data communications protocol

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DATA Communications DATA Communications ProtocolProtocol


IntroductionIntroduction A protocol is a set of rules which governs

how data is sent from one point to another. In data communications, there are widely accepted protocols for sending data. Both the sender and receiver must use the same protocol when communicating. One such rule is . . . .

BY CONVENTION, THE LEAST SIGNIFICANT BIT IS TRANSMITTED FIRST


Need For Protocol Architecture

E.g. File transfer Source must activate communications. Path or

inform network of destination Source must check destination is prepared to receive File transfer application on source must check

destination file management system will accept and store file for his user

May need file format translation Task broken into subtasks Implemented separately in layers in stack Functions needed in both systems Peer layers communicate


Key Elements of a Protocol Syntax

Data formats Signal levels

Semantics Control information Error handling

Timing Speed matching Sequencing


Protocol Architecture Task of communication broken up

into modules For example file transfer could use

three modules File transfer application Communication service module Network access module


Simplified File Transfer Architecture


A Three Layer Model Network Access Layer Transport Layer Application Layer


Network Access Layer Exchange of data between the

computer and the network Sending computer provides address

of destination May invoke levels of service Dependent on type of network used

(LAN, packet switched etc.)


Transport Layer Reliable data exchange Independent of network being used Independent of application


Application Layer Support for different user

applications e.g. e-mail, file transfer


Protocol Architectures and Networks


Addressing Requirements Two levels of addressing required Each computer needs unique

network address Each application on a (multi-tasking)

computer needs a unique address within the computer The service access point or SAP The port on TCP/IP stacks


Protocols in Simplified Architecture


Protocol Data Units (PDU) At each layer, protocols are used to

communicate Control information is added to user data at

each layer Transport layer may fragment user data Each fragment has a transport header added

Destination SAP Sequence number Error detection code

This gives a transport protocol data unit


Protocol Data Units


Network PDU Adds network header

network address for destination computer

Facilities requests


Standardized Protocol Architectures Required for devices to communicate Vendors have more marketable products Customers can insist on standards based

equipment Two standards:

OSI Reference model Never lived up to early promises

TCP/IP protocol suite Most widely used

Also: IBM Systems Network Architecture (SNA)


David Clark’s Theory of Standards

Time

Activit

y

ResearchBillion Dollars of Investments

Standards

Apocalypse of two elephants


THE NEED FOR STANDARDS

Over the past couple of decades many of the net-works that were built used different hardware and software implementations, as a result they were incompatible and it became difficult for networks using different specifications to communicate with each other.

To address the problem of networks being incompatible and unable to communicate with each other, the International Organisation for Standardisation (ISO) researched various network schemes.

The ISO recognised there was a need to create a NETWORK MODEL that would help vendors create

interoperable network implementations.


ISO - ORGANIZATION FOR STANDARDIZATION

The International Organisation for Standardisation (ISO) is an International standards organisation responsible for a wide range of standards, including many that are relevant to networking.

In 1984 in order to aid network interconnection without necessarily requiring complete redesign, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture. (ISO 7498)


THE OSI REFERENCE MODEL The model was developed by the International Organisation for

Standardisation (ISO) in 1984. It is now considered the primary Architectural model for inter-computer communications.

The Open Systems Interconnection (OSI) reference model is a descriptive network scheme. It ensures greater compatibility and interoperability between various types of network technologies.

The OSI model describes how information or data makes its way from appli-cation programmes (such as spreadsheets) through a network medium to another application programme located on another network.

The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems .

This separation into smaller more manageable functions is known as layering.


A LAYERED NETWORK MODEL

The OSI Reference Model is composed of seven layers, each specifying particular network functions.

The process of breaking up the functions or tasks of networking into layers reduces complexity.

Each layer provides a service to the layer above it in the protocol specification.

Each layer communicates with the same layer’s software or hardware on other computers.

The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network.

The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications.

Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.


THE SEVEN OSI REFERENCE MODEL LAYERS


LAYER 1: PHYSICAL The physical layer deals with the physical

characteristics of the transmission medium. It defines the electrical, mechanical,

procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

Examples :- EIA/TIA-232, RJ45, NRZ.


LAYER 2: DATA-LINK Responsible for communications between the

primary and secondary nodes within the network Provides the final framing of the information

envelope Facilitates the orderly flow of data between

nodes Allows error detection and correction Example of data link layer protocols are the

bisync and SDLC


LAYER 2: DATA LINK The data link layer provides access to the networking media

and physical transmission across the media and this enables the data to locate its intended destination on a network.

The data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses.

The data link layer uses the MAC address to define a hardware or data link address in order for multiple stations to share the same medium and still uniquely identify each other.

Concerned with network topology, network access, error notification, ordered delivery of frames, and flow control.

Examples :- Ethernet, Frame Relay, FDDI.


LAYER 3: NETWORK Defines end-to-end delivery of packets. Defines logical addressing so that any endpoint

can be identified. Defines how routing works and how routes are

learned so that the packets can be delivered. The network layer also defines how to fragment

a packet into smaller packets to accommodate different media.

