Exercises for the Lectures on Prof. Jochen Seitz & Dipl.-Ing. Karsten Renhak Department of University of Technology Ilmenau October 5, 010 [Jochen.Seitz Karsten.Renhak]@tu-ilmenau.de
Contents 1 Communication Services and Protocols 3 1.1 Terms and Definitions........................... 3 1. Classification of Communication Services................ 3 Protocol Mechanisms 4.1 Error Detection.............................. 4. Bit-Oriented Data Link Layer....................... 4.3 Acknowledged Data Transmission.................... 4.4 Flow Control............................... 5 3 Service Primitives 6 3.1 Service Access Point........................... 6 3. Transport Layer Connection Establishment............... 6 3.3 Layered Communication Architectures.................. 6 4 Switching and Multiplexing 7 4.1 Switching Techniques........................... 7 4. Multiplexing............................... 7 4.3 Code Division Multiple Access...................... 7 5 Cooperation of Layers 8 5.1 Network Layer and Data Link Layer................... 8 5. Satellite Communications......................... 8 6 Switching Technology 10 6.1 Switching Systems............................ 10 6. Routing.................................. 10 7 Integrated Services Digital Network (ISDN) 11 7.1 Bit Rate and Signal Rate......................... 11 7. Channel Capacity............................. 11 7.3 Pulse Code Modulation.......................... 11 7.4 Coding on the S 0 -Interface........................ 11 7.5 Coding on the U k0 -Interface........................ 1 8 The Internet Protocols 13 8.1 The Internet and the ISO/OSI Basic Reference Model.......... 13 8. Addresses in the Internet......................... 13 8.3 Detailed Flow through the Protocol Layers................ 14 9 Abbreviations 15
3 1 Communication Services and Protocols 1.1 Terms and Definitions 1. Explain the model of a communication service.. Define the term service interface. 3. How are communication services and communication protocols related? 1. Classification of Communication Services In telephone networks, communication services are classified as follows: bearer service tele service value-added service service attributes Explain the differences of these classes. appropriate class and give reasons for this: Assign the following services to the most 1. telephony. telefax 3. call waiting signal 4. toll-free phone call using 0800 5. call diversion 6. premium rate service using 0900 7. packet data transmission over the public telephone network
4 Protocol Mechanisms.1 Error Detection In order to detect bit errors which may occur during transmission due to interferences, protocols often stipulate check sums. 1. Which layers use this protocol mechanism?. Explain how a parity check works. What s the disadvantage of this kind of error detection mechanism? 3. A more sophisticated algorithm is the cyclic redundancy check. Let us assume the generator polynomial x 4 + x 3 + 1. The bit sequence 110011 contains important user information for which the frame check sequence (FCS) has to be computed. Please detail the FCS generation process and the FCS checking process. 4. What happens, if an error has been detected?. Bit-Oriented Data Link Layer One of the most important data link layer protocols is the High-Level Data Link Control (HDLC) protocol. It is bit-oriented, thus it may transport an arbitrary amount of bits. An HDLC frame starts with a start-of-frame bit pattern ( 01111110 ). Then, there is an address field and a control field (used for frame numbering and acknowledging). Thereafter, there is user data to be transported, which is secured by a frame check sequence. Finally, the frame is ended by an end-of-frame bit pattern (again 01111110 ). The frame structure is depicted in figure 1. Bits 8 8 8 n 8 8 start-of-frame address control user data frame check end-of-frame bit pattern field field sequence bit pattern Figure 1: HDLC frame Given this frame format, complete the frame to transmit the string Okay coded with the international 7-bit code (without parity bit). The generator polynomial for the frame check sequence is x 8 + x 4 + x 3 + x. Can you identify problems related to the frame format? How can these be solved?.3 Acknowledged Data Transmission Assume a character-oriented data link layer protocol that works in half-duplex mode. The communication partners are a primary station and a secondary station. The primary station initiates the communication by sending the control character ENQ. The secondary stations responds with the control character ACK, if it is ready to receive data. Otherwise, it sends a NAK control character. If the secondary station can receive data, the primary station transmits a data frame. Once this frame has been received correctly,
