Packet switching is a digital networking method that groups transmitted data into blocks called packets. It began in the 1960s and was first developed by Paul Baran. There are two main types: datagram packet switching treats each packet independently while virtual circuit packet switching establishes a logical connection before sending packets along the same path. X.25 is a standard protocol for packet switched networks that provides reliable data transfer and error control. It defines the connection between a terminal and a packet switched network using three device types: DTE, DCE, and PSE.

1. Packet Switching
(X.25 protocol)

2.  Packets
 Began around early 1960’s
 First developed by Paul Baran
 Network packet is a formatted unit
of data carried by a packet-switched network.
 Is a digital networking communications
method that groups all transmitted data
regardless of content, type, or structure into
suitably-sized blocks, called packets

3.  Advantages of Packet Switching
 Line efficiency
 Data rate conversion
 Packets are accepted even when network is
busy
 High data transmission quality
 When it comes to billing:
 Fee is just the duration of connectivity

4.  Disadvantages of Packet Switching
 More complex
 Data Transmission Delays
 Packets may be lost on their route (Datagram)

5.  Types of Packet Switching
 Datagram Packet Switching
 Virtual Circuit Packet Switching

6.  Datagram Packet Switching
 Each packet is treated independently
 The packets may take different paths to the
destination
 The packets might arrive in a different
sequence from the order in which they were
sent
 The packets may have to be reordered at the
destination

7.  Datagram Approach

8.  Virtual Circuit Packet Switching
 A logical connection is established before any
packets are sent
 All packets follow the same path through the
network
 This does not mean that there is a dedicated
path, as in circuit switching
 There is a call set up before the exchange of
data (handshake).

9.  Virtual Circuit Approach

10.  Datagram vs. Virtual Circuits

11.  Packet Switching Service Protocols
 There are five protocols in use for packet-
switched services:
 X.25
 Asynchronous Transfer Mode (ATM)
 Frame Relay
 Switched Multimegabit Data Service (SMDS)
 Ethernet/IP packet networks

12.  X.25
 is a standard that defines the connection
between a terminal and a PSN.
 is a reliable protocol, meaning it performs
error control and retransmits bad packets.

13.  X.25 network devices fall into three general
categories:
 Data Terminal Equipment (DTE),
 Data Circuit-terminating Equipment (DCE),
 Packet-switching Exchange (PSE)

14.  Data Terminal Equipment (DTE)
 Are end systems that communicate across
the X.25 network.

15.  Data Circuit-terminating Equipment (DCE),
 are communications devices that provide the
interface between DTE devices and a PSE.

16.  Packet-switching Exchange (PSE)
 Are switches that compose the bulk of the
carrier's network. They transfer data from one
DTE device to another through the X.25 PSN

18.  X.25 & OSI Model

19.  X.25 – Physical
 Defines physical, mechanical and electrical
characteristics required to establish point to
point synchronous circuit between DTE and
DCE.

20.  X.25 – Link
 Link Access Protocol Balanced (LAPB)
 Provides reliable transfer of data over link
 Sending as a sequence of frames
 Has the responsibility to make sure that the
information is transmitted across the link
without errors

21.  X.25 – Packet
 Provides a logical connections (virtual circuit)
between subscribers
 All data in this connection form a single
stream between the end stations

22.  Packet Assembler/Disassembler
 The PAD is located between a DTE device
and a DCE device, and it performs three
primary functions:
 Buffering (storing data until a device is ready
to process it),
 Packet assembly,
 Packet disassembly.

23.  Switched Virtual Circuit (SVC)
 Permanent Virtual Circuit (PVC)
 A PVC (Permanent) is similar to a leased line.
PVCs use no call setup or call clear that is
apparent to the subscriber. Any provisioned PVCs
are always present, even when no data traffic is
being transferred.
 An SVC exists only for the duration of the session.
Three phases are associated with X.25 SVCs:
 Call setup.
 Information transfer.
 Call clear.

24.  Configuration of X.25
 When you select X.25 as a WAN protocol, you
must set appropriate interface parameters. The
interface configuration tasks include:
 Define the X.25 encapsulation (DTE is the default
(Cisco's encapsulation type)).
 Assign the X.121 address.(PDN – Public Data
Numbers)
 Define map statements to associate X.121
addresses with higher-level protocol addresses.
 Other configuration tasks can be performed to
control data throughput and to ensure
compatibility with the X.25 network service
provider.