Routers operate at Layer 3. Examples :- IP, IPX, AppleTalk.


LAYER 3: NETWORKLAYER 3: NETWORK Determines which network configuration

(dial-up, leased, or packet) is most appropriate for the function provided by the network

Defines the mechanisms in which messages are broken into data packets and routed from a sending node to a receiving node within the communications network


LAYER 4: TRANSPORT Controls the end-to-end integrity of

the message, which includes routing, segmenting and error recovery

It is the highest layer in terms of communications

Acts as the interface between the session and the network layer


LAYER 4: TRANSPORT The transport layer regulates information flow to ensure end-to-

end connectivity between host applications reliably and accurately.

The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system.

The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues.

Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).


LAYER 5: SESSION Responsible for network availability

(I.e. buffer storage and processor capacity)

Responsible for log-in and log-off procedures and user authentication

Determines the type of transmission modes used


LAYER 5: SESSION The session layer defines how to start, control

and end conversations (called sessions) between applications.

This includes the control and management of multiple bi-directional messages using dialogue control.

It also synchronizes dialogue between two hosts' presentation layers and manages their data exchange.

The session layer offers provisions for efficient data transfer.

Examples :- SQL, ASP(AppleTalk Session Protocol).


LAYER 6: PRESENTATION Functions include data file formatting,

encoding, encryption and decryption of messages, dialogue procedures, data compression, synchronization, interruption and termination

Performs code and character set translation and determines the display mechanism for messages


LAYER 6: PRESENTATION The presentation layer ensures that the

information that the application layer of one system sends out is readable by the application layer of another system.

If necessary, the presentation layer translates between multiple data formats by using a common format.

Provides encryption and compression of data. Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.


LAYER 7: APPLICATION The general manager of the network Controls the sequence of activities

within an application and also the sequence of events between the computer application and the user of another application

Communicates directly with the user’s application program


LAYER 7: APPLICATION The application layer is the OSI layer that is closest to the

user.

It provides network services to the user’s applications.

It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model.

Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs.

The application layer establishes the availability of intended communication partners, synchronizes and establishes agreement on procedures for error recovery and control of data integrity.


SENDING DATA via the OSI Model


Asynchronous ProtocolsAsynchronous Protocols A character-oriented protocol. It uses VRC as the

only type of error detection. Symbol substitution and ARQ (retransmission) are used for error correction. The polling sequence usually encompasses sending one or two data link control characters, then a station polling address.

Each secondary station is generally limited to a single terminal/ printer pair

More than one station can be selected simultaneously with group or broadcast addresses

There is a single broadcast address that is used to simultaneously poll all the remote stations (usually FFH)


Example of Asynchronous Protocol Selective Calling System Asynchronous Data Link Protocol


Operating Modes of Remote Stations Line monitoring mode – the LCU is cleared, a

secondary is in the “listen” mode Transmit mode – a secondary station is in the

transmit mode whenever it has been designated master

Receive mode – a secondary station is in the receive more whenever it is selected by a primary as its receiver

Local mode – For a terminal operator to enter information into his computer terminal, the terminal must be in the local mode. A terminal can be placed in the local mode through software commands sent from primary or the operator can do it manually from the keyboard


Typical Polling Sequence

EOT

DC3

A


Poll Response

Acknowledgment

Function

A

\C No message to transmit, ready to receive

K

\\ No message, not ready to receive


Selection sequenceEOT XY

Acknowledgment

Function

A

\C Ready to receive

K

\\ Not ready to receive, terminal in local, or printer out of paper

** Not ready to receive, have a formatted message to transmit

Acknowledgements in response to a selection poll.


Asynchronous data format

SOH

HeadingSTX

Message dataEOT

Format for transmitting heading information together with message data


Synchronous Protocols With synchronous protocols, a

secondary station can have more than a single terminal/ printer pair.

The group of devices is commonly called a cluster.

A single LCU can serve a cluster with as many as 50 devices.

Synchronous protocols can either character or bit-oriented.


Synchronous ProtocolsSynchronous Protocols1. IBM’s Bisync Protocol2. SDLC3. HDLC

ISO 3309-1976(E) ISO 4335-1979(E) ISO 7809-1985(E)


IBM’s Bisync Protocol A character-oriented synch protocol. Each

transmission is preceded by a unique SYN character (16H for ASCII, 32H for EBCDIC).

The SYN character places the receive USRT in the character or byte mode and prepares it to receive data in 8-bit groupings.

With bisync, SYN character are always transmitted in pairs. A secondary response to a poll with a formatted message or with a handshake[1].

Group and broadcast selections are not allowed.

[1] Handshake is a negative acknowledgement to a poll


Bisync

Polling Formats General

Specific

PAD

SYN

SYN

EOT

PAD

SYN

SYN

SPA

SPA

“ “ENQ

PAD

PAD

SYN

SYN

EOT

PAD

SYN

SYN

SPA

SPA

DA

DA

ENQ

PAD

A specific poll is an invitation for a specific device at a given station to transmit its message.


