PSDN and Data Communication Systems

Chapter 23

Electronic Communications Sytems , Fifth Editon

By: Wayne Tomasi

ECE @Saint Louis University, Baguio City1

Prepared by: Engr. Jeffrey Des B. Binwag

PUBLIC SWITCHED DATA NETWORKS• A switched communications network similar

to the public telephone network except that it is designed for transferring data only.

• Transports data from source to destination through a network of intermediate switching nodes and transmission links.

• COMPONENTS OF PSDN:

– END STATIONS

– SWITCHING NODES

– TRANSMISSION LINKS

ECE @Saint Louis University, Baguio City 2

PUBLIC SWITCHED DATA NETWORKS

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VALUE ADDED NETWORK• “Adds value” to the services or facilities

provided by a common carrier to provide new types of communication services.

• Comprises an organization that leases communications lines from common carriers and adds new types of communications services to those lines.

• Examples Value Added Services:– Error Control, Enhanced Connection Reliability,

Dynamic Routing, Failure Protection, Logical Multiplexing, and Data Format Conversion

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PSDN SWITCHING TECHNIQUES

• CIRCUIT SWITCHING

• MESSAGE SWITCHING

• PACKET SWITCHING

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PSDN SWITCHING TECHNIQUES• CIRCUIT SWITCHING

– The switching technique used for making a standard telephone call

– Involves establishing a call, information transfer (in real time), and call disconnection

– Blocking occurs due to a limited number of circuits and switching paths

• Blocking is the inability to complete a call because there are no facilities or switching paths available between the source and the destination

– A circuit switch is transparent to the data

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PSDN SWITCHING TECHNIQUES• MESSAGE SWITCHING

– A store-and-forward network where data is not transmitted in real time but rather passes through transactional switches capable of storing data and then transmitting them when it is convenient to do so or when the load on the network has decreased.

• Transactional switches can store data or change its format and bit rate, then convert the data back to their original form or an entirely different form at the receive end.

– Blocking cannot occur although delay time from message transmission to reception varies from call to call.

– Multiplexes data from different sources into a common facility.

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PSDN SWITCHING TECHNIQUES

• PACKET SWITCHING

– Involves dividing data into smaller segments called packets prior to transmission.

– Called a hold-and-forward network because unlike messages in message switching that are stored for a longer period, packets are usually stored (held) for shorter periods.

– Hold time is generally quite short and message transfer is near real time.

– Require complex and expensive switching arrangements and complicated protocols.

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PSDN SWITCHING TECHNIQUES

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CCITT X.25 USER-TO-NETWORK INTERFACE PROTOCOL

• Designated by the CCITT in 1976 as the international standard for packet network access.

• Addresses the ISO physical, data-link, and network layers.

• PHYSICAL LAYER STANDARDS:

– X.21, X.26, X.27

– EIA RS-232, RS-423A, RS-422A

• DATA-LINK LAYER STANDARDS:

– HDLC (High-Level Data Link Control Protocol)

– ANSI 3.66 ADCP (Advanced Data Communications Control Procedures)

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CCITT X.25 USER-TO-NETWORK INTERFACE PROTOCOL

• Link Access Procedure Balanced (LAPB)

– A subset of HDLC that provides for two-way, full-duplex communications between DTE and DCE at the packet network gateway.

• LAPB COMMANDS

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CCITT X.25 USER-TO-NETWORK INTERFACE PROTOCOL

• LAPB RESPONSES

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CCITT X.25 USER-TO-NETWORK INTERFACE PROTOCOL

X.25 SWITCHING SERVICES

PERMANENT VIRTUAL CIRCUIT

VIRTUAL CALL

DATAGRAM

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CCITT X.25 SWITCHING SERVICES

• PERMANENT VIRTUAL CIRCUIT

– Logically equivalent to a two-point dedicated private line circuit except slower because no hardwired circuit is provided.

– Appropriate switches and circuits must be established through the network to provide interconnection

– Identifies the routing between two predetermined network subscribers which will be used for all subsequent messages.

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CCITT X.25 SWITCHING SERVICES

• VIRTUAL CALL

– Logically equivalent to making a telephone call through the DDD network except no direct end-to-end connection is made.

– Temporary virtual connections that use common usage equipment and circuits.

– It is a one to many arrangement or point-to-multipoint.

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CCITT X.25 SWITCHING SERVICES• DATAGRAM

– Consists of self-contained packets that travel through the network independent of other packets of the same message by whatever means possible.

– The network does not acknowledge the packets nor ensure successful transmission.

– Reliable only when a message can fit into a single packet.

– Called a single-packet-per-segment protocol.

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CCITT X.25 PACKET FORMAT• CALL REQUEST PACKET

– Format Identifier

– Logical Channel Identifier (LCI)

– Packet Type

– Calling Address Length

– Called Address Length

– Called Address

– Calling Address

– Facilities Length Field

– Facilities Field

– Protocol Identifier

– User Data FieldECE @Saint Louis University, Baguio City 17

CCITT X.25 PACKET FORMAT• DATA TRANSFER PACKET

– Similar to a call request packet except for considerably less overhead and a much larger user data field.

– Also includes two additional fields namely:

• Send Packet Sequence Field

• Receive Packet Sequence Field

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ITU-T X SERIES OF RECOMMENDED STANDARDS

• X.1-X.39

– Deals with services and facilities, terminals, and interfaces.

• X.40-X.199

– Deals with Network architecture, transmission, signaling, switching, maintenance, and administrative arrangements.

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ITU-T X SERIES OF RECOMMENDED STANDARDS

End