Common Network Architecture: X.25 Networks, Ethernet (Standard and Fast): frame format and specifications, Wireless LAN’s – 802.11x, 802.3 Bluetooth etc.

Common Network Architecture
Chapter No. 2

Connection oriented Vs Connection less network
Connection-Oriented means that when devices
communicate, they perform handshaking to set up
an end-to-end connection.
In connectionless design every packet is addressed
and routed independently.

Connection oriented Protocols
Characteristics:
1. Handshaking (Setting up connection) between
communicating devices. Connections sometimes
are called as sessions, virtual circuits or logical
connections.
2. Acknowledgement procedure. This provides a
high level of network reliability.
3. It provides means of error control. Whenever
receiving station found that received data packet
consist of errors it request sender to retransmit that
packet.
4. It is a uni-cast (point-to-point) operation.

•Connection-oriented
• Setup data transfer ahead of time (through
handshaking)
• Internet’s connection-oriented service is TCP
(Transmission Control Protocol). It provides :
• reliable, in-order byte delivery
• flow control
• congestion control.
• Applications using TCP: Email (SMTP), web browsing
(HTTP), and file transfer (FTP)

Connectionless (Stateless) Protocols
Characteristics:
• It sends data with a source and destination
address without a handshake.
• Do not use any acknowledgment procedure.
• Usually do not support error control.
• Connectionless protocols are more efficient
than that of connection oriented protocols.
• It allows multicast and broadcast operations.

•Connectionless
Internet’s connectionless service is UDP (User
Datagram Protocol) . It provides
unreliable data transfer
no flow control
no congestion control
Applications using UDP: streaming media, video
conferencing, and IP telephony

A comparison of Connection-Oriented and Connectionless Network

Peer-to-Peer Network
• A peer-to-peer network is a distributed network
architecture composed of participants that make a
portion of their resources, such as processing power,
disk storage or network bandwidth directly to network
participants without the need for central coordination
instances.
• Used largely for sharing of content files such as audio,
video, data or anything in a digital format.
• Can be very large

• End-systems (or peers), are capable of behaving
as clients and servers of data, hence system is
scalable and reliable
• Peers participation is voluntary, membership is
dynamic, hence topology keeps changing
• Most popularly used for file sharing, hence peer-
to-peer systems have become synonymous with
peer-to-peer file sharing networks
Peer-to-Peer Network

A Peer
• Peers are both suppliers and consumers
while in the traditional client-server model,
the server supplies while the client only
consumes.

Advantages
• The more nodes that are part of the system,
demand increases and total capacity of the
system also increases. Where in client-server
network architectures as more clients are
added to the system, the system resources
decreases.
• There is no single point of failure, due to
robustness of the system.
• All clients provide to the system

Disadvantages
• Security is a major concern, not all shared files
are from beginning sources. Attackers may add
malware to p2p files as an attempt to take
control of other nodes in the network.
• Heavy bandwidth usage
• ISP speeding/slowing of P2P traffic.
• Potential legal/moral concerns

CO Vs. CL
• In connection oriented service authentication is
needed while connectionless service does not need
any authentication.
• Connection oriented protocol makes a connection
and checks whether message is received or not and
sends again if an error occurs connectionless service
protocol does not guarantees a delivery.
• Connection oriented service is more reliable than
connectionless service.
• Connection oriented service interface is stream
based and connectionless is message based.

Service Primitives
• A service is specified by a set of primitives. A
primitive means operation. To access the
service a user process can access these
primitives.
• These primitives are different for connection
oriented service and connectionless service.