Character Sequence for a Selection

PAD

SYN

SYN

EOT

PAD

SYN

SYN

SSA

SSA

DA

DA

ENQ

PAD


Character Sequence for a handshake

PAD

SYN

SYN

EOT

PAD

The PAD character at the beginning of the sequence is called a leading pad and is either a 55H or an AAH.


ACK in Response to a Poll The character sequence for a positive acknowledgment

A negative acknowledgment is called a reverse interrupt (RVI). The character sequence for an RVI is

PAD

SYN

SYN

DLE

0/1PAD

PAD

SYN

SYN

DLE

<PAD


Formatted Data Messages

PAD

SYN

SYN

SOH

headingSTX

messageETX

BCC

PAD


Synchronous Data Link Communications (SDLC)


SDLC An example of bit-oriented synch protocol. With

BOP, there is a single control field that performs essentially all the data link control functions.

The character language used with SDLC is EBCDIC and data are transferred in groups called frames.

There are three transmission states in SDLC: transient,

The transient state exists before and after the initial transmission and after each line turnaround.

idle The idle state is presumed after 15 seconds or more

consecutive 1’s have been received. active

The active state exists whenever either the primary or secondary station is transmitting information or control signals.


Five Fields used with SDLC Information field

contains information transmitted in an SDLC frame. The number of bits in the information field must be a multiple of 8.

Flag field used for delimiting sequence and to achieved character

synchronization. The delimiting sequence sets the limits of the frame. The sequence for a flag is 7EH (01111110).

Address field contains the address of the destination station, and has 8 bits. The

address 00H is called the null or void address and is never assigned to a secondary. The null address is used for network testing. The address FFH is the broadcast address and is common to all secondaries.

Control field an 8-bit field that identifies the type of frame it is; used for polling,

confirming previously received information frames Frame Check Sequence (FCS) field

contains the error detection mechanism for SDLC. It is equivalent to BCC use with bisync. SDLC uses CRC-16.


Three Frame Formats with SDLC Information frame

used for transmitting sequenced information; identified by a 0 (zero) in the LSB position

Supervisory frame used to assist in the transfer of information;

used to confirm previously received information frames, convey ready or busy conditions, and to report frame numbering errors; identified by a 01 in bit position b6 and b7

Unnumbered frame identified by making bits b6 and b7 in the

control field a 11.


High-level Data Link Control (HDLC) HDLC is a superset of SDLC Comprises three standards that

outlined the frame structure, control standards, and class of operation for a bit-oriented data link control. ISO 3309-1976 (E) ISO 4335-1979 (E) ISO 7809-1985 (E)


ISO 3309-1976(E)ISO 3309-1976(E) defines the frame structure,

delimiting sequence and transparency mechanism used with HDLC


ISO 4335-1979(E)ISO 4335-1979(E) Defines the elements of procedure for HDLC.

The control field, information field and supervisory format have increased capabilities over SDLC. Control field

With HDLC, the control field can be extended to 16 bits. Information field

HDLC permits any number of bits in the information field of an information command or response.

Supervisory format With HDLC, the supervisory format includes a fourth

status condition: selective reject (SREJ)


ISO 7809-1985(E)ISO 7809-1985(E) combines previous standards 6159 (E),

unbalanced and 6256 (E), balanced and outlines the class of operation necessary to establish the link-level protocol/

Unbalanced operation – logically equivalent to a multipoint private line circuit with a polling environment. There is a single primary station responsible for central control of the network. Data transmission may be either half- or full-diplex

Balanced operation – logically equivalent to a two-point private line circuit.


Operational Modes1. Asynchronous Response Mode (ARM)

Secondary stations are allowed to send unsolicited responses; to transmit, a secondary does not need to have received a frame from the primary with the P bit set

2. Asynchronous Disconnect Mode (ADM) Identical to the normal disconnect mode

except that the secondary can initiate a DM or RIM response anytime


Sliding Window Protocol Because frames are numbered, it is

possible for a primary station to transmit a number of frames without receiving an acknowledgement for each frame. The secondary can store the incoming frames and reply using a supervisory frame with the sequence number bits in the control field set to acknowledge a group of received frames.


Sliding Window Protocol If the secondary runs out of buffer space to

store incoming Information frames, it can transmit a supervisory frame informing primary stations of its status. Primary stations will thus keep their Information frames and wait till the secondary is again able to process Information frames.

When a secondary cannot process Information frames, it must still be able to process incoming supervisory and unnumbered frames (because of status requests).


Public Data NetworkPublic Data NetworkA switched data communications

network similar to the Public Telephone Network (PTN) except that a PDN is designed for transmitting data only; it combines the concepts of both Value-Added Networks (VANs) and Packet-Switching Networks


Value-Added Network “Adds value” to the services or facilities

provided by a common carrier to provide new types of communications services

Examples of “Added Values”: Error control Enhanced connection reliability Dynamic routing Failure protection Logical multiplexing Data format conversions


Packet-Switching NetworkPacket switching involves dividing data messages into Packet switching involves dividing data messages into small bundles of information and transmitting them small bundles of information and transmitting them through communications network to their intended through communications network to their intended destinations using computer-controlled switches.destinations using computer-controlled switches.

