Tutorial Questions EG/ES The tutorial questions illustrate the style of examination questions for EG/ES 3567.

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1 The tutorial questions illustrate the style of examination questions for EG/ES The paper will be of 3 hours duration, and each student should attempt four questions during this time. You should aim to spend the initial part of an exam reading the questions. Make sure that you understand where the marks are allocated. You are expected to have an understanding of the material which was presented in the lectures, to have done some personal reading based on the syllabus, and to have completed the set of practical exercises and questions located on the course world wide web pages. Ensure that your answers provide sufficient detail to justify the allocated marks. As a guideline, a question part worth ten marks is expected to take minutes to complete. Do draw diagrams which you may feel appropriate to support your answer, ensuring that each diagram is clearly labelled and that you write an explanation underneath describing what the diagram shows. An answer consisting of one or two diagrams and three or four paragraphs describing what the diagrams show and touching on additional details is likely to score a high mark. A full set of notes summarising the answers to these questions will be provided in the final weeks of the course. The class demonstrator(s) will help work out your answers and check the results do ask. TUTORIAL 1: Ethernet Link and Physical Layers 1.1 (a) The IEEE 802.x family of LAN specifications support many physical media, explain with the aid of diagrams the differences between 10BT and 10B5 media. (b) Suggest two situations where optical fibre is preferable to copper cabling. [2 marks ] (c) (i) Explain the function of the Ethernet Preamble. (ii) Explain the function of the Ethernet Cyclic Redundancy Check (CRC). (d) The Ethernet standard states that each frame should carry two addresses. (i) What is the Ethernet destination address used for? [1 marks ] (ii) How does a manufacturer decide which address to use? [3 marks ] (iii) Why is the first bit never set in an Ethernet source address? [2 marks ] 1.2 (a) Sketch the format of an Ethernet frame carrying 1000 B of data. Your answer should show all the protocol fields (headers) present in the frame. [3 marks ] (b) Sketch an outline block diagram of the process by which a byte is encoded by an Ethernet processor prior to transmission by the physical layer. [5 marks ] (c) Plot the waveform which you would observe on an oscilloscope when a byte with the hexadecimal value of 0x57 is transmitted along an Ethernet coaxial cable. [3 marks] (d) The Ethernet Local Area Network (LAN) uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to share the transmission medium. Define the following terms: (i) Carrier Sense (ii) Collision Detection (iii)collision Domain (e) Describe how Ethernet Capture allows a single computer to dominate the use of the medium when it has excessive amounts of data to transmit. [ 3 marks ] (c) G. Fairhurst, 2011 v 12 page 1

2 TUTORIAL 2: LAN Interconnection 2.2 (a) Explain in detail the operation of an Ethernet bridge, and describe how the bridge filters frames which need not be forwarded between Ethernet segments. [10 marks] (b) When considering repeaters, bridges and routers, which type of equipment is most suited for the connection of a LAN to the Internet? Why? (c) What is the role of the Ethernet Source Address? (d) Explain the concept of a collision domain (provide appropriate diagrams in your answer) TUTORIAL 3: Calculating the Utilisation of LANs 3.1 (a) A small Local Area Network (LAN) is shown in the figure: Bridge LAN X LAN Y Computer A Computer B Computer C Computer D Four computers, A, B, C and D connected to a LAN. The LAN is formed from two shared Ethernet segments joined via a bridge. The computer A sends three simultaneous Unicast file transfer packets. each to computers B, C, and D. Calculate the size of a frame, given that it carries 1032 B of IP network payload data. Using this information, calculate the Utilisation of LAN X, assuming that the transmission continues at 50 packets per second to each of the three destinations. (b) What is the utilisation on LAN Y? (c) How does Multicast transmission differ from Unicast transmission? (d) Calculate the utilisation for LAN Y when the file is sent using multicast packets instead of the unicast packets used in section (a). Q4 not used in (c) G. Fairhurst, 2011 v 12 page 2

