View
214
Download
0
Category
Tags:
Preview:
Citation preview
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
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.
Recommended