Date of Issue: 02-01-2023 Rate this Study Guide

X.25: An Oldie but a Goodie

by Howard Berkowitz

Origin and Design Assumptions
  Designed as Access/Signaling Protocol
  X.25 Stacks
  PtP or NBMA
  X.25 Addressing
    Persistent Addressing
    X.121 Extensions and Manipulations
    X.25 Transient/Channel Addressing
Frame Level (LAP-B) Protocol Operation
  Connection Establishment
  User Information Transfer
  Basic Interface Configuration
  Tuning and LAPB Parameters
    Modulo and Window Size (K Parameter)
    N1 Parameter: Number of Bits per Frame
    N2 Parameter: Transmit Failures before Declaring Link Down
    T1 Timer: Retransmission Delay
    LAPB Interface Outage
    T4 timer: Unsignaled Link Failure
X.25 Packet Layer Protocol Operation
  Connection Establishment
    The Strange Case of Fast Select
  Information Transfer
    The Difference between Frame and Packet Flow Control
    Fragmentation Mechanism
    Contemporary Fragmentation Application
  Global Decisions in Configuring the Packet Layer
    Tuning with LCN Values
  Interface-level Configuration Decisions - Basic Parameters
    Independence of DTE/DCE Status by Level
    X.25 Address
    X.25 Map
  X.25 Packet Layer Protocol Parameters
    Window and Packet Size
    RFC 1356 advises:
    Protocol Timers
Higher-Layer Protocol Family Mapping to X.25
    Pros and Cons: Single or Multiple Protocols per VC
    Multiplexing versus Multiple Virtual Circuits
  Higher Layer Address Mapping to X.121
X.25 Switching and Tunneling
  X.25 Routes
    Selection Options
    Disposition Options
  XOT Tunneling
Connecting to X.25 PDNs
  Closed User Groups
    Preferential CUGs and Actions on Incoming Calls
Associated Protocols
    The Lovely and Sneaky nvc Parameter

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