X25, Benefits of Packet Switched Networks

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X-25, Benefits of Packet Switched Networks

• PSNs, packet-switched networks, provide remote
  offices with either permanent or switched
  connections that feature high-levels of
  throughput (typically up to DS1).
• An important advantage of PSNs is that they offer
  customers a way to share facilities with other
  customers, thereby reducing the cost of WAN
  service.

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X-25, Benefits of Packet Switched Networks

• Paths through the PSN are called virtual
  circuits (VCs). A virtual circuit is a logical
  path, not a physical one.
• Virtual circuits make it possible for a remote
  site to maintain connections to multiple sites
  over the same physical interface.

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X-25, Benefits of Packet Switched Networks

• A site can send data directly to several other
  remote sites via different virtual circuits in
  the carrier network. This requires that the
  customer mark, or tag, each unit of data in some
  way so the provider's WAN switch can determine
  which destination to route the traffic through
  the cloud (refer to the figure).

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X-25, Benefits of Packet Switched Networks

• In frame relay networks, the VC information is
  called a data link control identifier (DLCI) and
  is included in the frame header. In X.25
  networks, the VC information is called the
  logical channel identifier (LCI) and is included
  in the packet header.

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X-25, Benefits of Packet Switched Networks

• PSNs allow providers to charge their customers on
  the basis of the number of packets transmitted.
  Because the customer can "pay as they go," PSNs
  can provide optimal cost-effectiveness.

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X-25, Benefits of Packet Switched Networks

• X.25 was one of the earliest packet switched
  technologies and the first to be deployed
  worldwide. In fact, since X.25 is still
  frequently used in developing countries and for
  legacy equipment, X.25 continues to be the
  world's most common packet-switched technology,
  and can be found in virtually every region that
  supports data communications.

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X.25

• X.25 is a standard that defines the connection
  between a terminal and a PSN. In other words,
  X.25 is an interface specification. It does not
  specify the characteristics of the PSN itself.
  Despite this, the networking industry commonly
  uses the term X.25 to refer to the entire suite
  of X.25 protocols.

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X.25

• Developed in the early 1970s, X.25 was designed
  to transmit and receive data between alphanumeric
  "dumb" terminals through analog telephone lines.
  X.25 enabled these terminals to remotely access
  applications on mainframes or minicomputers.
  Later, X.25's capability was expanded to support
  a variety of networking protocols, including
  TCP/IP, Novell IPX, and AppleTalk.

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X.25

• The X.25 suite of protocols includes Packet Layer
  Protocol (PLP), Link Access Procedure, Balanced
  (LAPB), and various physical-layer serial
  interfaces (e.g., X.21bis, EIA/TIA-232,
  EIA/TIA-449, EIA-530, and G.703). The figure maps
  the key X.25 protocols to the layers of the OSI
  reference model.

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X.25

• Both PLP and LAPB include mechanisms for error
  checking, flow control, and reliability. By
  including these mechanisms at both Layer 2 and
  Layer 3, X.25 provides a high level of
  reliability.
• If a network is built on unreliable circuits,
  error checking at the hardware level (the Data
  link layer) can handle transmission errors more
  efficiently than processes in software (the
  Network layer and above).

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X.25

• X.25 is "over-engineered" when implemented over
  modern WAN links. Newer technologies, such as
  Frame Relay, take advantage of lower error rates
  by providing a stripped-down, unreliable data
  link. Such technologies rely on error detection
  and correction in the upper layers (typically the
  Transport layer).

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X.25 Network Devices

• X.25 network devices fall into three general
  categories
• Data terminal equipment (DTE).
• Data circuit-terminating equipment (DCE).
• Packet switching exchange (PSE).

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X.25 Network Devices

• DTE devices are end systems that communicate
  across the X.25 network. They are usually
  terminals, routers, or network hosts, and are
  located on the premises of individual
  subscribers.
• DCE devices are communications devices such as
  modems and packet switches. They provide the
  interface between DTE devices and a PSE. X.25
  DCEs are typically located in the carrier's
  facilities.

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X.25 Network Devices

• 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.
  Figure illustrates the relationships between
  the three types of X.25 network devices.