Three Common Switching Techniques Used with PDN:

1. Circuit Switching

2. Message Switching

3. Packet Switching


Circuit Switching the telephone is establish, information is transferred

and then the call is disconnected The time required to establish the call is called setup

time After a call has been established, information is

transferred in real time BlockingBlocking can occur since there are limited number of

circuits & switching paths available Blocking is the inability to complete a call because there are

no facilities or switching paths available between the source and the destination locations

The terminal equipment at the source & destination must be compatible, they must use modems with same bit rate, character set and protocol

Used for making a standard telephone call on the PTN


Message SwitchingMessage SwitchingA form of store-and-forward network

- Each switch within the network has message storage capabilities

- The terminal equipments need not be compatible

- A message switch can store data and change its format and bit rate, then convert the data back to their original form

- Multiplexes data from different sources onto a common facility


Packet Switching data are divided into smaller

segments called packets prior to transmission through the network; it is called a hold-and-forward network


ComparisonCircuit switching Message switching Packet switching

Dedicated transmission path No dedicated transmission path No dedicated transmission path

Continuous transmission of data Transmission of messages Transmission of packets

Operation in real time Not real time Near real time

Messages not stored Messages stored Messages held for short time

Path establish for entire message Route establish for each message Route establish for each packet

Call setup delay Message transmission delay Packet transmission delay

Busy signal if called party busy No busy signal No busy signal

Blocking may occur Blocking can not occur Blocking can not occur

User is responsible for message loss protection

Network responsible for loss messages Network responsible for each packet but not for entire message

No speed or code conversion Speed and code conversion Speed and code conversion

Fixed bandwidth transmission Dynamic use of bandwidth Dynamic use of bandwidth

No overhead bits after initial setup delay

Overhead bits in each message Overhead bits in each packet


CCITT X.1CCITT X.1 International User Class of ServiceInternational User Class of Service

Three modes of transmissionStart/Stop mode data are transferred from the source to the network

and from the network to the destination in an asynchronous data format. Call control signaling is done in International Alphabet No. 5 (ASCII-77).

Two protocols used in this mode: IBM’s 83B protocol AT&T’s 8A1/B1 selective calling arrangement

Synchronous modePacket mode



Three modes of transmissionStart/Stop modeSynchronous mode data are transferred from the source to the network

and from network to the destination in a synchronous data format.

Common protocols used: IBM’s 3270 bisync Burrough’s BASIC UNIVAC’s UNISCOPE

Packet mode



Three modes of transmissionStart/Stop modeSynchronous modePacket mode Data are transferred in a frame format;

data are divided into smaller packets and transferred in accordance with the CCITT X.25 – User-to-Network interface protocol


Packetmode

Synchronous mode

Asynchronous mode

Asynchronous mode

Packet mode

Synchronous mode

PDN

PDN

Public data network


CCITT X.25 User-to-Network CCITT X.25 User-to-Network Interface ProtocolInterface Protocol

Three switching services offered in a switched data network.

Permanent Virtual CircuitVirtual CallDatagram

In 1976, X.25 user interface is designated as the international standard for packet network access.Addresses only the physical, data-link and network layers of the OSI.


Permanent Virtual Circuit Logically equivalent to a two-point dedicated

private line circuit except slower. A PVC is slower because a hardwired end to end connection is not provided.

The first time a connection is requested, the appropriate switches and circuits must be established through the network to provide the interconnection.

A PVC identifies the routing between two predetermined subscribers of the network that is used for all subsequent messages.

With a PVC, a source and destination address is unnecessary because the two users are fixed.


Virtual Call Logically equivalent to making a telephone

call through the DDD network except no direct end-to-end connection is made.

A VC is a one-to-many arrangement. VCs are temporary virtual connections that

use common usage equipment and circuits. The source must provide its address and

the address of the destination before a VC can be completed.


Datagram Users send small packets of data into

the network. The network does not acknowledge

packets nor does it guarantee successful transmission. However, if a message will fit into a single packet, a DG is somewhat reliable.

It is a “single-packet-per-segment” protocol.


X.25 Packet Format There are two packet formats used with

virtual calls: A call request packet A data transfer packet


Call Request Packet Format

FlagLink

Addressfield

LinkControl

field

Formatidentifier

LogicalChannel identifier

Packet type

CallingAddresslength

CalledAddresslength

Calledaddress

Callingaddress

0Facilities

FieldLength

FacilitiesProtocol

IDUser data

FrameCheck

SequenceFlag

Point mouse over the field to see description.