• LISTEN : When a server is ready to accept an incoming
connection it executes the LISTEN primitive. It blocks waiting
for an incoming connection.
• CONNECT : It connects the server by establishing a
connection. Response is awaited.
• RECIEVE: Then the RECIEVE call blocks the server.
• SEND : Then the client executes SEND primitive to transmit its
request followed by the execution of RECIEVE to get the reply.
Send the message.
• DISCONNECT : This primitive is used for terminating the
connection. After this primitive one can’t send any message.
When the client sends DISCONNECT packet then the server
also sends the DISCONNECT packet to acknowledge the client.
When the server package is received by client then the process is
terminated.
Connection Oriented Service Primitives

Primitives for Connectionless Oriented Service
• UNIDATA - This primitive sends a packet of data
• FACILITY, REPORT - Primitive for enquiring
about the performance of the network, like
delivery statistics.

X.25 Networks
• X.25 is a standard used by many older public
networks specially outside the U.S.
• This was developed by CCITT for providing an
interface between public packet-switched
network and their customers.
• The packet switching networks use X.25 protocol
. The X.25 recommendations were first prepared
in 1976 and then revised in 1978, 1980 and 1984.
• X.25 was developed for computer connections,
used for terminal/timesharing connection.

X.25 Networks
• This protocol is based on the protocols used in
early packet switching networks such as
ARPANET, DATAPAC, and TRANSPAC etc.
• A protocol X.21 which is a physical layer
protocol is used to specify the physical electrical
and procedural interface between the host and
network.
• The problem with this standard is that it needs
digital signal rather than analog signals on
telephone lines.

• So not many networks support this standard.
Instead RS 232 standard is defined.
• The data link layer standard has a number of
variations. It is designed for error detection and
corrections.
• The network layer protocol performs the
addressing, flow control, delivery confirmation etc.
• It allows the user to establish virtual circuits and
send packets on them. These packets are delivered
to the destination reliably and in order.
• X.25 is a connection oriented service. It supports
switched virtual circuits as well as the permanent
circuits.

• Packet Switching is a technique whereby the
network routes individual packets of HDLC data
between different destinations based on addressing
within each packet.
• A switched virtual circuit is established between a
computer and network when the computer sends a
packet to the network requesting to make a call to
other computer.
• Packets can then be sent over this connection from
sender to receiver.
• X.25 provides the flow control, to avoid a fast
sender overriding a slow or busy receiver.

• A permanent virtual circuit is analogous to-a leased
line. It is set up in advance with a mutual agreement
between the users.
• Since it is always present, no call set up is required
for its use.
• In order to allow the computers which do not use the
X.25 to communicate with the X.25 network a packet
assembler dis-assembler (PAD) is used.
• PAD is required to be installed along with each
computer which does not use X.25.
• X.25 defines the interface for exchange of packets
between a DTE and switch data sub-network node.

• X.25 Most widely used today
• X.25 is an interface between DTE ( Data Terminal
Equipment) and DCE (Data Communication
Equipment) for terminal operation at the packet mode
on public data network.
– A packet switching protocol used in a wide area
network
– use virtual circuit and asynchronous TDM

• DTE stands for Data Terminal Equipment,
and DCE stands for Data Communications
Equipment.
• DTE is typically either a dumb terminal or the
serial port on a computer/workstation.
• DCE is typically a modem, or other piece of data
communications equipment, hence the names.

Three Layers of X.25
The X.25 interface is defined at three levels:
The three levels are:
(i) Physical layer (level 1)
(ii) Data link layer (level 2)
(iii) Packet layer (level 3).

• These three layers correspond to the three lower
most layers of the ISO-OSI reference model. The
physical layer takes care of the interface between
a computer terminal and the link which attaches it
to the packet switching node.
• The X.25 defines the interface for exchange of
packets between the user's machine (DTE- Data
Terminal Equipment) and the packet switching
node to which this DTE is attached which is
called as DCE(Data Circuit Terminating
Equipment).

• The three layers of X.25 interface are as
shown in Fig..
• At the physical level X.21 physical interface is
being used which is defined for circuit
switched data network. At the data link level,
X.25 specifies the link access procedure-B
(LAP-B) protocol.

Common Network Architecture: X.25 Networks, Ethernet (Standard and Fast): frame format and specifications, Wireless LAN’s – 802.11x, 802.3 Bluetooth etc.

