PACKET SWITCHING

❑Packet Switching

❑ Packets

❑ Packet switching approaches

❑ Datagram

❑ Virtual Circuit

❑ Store and Forward Transmission

❑ Queuing Delay and Packet Loss

❑ Forwarding Table and Routing Protocols

 Packet Switching
• Packet switching is a method of transferring the data to a network in form of packets.
• In
order to transfer the file fast and efficiently manner over the network the data is
broken into chunks, called Packet.
• A packet composes of payload and various control information.

• No pre-setup or reservation of resources is needed. Each packet is sent individually.

• Each packet will have source and destination IP address with sequence number.

• Sequence number will help the receiver to

• Reorder the packets at the destination side
• Detect the missing packets
• Send acknowledgement
• At the
destination, all these small parts (packets) have to be reassembled, belonging to
the same file.
• Two Approaches to packet Switching

• Datagram Approach

• Virtual Circuit Approach

Packet Switching – Datagram Approach
• Datagram Packet Switching is also known as connectionless switching.

• Each independent entity is called as datagram.

• Datagrams contain destination information and the intermediary devices uses this
information to forward datagrams to right destination.

• In Datagram packet switching approach the path is not fixed.

• Intermediate nodes take the routing decisions to forward the packets.

• Packet delivery is not guaranteed in connectionless packet switching, so reliable

delivery must be provided by end systems using additional protocols.
 Packet Switching – Virtual Circuit Switching
• Virtual Circuit Switching is also known as connection-oriented switching.

• In the case of virtual circuit switching , a pre-planned route is established before the
messages are sent.

• Call request and call accept packets are used to establish the connection between
sender and receiver.

• In this approach the path is fixed for the duration of a logical connection.

• All address information is only transferred during the setup phase. Once the route to a
destination is discovered, entry is added to the switching table of each intermediate node.
During data transfer, packet header (local header) may contain information such as length,
timestamp, sequence number, etc.
 Packet Switching - Store-and-Forward Transmission

• Most packet switches use store-and-forward

transmission at the inputs to the links.
• Store-and-forward transmission means that the
packet switch must receive the entire packet
before it can begin to transmit the first bit of the
packet onto the outbound link.
• To explore store-and-forward transmission in
more detail
• Consider a simple network consisting of two end
systems connected by a single router, as shown in
Figure
• A router will typically have many incident
links, since its job is to switch an incoming
packet onto an outgoing link

• In this example, the source has three

packets, each consisting of L bits, to send to
the destination.
• Here source has transmitted packet 1, and
the front of packet 1 has already arrived at
the router.
• Because the router employs store-and-
forwarding, at this instant of time, the
router cannot transmit the bits it has
received.
• Insteadit must first buffer (i.e., “store”)
the packet’s bits.
• Only after the router has received all of
the packet’s bits can it begin to transmit
(i.e., “forward”) the packet onto the
outbound link.
 Queuing Delays and Packet Loss
• Each packet switch has multiple links
attached to it.
• For each attached link, the packet switch
has an output buffer (also called an output
queue), which stores packets that the
router is about to send into that link.
• The output buffers play a key role in packet
switching.
• If an arriving packet needs to be
transmitted onto a link but finds the link
busy with the transmission of another
packet, the arriving packet must wait in the
output buffer.
• Thus, in addition to the store-and-forward delays,
packets suffer output buffer queuing delays.

• These delays are

• variable
• depend on the level of congestion in the network.

• Since the amount of buffer space is finite, an

arriving packet may find that the buffer is
completely full with other packets waiting for
transmission.

• In this case, packet loss will occur—either the

arriving packet or one of the already-queued
packets will be dropped.
 Forwarding Tables and Routing Protocol
• Packet forwarding
• In the Internet, every end system has an address called an IP address.
• When a source end system wants to send a packet to a destination end system, the source
includes the destination’s IP address in the packet’s header.
• Each router has a forwarding table that maps destination addresses (or portions of the
destination addresses) to that router’s outbound links.
• When a packet arrives at a router, the router examines the address and searches its forwarding
table, using this destination address, to find the appropriate outbound link.
• The router then directs the packet to this outbound link.
Thank You