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Computer Communication Network

Sheet 5

1- What are the X.25 layers? How does each relate to OSI model?

2- How does the frame layer address field differ from HDLC address field? Address field in HDLC: Has the address of the secondary station Address field in X.25 : is 00000001 for command from DTE or Response to DTE Or 00000011 for command from DCE or Response to DCE 3- Name the X.25 frame types and their primary functios? 1- I: information frame and its control filed start by 0 2- S: supervisory frame and its control filed start by 10 3- U: unnumbered frame and its control filed start by 11 4- What the purpose of an LCN? LCN (Logical channel Number) is the identifier of the virtual circuit in X.25 5- What kind of virtual circuits does X.25 use? X.25 uses permanent and switched virtual circuits. 6- What are the two general PLP packet types? 1- Data Packet 2- Control Packet 7- What is the purpose of the X.21 protocol? It is the protocol which has been specifically defined for X.25 similar to other physical layer protocols. 8- A DTE with an X.121 address of 445 6 141121312 wants to make a connection to another DTE with an X.121 address of 231 1 3178121132. Is this connection inter-country or intra-country? Inter-country (different country) What are the country codes of the source and destination ? Source: 445 Destination: 231 What are the network numbers? Source: 6 Destination: 1 What are the terminal numbers? Source: 141121312 Destination: 3178121132 9- HDLC has three types of frames (I-frame, S-frame, U-frame). Which one corresponds to the Frame Relay frame? I-Frame

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Computer Communication Network

Sheet 5

10- There are no sequence numbers in Frame Relay. Why? No, Frame relay isn't provide error control nor flow control. 11- Why is Frame Relay a better solution for connecting LANs than T-1 lines? - Frame relay provide the same type of services at lower cost. - Frame relay support burst data (Band width on demand) while T-1 lines support fixed data only. 12- Name some advantage of Frame Relay over X.25. 1 2 3 4 5

FR operate at higher speed (1.544 Mbps & 44.376 Mbps) FR operate just in two layer (PHYL+DDL) this enable it to provide services to protocols that already has a network layer (TCP/IP) FR allows burst data FR frame size of 9000 bytes this enable it to handle all local area network frames FR is less expensive than other traditional WANs.

13- How is flow control handled in Frame Relay? FR doesn't provide flow control instead all flow control is left to protocols at the network & transport layer. 14- Why is the control field from HDLC totally dropped from Frame Relay? Because FR doesn't provide flow control. 15- Why is Frame Relay unsuitable for real-time communication such as teleconferencing? - FR is variable frame size hence varying delays which is not under user control - So it is not suitable for sending delay sensitive data such as real time voice or video such as teleconferencing 16- How are Frame Relay assemblers/dissemblers (FRADs) used in a Frame Relay backbone? - FRADs allow other protocols frames to be carried by FR frames by assembling and disassembling these frames to & from FR frames. 17- Compare an SVC with a PVC. SVC (Switched virtual circuit) DLCIs are temporary and assigned by FR during the connection phase. Connection needed to be established with each DTE wants to make a connection with another. Need ( Setup - Data transfer - Release )

PVC (permanent virtual circuit) DLCIs are permanent and assigned by FR network provider. DTEs connected permanently through a virtual connections No need

- They all need DLCIs assigned to UNI interface

18- The address field of a Frame Relay is 1011000100010110. What is the DLCI (in decimal)?

101100 01 0001 01 10

DLCI: (1011000001) = 705 in decimal

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Computer Communication Network

Sheet 5

19- The address field of a Frame Relay is 1011000100010110. Is there any congestion in the forward direction? Is there any congestion in the backward direction? 101100 01 0001 01 10 No congestion in forward direction. There in congestion in backward direction. 20- A frame goes from DTE A to DTE B. There is congestion in the A to B direction

Is the FECN bit set? Is the BECN bit set? FECN:

BECN: 0

21- A frame goes from DTE B to DTE A. There is congestion in the A to B direction

Is the FECN bit set? Is the BECN bit set? FECN:

BECN: 1

22- A frame goes from DTE B to DTE A. There is congestion in both direction Is the FECN bit set? Is the BECN bit set? FECN:

BECN: 1

23- In a leaky bucket, what should be the capacity of the bucket if the output rate is 5 gallons per minute, and there is an input burst of 100 gallons per minute for 12 seconds and there is no input for 48 seconds? C=100 *12 /60 - 5 = 15 gallons

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