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S6C6 – X.25

S6C6 – X.25. The “Grandfather” Technology. Disadvantages – On-Demand Connections. Bandwidth is limited Connectivity is not constant Alternatives are: "always on," high-speed Internet connections using DSL or cable packet-switched network (PSN) services “always on" connections or

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S6C6 – X.25

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  1. S6C6 – X.25 The “Grandfather” Technology

  2. Disadvantages – On-Demand Connections • Bandwidth is limited • Connectivity is not constant • Alternatives are: • "always on," high-speed Internet connections using DSL or cable • packet-switched network (PSN) services • “always on" connections or • temporary "switched" circuits

  3. Packet Switched Networks • Can use same interface to send to several routers • Use shared transmission facilities to provide customers with cost-effective WAN services • Circuits are built on demand • Good for bursty traffic • Charge based on the duration of a call and the number of attempts

  4. Dedicated Links • Provide guaranteed bandwidth and constant connectivity • created by switching several of the provider's links to form an end-to-end path • often not the most efficient solution for a customer. • Can result in unused time or inadequate bandwidth • not efficient or cost-effective.

  5. Packet Switched Network Benefits • Offer customers a way to share facilities with other customers • Paths through the PSN are called virtual circuits • virtual circuit is a logical path, not a physical one • Frame Relay • VC information is called a data link control identifier (DLCI) • X.25 • VC information is called the logical channel identifier (LCI) and is included in the packet header.

  6. X.25 • Standard that defines the connection between a terminal and a PSN • An interface specification. • First to be deployed worldwide • Designed to transmit and receive data between alphanumeric "dumb" terminal • Now supports a variety of networking protocols, including TCP/IP, Novell IPX, and AppleTalk. • Used in developing countries • Used for legacy equipment • Continues to be the world's most common packet-switched technology

  7. X.25 Protocol Suite • Packet Layer Protocol (PLP) (L3) • Link Access Procedure, Balanced (LAPB) (L2) • Various physical-layer serial interfaces (L1) • (e.g., X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, and G.703) • high level of reliability • relatively high overhead • Typically implemented when supporting a legacy application or when more modern technologies are not available.

  8. X.25 Network Devices • Data terminal equipment (DTE) • Terminals, routers, or network hosts • Data circuit-terminating equipment (DCE) • modems and packet switches • For X. 25, DTE and DCE are independent of the plug-gender and clock-source definitions • Packet switching exchange (PSE) • compose the bulk of the carrier's network • transfer data from one DTE device to another through the X.25 PSN

  9. Packet Assembler/DisassemblerPAD • Located between a DTE device and a DCE device • Performs three primary functions: • buffering • packet assembly • packet disassembly • ITU-T Standards • X.3 - Specifies the parameters for terminal-handling functions • X.28 - Specifies the user interface for locally controlling a PAD • X.29 - Specifies a protocol for setting the X.3 parameters via a network connection

  10. Virtual Circuit (VC) • Logical channel identifier (LCI) • Virtual circuit number (VCN) • Logical channel number (LCN • Virtual channel identifier (VCI) • Permanent virtual circuit (PVC) or a switched virtual circuit (SVC). • Three phases for SVC • Call setup • Information transfer • Call clear

  11. X.25 Encapsulation • Layer 3 packet must include X.25 Packet Layer Protocol (PLP) • Layer 3 PLP header provides reliability through sequencing, and manages packet exchanges between DTE devices • virtual circuit information (the LCI) is carried in the Layer 3 header • Encapsulation occurs twice in an X.25 TCP/IP packet • once for the IP datagram • once for X.25 PLP • Cisco or IETF

  12. X.25 Addressing Standard • International data numbers, or IDNs vary in length and can be up to 15 decimal digits long. • DNIC --first four digits of an IDN are called the data network identification code • National terminal number (NTN) • identify the specific DTE on the PSN • Uses Mapping • an IP network layer address is mapped to an X.121address to identify the next-hop host • Must be configured manually • Layer 3 address mapped to layer 2 address (ARP)

  13. Configuring X.25Flow-controlled Protocol • Both ends of flow control must match • Define the X.25 encapsulation (DTE is the default). • Assign the X.121 address (usually supplied by the PDN service provider). • Define map statements to associate X.121 addresses with higher-level protocol addresses. • Optional parameters include • the number of VCs allowed • VC ranges • Packet sizes • Window parameters

  14. X.25 Configuration Steps • Define Encapsulation Type • Router(config-if)#encapsulation x25 [dte | dce] [ddn | bfe] | [ietf] • Configure X.25 address • Router(config-if)#x25 address x.121-address • Configure X.25 map • Router(config-if)#x25 map protocol address x.121-address [options] • common option used with this command is the broadcast keyword

  15. Configuring PVCs • configure the interface using the encapsulation x25 command. • assign an X.121 address using the x25address command • use the x25 pvc command instead of a map • Router(config-if)#x25 pvc circuit protocol address [protocol2 address2 [...[protocol9 address9]]] x121-address [options]

  16. PVC Example • Central(config)#interface serial 1 • Central(config-if)#encapsulation x25 • Central(config-if)#x25 address 311082194567 • Central(config-if)#ip address 10.60.8.1 255.255.248.0 • Central(config-if)#x25 pvc 4 ip 10.60.8.2 311082191234 broadcast

  17. VC Ranges • Permanent virtual circuits (PVCs) • Incoming-only circuits • Two-way circuits • Outgoing-only circuits

  18. DCE-DTE Rules • Only the DCE can initiate a call in the incoming-only range. • Only the DTE can initiate a call in the outgoing-only range. • Both the DCE and DTE can initiate a call in the two-way range. • Six X.25 parameters define the upper and lower limit of each of the three SVC ranges • Ranges can’t overlap

  19. Command Abbreviations • i incoming • t two-way • o outgoing • l low • h high • c circuit

  20. Packet Sizes • Router(config-if)#x25 ips bytes • Router(config-if)#x25 ops bytes • default byte value is 128 • Supported values are: 16,32, 64, 128, 256, 512, 1024, 2048, and 4096 • Fragmentation is a feature of X.25. The PAD willreassemble the IP packet at the destination.

  21. Window Size • Router(config-if)#x25 modulo modulus • Router(config-if)#x25 win packets • Router(config-if)#x25 wout packets

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