High Level Data Link Control (HDLC)

 HDLC was developed by ISO and has become the most widely accepted data link protocol. It offers a high level of flexibility, adaptability, reliability and efficiency of operation for today as well as tomorrow's synchronous data communication needs. ADCCP developed by ANSI is almost similar to HDLC, IBM'S SDLC is a proper subset of HDLC and level 2 of X-25 is a permissible option of HDLC.

In this chapter, we shall study the basic features and operation of HDLC protocol. Certain liberties have been taken in the level of completeness of description so as not to cloud the overall picture with the details.

GENERAL FEATURES of hdlc  

HDLC is a bit oriented data link control protocol which satisfies wide variety of data link control requirements including:

·         Point-to-point and point-to-multipoint links.

·         Two way simultaneous communication over full duplex circuits.

·         Two way alternate operation over half duplex or full duplex circuits.

·         Synchronous and asynchronous communication.

·         Communication between primary stations and between primary and secondary stations.

·         Full data transparency.

Types of Stations

To make HDLC protocol applicable in various possible network configurations, three types of stations have been defined.

·         Primary station

·         Secondary station

·         Combined station

Communication can be between a primary station and one or more secondary stations . The primary station has the responsibility of link management, i.e. activating and disconnecting the communication link. The secondary stations operate under the control of the primary station. The frames sent by a primary station are called commands and the frames sent by secondary station are called responses. as showing in fig.

A combined station can act as a primary as well as secondary station, i.e. it is capable of link management function, sending and receiving both commands and responses. Such a communication situation occurs when it is between two logical equal stations. 

ERROR DETECTION & COORECTION

ERROR DETECTION

When a code word is transmitted, one or more of its bits may be reversed due to signal impairment. The receiver can detect these errors if the received code word is not one of the valid code word of the code set. If the corrupted received word becomes another valid code word, the error cannot be detected.

When error occurs, the distance between the transmitted and received code words is equal to the number of erroneous bits . as showing in given below figure.

TRANSMITTED CODE WORD	RECEIVED CODE WORD	NUMBER OF ERRORS	DISTANCE
11001100	11001110	1	1
10010010	00011010	2	2
10101010	10100100	3	3

In other words the valid code words must be separated by a distance more than 1 else even a single bit error will generate another valid code word and the error will not be detected. The number of errors which can be detected depends on the distance between any two valid code words. For example, if the valid code words are separated by a distance 4, upto three errors in a code word can be detected. By adding certain number of redundant bits and properly choosing the algorithm for generating them, we ensure some minimum distance between any two valid code words and, therefore, the error detection capability.

ERROR CORRECTION

After an error is detected, there are two approaches to correction of errors :

·          Reverse Error Correction (REC).

·          Forward Error Correction (FFC).

In the first approach, the receiver requests for retransmission of the code word. In the second approach, the code set is so designed that it is possible for the receiver to not only detect but correct the errors also without requesting for retransmission. The receiver either locates the errors by analysing the received code word and reverses the erroneous bits. An alternative way is to search the most likely correct code word. When an error is detected, the distances of all the valid code words from the received invalid code word are measured. The nearest valid code word is the most likely correct version of the received word (Fig.3). If the minimum distance between valid code words is D, upto D/2-1 errors can be corrected. More than D/2-1 errors will cause the received code word to be nearer to the wrong valid code word.

TRANSMISSION ERRORS

Errors are introduced in the data bits during their transmission across a sub network. These errors can be categorised into :

·          Content errors

·          Flow integrity errors

Content errors are the errors in the content or a message, e.g. a "1" may be received as "0". This type of errors gets introduced due to impairment of the electrical signal in the transmission media.

Flow integrity errors refer to missing blocks of data. For example, a data block may be lost in the sub-network due to its having been delivered to a wrong destination.