5 the secondary station again sends an ACK character. In order to provide a means for synchronization, each transmission must start with a SYN character. Develop protocol mechanisms dealing with the following errors: 1. Loss of ENQ. Loss of the ACK character stating that the secondary station is ready to receive. 3. Permanent errors on the physical line. 4. Bit error within the data frame. 5. Loss of an acknowledgment (ACK) for a data frame. Which problem may arise if you consider the points 4 and 5? How can you cope with these problems..4 Flow Control Describe how credit-based flow control works. Given is a maximum window size of 4. Acknowledgments are sent within every packet (piggybacked acknowledgments). Complete the chart in figure with the according credit and the sending and receiving sequence numbers. S 0 Sender Sliding Window (Credit 4) Receiver 6 0 4 C 6 0 4 6 4 0 0 6 6 4 4 0 0 6 6 4 time 4 S: Sequence number of the last packet sent R: Expected number of the next packet to be received (all packets with a number less than R are acknowledged) C: Upper window edge (maximum number of the packet to be sent) Figure : Sliding Window
6 3 Service Primitives 3.1 Service Access Point A specific protocol entity can use the service of the underlying layer by exchanging service primitives at the service access point (SAP). 1. Discuss the characteristics of such a service access point.. Give examples for a service access point: a) in the public telephone network b) in the Internet 3. Transport Layer Connection Establishment To illustrate the quite abstract terms service, protocol, and service primitive, the example of establishing a connection on the transport layer shall be worked out in detail. 1. For connection establishment on the transport layer, distinguish between service provider and service user.. Which service primitives have to be used for connection establishment on the transport layer? 3. Specify the connection establishment service a) using a finite state machine b) using message sequence charts 4. Specify the according protocol. Assume a datagram-oriented network layer that is very reliable. 5. Describe successful connection establishment with a message sequence chart. 6. What would have to be changed in the establishment procedure if the network layer service were connection-oriented? Sketch another message sequence chart depicting this case. 7. Now consider a quite unreliable network layer service. The network layer is able to avoid phantom packets, erroneous packets and packets that are out of sequence. Which problems may occur nevertheless and what would you build into the transport layer protocol to cope with these problems. 3.3 Layered Communication Architectures Generally, communication systems are described by layered communication architectures. What is the advantage of this? Discuss the optimal number of layers in such an architecture.
7 4 Switching and Multiplexing 4.1 Switching Techniques Describe the switching technique used in the following communication networks. Identify those networks that provide a physical channel that is connected through from the sender to the receiver. 1. plain old (analogue) telephone network. Internet 3. public digital telephone network (e.g. ISDN) 4. snail mail (letter) 4. Multiplexing Complete the following statements or check the correct answers: 1. Multiplexing is used to improve the symbol rate to better exploit the physical channel to allow different transmissions over one physical channel in parallel. Using... multiplex, each transmission activity gets the complete channel bandwidth. 3. Using... multiplex, each transmission activity can use only a fraction of the channel bandwidth. 4. One physical channel with bandwith B shall be used for many transmission activities synchronistically. Which multiplex technique is to be used? space multiplex time multiplex frequency multiplex 4.3 Code Division Multiple Access Explicate the functionality of code division multiple access (CDMA) with the following example: Two senders A and B share the same physical medium using CDMA. A uses chipping sequence CS A = + ++, B uses chipping sequence CS B = + + + +. A wants to send 101, while B has to send 100. Detail the transmission by showing which (physical) signals are sent out by the two senders, how these signals interfere with each other, and how the mixture of these two signals must be decoded by the receivers accordingly, which know the chipping sequence of the senders.
