Biblio5

10.10 WHAT ARE DATA PROTOCOLS?

291

Figure 10.20 The OSI reference model.

col. For example, the transport layer of system A communicates with its peer transport layer at system B. It is important to note that there is no direct communication between peer layers except at the physical layer (layer 1). That is, above the physical layer, each protocol entity sends data down to the next lower layer, and so on to the physical layer, then across and up to its peer on the other side. Even the physical layer may not be directly connected to its peer on the other side of the “connection” such as in packet communications. This we call connectionless service when no physical connection is set up.14 However, peer layers must share a common protocol in order to communicate.

There are seven OSI layers, as shown in Figure 10.20. Any layer may be referred to as an N-layer. Within a particular system there are one or more active entities in each layer. An example of an entity is a process in a multiprocessing system. It could simply be a subroutine. Each entity communicates with entities above it and below it across an interface. The interface is at a service access point (SAP).

The data that pass between entities are a bit grouping called a protocol data unit (PDU). Data units are passed downward from a peer entity to the next OSI layer, called the (N − 1) layer. The lower layer calls the PDU a service data unit (SDU). The (N − 1) layer adds control information, transforming the SDU into one or more PDUs. However, the identity of the SDU is preserved to the corresponding layer at the other end of the connection. This concept is illustrated in Figure 10.21.

OSI has considerable overhead. By overhead, we mean bit sequences that are used for logical interfaces or just simply to make the system work. Overhead does not carry revenue-bearing traffic. Overhead has a direct bearing on system efficiency: as overhead increases, system efficiency decreases.

OSI layering is widely accepted in the world of data communications even with its considerable overhead. Encapsulation is a good example. Encapsulation is used on all OSI layers above layer 1, as shown in Figure 10.22.

14Connectionless service is a type of delivery service that treats each packet, datagram, or frame as a separate entity containing the source and destination address. An analogy in everyday life is the postal service. We put a letter in the mail and we have no idea how it is routed to its destination. The address on the letter serves to route the letter.

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Figure 10.21 An illustration of mapping between data units on adjacent layers.

10.10.2.2 Functions of the First Four OSI Layers. Only the first four OSI layers are described in the following paragraphs. Layers 5, 6, and 7 are more in the realm of software design.

Physical layer. The physical layer is layer 1, the lowest OSI layer. It provides the physical connectivity between two data terminals who wish to communicate. The services it provides to the data link layer (layer 2) are those required to connect, maintain the connection, and disconnect the physical circuits that form the physical connectivity. The physical layer represents the traditional interface between the DCE and DTE, described in Section 10.8.

Figure 10.22 Buildup and breakdown of a data message based on the OSI model. OSI encapsulates at every layer except layer 1.

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293

The physical layer has four important characteristics:

1. Mechanical;

2. Electrical;

3. Functional; and

4. Procedural.

The mechanical aspects include the actual cabling and connectors necessary to connect the communications equipment to the media. Electrical characteristics cover voltage and impedance, balanced and unbalanced. Functional characteristics include connector pin assignments at the interface and the precise meaning and interpretation of the various interface signals and data set controls. Procedures cover sequencing rules that govern the control functions necessary to provide higher-layer services such as establishing a connectivity across a switched network.

Data-link layer. The data-link layer provides services for reliable interchange of data across a data link established by the physical layer. Link-layer protocols manage the establishment, maintenance, and release of data-link connections. These protocols control the flow of data and supervise error recovery. A most important function of this layer is recovery from abnormal conditions. The data-link layer services the network layer or logical link control (LLC; in the case of LANs) and inserts a data unit into the INFO portion of the data frame or block. A generic data frame generated by the link layer is illustrated in Figure 10.7.

Several of the more common data-link layer protocols are: CCITT LAPB, LAPD; IBM SDLC; and ANSI ADCCP (also the U.S. government standard).

Network layer. The network layer moves data through the network. At relay and switching nodes along the traffic route, layering concatenates. In other words, the higher layers (above layer 3) are not required and are utilized only at user end-points.

