Download 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
• 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 packetswitched 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/Disassembler
PAD
• 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.25
Flow-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
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.121address [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 outgoingonly 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
t
o
l
h
c
incoming
two-way
outgoing
low
high
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