1. S6C6 – X.25 The “Grandfather” Technology

2. 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

3. 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

4. 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.

5. 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.

6. 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

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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)

13. 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 –Window parameters

14. 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

15. 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]

16. 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

17. VC Ranges Permanent virtual circuits (PVCs) Incoming-only circuits Two-way circuits Outgoing-only circuits

18. 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

19. Command Abbreviations i incoming t two-way o outgoing l low h high c circuit

20. 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.

21. Window Size Router(config-if)#x25 modulo modulus Router(config-if)#x25 win packets Router(config-if)#x25 wout packets