The concept of relay open system is shown in Figure 10.23. At the relay switching point, only the first three layers of OSI are required.

Layer					Relay open system
		Open system				Open system
Application
Presentation
Session
Transport
Network
Data Link
Physical

Physical media for OSI

Figure 10.23 Only the first three OSI layers are required at an intermediate relay (switching) point.

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The network layer carries out the functions of switching and routing, sequencing, logical channel control, flow control, and error-recovery functions. We note the duplication of error recovery in the data-link layer. However, in the network layer error recovery is network-wide, whereas on the data-link layer error recovery is concerned only with the data link involved.

The network layer also provides and manages logical channel connections between points in a network such as virtual circuits across the public switched network (PSN). It will be appreciated that the network layer concerns itself with the network switching and routing function. On simpler data connectivities, where a large network is not involved, the network layer is not required and can be eliminated. Typical of such connectivities are point-to-point circuits, multipoint circuits, and LANs. A packet-switched network is a typical example where the network layer is required.

The best-known layer 3 standard is CCITT Rec. X.25. (Ref. 22).

Transport layer. The transport layer (layer 4) is the highest layer of the services associated with the provider of communication services. One can say that layers 1–4 are the responsibility of the communication system engineer. Layers 5, 6, and 7 are the responsibility of the data end-user. However, we believe that the telecommunication system engineer should have a working knowledge of all seven layers.

The transport layer has the ultimate responsibility for providing a reliable end-to- end data-delivery service for higher-layer users. It is defined as an end-system function, located in the equipment using network service or services. In this way its operations are independent of the characteristics of all the networks that are involved. Services that a transport layer provides are as follows:

•Connection Management. This includes establishing and terminating connections between transport users. It identifies each connection and negotiates values of all needed parameters.

•Data Transfer. This involves the reliable delivery of transparent data between the users. All data are delivered in sequence with no duplication or missing parts.

•Flow Control. This is provided on a connection basis to ensure that data are not delivered at a rate faster than the user’s resources can accommodate.

The TCP (transmission control protocol) was the first working version of a transport protocol and was created by DARPA for DARPANET.15 All the features in TCP have been adopted in the ISO version. TCP is often combined with the Internet protocol (IP) and is referred to as TCP/ IP.

10.10.3 High-Level Data-Link Control: A Typical Link-Layer Protocol

High-level data-link control (HDLC) was developed by the ISO. It has spawned many related or nearly identical protocols. Among these are ANSI ADCCP, CCITT LAPB and LAPD, IEEE Logical Link Control (LLC), and IBM SDLC. (Refs. 20–25).

The following HDLC definitions include stations, configurations, and three modes of operation.

15DARPA stands for Defense Advanced Research Projects Agency, under the U.S. Department of Defense.

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295

Figure 10.24 HDLC link configurations. (a) Unbalanced configuration; (b) balanced configuration.

Primary Station. A logical primary station is an entity that has primary link control responsibility. It assumes responsibility for organization of data flow and for link level error recovery. Frames issued by the primary station are called commands.

Secondary Station. A logical secondary station operates under control of a primary station. It has no direct responsibility for control of the link, but instead responds to primary station control. Frames issued by a secondary station are called responses.

Combined Station. A combined station combines the features of primary and secondary stations. It may issue both commands and responses.

Unbalanced Configuration. An unbalanced configuration consists of a primary station and one or more secondary stations. It supports full-duplex and half-duplex operation, point-to-point, and multipoint circuits. An unbalanced configuration is illustrated in Figure 10.24a.

Balanced Configuration. A balanced configuration consists of two combined stations in which each station has equal and complementary responsibility of the data link. A balanced configuration, shown in Figure 10.24b, operates only in the point-to- point mode and supports full-duplex operation.