• At the network level (3rd
level), X.25 defines a
protocol for an access to packet data subnetwork.
• This protocol defines the format, content and
procedures for exchange of control and data
transfer packets. The packet layer provides an
external virtual circuit service.
• Next Fig. shows the relationship between the
levels of x.25. User data is passed down to X.25
level 3.
• This data then appends the control information as
a header to form a packet. This control information
is then used in the operation of the protocol.

• The entire X.25 packet formed at the packet level
is then passed down to the second layer i.e. the
data link layer.
• The control information is appended at the front
and back of the packet forming a LAP-B frame.
The control information in LAP-B frame is
needed for the operation of the LAP-B protocol.
• This frame is then passed to the physical layer
for transmission.

Packet Layer Protocol
• PLP packets : Information packets and Control packets
• Information Packets (I-packets)
– transmit user data
– consists of a header and a user data field
– short and long packets: long packets to support
facilities with long delays
– The last bit in the header = 0 for I-packets

• I-packet fields
– General format identifier (GFI) field: 4 bits
• Q bit : not defined in the standard. Users define
two types of data.
• D bit: for packet sequencing.
• Mod: the length of the header. (01: the header
short, 10: long)
– Virtual circuit identification fields
• Logical channel group number (LCGN) : 4 bits
• Logical channel number (LCN) : 8 bits
• total 12 bits to identify the virtual circuit for a
transmission

• I-packet fields
• P(S) and P(R) : packet sequence numbers for
flow and error control
• P(S) for packet send, P(R) for packet receive:
sliding window ARQ

• Control Packets (C-packets)
~ flow and error control
~ connection, termination and management control
– Category I (for flow and error control)
• RR: Receive Ready - the station is ready to
receive more packets
• RNR: Receive Not Ready
• REJ: Reject - an error in the packet (go-back-n
ARQ)

• Control Packet (C-packet)
– Category II
• for connection, termination and management control
• control packet types

Control Packets
• Category II
– Call Request/Incoming Call
• request the establishment of a connection between
two DTEs
– Call Accepted/Call Connected
• indicate the acceptance of the requested connection
by the called system

Control Packets
• Category II (cont’d)
– Clear Request/Clear Indication
• to disconnect the connection at the end of an exchange
– Clear Confirm
• sent in response to the clear indication
– and More...

• Complete Packet Sequence
– A message size may be too small or too large for
a network and not compatible with a network.
– X.25 mechanism to break up a long message
among multiple packets but still keep those
packets as a single contiguous transmission
– A packets: at least one additional packets needed
to carry the remainder of a message (M=1,D=0)
- B packets: either stand alone or final packet
(M=0, D=1)
– final packet in the sequence need
ACK( M=1,D=1)

• Complete Packet Sequence (cont’d)

• Virtual Channel ID Numbers
– up to 4096 multiplexed channels to be identified
between each DTE and DCE
– not permanent, allocated dynamically
– the calling and called hosts use different numbers

Virtual Circuit Service
• With the X.25 packet layer, data are
transmitted in packets over external virtual
circuits, The virtual circuit service of X.25
provides for two types of virtual circuits.
• The virtual circuit service of X.25 provides for
two types of virtual circuits i.e. "virtual call"
and "permanent virtual circuit".

Virtual Circuit Service
• A virtual call is a dynamically established
virtual circuit using a call set up and call
clearing procedure.
• A permanent virtual circuit is a fixed, network
assigned virtual circuit. Data transfer takes
place as with virtual calls, but no call set up or
clearing is required.

Characteristics of X.25
• Multiple logical channels can be set on a single
physical line
• Terminals of different communication speeds can
communicate
• The procedure for transmission controls can be
changed.

Common Network Architecture: X.25 Networks, Ethernet (Standard and Fast): frame format and specifications, Wireless LAN’s – 802.11x, 802.3 Bluetooth etc.

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