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Communication Protocols:

Introduction:A communications protocol is a formal description of digital message formats and the rules for

exchanging those messages in or between computing systems and in telecommunications.

Protocols may include signaling, authentication and error detection and correction capabilities. A protocol

defines the syntax, semantics, and synchronization of communication, and the specified behavior is

typically independent of how it is to be implemented. A protocol can therefore be implemented as

hardware or software or both. In computer science the word algorithm is a synonym for the word

procedure so a protocol is to communications what an algorithm is to mathematics.

Communicating systems use well-defined formats for exchanging messages. Each message has an exact

meaning intended to provoke a defined response of the receiver. A protocol therefore describes the

syntax, semantics, and synchronization of communication. A programming language describes the same

for computations, so there is a close analogy between protocols and programming languages: protocols

are to communications what programming languages are to computations. (A less technical reader mightappreciate this similar analogy: protocols are to communications what grammar is to writing.)

The communications protocols in use on the Internet are designed to function in very complex and diversesettings. To ease design, communications protocols are structured using a layering scheme as a basis.

Instead of using a single universal protocol to handle all transmission tasks, a set of cooperating protocols

fitting the layering scheme is used. The layering scheme in use on the Internet is called the TCP/IP model.

The actual protocols are collectively called the Internet protocol suite. The group responsible for thisdesign is called the Internet Engineering Task Force (IETF).

The information exchanged between devices on a network or other communications medium is governed

by rules or conventions that can be set out in a technical specification called a communication protocol

standard. The nature of the communication, the actual data exchanged and any state-dependent behaviors

are defined by the specification. Communicating systems have to communicate with each other using

shared transmission media, because there is no common memory. Transmission is not necessarily reliable

and can involve different hardware and operating systems on different systems.

To implement a networking protocol, the protocol software modules are interfaced with a frameworkimplemented on the machine's operating system. This framework implements the networking

functionality of the operating system. The best known frameworks are the TCP/IP model and the OSI

model.

In communications, message values are transferred using transmission media. By analogy, the equivalent

of a store would be a collection of transmission media, instead of a collection of memory locations. A

valid assignment in a protocol (as an analog of programming language) could be Ethernet:='message' ,

meaning a message is to be broadcast on the local Ethernet.

On a transmission medium there can be many receivers. For instance a mac-address identifies an ether

network card on the transmission medium (the 'ether'). In our imaginary protocol, the assignment

Ethernet[mac-address]:=message value could therefore make sense.

Basic requirements of protocols:Messages are sent and received on communicating systems to establish communications. Protocols should

therefore specify rules governing the transmission. In general, much of the following should be addressed.

Data formats for data exchange. Digital message bit strings are exchanged. The bit strings are dividedin fields and each field carries information relevant to the protocol. Conceptually the bit string is divided

into two parts called the header area and the data area. The actual message is stored in the data area, so

the header area contains the fields with more relevance to the protocol. Bit strings longer than the

maximum transmission unit (MTU) are divided in pieces of appropriate size.

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Address formats for data exchange. Addresses are used to identify both the sender and the intended

receiver(s). The addresses are stored in the header area of the bitstrings, allowing the receivers to

determine whether the bitstrings are intended for themselves and should be processed or should be

ignored. A connection between a sender and a receiver can be identified using an address pair (sender

address, receiver address). Usually some address values have special meanings. An all-1s address could

be taken to mean an addressing of all stations on the network, so sending to this address would result in a

broadcast on the local network. The rules describing the meanings of the address value are collectivelycalled an addressing scheme.

Address mapping. Sometimes protocols need to map addresses of one scheme on addresses of another

scheme. For instance to translate a logical IP address specified by the application to an Ethernet hardware

address. This is referred to as address mapping.

Routing. When systems are not directly connected, intermediary systems along the route to the intended

receiver(s) need to forward messages on behalf of the sender. On the Internet, the networks are connected

using routers. This way of connecting networks is called internetworking.

Detection of transmission errors is necessary on networks which cannot guarantee error-free operation.

