITU-T standard protocol suite for connection to packet switched wide area networks using leased lines, the phone or ISDN system as the networking hardware. X.25 is designed to operate effectively regardless of the type of systems connected to the network. It is typically used in the packet-switched networks (PSNs) of common carriers, such as the telephone companies. Subscribers are charged based on their use of the network. The development of the X.25 standard was initiated by the common carriers in the 1970s. At that time, there was a need for WAN protocols capable of providing connectivity across public data networks (PDNs). X.25 is now administered as an international standard by the ITU-T.

X.25 Overview Next PAD In X.25 Overview Page 30% X.25 network devices fall into three general categories: 1.data terminal equipment (DTE) - Data terminal equipment devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers. 2. data circuit-terminating equipment (DCE) -DCE devices are communications devices, such as modems and packet switches, that provide the interface between DTE devices and a PSE, and are generally located in the carrier’s facilities. 3. packet-switching exchange (PSE) - PSEs are switches that compose the bulk of the carrier’s network. They transfer data from one DTE device to another through the X.25 PSN. 06

X.25 Overview Next PAD In X.25 Overview Page 35% 07

X.25 Overview Next Session Establishment In X.25 Overview Page 40% 08 The PAD is located between a DTE device and a DCE device, and it performs three primary functions: buffering (storing data until a device is ready to process it), packet assembly, and packet disassembly. The PAD buffers data sent to or from the DTE device. It also assembles outgoing data into packets and forwards them to the DCE device. (This includes adding an X.25 header.) Finally, the PAD disassembles incoming packets before forwarding the data to the DTE. (This includes removing the X.25 header.)

X.25 Overview Next X.25 Virtual Circuits In X.25 Overview Page 45% 09 X.25 sessions are established when one DTE device contacts another to request a communication session. The DTE device that receives the request can either accept or refuse the connection. If the request is accepted, the two systems begin full-duplex information transfer. Either DTE device can terminate the connection. After the session is terminated, any further communication requires the establishment of a new session.

X.25 Overview Next X.25 Protocol Suite In X.25 Overview Page 50% 10 A virtual circuit is a logical connection created to ensure reliable communication between two network devices. A virtual circuit denotes the existence of a logical, bidirectional path from one DTE device to another across an X.25 network. Physically, the connection can pass through any number of intermediate nodes, such as DCE devices and PSEs. Multiple virtual circuits (logical connections) can be multiplexed onto a single physical circuit (a physical connection). Virtual circuits are demultiplexed at the remote end, and data is sent to the appropriate destinations.

X.25 Overview Next X.25 Protocol Suite In X.25 Overview Page 55% 11

X.25 Overview Next X.25 Protocol Suite In X.25 Overview Page 60% 12 Two types of X.25 virtual circuits exist: 1.Switched virtual circuits (SVCs) -SVCs are temporary connections used for sporadic data transfers. They require that two DTE devices establish, maintain, and terminate a session each time the devices need to communicate. 2. Permanent virtual circuits(PVCs) -PVCs are permanently established connections used for frequent and consistent data transfers. PVCs do not require that sessions be established and terminated. Therefore, DTEs can begin transferring data whenever necessary because the session is always active.

X.25 Overview Next Packet Layer Protocol In X.25 Overview Page 65% 13

X.25 Overview Next LAPB In X.25 Overview Page 70% 14 PLP manages packet exchanges between DTE devices across virtual circuits. PLPs also can run over Logical Link Control 2 (LLC2) implementations on LANs and over Integrated Services Digital Network (ISDN) interfaces running Link Access Procedure on the D channel (LAPD).

X.25 Overview Next LAPB In X.25 Overview Page 75% 15 The PLP operates in five distinct modes: 1.Call Setup - Call setup mode is used to establish SVCs between DTE devices. A PLP uses the X.121 addressing scheme to set up the virtual circuit. The call setup mode is executed on a per-virtual- circuit basis, which means that one virtual circuit can be in call setup mode while another is in data transfer mode. This mode is used only with SVCs, not with PVCs. 2.Data Transfer -Data transfer mode is used for transferring data between two DTE devices across a virtual circuit. In this mode, PLP handles segmentation and reassembly, bit padding, and error and flow control. This mode is executed on a per-virtual-circuit basis and is used with both PVCs and SVCs. 3.Idle -Idle mode is used when a virtual circuit is established but data transfer is not occurring. It is executed on a per-virtual-circuit basis and is used only with SVCs.

X.25 Overview Next LAPB In X.25 Overview Page 80% 16 4.Call Clearing - Call clearing mode is used to end communication sessions between DTE devices and to terminate SVCs. This mode is executed on a per-virtual-circuit basis and is used only with SVCs. 5.Restarting -Restarting mode is used to synchronize transmission between a DTE device and a locally connected DCE device. This mode is not executed on a per-virtual-circuit basis. It affects all the DTE device’s established virtual circuits. Four types of PLP packet fields exist: General Format Identifier (GFI) -Identifies packet parameters, such as whether the packet carries user data or control information, what kind of windowing is being used, and whether delivery confirmation is required.

X.25 Overview Next LAPB In X.25 Overview Page 85% 17 Logical Channel Identifier (LCI) -Identifies the virtual circuit across the local DTE/DCE interface. Packet Type Identifier (PTI) -Identifies the packet as one of 17 different PLP packet types. User Data -Contains encapsulated upper-layer information. This field is present only in data packets. Otherwise, additional fields containing control information are added.

X.25 Overview Next The X.21bis Protocol In X.25 Overview Page 90% 18 LAPB is a data link layer protocol that manages communication and packet framing between DTE and DCE devices. LAPB is a bit-oriented protocol that ensures that frames are correctly ordered and error-free. Three types of LAPB frames exist: 1.Information(I-frame) -carries upper-layer information and some control information. I-frame functions include sequencing, flow control, and error detection and recovery. I-frames carry send- and receive- sequence numbers. 2.Supervisory(S-frame) -carries control information. S-frame functions include requesting and suspending transmissions, reporting on status, and acknowledging the receipt of I-frames. S-frames carry only receive- sequence numbers. 3.Unnumbered (U-frame) -The unnumbered frame (U frame) carries control information. U-frame functions include link setup and disconnection, as well as error reporting. U frames carry no sequence numbers.

X.25 Overview Next X.121 Address Format In X.25 Overview Page 90% 19

X.25 Overview Next In X.25 Overview Page 95% 20 The X.121 Address field includes the International Data Number (IDN), which consists of two fields: 1. Data Network Identification Code (DNIC): DNIC is an optional field that identifies the exact PSN in which the destination DTE device is located. This field is sometimes omitted in calls within the same PSN. The DNIC has two subfields: Country: subfield specifies the country in which the destination PSN is located. PSN: specifies the exact PSN in which the destination DTE device is located. 2. National Terminal Number (NTN): identifies the exact DTE device in the PSN for which a packet is destined. This field varies in length.

X.25 Overview Next In X.25 Overview Page 100% 21