Modes of Operation. With normal response mode (NRM) a primary station initiates data transfer to a secondary station. A secondary station transmits data only in response to a poll from the primary station. This mode of operation applies to an unbalanced configuration. With asynchronous response mode (ARM) a secondary station may initiate transmission without receiving a poll from a primary station. It is useful on a circuit where there is only one active secondary station. The overhead of continuous polling is thus eliminated. Asynchronous balanced mode (ABM) is a balanced mode that provides symmetric data transfer capability between combined stations. Each station operates as if it were a primary station, can initiate data transfer, and is responsible for error recovery. One application of this mode is hub polling, where a secondary station needs to initiate transmission.

10.10.3.1 HDLC Frame. Figure 10.25 shows the HDLC frame format. Note the similarity to the generic data-link frame illustrated in Figure 10.8. Moving from left to right in the figure, we have the flag field (F), which delimits the frame at both ends with the unique pattern 01111110. This unique field or flag was described in Section 10.7.2.

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Figure 10.25 The HDLC frame format (Ref. 25).

The address field (A) immediately follows the opening flag of a frame and precedes the control field (C). Each station in the network normally has an individual address and a group address. A group address identifies a family of stations. It is used when data messages must be accepted from or destined to more than one user. Normally the address is 8 bits long, providing 256 bit combinations or addresses (28 c 256). In HDLC (and ADCCP) the address field can be extended in increments of 8 bits. When this is implemented, the least significant bit is used as an extension indicator. When that bit is 0, the following octet is an extension of the address field. The address field is terminated when the least significant bit of an octet is 1. Thus we can see that the address field can be extended indefinitely.

The control field (C) immediately follows the address field (A) and precedes the information field (I). The control field conveys commands, responses, and sequence numbers to control the data link. The basic control field is 8 bits long and uses modulo 8 sequence numbering. There are three types of control field: (1) I frame (information frame), (2) S frame (supervisory frame), and (3) U frame (unnumbered frame). The three control field formats are illustrated in Figure 10.26.

Consider the basic 8-bit format shown in Figure 10.26. The information flows from left to right. If the frame shown in Figure 10.25 has a 0 as the first bit in the control field, the frame is an I frame (see Figure 10.26a). If the bit is a 1, the frame is an S or a U frame, as illustrated in Figure 10.26b and 10.26c. If the first bit is followed by a 0, it is an S frame, and if the bit again is a 1 followed by a 1, it is a U frame. These bits are called format identifiers.

Turning now to the information (I) frame (Figure 10.26a), its purpose is to carry user data. Bits 2, 3, and 4 of the control field in this case carry the send sequence number N(S) of the transmitted messages (i.e., I frames). N(S) is the frame sequence number of the next frame to be transmitted and N(R) is the sequence number of the frame to be received.

Each frame carries a poll/ final (P/ F) bit. It is bit 5 in each of the three different types of control fields shown in Figure 10.26. This bit serves a function in both command and response frames. In a command frame it is referred to as a poll (P) bit; in a response frame as a final (F) bit. In both cases the bit is sent as a 1.

The P bit is used to solicit a response or sequence of responses from a secondary or balanced station. On a data link only one frame with a P bit set to 1 can be outstanding at any given time. Before a primary or balanced station can issue another frame with a P

10.10 WHAT ARE DATA PROTOCOLS?

297

Figure 10.26 The three control field formats of HDLC.

bit set to 1, it must receive a response frame from a secondary or balanced station with the F bit set to 1. In the NRM mode, the P bit is set to 1 in command frames to solicit response frames from the secondary station. In this mode of operation the secondary station may not transmit until it receives a command frame with the P bit set to 1.

Of course, the F bit is used to acknowledge an incoming P bit. A station may not send a final frame without prior receipt of a poll frame. As can be seen, P and F bits are exchanged on a one-for-one basis. Thus only one P bit can be outstanding at a time. As a result the N(R) count of a frame containing a P or F bit set to 1 can be used to detect sequence errors. This capability is called check pointing. It can be used not only to detect sequence errors but to indicate the frame sequence number to begin retransmission when required.