In voice communication, the listener can tolerate a good deal of signal corruption during transmission. But data is very sensitive to errors. Measures are, therefore, built into a data communication system to counteract the effect of errors. These measures include:

·                     Introduction of additional check bits in the data bits to detect and correct content errors.

·                     Establishing procedures of data exchange which enable recovery of corrupted/lost messages.

BIT ERROR RATE (BER)

In analog transmission, signal quality is specified in terms of Signal to Noise ratio (S/N) which is usually expressed in decibels. In digital transmission, the quality of received digital signal is expressed in terms of Bit Error Rate (BER) which is number of errors in a fixed number of transmitted bits. A typical error rate on a high quality leased telephone line is as low as 1 error in 10⁶ bits or simply 1 x 10^-6.

OSI REFERENCE MODEL

The International Organization introduced the OSI layer for Standardization (ISO) in 1984 in order to provide a reference model to make sure products of different vendors would interoperate in networks. OSI is short for Open System Interconnection.

The OSI layer shows WHAT needs to be done to send data from an application on one computer, trough a network, to an application on another computer, not HOW it should be done.  A layer in the OSI model communicates with three other layers: the layer above it, the layer below it, and the same layer at its communication partner. Data transmitted between software programs passes all 7 OSI layers. The Application, Presentation and Session layers are also known as the Upper Layers.

The Data Link and Physical layers are often implemented together to define LAN and WAN specifications. 

Application Layer (Layer 7)

Application Layer provides network services directly to applications. Type of software programs vary a lot: from groupware and web browser to Tactical Ops (video game). Software programs itself are not part of the OSI model. It determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication. This layer is closest to the user. Gateways operate at this layer. Following are the examples of Application layer protocols:

i)                 Telnet

ii)               SMTP

iii)             FTP

iv)             SNMP

v)               NCP

vi)             SMB

Presentation Layer (Layer 6)

Presentation Layer defines coding and conversion functions. It ensures that information sent from the application layer of one system is readable by the application layer of another system. It includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:

i)          MPEG, QuickTime

ii)        ASCII, EBCDIC

iii)      GIF, TIFF, JPEG

Gateways operate at this layer. It transmits data to lower layers.

Session Layer (Layer 5)

The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes. It provides error reporting for the Application and Presentation layer. Examples of Session layer protocols are:

i)                 NFS

ii)               SQL

iii)             RPC

iv)             Zone Information Protocol (ZIP)

Gateways operate at this layer. It transmits data to lower layers.

Transport Layer (Layer 4)

The main purpose of this layers is making sure that the data is delivered error-free and in the correct sequence. It establishes, maintains and terminates virtual circuits. It provides error detection and recovery. It is concerned with reliable and unreliable transport. When using a connection-oriented, reliable transport protocol, such as TCP, acknowledgments is send back to the sender to confirm that the data has been received. It provides Flow Control and Windowing. It provides multiplexing; the support of different flows of data to different applications on the same host. Examples of Transport layer protocols are:

i)                 TCP (connection-oriented, reliable, provides guaranteed delivery.)

ii)               UDP (connectionless, unreliable, less overhead, reliability can be provided by the Application layer)

iii)             SPX

Gateways operate at this layer. It transmits data to lower layers.

Network Layer (Layer 3)

This layer defines logical addressing for nodes and networks/segments. It enables internetworking, passing data from one network to another. It defines the logical network layout so routers can determine how to forward packets trough an internet-work. Routing occurs at this layer, hence Routed and Routing protocols reside on this layer. Routed protocols are used to encapsulate data into packets. The header added by the Network layer contains a network address so it can be routed trough an internet-work. Examples of Network layer Routed protocols are:

i)        IP

ii)      IPX

iii)    AppleTalk

Routing protocols are used to create routing tables; routing tables are used to determine the best path / route. Routing protocols provide periodic communication between routers in an Internet work to maintain information on network links in a routing table. It transmits Packets. Routers operate at this layer. Examples of Network layer Routing protocols are:

i)                 OSPF

ii)               IGRP/EIGRP

iii)             RIP

iv)             BGP

v)               NLSP

Data Link Layer (Layer 2)