Data Transfer Packet format

FlagLink

Addressfield

LinkControl

field

Formatidentifier

LogicalChannel identifier

Send packetSequenceNumber

P(s)

0

Receive packetSequence Number

P(r)

0User data

FrameCheck

SequenceFlag


CCITT X Series StandardsX.1

International user classes of service in public data networks. Assigns numerical class designation to different terminal speeds and types

X.2International user services and facilities in public data networks. Specifies essential and additional services and

facilities

X.3Packet assembly/disassembly facility (PAD) in a public data network. Describe the packet

assembler/disassembler, which is normally used at a network gateway to allow connection of a start/stop terminal to a packet network

X.20-bisUsed in public data networks of DTE designed for interfacing to asynchronous full-duplex V-series modems.

Allows used of V.24/V.28

X.21-bisUsed in public data networks of DTE designed for interfacing to synchronous full-duplex V-series modems.

Allows used of V.24/V.28

X.25Interface between DTE and DCE for terminals operating in the packet mode on public data networks. Defines

the architecture of three levels of protocols existing in the serial interface cable between a packet-mode terminal and a gateway to a packet network

X.28DTE/DCE interface for a start/stop mode DTE accessing the PAD in a public data network situated in the same

country. Defines the structure of protocols existing in a serial interface cable between a start/stop terminal and an X.3 PAD

X.29Procedures for exchange of control information and user data between a PAD and a packet mode DTE or

another PAD. Defines the architecture of protocols behind the X.3 PAD, either between two PADs or between a PAD and a packet-mode terminal on the other side of the network

X.75Terminal and transit call procedures and data transfer system on international circuits between packet-

switched data networks. Defines the architecture of protocols between two public packet networks

X.121International numbering plan for public data networks. Defines numbering plan including code assignments for

each nation


Local Area NetworkA data communications network that is designed to provide two-way communications between a large variety of data communications terminal equipment within a relatively small geographic area


Local Area Networks (LAN)Local Area Networks (LAN) LAN System Configuration

Topology (or physical architecture of a LAN) identifies how the stations are interconnected. Mostly used topologies are mesh, star, bus and ring.

Connecting Medium used in most LANs are coaxial cable (overall length of 1500 m) and optical fiber.

Transmission Format Baseband – uses the connecting medium as a single-

channel device. Only one station can transmit at a time and all stations must transmit and receive the same type of signal.

Broadband – uses the connecting medium as a multi-channel device. It uses frequency-division multiplexing technique.


Transmission Format Summary

Baseband Broadband

Characteristics

Digital signaling Analog signaling (requires RF modem)

Entire bandwidth used by signal FDM possible (i.e., multiple data channel)

Bidirectional Unidirectional

Bus topology Bus topology

Maximum length approx. 1500 meters Maximum length up to tens of kilometers

Advantages

Less expensive High capacity

Simpler technology Multiple traffic types

Easy and quick to install More flexible circuit configuration

Larger area coverage

Disadvantages

Single channel Modem required

Limited capacity Complex installation and maintenance

Grounding problems Double propagation delay

Limited distance


Channel Accessing in LAN CSMA/CD

A station monitors the line to determine if the line is busy. A station is not guaranteed access to the network. Used by most baseband LANs in the bus topology

Token Passing Best suited for a ring topology with either a

baseband or broadband network. An electrical token is circulated around the ring from station to station. In order to transmit, a station must possess the token.


CSMA/CD Carrier sense — Each station continuously

listens for traffic on the medium to determine when gaps between frame transmissions occur.

Multiple access —Stations may begin transmitting any time they detect that the network is quiet (there is no traffic).

Collision detect —If two or more stations in the same CSMA/CD network (collision domain) begin transmitting at approximately the same time, the bit streams from the transmitting stations will interfere (collide) with each other, and both transmissions will be unreadable. If that happens, each transmitting station must be capable of detecting that a collision has occurred before it has finished sending its frame. Each must stop transmitting as soon as it has detected the collision and then must wait a quasi-random length of time (determined by a back-off algorithm) before attempting to retransmit the frame.


Csma/ca Short for Carrier Sense Multiple Access/Collision

Avoidance, A network contention protocol that listens to a

network in order to avoid collisions, unlike CSMA/CD that deals with network transmissions once collisions have been detected.

CSMA/CA contributes to network traffic because, before any real data is transmitted, it has to broadcast a signal onto the network in order to listen for collision scenarios and to tell other devices not to broadcast.


Classification of LAN IEEE 802.3

Ethernet environment, bus or star topology, uses CDMA/CD

IEEE 802.4 Bus topology using Token Passing (Token

Bus) LAN IEEE 802.5

Ring topology using Token Passing (Token Ring) LAN

IEEE 802.11 Wireless LAN


A base band system which uses a bus topology originally developed by Xerox Corporation and runs at 3 Mbps data rate.

Refers to the family of local-area network (LAN) products covered by the IEEE 802.3 standard that defines what is commonly known as the CSMA/CD protocol.