In a common approach, CRCs of the data area are added to the end of packets, making it possible for the

receiver to detect differences caused by errors. The receiver rejects the packets on CRC differences and

arranges somehow for retransmission.

Acknowledgements of correct reception of packets is required for connection oriented communication.Acknowledgements are sent from receivers back to their respective senders.

Loss of information - timeouts and retries. Packets may be lost on the network or suffer from long

delays. To cope with this, under some protocols, a sender may expect an acknowledgement of correct

reception from the receiver within a certain amount of time. On timeouts, the sender must assume thepacket was not received and retransmit it. In case of a permanently broken link, the retransmission has no

effect so the number of retransmissions is limited. Exceeding the retry limit is considered an error.

Direction of information flow needs to be addressed if transmissions can only occur in one direction at

a time as on half-duplex links. This is known as Media Access Control. Arrangements have to be made to

accommodate the case when two parties want to gain control at the same time.

Sequence control. We have seen that long bitstrings are divided in pieces, that are send on the network

individually. The pieces may get lost or delayed or take different routes to their destination on some types

of networks. As a result pieces may arrive out of sequence. Retransmissions can result duplicate pieces.By marking the pieces with sequence information at the sender, the receiver can determine what was lost

or duplicated, ask for necessary retransmissions and reassemble the original message.

Flow controlis needed when the sender transmits faster than the receiver or intermediate network

equipment can process the transmissions. Flow control can be implemented by messaging from receiver

to sender.

Getting the data across a network is only part of the problem for a protocol. The data received has to be

evaluated in the context of the progress of the conversation, so a protocol has to specify rules describing

the context. These kind of rules are said to express the syntax of the communications. Other rules

determine whether the data is meaningful for the context in which the exchange takes place. These kind of

rules are said to express the semantics of the communications. Both intuitive descriptions as well as moreformal specifications in the form of finite state machine models are used to describe the expected

interactions of the protocol. Formal ways for describing the syntax of the communications are Abstract

Syntax Notation One (a ISO standard) or Augmented Backus-Naur form (a IETF standard).

Protocol designCommunicating systems operate in parallel. The programming tools and techniques for dealing withparallel processes are collectively called concurrent programming. Concurrent programming only deals

with the synchronization of communication. The syntax and semantics of the communication governed by

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a low-level protocol usually have modest complexity, so they can be coded with relative ease. High-level

protocols with relatively large complexity could however merit the implementation of language

interpreters. An example of the latter case is the HTML language.

Concurrent programming has traditionally been a topic in operating systems theorie texts. Formal

verification seems indispensable, because concurrent programs are notorious for the hidden and

sophisticated bugs they contain. A mathematical approach to the study of concurrency and

communication is referred to as Communicating Sequential Processes (CSP). Concurrency can also bemodelled using finite state machines like Mealy- and Moore machines. Mealy- and Moore machines are

in use as design tools in digital electronics systems, which we encounter in the form of hardware used in

telecommunications or electronic devices in general.

This kind of design can be a bit of a challenge to say the least, so it is important to keep things simple. For

the Internet protocols, in particular and in retrospect, this meant a basis for protocol design was needed to

allow decomposition of protocols into much simpler, cooperating protocols

The OSI ModelThe Open Systems Interconnection (OSI) is a seven-layer protocol stack model used as a reference

throughout the computer industry. The ISO was developed as a functional reference model only. One

must note that some well known protocols such as TCP/IP and Signaling System 7 (SS7) were developedbefore the OSI model, and therefore do not exactly align with the defined OSI boundaries, where protocol

stacks like IBMs SNA have a very close relationship. However, when used as a reference, the OSI modelgives us a general idea of what to expect from the various layers of a given protocol stack.

OSI Stack Protocol Model;

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HDLC Protocol:HDLC (High Level Data Link Control) is one of the most common data link layer (layer 2) protocols.

Many other common layer 2 protocols are heavily based on HDLC, particularly its framing structure. The

HDLC protocol is defined by ISO for use on both point-to-point and multipoint (multidrop) data links. It

supports full duplex transparent-mode operation and is now extensively used in both multipoint and

computer networks.