Supervisory frames, shown in Figure 10.26b, are used for flow and error control. Both go-back-n and continuous (selective) ARQ can be accommodated. There are four types of supervisory frames:

1. Receive ready (RR): 1000 P/ F N(R);

2. Receive not ready (RNR): 1001 P/ F N(R); 3. Reject (Rej): 1010 P/ F N(R); and

4. Selective reject (SRej): 1011 P/ F N(R).

The RR frame is used by a station to indicate that it is ready to receive information and acknowledge frames up to and including N(R) − 1. Also, a primary station may use the RR frame as a command with the poll (P) bit set to 1.

The RNR frame tells a transmitting station that it is not ready to receive additional incoming I frames. It does acknowledge receipt of frames up to and including sequence number N(R) − 1. I frames with sequence number N(R) and subsequent frames, if any, are not acknowledged. The Rej frame is used with go-back-n ARQ to request retransmission of I frames with frame sequence number N(R), and N(R) − 1 frames and below are acknowledged.

Unnumbered frames are used for a variety of control functions. They do not carry

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sequence numbers, as the name indicates, and do not alter the flow or sequencing of I frames. Unnumbered frames can be grouped into the following four categories:

1. Mode-setting commands and responses;

2. Information transfer commands and responses;

3. Recovery commands and responses; and

4. Miscellaneous commands and responses.

The information field follows the control field (Figure 10.25) and precedes the frame check sequence (FCS) field. The I field is present only in information (I) frames and in some unnumbered (U) frames. The I field may contain any number of bits in any code, related to character structure or not. Its length is not specified in the standard (ISO 3309, Ref. 25). Specific system implementations, however, usually place an upper limit on I field size. Some versions require that the I field contain an integral number of octets.

Frame check sequence (FCS). Each frame includes an FCS field. This field immediately follows the I field, or the C field if there is no I field, and precedes the closing flag (F). The FCS field detects errors due to transmission. The FCS field contains 16 bits, which are the result of a mathematical computation on the digital value of all bits excluding the inserted zeros (zero insertion) in the frame and including the address, control, and information fields.

REVIEW EXERCISES

1. What is the basic element of information in a binary system? How much information does it contain?

2. How does one extend the information content of that basic information element (question 1), for example, to construct a binary code that represents, as a minimum, our alphabet?

3. There are many ways we can express the binary 1 and the binary 0. Give at least four. How do we remove ambiguity about meaning (reversing the sense)?

4. How many distinct characters or symbols can be represented by a 4-unit binary code? a 7-unit binary code? an 8-unit binary code? (Hint: Consider a unit as a bit for this argument.)

5. How many information elements (bits) are there is an ASCII character?

6. Give two causes of burst errors.

7. Give the two generic methods of correcting errors on a datalink.

8. Name the three different types of ARQ and define each.

9. Describe the difference between neutral and polar transmission.

10. On a start–stop circuit, where does a receiver start counting information bits?

11. There are three major causes of error on a data link. Name two of them.

12. On start–stop transmission, the mark-to-space transition on the start element tells

REFERENCES 299

the receiver when to start counting bits. How does a synchronous data system know when to start counting bits in a frame or packet?

13. How does a synchronous data receiver keep in synchronization with an incoming bit stream?

14. A serial synchronous NRZ bit stream has a data rate of 19.2 kbps. What is the period of one bit?

15. What is notably richer in transitions per unit time: RZ or NRZ coding?

16. The CCITT V.29 modem operates on the standard analog voice channel at 9600 bps. How can it do this on a channel with a 3100-Hz bandwidth?

17. Name the three basic impairments for data transmission.

18. Name the four types of noise. Indicate the two that a data circuit is sensitive to and explain.

19. Phase distortion, in general, has little effect on speech transmission. What can we say about it for data transmission?

20. Shannon’s formula for capacity (bps) for a particular bandwidth was based only on one parameter. What was/ is it?

21. We usually equalize two voice channel impairments. What are they and how does the equalization work?

22. Why do higher-speed modems use a center frequency around 1800 Hz?

23. Why is the PSTN digital network not compatible with data bit streams?

REFERENCES

1. IEEE Standard Dictionary of Electrical and Electronic Terms, 6th ed, IEEE Std. 100-1996, IEEE, New York, 1996.

2. Equivalence between Binary Notation Symbols and the Significant Conditions of a Two-Con- dition Code, CCITT Rec. V.1, Fascicle VIII.1, IXth Plenary Assembly, Melbourne 1988.