3 TUTORIAL 4: Network Layer 5.1 (a) The Internet uses connectionless packet switching. Explain the difference between the operation of a packet switching network and a message switching network, paying attention to the transmission of long messages. (b) Define the following terms: (i) The IP Version number (ii) The IP network number (iii) The Network Layer address (c) Explain the term connectionless, describing the services which are provided by a connectionless network layer protocol, such as the Internet Protocol. 5.2 (a) The Domain Name System (DNS) provides resolution of Internet names and addresses. Explain the difference between a name and an address, giving examples of each. (b) Addresses may be assigned in a hierarchical or a flat structure. Describe (giving examples) the addressing structure using in the following types of network: (i) Ethernet (ii) a telephony network (iii) the Internet (c) A computer Z is the DNS server for two computers X and Y. Explain what is meant by the terms client and server. (d) Using the example of a login from the computer X to a remote computer Y, explain how the DNS server (Z) operates. TUTORIAL 5: The Address Resolution Protocol 6.1 (a) What is the purpose of the source and destination addresses in Ethernet frames? Describe in detail how a computer connected to an Ethernet LAN determines its own Ethernet address and how it determines the Ethernet address of other computers with which it needs to communicate. [10 marks] (b) An Ethernet Local Area Network (LAN) connects two workstations (A and B). The following information is provided about the IP interfaces of the computers connected to the LAN, also giving the hardware address (ha) for each interface. A: IP = ; h.a..= 08:00:20:02:b7:f9 B: IP = ; h.a..= 08:00:20:ff:00:00 The computer A has not previously sent any packets to the Ethernet LAN. The user at A runs a program which sends one IP packet and has a destination address of B. Explain the hardware address values in the first frame which is sent by the computer A. (c) What are the hardware source and destination addresses and the source and destination IP addresses of the first network layer packet which is received by B? (c) G. Fairhurst, 2011 v 12 page 3

4 6.2 (a) Three styles of transmission are commonly supported in networks: Unicast, Broadcast and Multicast What is the difference between each of these styles of transmission? (b) Redraw the diagram above to provide a detailed explanation of the operation of the Address Resolution Protocol (ARP). (c) For each packet, specify which computers addresses are used for the source and destination address at both the link layer AND the network layer. (d) Calculate the time taken to transmit one ARP packet using an Ethernet LAN operating at 10 Mbps. Your answer should first calculate the total size of the ARP packet including ALL overhead introduced by each layer during the transmission process. 6.3 Two computers, A and B are connected by an Ethernet LAN. (a) A has not previously sent any packets to the LAN. It sends 20B of data to B in a single IP datagram. (i) What is the type of the first Ethernet frame sent by A? (ii) To which MAC address is the first frame sent? [1 mark] (iii) What is the IP source address of the first IP packet received by B? (b) Computer B responds by sending an 8K byte datagram with a destination of A. (i) How many IP packets are sent by B? (ii) How many IP packets are received by the network layer at A? (iii) How many IP packets are received by the user program at A? (c) Now suppose a router, R, was inserted in the LAN between A and B, and that the Maximum Transmission Unit (MTU) of the LAN connected to B was set to 576 B. (i) How many IP packets are received by the network layer at A? (ii) How many IP packets are received by the user program at A? (iii) What would be the result if the first packet sent by B was lost due to an overflow of the buffer in the router R? [5 marks] TUTORIAL 6: Transport Layer 7.1 (a) List the functions of a reliable communication service. (b) What type of service is provided by the User Datagram Protocol (UDP)? (c) Explain the function of each of the fields in the UDP packet header. (d) A UDP packet containing 50 B of payload data is transmitted using IP over an Ethernet LAN. Draw a diagram showing the complete packet, including all protocol headers. What is the total size of the frame transmitted on the Ethernet LAN 7.2 (a) An end to end connection may be checked in an IP internet using the ping program which uses the Internet Control Management Protocol, ICMP. Describe the operation of the ICMP echo request and ICMP echo reply to perform this check, and how this may measure the round trip delay across the network. (b) Provide a step by step explanation using diagrams to show the way an Ethernet network interface card and the network layer protocol process an IP frame received from an Ethernet transceiver. [12 marks] (c) G. Fairhurst, 2011 v 12 page 4