Ethernet


ETHERNETSoftware description

Information is transmitted from one station to another in the form of packets

Hardware description the transmission used is the coaxial cable a cable segment may have a maximum distance of 500 m

(1640 ft) each segment may have up to 100 transceiver, attach by way

of pressure taps maximum of three segments may be connected end to end by

way of repeaters to extend the system length to 1500 metersSystem Operation Transmission – the data link control is the CSMA/CD Reception

the line is monitored until the station’s address is detected the controller strips the preamble, checks the CRC, and convert the

Manchester code back to digital format the decoding is accomplished through a phased-lock loop


Ethernet Data Rates 10 Mbps—10Base-X Ethernet (Basic

Ethernet) 100 Mbps—Fast Ethernet 1000 Mbps—Gigabit Ethernet 10000 Mbps—10 Gigabit Ethernet

(IEEE 802.3ae)


Ethernet Protocol Characteristics Is easy to understand, implement,

manage, and maintain Allows low-cost network

implementations Provides extensive topological flexibility

for network installation Guarantees successful interconnection

and operation of standards-compliant products, regardless of manufacturer


Logical Relationship to OSI Reference Model


MAC-Client Sublayer Maybe one of the following:

Logical Link Control (LLC), if the unit is a DTE. This sublayer provides the interface between the Ethernet MAC and the upper layers in the protocol stack of the end station. The LLC sublayer is defined by IEEE 802.2 standards.

Bridge entity, if the unit is a DCE. Bridge entities provide LAN-to-LAN interfaces between LANs that use the same protocol (for example, Ethernet to Ethernet) and also between different protocols (for example, Ethernet to Token Ring). Bridge entities are defined by IEEE 802.1 standards.


MAC Layer The MAC layer controls the node's access to the

network media and is specific to the individual protocol.

All IEEE 802.3 MACs must meet the same basic set of logical requirements, regardless of whether they include one or more of the defined optional protocol extensions.

The only requirement for basic communication (communication that does not require optional protocol extensions) between two network nodes is that both MACs must support the same transmission rate.


Functions of MAC Sublayer Data encapsulation, including frame

assembly before transmission, and frame parsing/error detection during and after reception

Media access control, including initiation of frame transmission and recovery from transmission failure


Physical Layer The 802.3 physical layer is specific to the

transmission data rate, the signal encoding, and the type of media interconnecting the two nodes.

Gigabit Ethernet, for example, is defined to operate over either twisted-pair or optical fiber cable, but each specific type of cable or signal-encoding procedure requires a different physical layer implementation.


Frame Transmission The preamble and start-of-frame delimiter are

inserted in the PRE and SOF fields. The destination and source addresses are

inserted into the address fields. The LLC data bytes are counted, and the number

of bytes is inserted into the Length/Type field. The LLC data bytes are inserted into the Data

field. If the number of LLC data bytes is less than 46, a pad is added to bring the Data field length up to 46.

An FCS value is generated over the DA, SA, Length/Type, and Data fields and is appended to the end of the Data field.


Standard Ethernet 10base2 – uses bus topology with

thin coaxial cable 10base5 – uses bus topology with

thick coaxial cable 10baseT – uses star topology with

twisted pair medium 10baseF – uses star topology with

fiber cable


10Base-2 Uses coaxial cable with 0.25 inch diameter,

T-connector, repeater, terminator, BNC cable.

Maximum number of repeaters is 4 Maximum cable length = 925 meters Maximum segment length before a repeater

is needed = 185 m (rounded to 200 m ) Uses 50-ohm terminator Minimum spacing


10Base-5 Uses coaxial cable with 0.50 inch

diameter, Vampire taps, N-connector, repeater, terminator

Maximum number of repeaters is 4 Maximum cable length = 2.5 km Maximum segment length before a

repeater is needed = 500 m Maximum number of transceivers = 100 Minimum spacing between 2 transceivers

is 2.50 m, maximum is 50 m


10Base-T provides Manchester-encoded 10-

Mbps bit-serial communication over two unshielded twisted-pair cables.

use two pair of a four-pair Category 3 or 5 cable, terminated at each NIC with an 8-pin RJ-45 connector (the MDI)


10Base-T


10Base-F Uses a fiber hub connector Uses an adapter to connect the

transceiver to a fiber optic cable


100 Mbps – Fast Ethernet 100Base-X 100Base-T4 100Base-T2


1000 Mbps – Gigabit Ethernet 1000Base-T 1000Base-X


MAC Data Frame Format


Basic Data Frame Format Preamble (PRE)—Consists of 7 bytes. The PRE is an

alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.

Start-of-frame delimiter (SOF)—Consists of 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.

Destination address (DA)—Consists of 6 bytes. The DA field identifies which station(s) should receive the frame. The left-most bit in the DA field indicates whether the address is an individual address (indicated by a 0) or a group address (indicated by a 1). The second bit from the left indicates whether the DA is globally administered (indicated by a 0) or locally administered (indicated by a 1). The remaining 46 bits are a uniquely assigned value that identifies a single station, a defined group of stations, or all stations on the network.