3. R. L. Freeman, Practical Data Communications, Wiley, New York, 1995.

4. R. L. Freeman, Telecommunication Transmission Handbook, 4th ed., Wiley, New York, 1998.

5. Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, EIA/ TIA-232E, Electronics Industries Assoc., Washington, DC, July 1991.

6. Error Performance on an International Digital Connection Forming Part of an Integrated Services Digital Network, CCITT Rec. G.821, Fascicle III.5, IXth Plenary Assembly, Melbourne, 1988.

7. High-Speed 25-Position Interface for Data Terminal Equipment and Data Circuit Terminating Equipment Including Alternative 26-Position Connector, EIA/ TIA-530-A, Electronic Industries Assoc., Washington, DC, June 1992.

8. W. R. Bennett and J. R. Davey, Data Transmission, McGraw-Hill, New York, 1965.

9. Attenuation Distortion, CCITT Rec. G132, Fascicle III.1, IXth Plenary, Melbourne, 1988. 10. C. E. Shannon, “A Mathematical Theory of Communications.” BSTJ 27, 1948.

11. 1200 bps Duplex Modem Standardized for Use in the General-Switched Telephone Network

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and on Point-to-Point 2-Wire Leased Telephone Type Circuits, CCITT Rec. V.22, Fascicle VIII.1, IXth Plenary Assembly, Melbourne, 1988.

12. 9600 bps Modem Standardized for Use on Point-to-Point 4-Wire Leased Telephone Type Circuits, CCITT Rec. V.29, Fascicle VIII.1, IXth Plenary Assembly, Melbourne, 1988.

13. K. Pahlavan and J. L. Holsinger, “Voice Band Communication Modems: An Historical Review, 1919–1988,” IEEE Communications Magazine, 261(1), 1988.

14. Transmission Systems for Communications, 5th ed., Bell Telephone Laboratories, Holmdel, NJ, 1982.

15. Standardization of Data Signaling Rates for Synchronous Data Transmission in the General Switched Telephone Network, CCITT Rec. V.5, Fascicle VIII.1, IXth Plenary Assembly, Melbourne, 1988.

16. Standardization of Data Signaling Rates for Synchronous Data Transmission on Leased Telephone-Type Circuits, CCITT Rec. V.6, IXth Plenary Assembly, Melbourne, 1988.

17. Synchronous Signaling Rates for Data Transmission, EIA-269A, Electronic Industries Assoc., Washington, DC, May 1968.

18. Support of Data Terminal Equipments with V-Series Type Interfaces by an Integrated Services Digital Network, CCITT Rec. V.110, ITU Geneva, 1992.

19. Digital Data System Data Service Unit Interface Specification, Bell System Reference 41450, AT&T, New York, 1981.

20. Information Processing Systems Open Systems Interconnection—Basic Reference Model, ISO 7498, Geneva, 1984.

21. Advanced Data Communications Control Procedures, X.3.66, ANSI, New York, 1979.

22. Interface between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode and Connected to Public Data Networks by a Dedicated Circuit, ITU-T Rec. X.25, ITU Geneva, March 1993.

23. ISDN User Network Interface—Data Link Layer, CCITT Rec. Q.921, Fascicle VI.10, IXth Plenary Assembly, Melbourne, 1988.

24. Information Processing Systems—Local Area Networks, Part 2, Logical Link Control, IEEE Std. 802.3, 1994 ed., IEEE, New York, 1994.

25. High-Level Data Link Control Procedures—Frame Structure, ISO 3309, International Standards Organization, Geneva, 1979.

26. “Military Communication System” Technical Standard, MiL-STD-188C, U.S. Dept. of Defense, Washington, DC 1966.

27. Reference Data for Engineers: Radio, Electronics, Computers and Communications, 8th ed, SAM Publishing, Carmel, IN, 1993.