The rule of HDLC is to ensure that the data has been received without any loss or errors and in the correct

order. HDLC has a flow control function which ensures that the data is transmitted as fast as the receiver

can receive it.

High-level Data Link Control (HDLC) is an enhanced derivative of SDLC from IBM. HDLC is the

default serial link protocol for Cisco routers. Nearly all other routers on the market use PPP for link

control. HDLC is a Data Link layer protocol. These protocols allow computers on either end of a physical

connection (end stations) to communicate.

There are two distinct HDLC implementations: HDLC NRM (also known as SDLC) and HDLC Link

Access Procedure Balanced (LAPB), the later is a more popular implementation. HDLC is usually usedby X.25.

LAPB is a bit-oriented synchronous protocol that provides complete data transparency in a full-duplex

point-to-point operation. It supports a peer-to-peer link in that neither end of the link plays the role of the

permanent master station. HDLC NRM, on the other hand, has a permanent primary station with one or

more secondary stations.

HDLC LAPB is a very efficient protocol, which requires a minimum of overhead to ensure flow control,

error detection and recovery. If data is flowing in both directions (full duplex), the data frames themselves

carry all the information required to ensure data integrity.

The concept of a frame window is used to send multiple frames before receiving confirmation that the

first frame has been correctly been received. This means that data can continue to flow in situations where

there may be long "turn-around" time lags without stopping to wait for an acknowledgement. This kind of

situation occurs, for instance in satellite communication.

Types of HDLC Stations:

PrimaryPrimary stations manage the connection to all secondary sessions by sending commands. Primary

stations open and close connections and poll remote stations for data, or availability. Primary stations

are used primarilly in multi-point networks or with mainframe applications where the mainframe is

the primary station. The primary station uses unbalanced mode to communicate with the secondary

stations. The primary station is responsible for all error detection, line control and data flow control.

SecondarySecondary machines respond to commands sent from the Primary. Secondary stations are

usually terminals attatched to mainframes. Communication between secondary stations and

primary stations is in unbalanced mode. The secondary station can only respond to the primary

station and only when signaled to do so by the primary station.

CombinedCombined stations can establish and tear down their own connections. A combined station has the

capabilities of both primary and secondary stations. These station types are usually used in point-to-pointserial links such as a V.35 link to a CSU/DSU or between routers connected by a T1 or frame relay.

Communication between two combined stations is in balanced mode.

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Configurations of HDLC:HDLC also defines three types of configurations for the three types of stations. The word configuration

refers to the relationship between the hardware devices on a link. Following are the three configurations

defined by HDLC:

Unbalanced Configuration Balanced Configuration Symmetrical ConfigurationUnbalanced ConfigurationThe unbalanced configuration in an HDLC link consists of a primary station and one or more secondary

stations. The unbalanced condition arises because one station controls the other stations. In an unbalanced

configuration, any of the following can be used:

Full-Duplex or Half-Duplex operation Point to Point or Multi-point networksAn example of an unbalanced configuration can be found below in Fig

Balanced ConfigurationThe balanced configuration in an HDLC link consists of two or more combined stations. Each of the

stations has equal and complimentary responsibility compared to each other. Balanced configurations can

use only the following:

Full - Duplex or Half - Duplex operation Point to Point networks

An example of a balanced configuration can be found below in Fig

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Symmetrical ConfigurationThis third type of configuration is not widely in use today. It consists of two independent point-

to-point, unbalanced station configurations as shown in Fig.In this configuration, each station has a

primary and secondary status. Each station is logically considered as two stations.

HDLC Operation Modes;HDLC has three operational modes:

1. Normal Response Mode (NRM)Normal Response Mode is used in unbalanced configurations, a configuration is which there is only one

primary, controlling station. In this mode, slave stations (or secondary) can only transmit when specially

instructed by the master (primary station). The link may be point-to-point or multipoint. In the latter case

only one primary station is allowed.