8 5 Cooperation of Layers 5.1 Network Layer and Data Link Layer Assume a network layer entity utilizing the services of the data link layer that features the following constraints: The maximum amount of data to be handled by the data link layer entity is 1, 500byte. The data link layer entities work at a transmission speed of 4Mbit/s. At the data link layer, frames are sent according to the stop-and-wait principle. The data link layer guarantees delivery in sequence. The transport layer entity delivers a 0.6 MByte service data unit (SDU) to the network layer entity. 1. Which protocol mechanisms must be implemented by the network layer entities?. Draw a message sequence chart depicting data link and network link layer entities that describes this data transfer. 3. An acknowledgment frame on the data link layer is 1kbit long. How long does the complete transfer from the delivery of the transport layer SDU at the sender s N-SAP to the indication of the received SDU at the receiver s N-SAP? Ignore the processing time within the entities and the protocol control information that has to be attached to the data. Consider only the error-free case. 5. Satellite Communications Let us consider a communication network with satellite links as depicted in figure 3. Both sender and receiver possess entities of all seven layers of the ISO/OSI basic reference model. The other three network components are equipped with entities of layers one to three only. The following constraints must be considered when analyzing the communication in this network: The transport layer entity on the sender side would like to send 5.5Kbyte data to the receiver. The network layer entity on the sender side has to fragment the data into smaller portions, because the data link layer can only cope with a Kbyte SDU per frame. The protocol overhead to be added by the network layer is 00byte. The data link layer works according to the stop-and-wait principle, which means that a sending data link layer entity has to wait for an acknowledgment by the receiving entity before it can transmit the next frame.
9 50 ms 50 ms Sender Receiver 5 ms 5 ms Figure 3: Scenario The durations given in figure 3 refer to the data link layer and specify how long it takes until the frame has reached the receiving data link layer entity. These durations are the same in both directions and are constant, since we assume a frame of constant length on the data link layer. To simplify the computation we do not care about the processing time in the entities. How long does the transmission of the aforementioned 5.5Kbyte take? [Hint: Try to illustrate the process with a message sequence chart!]
10 6 Switching Technology 6.1 Switching Systems Switching systems can be classified according to the technology they implement. 1. Which technology is best suited for the following use cases? a) transmission of a huge amount of data in a limited time; b) transmission of a huge amount of data without time limits; c) transmission of a quite small amount of data without time limits; d) transmission of a quite small amount of data in a limited time.. Discuss the quality of service degree each of the switching technologies is able to guarantee. 6. Routing Look at the network depicted in figure 4. It consists of six nodes. The links between nodes have different costs that are given in the figure, too. The routing algorithm that is used to build the routing tables for all nodes aims at taking the most cost-effective path. If two paths have equal costs, the shortest path (with the least number of nodes) is chosen. 5 Router 3 Router 3 5 Router 1 3 1 Router 6 1 Router 4 1 Router 5 Figure 4: Sample Network 1. Construct the routing tables for all six nodes.. Assume that the link between routers 4 and 5 breaks down. How does this affect the six routing tables?
11 7 Integrated Services Digital Network (ISDN) 7.1 Bit Rate and Signal Rate The unit of bit rate is bit/s, whereas the signal rate is given in baud. 1. How are these two units related?. Under which circumstances are signal rate and bit rate equal? 3. Is it possible that the bit rate is smaller than the signal rate? 4. Why can a signal not carry an arbitrary amount of bits? 7. Channel Capacity A transmission channel (approximated by an ideal low pass) has a cut-off frequency of f Chmax = 4kHz and a signal-to-noise ratio of 40dB. Which bit rate can theoretically be achieved over this channel? 7.3 Pulse Code Modulation 1. Which kind of multiplexing is the basis for PCM?. A PCM30 frame is depicted in figure 5. Knowing that an ISDN-B-channel works at 64kbit/s, compute the duration of one single time slot and of one complete PCM30 frame. 3. What is the effective bit rate to transmit user data (voice) using PCM30? How much overhead has to be transmitted (in percent)? 0 1 15 16 17 31 Frame Alignment/ Synchr. Telephone Channel 1 Telephone Channel... Telephone Channel Signaling Channel Channel... 15 16 Telephone Telephone Channel 30 Slot 1 3 4 5 6 7 8 t 3 x 8 bit = 56 bit PCM30 frame Figure 5: PCM30 frame structure 7.4 Coding on the S 0 -Interface At the subscriber installation, ISDN uses the modified Alternate Mark Inversion (AMI) code. This is a pseudo-ternary code that codes a binary 1 by zero voltage and a binary 0 by alternating impulses of +0.75V or 0.75V respectively.
