Packet Switching
Content:
Packet Switching Principles:
Until before the 1970s, the most used method was Circuit Switching, which, due to the inefficient qualification it received from many people who argued that it was not agile for data connections and especially for what two devices connected in a network have to transmit and receive data at the same speed, which limits the usefulness of the network, then the Packet Switching appears and with it its respective techniques.
A Packet is a group of information that consists of two parts: the data itself and the control information, in which the path to follow along the network to the packet's destination is specified. One thousand octets is the upper limit of packet length, and if the length is greater the message is fragmented into other packets.
General advantages:
- Packets form a queue and are transmitted as fast as possible.
- Allow conversion at data speed.
- The network can continue to accept data although the transmission will be slow.
- There is the possibility of managing priorities (if a group of information is more important than the others, it will be transmitted before said others).
Switching Techniques:
Two types of techniques have been defined for the use of Packet Switching: Datagrams and Virtual Circuits.
Datagrams:
- Considered the most sensitive method.
- It does not have a call establishment phase.
- Data passing is more secure.
- Not all packets follow the same route.
- Packets can arrive at the destination out of order because their treatment is independent.
- A packet can be destroyed on the way, whose recovery is the responsibility of the destination station. (This implies that the rest of the packets are intact)Virtual Circuits:
- They are the most used.
- Its operation is similar to that of circuit switching networks.
- Prior to the transmission, the route prior to the transmission of the packets is established by means of Call Request packets (requests a logical connection to the destination) and Accepted Call packets (if the destination station is suitable for transmission, it sends this type of package ); Once the transmission is established, the data exchange takes place, and once finished, the Release Request packet is presented (notification that the network is available, that is, that the transmission has come to an end).
- Each packet has a virtual circuit identifier instead of the destination address.
- The packages will be received in the same order in which they were sent.
It is related to the transmission time, that is: Suppose we have to transmit a packet of forty octets with three header octets from station X to station Y via nodes a and b; then the packet will first go from station X to node a, and once fully received at node a, it will be sent to node b and when node b has fully received a packet it will be sent to station Y. The transmission time (ignored switching time) will be: 129 (43 octets * 3 packet transmissions).
Technical Comparisons Between Circuit and Packet Switching
To refer to this topic, we will first address what is known as Benefits and then other characteristics will be analyzed:
Features:
- Propagation Delay: Time in the passage of information
between node and node.
- Transmission time: Time that the transmitter takes to send the block.
- Node delay: Time that a node takes for commutation.Other features in:
Circuit switching:
- Transparent service and constant speed.
- Suffer delay due to call establishment.
- Analog or Digital data goes from the origin to the destination.Packet Switching:
- The data must be converted from Analog to Digital by means of a virtual circuit before transmission.
- They have relative supplementary bits.
- There is delay prior to transmission.datagrams:
- Their arrival is in a different order.
- There is no call establishment (they are fast for short messages).
External and Internal Operation
The choice of an internal or external virtual circuit depends on the specific objectives in the design of the network and the priority cost.
One of the most important features of a packet-switched network is the use of datagrams or virtual circuits.
- external virtual circuit: A logical connection is established between two stations. Packets are marked with a virtual circuit number and a sequence number; packets are received in order.
- external datagram: Each packet is transmitted independently marked with a destination address and received out of order.
- internal virtual circuit: A route for packets between two stations is defined and marked. All the packets of said virtual circuit follow the same route and are received at the destination in the same order.
- internal datagram: The network treats each packet independently. Packets are marked with a destination address and may be received out of order at the destination node.
routing
Your requirements:
- Accuracy
- Impartiality
- Optimization
- Robustness
- Efficiency
- Simplicity
- Stability
It is the most complex and crucial part that must be analyzed prior to a data transmission to achieve the most effective performance.
You can choose to:
Choice of route:
- Path with the least number of jumps.
- Way that implies the minimum cost -> is the most used. (for this, a cost is associated to each link and for two stations the route with the minimum cost is chosen).