2. Asynchronous Response Mode (ARM)Asynchronous Response Mode is used in unbalanced configurations. It allows a secondary station to

initiate a transmission without receiving permission from the primary station. This mode is normally used

with point-to-point configurations and full duplex links and allows the secondary station to send framesasynchronously with respect to the primary station.

3. Asynchronous Balanced Mode (ABM)The Asynchronous Balanced Mode (ABM) is used mainly on full duplex point-to-point links for

computer to computer communications and for connections between a computer and a packed switched

data network, in this case each station has an equal status and performs the role of both primary and

secondary functions. This mode is used in the protocol set known as X.25.

HDLC Non-Operational Modes;HDLC also defines three non-operational modes. These three non-operational modes are:

Normal Disconnected Mode (NDM) Asynchronous Disconnected Mode (ADM) Initialization Mode (IM)The two disconnected modes (NDM and ADM) differ from the operational modes in that the secondary

station is logically disconnected from the link (note the secondary station is not physically disconnected

from the link). The IM mode is different from the operations modes in that the secondary station's data

link control program is in need of regeneration or it is in need of an exchange of parameters to be used in

an operational mode.

Protocol Structure - HDLC: High Level Data Link Control

Frame FormatThe standards frame of the HDLC protocol handles both data and control messages.

It has the following format:

Flag Address Control Information FCS Flag

1 Byte 1-2 Bytes 1 Byte variable 2 Bytes 1 Byte

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The Flag fieldEvery frame on the link must begin and end with a flag sequence field (F). Stations attached to the data

link must continually listen for a flag sequence. The flag sequence is an octet looking like 01111110.

Flags are continuously transmitted on the link between frames to keep the link active. Two other bit

sequences are used in HDLC as signals for the stations on the link. These two bit sequences are:

Seven 1's, but less than 15 signal an abort signal. The stations on the link know there is a problemon the link.

15 or more 1's indicate that the channel is in an idle state.

The time between the transmissions of actual frames is called the interframe time fill. The interframe

time fill is accomplished by transmitting continuous flags between frames. The flags may be in 8 bit

multiples.

HDLC is a code-transparent protocol. It does not rely on a specific code for interpretation of line control.

This means that if a bit at position N in an octet has a specific meaning, regardless of the other bits in the

same octet. If an octet has a bit sequence of 01111110, but is not a flag field, HLDC uses a technique

called bit-stuffing to differentiate this bit sequence from a flag field as we have discussed in the previous

lesson.

At the receiving end, the receiving station inspects the incoming frame. If it detects 5 consecutive 1's itlooks at the next bit. If it is a 0, it pulls it out. If it is a 1, it looks at the 8th bit. If the 8th bit is a 0, it

knows an abort or idle signal has been sent. It then proceeds to inspect the following bits to determine

appropriate action. This is the manner in which HDLC achieves code-transparency. HDLC is notconcerned with any specific bit code inside the data stream. It is only concerned with keeping flags

unique.

Address fieldThe address of the secondary station which is sending the frame or the destination of the frame sent by the

primary station. The address field contains Service Access Point (6bits), a Command/Response bit to

indicate whether the frame relates to information frames (I-frames) being sent from the node or received

by the node, and an address extension bit which is usually set to true to indicate that the address is of

length one byte. When set to false it indicates an additional byte follows. The address field may beextended to more than one byte by agreement between the involved parties.

Control field :It identifies the type of the frame. In addition, it includes sequence numbers, control features and error

tracking according to the frame type.

Information field Data is transmitted in the information field, which can vary in length depending upon the protocol using

the frame. Layer 3 frames are carried in the information field.

FCS The Frame Check Sequence (FCS) is in most cases - 16 bit wide and serves for errors detection, the

generator polynomial used is normally CRC-CCITT: x^16+x^12+x^5+1 .HDLC uses zero insertion/deletion process (commonly known as bit stuffing) to ensure that the bit pattern

of the delimiter flag does not occur in the fields between flags. The HDLC frame is synchronous and

therefore relies on the physical layer to provide method of clocking and synchronizing the transmission

and reception of frames.

