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

• temporary "switched" circuits

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

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.

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.

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

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.

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

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

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

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

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)

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

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

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]

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

VC Ranges

• Permanent virtual circuits (PVCs)

• Incoming-only circuits

• Two-way circuits

• Outgoing-only circuits

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

Command Abbreviations

• i incoming

• t two-way

• o outgoing

• l low

• h high

• c circuit

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.

Window Size

• Router(config-if)#x25 modulo modulus

• Router(config-if)#x25 win packets

• Router(config-if)#x25 wout packets