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Packet Assembler/disassembler (PAD

• The packet assembler/disassembler (PAD) is a
  device commonly found in X.25 networks. PADs are
  used when a DTE device, such as a character-mode
  terminal, is too simple to implement the full
  X.25 functionality.
• The PAD is located between a DTE device and a DCE
  device, and it performs three primary functions

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Packet Assembler/disassembler (PAD

• buffering.
• packet assembly.
• 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
  operation includes adding an X.25 header.

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Packet Assembler/disassembler (PAD

• Finally, the PAD disassembles incoming packets
  before forwarding the data to the DTE. This
  includes removing the X.25 header.

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Packet Assembler/disassembler (PAD

• Some ITU-T recommendations defining the PAD are
  as follows
• X.3 - Specifies the parameters for
  terminal-handling functions (e.g., baud rate,
  flow control, character echoing, and other
  functions) for a connection to an X.25 host. The
  X.3 parameters are similar in function to Telnet
  options or attention (AT) command set for modems.

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Packet Assembler/disassembler (PAD

• X.28 - Specifies the user interface for locally
  controlling a PAD. X.28 identifies the keystrokes
  that you would enter at a terminal to set up the
  PAD, similar to the AT command set for modems.

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Packet Assembler/disassembler (PAD

• X.29 - Specifies a protocol for setting the X.3
  parameters via a network connection. When a
  connection is established, the destination host
  can request that the PAD or terminal change its
  parameters by using the X.29 protocol. A PAD
  cannot tell the destination host to change its
  X.3 parameters, but it can communicate that its
  own parameters were changed.

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Packet Assembler/disassembler (PAD

• X.75 - Specifies the gateway between the clouds.
  It defines the signaling system between two PDNs.
  X.75 is essentially a Network-to-Network
  Interface (NNI).
• When discussing X.25, the term virtual circuit
  (VC) is used interchangeably with the following
  terms logical channel identifier (LCI), virtual
  circuit number (VCN), logical channel number
  (LCN), and virtual channel identifier (VCI).

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Virtual Circuits

• X.25 connection can be a permanent virtual
  circuit (PVC) or, more commonly, a switched
  virtual circuit (SVC).
• A PVC is similar to a leased line. Both the
  network provider and the attached X.25 subscriber
  must provision the VC. PVCs use no call setup or
  call clear that is apparent to the subscriber.
  Any provisioned PVCs are always present, even
  when no data traffic is being transferred.

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Switched Circuits

• An SVC exists only for the duration of the
  session. Three phases are associated with X.25
  SVCs
• Call setup.
• Information transfer.
• Call clear.

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Switched Circuits

• The X.25 protocol offers simultaneous service to
  many hosts. An X.25 network can support
  configurations of multiple SVCs and PVCs over the
  same physical circuit attached to the X.25
  interface.
• Cisco routers provide numbering for up to 4095
  VCs per X.25 interface. VCs are identified using
  the LCI.

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X.25 Encapsulation

• Delivery of Network layer data through the
  internetwork usually involves encapsulation of
  Layer 3 packets inside Layer 2 frames.
• In an X.25 environment, an LAPB frame is used.

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X.25 Encapsulation

• In an X.25 WAN, the Layer 3 packet must include
  X.25 Packet Layer Protocol (PLP). The Layer 3 PLP
  header provides reliability through sequencing,
  and manages packet exchanges between DTE devices
  across virtual circuits.
• Layer 3 encapsulation occurs twice in an X.25
  TCP/IP packet once for the IP datagram and once
  for X.25 PLP.

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X.25 Encapsulation

• When configuring X.25 on a Cisco router's
  interface, you can choose between Cisco's
  encapsulation type and the Internet Engineering
  Task Force (IETF) type. The Cisco encapsulation
  method is the default.
• The Layer 3 X.25 header is made up of the
  following components

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X.25 Encapsulation

• A general format identifier (GFI) - The GFI is
  4-bit field that indicates the general format of
  the packet header.
• A logical channel identifier (LCI) - The LCI is a
  12-bit field that identifies the virtual circuit.
  The LCI is locally significant at the DTE/DCE
  interface.
• A packet type identifier (PTI) - The PTI field
  identifies one of X.25's 17 packet types.

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X.25 Encapsulation

• Thus, in an X.25 environment, the virtual circuit
  information (the LCI) is carried in the Layer 3
  header. An end-to-end virtual circuit is
  established in the PSN via two logical channels,
  each with an independent LCI on two DTE/DCE
  interfaces.