Field Name Size(in bits)

Flag Field( F ) 8 bits

Address Field( A ) 8 bits

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Control Field( C ) 8 or 16 bits

Information Field( I ) OR Data Variable; Not used in some frames

Frame Check Sequence( FCS ) 16 or 32 bits

Closing Flag Field( F ) 8 bits

Categories of Frames;There are three categories of frames:

Information framestransport data across the link and may encapsulate the higher layers of the OSIarchitecture.

Supervisory framesperform the flow control and error recovery functions. Unnumbered framesprovide the link initialization and termination.

In the HDLC protocol three classes of frames are used:

1. Unnumbered framesUnnumbered frames are used for link management. . SNRM and SABM frames, for example, are used

both to set up logical link between the primary and the secondary station and to inform the secondary

station of the mode of operation to be used. A logical link is subsequently cleared by the primary station

sending a DISC frame. The UA frame is used as an acknowledgment to the other frames in this class.

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Unnumbered Format Commands and responses (U-Frame)The unnumbered format commands and responses are used to extend the number of data link control

functions. The unnumbered format frames have 5 modifier bits, which allow for up to 32 additionalcommands and 32 additional response functions. Below, 13 command functions, and 8 response functions

are described.

Set Normal Response Mode (SNRM) places the secondary station into NRM. NRM does not allow thesecondary station to send any unsolicited frames. Hence the primary station has control of the link.

Set Asynchronous Response Mode (SARM) allows a secondary station to transmit frames without a

poll from the primary station.

Set Asynchronous Balanced Mode (SABM) sets the operational mode of the link to ABM.

Disconnect (DISC) places the secondary station in to a disconnected mode.

Set Normal Response Mode Extended (SNRME) increases the size of the control field to 2 octets

instead of one in NRM. This is used for extended sequencing. The same applies for SARMEand SABME.

Set Initialization Mode (SIM) is used to cause the secondary station to initiate a station-specific

procedure(s) to initialize its data link level control functions.Unnumbered Poll (UP) polls a station without regard to sequencing or acknowledgment.

Unnumbered Information (UI) is used to send information to a secondary station.

Exchange Identification (XID) is used to cause the secondary station to identify itself and provide the

primary station identifications characteristics of itself.

Reset (RSET) is used to reset the receive state variable in the addressed station.

Test (TEST) is used to cause the addressed secondary station to respond with a TEST response at the

first response opportunity. It performs a basic test of the data link control.

Unnumbered Acknowledgment (UA) is used by the secondary station to acknowledge the receipt and

acceptance of an SNRM, SARM, SABM, SNRME, SARME, SABME, RSET, SIM, orDISCcommands.

Disconnected Mode (DM) is transmitted from a secondary station to indicate it is in disconnected

mode(non-operational mode.)

Request Initialization Mode (RIM) is a request from a secondary station for initialization to a primary

station. Once the secondary station sends RIM, it can only respond to SIM, DSIC, TEST or XID

commands.

Request Disconnect (RD) is sent by the secondary station to inform the primary station that it wishes to

disconnect from the link and go into a non-operational mode(NDM or ADM).

Frame Reject (FRMR) is used by the secondary station in an operation mode to report that a condition

has occurred in transmission of a frame and retransmission of the frame will not correct the condition.

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2. Information framesInformation frames are those who carry the actual data. The Information frames can be used to piggyback

acknowledgment information relating to the flow of Information frames in the reverse direction when the

link is being operated in ABM or ARM.

3. Supervisory framesSupervisory frames are use for error and flow control. They contain, send and receive sequence numbers.

There are four types of supervisory frames but only RR and RNR are used in both NRM and ABM. These

frames are used both to indicate the willingness of a secondary station to receive an information frame

from the primary station, and for acknowledgment purposes. REJ and SREJ frames are used only in ABM

which permits simultaneous two-way communication across a point to point link. The two frames areused to indicate to the other station that a sequence error has occurred, that is an information frame

containing an out of sequence N(s) has been received. The SREJ frame is used with a selective repeat

transmission procedure, whereas the REJ frame is used with a go back N procedure.