Basic Data Frame Format Source addresses (SA)—Consists of 6 bytes. The SA field identifies

the sending station. The SA is always an individual address and the left-most bit in the SA field is always 0.

Length/Type—Consists of 4 bytes. This field indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format. If the Length/Type field value is less than or equal to 1500, the number of LLC bytes in the Data field is equal to the Length/Type field value. If the Length/Type field value is greater than 1536, the frame is an optional type frame, and the Length/Type field value identifies the particular type of frame being sent or received.

Data—Is a sequence of n bytes of any value, where n is less than or equal to 1500. If the length of the Data field is less than 46, the Data field must be extended by adding a filler (a pad) sufficient to bring the Data field length to 46 bytes.

Frame check sequence (FCS)—Consists of 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames. The FCS is generated over the DA, SA, Length/Type, and Data fields.


Integrated Services Digital Network (ISDN)

A network which proposes to interconnect an unlimited number of independent users through a common communications network


Integrated Services Digital Network (ISDN) Introduction to ISDN CCITT I.120: Principles & Evolution of

ISDN ISDN Architecture ISDN System Connections & Interface

Units ISDN Protocol Broadband ISDN


CCITT I.120 Recommendation: Principles & Evolution of ISDN

Principles of ISDN: The main feature of the ISDN concept is to support a wide range of voice

(telephone) and non voice (digital data) applications in the same network using a limited number of standardized facilities.

ISDN’s support a wide variety of applications including both switched and non-switched (dedicated) connections. Switched connections include both circuit- and packet-switched connections and their concatenations.

Whenever practical, new services introduced into an ISDN should be compatible with 64 kbps switched digital connections. The 64 kbps digital connection is the basic building block of ISDN.

An ISDN will contain intelligence for the purpose of providing service features, maintenance, and network management functions. In other words, ISDN is expected to provide services beyond the simple setting up of switched circuit calls.

A layered protocol structure should be used to specify the access procedures to an ISDN and can be mapped into the Open System Interconnection (OSI) model. Standards already developed for OSI-related application can be used for ISDN, such as X.25 level 3 access to packet-switching services.

It is recognized that ISDNs may be implemented in a variety of configurations according to specific national situations. This accommodates both single-source or competitive national policy.


CCITT I.120 Recommendation: Principles & Evolution of ISDN Evolution of ISDN:

ISDNs will be based on the concepts developed for telephone ISDNs and may evolve by progressively incorporating additional functions and network features including those of any other dedicated networks such as circuit and packet switching for data so as to provide for existing and new services.

The transition from an existing network to comprehensive ISDN may require a period of time extending over one or more decades. During this period, arrangements must be developed for the internetworking of services on ISDNs and services in other networks.

In the evolution towards an ISDN, digital end-to-end connectivity will be obtain via plant and equipment used in existing networks, such as digital transmission, time-division multiplex and/or space-division multiplex switching. Existing relevant recommendations for these constituents for an ISDN are contained in the appropriate series of Recommendations of CCITT and CCIR.

In the early stages of the evolution of ISDNs, some interim user-network arrangements may need to be adopted in certain countries to facilitate early penetration of digital service capabilities.

An evolving ISDN may also include at later stages switched connections at bit rates higher than 64 kbps.


ISDN Architecture

Network user

Network user

ISDNswitch

ISDNswitch

Packet switching

Non-switched facilities

Circuit switching

Common channel signaling

ISDN Network


Three Basic Types of Channels Used with ISDN B channel: 64 kbps (bearer

channel) D channel: 16 or 64 kbps (data

channel) H channel: 384, 1536, or 1920

kbps


Basic Rate Interface (BRI) Requires bandwidth that can

accommodate 2 B channel (64 kbps) and one D channel (16 kbps) plus framing, synchronization, and other overhead bits for a total bit rate of 192 kbps.

BRI = 2B + D


Primary Rate Interface (PRI) Provides multiple 64 kbps channels,

consists of 23 64-kbps B channels and one 64-kbps D channel for a combined rate of 1.544 Mbps (USA).

Consists of 30 64-kbps B channels and one 64-kbps D channel for a combined bit rate of 2.048 Mbps (Europe).

PRI = 23B + D


Other Types of Channel H0 channel – this interface supports multiple

384-kbps H0 channels. These structures are 3H0 + D and 4H0 + D for the 1.544 Mbps interface and 5H0 + D for 2.048 Mbps interface

H11 channel – this interface consists of one 1.536 Mbps H11 channel

H12 channel – European version of H11 that uses 30 channels for a combined data rate of 1.92 Mbps

E channel – packet switched using 64 kbps


Projected ISDN ServicesService Transmission rate Channel

Telephone 64 kbps BC

System alarms 100 kbps D

Utility company metering 100 kbps D

Energy management 100 kbps D

Video 2.4-64 kbps BP

Electronic mail 4.8-64 kbps BP

Facsimile 4.8-64 kbps BC

Slow scan television 64 kbps BC


Advantages DisadvantagesCommon

Usage

Low fixed costNot available in all

centers or countries

Periodic Internet Access (for email etc)