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X.121, the X.25 Addressing Standard

• Addressing fields in PLP call setup packets
  provide source and destination DTE addresses.
  These are used to establish the virtual circuits
  that constitute X.25 communication.

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X.121, the X.25 Addressing Standard

• ITU-T recommendation X.121 specifies the source
  and destination address formats. X.121 addresses
  (also referred to as international data numbers,
  or IDNs) vary in length and can be up to 15
  decimal digits long.

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X.121, the X.25 Addressing Standard

• The first four digits of an IDN are called the
  data network identification code (DNIC). The DNIC
  is divided into two parts, the first three digits
  specifying the country and the last digit
  specifying the PSN itself.
• The remaining digits are called the national
  terminal number (NTN) and are used to identify
  the specific DTE on the PSN.

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X.121, the X.25 Addressing Standard

• For your specific DNIC code, consult your service
  provider. For a listing of ITU-T country code
  assignments, refer to the ITU-T recommendation
  X.121.

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X.121, the X.25 Addressing Standard

• For different network protocols to connect across
  X.25, mapping statements are entered on the
  router. These statements map the next-hop Network
  layer address to an X.121 address (refer to
  Figure ). For example, an IP network layer
  address is mapped to an X.121 address to identify
  the next-hop host on the other side of the X.25
  network.

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X.25 Configuration

• When you select X.25 as a WAN protocol, you must
  set appropriate interface parameters. The
  interface configuration tasks include
• Define the X.25 encapsulation (DTE is the
  default). .

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X.25 Configuration

• 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.

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X.25 Configuration

• Other configuration tasks can be performed to
  control data throughput and to ensure
  compatibility with the X.25 network service
  provider. Commonly used parameters include the
  number of VCs allowed and packet size negotiation.

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X.25 Configuration

• X.25 is a flow-controlled protocol. The default
  flow-control parameters must match on both sides
  of a link. Mismatches because of inconsistent
  configurations can cause severe internetworking
  problems.
• Before configuring X.25 parameters, you should
  enter interface configuration mode and assign a
  higher-layer address, such as an IP address to
  the interface.

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X.25 Configuration

• The following sections describe X.25 SVC
  configuration, X.25 PVC configuration, and
  optional configurations including
• VC ranges.
• Packet sizes.
• Window parameters.

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Configuring X.25 SVCs

• To activate X.25 on an interface, you must enter
  the encapsulation x25 command to specify the
  encapsulation type to be used
• Router(config-if)encapsulation x25 dte dce
  ddn bfe ietf.
• The router can be an X.25 DTE device, which is
  typically used when the X.25 PSN is used to
  transport various protocols.

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Configuring X.25 SVCs

• The router can also be configured as an X.25 DCE
  device, which is typically used when the router
  acts as an X.25 switch.
• You can choose between two encapsulation methods
  Cisco and IETF. The default is Cisco and is not
  specified by a keyword.

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Configuring X.25 SVCs

• The x25 address command defines the local
  router's X.121 address. Only one address per
  interface can be defined (refer to Figure ). The
  value specified must match the address designated
  by the X.25 PDN
• Router(config-if)x25 address x.121-address

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Configuring X.25 SVCs

• The x25 map command provides a static map of
  higher-level addresses to X.25 addresses. The
  command maps the network layer addresses of the
  remote host to the X.121 address of the remote
  host
• Router(config-if)x25 map protocol address
  x.121-address options

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Configuring X.25 PVCs

• When configuring a PVC, you must configure the
  interface using the encapsulation x25 command.
  You must also assign an X.121 address using the
  x25 address command. These tasks are the same,
  whether you are configuring an SVC or a PVC.
• However, instead of using the x25 map command to
  establish a PVC, you use the x25 pvc command.

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Configuring X.25 PVCs

• PVCs are the X.25 equivalent of leased lines
  they are never disconnected. You do not need to
  configure an address map before defining a PVC
  because the x25 command does the mapping for you,
  as follows
• Router(config-if)x25 pvc circuit protocol
  address protocol2 address2 ...protocol9
  address9 x121-address options.

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Configuring X.25 PVCs

• Multiple protocols can be routed on the same PVC.
  Multiple circuits can also be established on an
  interface by creating another PVC.