Supervisory Format commands and responses are as follows:

Receive Ready (RR) is used by the primary or secondary station to indicate that it is ready to receive an

information frame and/or acknowledge previously received frames.

Receive Not Ready (RNR) is used to indicate that the primary or secondary station is not ready to

receive any information frames or acknowledgments.

Reject (REJ) is used to request the retransmission of frames.

Selective Reject (SREJ) is used by a station to request retransmission of specific frames. An SREJ must

be transmitted for each erroneous frame; each frame is treated as a separate error. Only one SREJ can

remain outstanding on the link at any one time.

In a Nutshell;

Information Transfer Information Transfer

Format Commands Format ResponsesI - Information I - Information

Supervisory Format Supervisory Format

Commands ResponsesRR - Receive ready RR - Receive ready

RNR - Receive not ready RNR - Receive not ready

REJ - Reject REJ - Reject

SREJ - Selective reject SREJ - Selective reject

Unnumbered Format Unnumbered Format

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Commands CommandsSNRM - Set Normal Response Mode UA - Unnumbered Acknowledgm

SARM - Set Asynchronous Response Mode DM - Disconnected Mode

SABM - Set Asynchronous Balanced Mode RIM - Request Initialization Mod

DISC - Disconnect RD - Request Disconnect

SNRME - Set Normal Response Mode Extended UI - Unnumbered Information

SARME - Set Asynchronous Response Mode Extended XID - Exchange IdentificationSABME - Set Asynchronous Balanced Mode Extended FRMR - Frame Reject

SIM - Set Initialization Mode TEST - Test

UP - Unnumbered Poll

UI - Unnumbered Information

XID - Exchange identificationRSET - Reset

TEST - Test

The Poll/Final Bit (P/F)The 5th bit position in the control field is called the poll/final bit, or P/F bit. It can only be recognized

when it is set to 1. If it is set to 0, it is ignored. The poll/final bit is used to provide dialogue between theprimary station and secondary station. The primary station uses P=1 to acquire a status response from the

secondary station. The P bit signifies a poll. The secondary station responds to the P bit by transmitting a

data or status frame to the primary station with the P/F bit set to F=1. The F bit can also be used to signal

the end of a transmission from the secondary station under Normal Response Mode.

Protocol Operation

The two basic functions in the protocol are link management and data transfer (which includes error and

flow control).

Link management

Prior to any kind of transmission (either between two stations connected by a point to point link or

between a primary and secondary station connected with a multidrop link) a logical connection between

the two communication parties must be established.

A logical connection between the two communication parties is accomplished by the exchange of two

unnumbered frames. The procedure for setting a logical connection (In a multidrop link) is given below:

The SNRM is first sent by the primary station with the poll bit set to 1 and the address of the appropriate

secondary in the address field. The secondary responds with a UA frame with the final bit set and its own

address in the address field. After all the data has been transferred, the link is cleared by the primary

sending a DISC frame and the secondary responding with a UA.

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Data transfer

In NRM all data (information frames) istransferred under the control of the primarystation. The unnumbered poll frame with the

P bit set to 1 is normally used by the primary

to poll a secondary. If the secondary has no

data to transmit, it returns an RNR frame

with the F bit set. If data is waiting, ittransmits the data, typically as a sequence of

information frames.

The two most important aspects associated with the data transfer phase are error control and flow control.

Essentially, error control uses a continuous RQ procedure with either a selective repeat or a go back N

transmission strategy, while flow controls based on a window mechanism.

TransparencyThe flag sequence must never occur within the content of a frame otherwise it could be confused with an

intentionally sent flag. A technique known as 0-bit insertion is used to prevent random data synthesising a

flag. The technique is said to make HDLC transparent, since any stream of bits may be present between

the open and closing flag of a frame. The transparency is achieved by encoding the data by inserting a 0-

bit after any sequence of 5 consecutive 1's within the payload, as shown:

Insertion of a "zero-bit" into the content of a frame to ensure transparency

Bit stuffing and removal

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FSM of Transparency: