1. PSDN and Data
Communication Systems
Chapter 23
Electronic Communications Sytems , Fifth Editon
By: Wayne Tomasi
ECE @Saint Louis University, Baguio City 1
Prepared by: Engr. Jeffrey Des B. Binwag
2. 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
4. 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
ECE @Saint Louis University, Baguio City 4
5. PSDN SWITCHING TECHNIQUES
• CIRCUIT SWITCHING
• MESSAGE SWITCHING
• PACKET SWITCHING
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6. 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
ECE @Saint Louis University, Baguio City 6
7. 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.
ECE @Saint Louis University, Baguio City 7
8. 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.
ECE @Saint Louis University, Baguio City 8
10. 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)
ECE @Saint Louis University, Baguio City 10
11. 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
ECE @Saint Louis University, Baguio City 11
14. 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.
ECE @Saint Louis University, Baguio City 14
15. 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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16. 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.
ECE @Saint Louis University, Baguio City 16
17. 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 Field
ECE @Saint Louis University, Baguio City 17
18. 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
ECE @Saint Louis University, Baguio City 18
19. 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.
ECE @Saint Louis University, Baguio City 19