Scalable (B circuits can

be combined for greater

speeds)

Not suited to mobile users (users dialing in via remote

access)

LAN-LAN remote connections

which are not permanent

Fast call set up times


ISDN Protocol LAP-D (Link Access Protocol for D-

channel) – CCITT Q.920 and Q.921 Two types of services:

1. Unacknowledged information transfer Provides for the transfer of data with no

acknowledgement Supports both point-to-point or broadcast

transmission Does not guarantee successful transmission of data Does not inform the sender if a transmission fails Does not provide for any type of data flow control

or error control mechanism


ISDN Protocol2. Acknowledged information transfer

A logical connection is established between two subscribers prior to the transfer of any data

Data are transferred in sequentially numbered frames that are acknowledged either individually or in groups

Both error and flow control are present Sometimes referred as “multiple-frame

operation”


Broadband ISDN (BISDN) Service that provides transmission channel

capable of supporting transmission rates greater than the primary data rate.

Broadband channel rates: H21: 32.768 Mbps H22: 43 to 45 Mbps H4: 132 to 138.24 Mbps H21 and H22 are used for full-motion video

transmission for video conferencing, video telephone, and video messaging

H4 is intended for bulk data transfer of text, facsimile and enhanced video information


Asynchronous Transfer Mode (ATM) A means by which data can enter and exit the BISDN

network in an asynchronous (time independent) fashion. ATM breaks data into small chunks of fixed size cells (48

bytes of data plus a 5 byte overhead). ATM is designed for handling large amounts of data across long distances using a high-speed backbone approach. Rather than allocating a dedicated virtual circuit for the duration of each call, data is assembled into small packets and statistically multiplexed according to their traffic characteristics.

One problem with other protocols which implement virtual connections is that some time slots are wasted if no data is being transmitted. ATM avoids this by dynamically allocating bandwidth for traffic on demand. This means greater utilization of bandwidth and better capacity to handle heavy load situations.


ATM When an ATM connection is requested, details

concerning the connection are specified which allow decisions to be made concerning the route and handling of the data to be made.

Typical details are the type of traffic [video requires higher priority], destination, peak and average bandwidth requirements [which the network can use to estimate resources and cost structures], a cost factor [which allows the network to chose a route which fits within the cost structure] and other parameters.


ATM Cell Header Format

Virtual channel

identifier

Header error detection character

Undefined Channel data


BISDN Configuration

Workstation

Telephone

Subscriber local

network

BDT

BDT

Access node

Feeder point

Service node

Transport network

node


Digital Subscriber Line (xDSL) xDSL is a high speed solution that allows

megabit bandwidth from telecommunications to customers over existing copper cable, namely, the installed telephone pair to the customers premises (called the local loop). With the high penetration and existing infrastructure of copper cable to virtually everyone's home (for providing a voice telephone connection), xDSL offers significant increases in connection speed and data transfers for access to information.


Digital Subscriber Line (xDSL) In many cases, the cost of relaying fiber optic

cable to subscriber premises is prohibitive. As access to the Internet and associated

applications like multi-media, teleconferencing and on demand video become pervasive, the speed of the local loop (from the subscriber to the telephone company) is now a limiting factor.

Current technology during the 1980's and most of the 1990's has relied on the use of the analog modem with connection rates up to 56Kbps, which is too slow for most applications except simple email.


Digital Subscriber Line (xDSL) xDSL is a number of different technologies that

provide megabit speeds over the local loop, without the use of amplifiers or repeaters. This technology works over non-loaded local loops (loaded coils were added by telephone companies on some copper cable pairs to improve voice quality). xDSL coexists with existing voice over the same cable pair, the subscriber is still able to use their telephone, at the same time. This technology is referred to seamless.

To implement xDSL, a terminating device is required at each end of the cable, which accepts the digital data and converts it to analogue signals for transmission over the copper cable. In this respect, it is very similar to modem technology.


Digital Subscriber Line (xDSL) xDSL provides for both symmetric and

asymmetric configurations.

Asymmetric Symmetric

Bandwidth is higher in one direction

Bandwidth same in both directions

Suitable for Web Browsing

Suitable for video-conferencing


Variations of xDSLxDSL

Technology

Meaning Rate

DSLDigital Subscriber

Line

2 x 64Kbps circuit switched1 x 16Kbps packet

switched(similar to ISDN-BRI)

HDSL High-bit-rate DSL2.048Mbps over two pairs at

a distance up to 4.2Km

S-HDSL/SDSL

Single-pair or Symmetric High-bit-rate

DSL

768Kbps over a single pair

ADSL Asymmetric DSL up to 6Mbps in one direction

RADSL Rate Adaptive DSL

An extension of ADSL which supports

a variety of data rates depending upon

the quality of the local loop

VDSLVery High-bit-rate

asymmetric DSL

Up to 52Mbps in one direction and

2Mbps in the other direction.


EndEnd

data communications protocol

Documents