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Chapter 17 Homework

Chapter 17 Homework. 1,7,15,28, 35, 36. Chapter 17 X.25. X.25 Layers X.25 Operation. X.25. X.25 : A packet switching WAN developed by ITU in 1976 ITU recommends packet size of 128 bytes It can support larger packets up to 1024 bytes Normal speed is 64 Kbps .

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Chapter 17 Homework

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  1. Chapter 17Homework • 1,7,15,28, 35, 36

  2. Chapter 17X.25 • X.25 Layers • X.25 Operation

  3. X.25 • X.25 : A packet switching WAN developed by ITU in 1976 • ITU recommends packet size of 128 bytes • It can support larger packets up to 1024 bytes • Normal speed is 64 Kbps. • European standards support 2.048 Mbps • It provides connection oriented service • Data transmission rate are fixed

  4. X.25 (continued) • User machines (terminal or computer) are called Data Terminal Equipment (DTE) • Packet switching nodes are called (Data Communications Equipment or Data Circuit-terminating Equipment (DCE)) • A device that establishes, maintains and terminates a session on a network. It may also convert signals for transmission.

  5. X.25 (continued) • All control packets and data are carried on the same channel. (inband signaling used) • Intermediate nodes must process control data as well. • X.25 functions are implemented at 3 layers. • Both layer 2 and layer 3 include error control and flow control functions. • Multiplexing of VCs takes place at layer 3.

  6. X.25 Implementation • X.25 is implemented at three layers. • Physical layer • Data link layer (Frame Layer) • Packet (network) layer

  7. Physical Layer • Standards (called X.21) provides for: • Electrical, mechanical, procedural interface between the host and the X.25 network. • It requires digital signaling

  8. Data Link Layer(Frame Layer (LAPB)) • Link layer standards called (link access procedure- balanced) provide: • Error and flow control functions for the connection between DTE and DCE.

  9. Packet (network) layer • Receives user data from the higher layers • Link setup between DTE and DCE • Transferring data • Disconnecting the Link • Establishing virtual connections between DTEs. (end-to-end connection)

  10. Virtual Circuits in X.25 Networks • Created at the network layer. • Identified by virtual circuit identifier (called logical channel number LCN) • Permanent virtual circuit (PVC) • No call setup is required • Analogous to leased line • Switched virtual circuit (SVC) • Call setup is required

  11. Addressing in X.25 networks • X.121 protocol defines global addressing in X.25 networks. • Address field is 14 digits. • The first 4 digits define the country (3 digits) and the network within that country (1 digits). • Next ten digits called (NTN, National Terminal Number) refer to DTEs in a particular network.

  12. Triple-X Protocols • X.3, X.28, and X.29 are called triple-X protocols. • X. 121 protocol defines global addressing in X.25 networks. • X.3 defines a PAD. • Used to set connection between a dumb terminal and an X.25 network • X.28 defines the comm. rules between dumb terminal and a PAD. • X.29 defines the communication terms between a PAD and a remote DTE.

  13. Flow and Error Control • X.25 uses sliding-window flow control. • Flow and error control are implemented at data link layer and network layer. • For error control: • For each packet, there must be an acknowledgment received. • Go-back-N (ARQ) is used.

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