It defines psychical addressing, network topology, and is also concerned with error notification, sequencing of frames and flow control. Examples of network topologies are:

i)                 Star

ii)               Bus

iii)             Ring 

Physical Layer (Layer 1)

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. It transmits and receives bits (bit stream) to transmission media. Physical layer specifications define characteristics such as:

• Voltage levels
• Timing of voltage changes
• Physical data rates
• Maximum transmission distances
• Physical connectors 

different between TRANSMISSION AND COMMUNICATION

Let us now understand the difference between transmission and communication. Transmission means physical movement of information from one point to another. Communication means meaningful exchange of information between the communicating devices.

Example

Two persons, one knowing English language only and the other knowing French language only cannot communicate with each other.

Here transmission is taking place, but communication is not there. Therefore, for communication, we need much more than the transmission. For communication, we must have the same language, i.e. Data codes should be understood both by transmitter and the receiver. Moreover, receiver should be in a position to receive, i.e. Timing is also very important.

We have two types of communication :

(1)               Synchronous Communication.

(2)               Asynchronous Communication.

Synchronous Communication

In Synchronous communication the exchange of information is in a well disciplined manner, e.g. if A want to send some information to B, it can do so only when B permits it to send. Similarly, vice-versa is true. There is complete synchronisation of dialogues, i.e. each message of the dialogue is either a command or a response. Physical transmission of data may be in synchronous or asynchronous mode already decided between A and B.

Asynchronous Communication

In Asynchronous communication the exchange of information is in less disciplined manner, e.g. A and B can send messages whenever they wish to do so. Physical transmission of data may be in synchronous / asynchronous mode.

Thus, we see that Simplex Transmission is one way communication (OW), Half Duplex Transmission is two way Alternate Communication (TWA), and Full Duplex Transmission is two way Simultaneously Communication (TWS).

Remote OMT and Remote OMT over IP

The features Remote OMT (Operation and Maintenance Terminal) and Remote OMT over IP are updated to support the new RBS 6000 DUG-20/RUS-01 configurations. In MCPA backwards compatible mode, having a BTS G11A or newer in a BSS 07B-G10B network, the configuration of an MCPA is made using OMT. Configuration in MCPA single mode (BTS G11B with BSS G10B or newer) is made from the BSC, refer to Section 5.7 on page 43.

All TRXs in a DUG are connected to one or several RUSs. It is the connections between RUSs and antennas that will decide which MCPAs to use for which antenna sectors (cells).

Each antenna sector is configured in the OMT with the default configuration of 3*20W (3*43.0 dBm) per MCPA. If desired it is possible to choose a different number of TRXs per MCPA, and it is possible to choose some configurations where the total MCPA mean power will end up on less than 60 W (47.8 dBm). Also the levels 40W (46.0 dBm) and 20W (43.0 dBm) are available.

The chosen number of TRXs and MCPA maximum mean power corresponds to different GSM RUS HW activation codes, although there is no licensing mechnism involved in the OMT based configuration.

Antenna Control - Standard TMA

The RBS 6000 Radio Units have built-in functionality for what would otherwise require external equipment so there is no need for installation of extra hardware for supervision and power feed for TMAs. The radio units in RBS 6000 will provide the power and also supervise standard TMAs by monitoring the current consumed by the TMA.

Commands:

• RXBFC, Radio X-ceiver Administration, BTS Feature Data, Change

The parameter to activate Standard TMA (CSTMA) is added to this existing

command.

Printouts:

• RADIO X-CEIVER ADMINISTRATION BTS FEATURE DATA (command RXBFP)

• RADIO X-CEIVER ADMINISTRATION MANAGED OBJECT

CONFIGURATION DATA (command RXCDP)