5 TUTORIAL 7: Packet Decodes 8.1 (a) Define the functions of the transport layer in the OSI Reference Model (b) Define the following terms (i) Ethernet Source Address (ii) Service Access Point provided by the Medium Access Control (MAC) layer (iii) IP Header Checksum (iv) UDP Checksum (c) An Ethernet protocol analyser observes the following frame: DF F E 4A C B 85 CC 16 8B 85 CC B A By decoding the hexadecimal bytes of this frame using the header chart supplied, determine the values for each of the parts (i)- (iv) in the preceding question. Students are expected to know how to decode packets from the sequence of hexadecimal bytes passing over a physical media. The packets may be decoded using the packet header chart provided which indicates the format of the Protocol Control Information (PCI) for a set of well-known protocols. A table of Service Access Point (SAP) codes is also provided below: Ethernet: (in hexadecimal):0x0800 = IP; 0x0806 = arp IP: (in decimal) 1 = ICMP; 2= IGMP; 6 = TCP; 17= UDP TCP ports: (in decimal) 23 = Telnet; 25 = Mail; 69 = TFTP; 80 = WWW (http) You should assume the packet starts with a Medium Access Control (MAC) header. Given the MAC header format, you should be aware that the first six bytes are the destination MAC address, the next six are the MAC source address, and the final two bytes are the MAC protocol type. By identifying the protocol type using the table of SAPs, you should now identify the type of network layer header is expected to follow. The following information is also provided about the two communicating nodes: Node Name: gateway client Network Mask (netmask): Ethernet Physical Address: 00:e0:f7:26:3f:e9 08:00:20:86:35:4b 10.1) 00e0 f726 3fe b ab ff11 f700 8b85 d96e 8b85 e902 99d0 043f c6c 6f68 656c 6c6f 10.2) 00e0 f726 3fe b c 08b ff b85 d96e 8b85 e f a92c b4 10.3) b 00e0 f726 3fe aafb 4000 fc01 fa30 8b85 e902 8b85 d96e da 1e e 3ab ac a0b 0c0d 0e0f a1b 1c1d 1e1f a2b 2c2d 2e2f Further examples are available on the web in eg3567/inet-pages/packetdecode.html, including a complete decode of these three Ethernet frames. (c) G. Fairhurst, 2011 v 12 page 5

6 Additional Questions: 2.1 (a) 4 computers (W,X,Y,Z) are connected by Ethernet segments using a Repeater (R) and Bridges (B1,B2). Which computers receive (at the network level) the following frames? (Make sure your answer also shows which LAN segments carry each frame.) W -> Broadcast address X-> Z Y-> Z Y-> Broadcast address (b) The computers W and X have MAC addresses of W = 0x and X = 0x They are both also members of the multicast group 0x23. Sketch the MAC header for a multicast frame sent from X to the multicast group 0x23. (c) Which LAN segments carry a multicast frame? (explain your answer) (d) Which of the following may be used to extend a LAN to allow computers to be connected by more than 5 cable segments? (i) Transceiver (ii) Repeater (iii) Hub (iv) Bridge (v) Switch 3.2) (a) Define the term Utilisation. (b) An application sends 50 packets per second over an Ethernet Local Area Network. Each packet has a total size of 1480 B (excluding the preamble and cyclic redundancy check (CRC)). Determine the total number of bits sent by the transmitter, and hence the utilisation of the link given that the cable used is a 10BT shielded twisted pair cable. (c) Ethernet transmission over 10B2 cable may be described as Half Duplex. Explain this term. (d) A client on a Ethernet LAN sends a series of queries to a server on the same LAN. Each request is contained in an Ethernet frame with a payload of 100 B of data. The server transmits a response to each query by sending an Ethernet frame with 573 B of data. Given that there are 100 requests per second, calculate the size of each Ethernet frame and the utilisation of the Ethernet LAN. (c) G. Fairhurst, 2011 v 12 page 6