1 1. Explain the term pseudo-ternary.. Compare the bit rate to the signal rate. 3. Encode the bit sequence 0011110100 using modified AMI. 4. Describe the characteristics of this code. 7.5 Coding on the U k0 -Interface 1. On the subscriber line from the telephone exchange to the subscriber s home, 4B3T-coding was used (cf. table 1). Use this coding scheme to code the bit sequence 1001111111110010. Bit Sequence S1 Sn S Sn S3 Sn S4 Sn 0001 0 - + 1 0 - + 0 - + 3 0 - + 4 0111-0 + 1-0 + - 0 + 3-0 + 4 0100 - + 0 1 - + 0 - + 0 3 - + 0 4 0010 + - 0 1 + - 0 + - 0 3 + - 0 4 1001 + 0-1 + 0 - + 0-3 + 0-4 1110 0 + - 1 0 + - 0 + - 3 0 + - 4 1011 + - + + - + 3 + - + 4 - - - 1 0011 0 0 + 0 0 + 3 0 0 + 4 - - 0 1101 0 + 0 0 + 0 3 0 + 0 4-0 - 1000 + 0 0 + 0 0 3 + 0 0 4 0 - - 0110 - + + - + + 3 - - + - - + 3 1010 + + - + + - 3 + - - + - - 3 1111 + + 0 3 0 0-1 0 0-0 0-3 0000 + 0 + 3 0-0 1 0-0 0-0 3 0101 0 + + 4-0 0 1-0 0-0 0 3 1100 + + + 4 - + - 1 - + - - + - 3 Table 1: 4B3T-Coding. With the introduction of Euro-ISDN, the coding scheme on the subscriber line was changed to the B/1Q-code. According to table, two bits are coded by one signal that can have four different voltage levels, Hence, this signal is called quaternary. Sketch the signal of exercise 1 coded with the B/1Q-code. Bit Sequence Quart 1 0 +3 1 1 +1 0 1-1 0 0-3 Table : Coding Table for B/1Q-Code
13 8 The Internet Protocols 8.1 The Internet and the ISO/OSI Basic Reference Model In the Internet, the communication systems are defined in layers, too. However, the resulting architecture is much simpler than the ISO/OSI Basic Reference Model. 1. Give the full names for the following Internet protocols and classify them into the according ISO/OSI layer: a) TCP b) IP c) HTTP d) UDP e) ARP f) FTP g) DHCP Explain the functions of each of the given protocols.. Discuss the advantages and disadvantages of the reduction of the number of protocols in the Internet reference model. 3. Where does Internet telephony, a.k.a. Voice over IP, fit into the Internet reference model. Explain your answer. 8. Addresses in the Internet The Internet reference model also incorporates service access points between different layers. These SAPs have to be addressed in a unique way. 1. Which types of addresses have to be differentiated in the Internet on the different layers?. An Internet user should only handle addresses on the application layer. In order to communicate, this addresses have to be mapped onto addresses of the layers below. Explain how this mapping is done. 3. Let us assume a PC that is located within the university s WLAN network. Its assigned IP address is 141.4.15.111. The PC user would like to retrieve the web site of the Internet Engineering Task Force (www.ietf.org). Try to identify the addresses on each layer this PC has to use in order to send an HTTP request to the IETF s web server.
14 8.3 Detailed Flow through the Protocol Layers The last part of exercise 8. shall now be used to illustrate how all the layers cooperate. Answer the following questions for this. 1. Which protocols are needed on which layer?. Which protocol entities are necessary and where do you find them? 3. Which functionalities are provided by the network layer? 4. What would happen, if the IETF switched off the web server for maintenance?
15 9 Abbreviations A AMI Alternate Mark Inversion C CDMA CS Code Division Multiple Access Chipping Sequence F FCS Frame Check Sequence H HDLC High-Level Data Link Control I IETF ISDN ISO Internet Engineering Task Force Integrated Services Digital Network International Organization for Standardization N N-SAP Network Layer Service Access Point O OSI Open Systems Interconnection P PCM Pulse Code Modulation S SAP SDU Service Access Point Service Data Unit 0... 9 B1Q 4B3T Two Binary One Quaternary line encoding scheme Four Binary Three Ternary line encoding scheme