The routing decision is made on a packet or virtual circuit basis. When it is done based on datagrams, the routing decision is made individually for each packet. In the case of internal virtual circuits, the packets directed to this circuit will follow the same route.
There is a third case which is Origin routing, in which the origin station is the one that makes the decisions and not the network nodes.
Important:
The node or nodes are responsible for the routing decision which can be of two types:
- Distributed: It is the most common with greater robustness in which each node has the responsibility to select an output link
- Centralized: A single designated node makes the decision (this could be in the Network Control Center); a great disadvantage is that a failure of this node causes a total blockage of the network.
One can also speak of Origin Routing, in which the origin station makes any type of decision.
Routing Strategies:
1.- Static:
- Specification for each pair of origin-destination nodes, the identity of the next node in the route.
- It is not necessary to know the full path, but only the path to the next node.
- There is no differentiation between Datagrams and Virtual Circuits since all packets will follow the same route.
- Advantages: Simplicity and good operation.
- Disadvantage: Lack of flexibility (there is no reaction to failures or congestion).
2.- Floods:
- For high priority messages.
- Information about the network is not required.
- The source node sends a copy of the packet to the neighboring node, and these through links will send it to the rest of the nodes until a copy reaches the destination.
- Properties:
a.- The possible paths between the nodes are tested
origin and destination.
b.- A copy of the package will use the least number of
jumps.
c.- All the nodes that are directly or
indirectly connected to the origin.
d.- Generation of too much traffic.
3.- Random:
- Improvement of the previous one (greater congestion control).
- Selection of a single output path to transmit the incoming packet.
- The exit link is chosen randomly without taking into account the previous link.
4.- Adaptable:
- Decisions change according to network conditions (if the
network has faults the node or line already knows that it cannot be part of the
route and; in case of congestion, the parking area will be surrounded).
- The nodes exchange information about the state of the network.
- This technique is more used than Static for:
1.- Improvement of Performance.
2.- It delays the appearance of serious situations of
overcrowding.
X.25
· Standard Protocol
· Approved in 1976
· Interface
· Interact with packet switching networks.
Specifies 3 layers
PHYSICAL:
LINK:
PACK LEVEL:
The data can be segmented into various packets, the control information included in the packet has several purposes:
- Identification of a Virtual Circuit
- Definition of sequence numbers
VIRTUAL CIRCUIT SERVICE:
Virtual Calls: - Virtual Circuit
- Establishes a request and a call release
Permanent Virtual Circuits: - Fixed Virtual Circuit
- Data Transfer is the same as Virtual Calls; but neither the establishment nor the closing of the Calls is needed.
SEQUENCE OF EVENTS:
- Call Request Packet
- Package Incoming Call (In Coming Call)
- Call Accepted Packet
- Call Established Package (Call Connected)
- Release Request Packet (Clear Request)
- Release Confirmation Package (Clear Confirmation).
- Clear Indication Packet -
- Release Confirmation (Clear Confirmation).
- Interrupt Packet
- Diagnosis
- Registration Packages
PACKAGE FORMAT:
MULTIPLEXATION:
- Most important service of X.25.
- A DTE can establish up to 4095 Virtual Circuits simultaneously with other DTEs on the same physical link, the DTE can internally assign these circuits in any way, each circuit corresponds eg. Application, process, terminal, etc.
- The DTE-DCE line allows multiplexing (full duplex) (a packet associated with a Virtual Circuit can be transmitted in both directions at any time).
PACKAGE SEQUENCES:
RESTART AND REBOOT
They are facilities for error recovery.
RESET:
Restart a Virtual Circuit:
- Sequence number are made 0 at both ends.
- Data or interrupt packets are lost in transit.
- Function of a higher level protocol: recovery of lost packets.
Error Conditions that cause Reboot:
- Packet Loss.
- Errors in the sequence number.
- Congestion.
- Loss of a Virtual Circuit internal to the network.
The DTE or the DCE can originate a reset through the use of a Reset Request Packet or a Reset Indication Packet, to which the receiver will respond with a Reset Confirmation Packet, it is the responsibility of the DCE involved inform the other end.
RESTART:
· By more serious error situation Eg.