Switching Techniques

Question 1

Circuit Switching

Answer 1

• physical path between communicating parts achieved using circuit switching - switches (relays) - in the network nodes
• three phase communication:
  1. circuit establishment (setup), establish an optimum path between parts; both parts agree communication
  2. effective data transmission (signal transfer), on this route
  3. circuit release (disconnection), initiative of one part
• drawbacks:
• not efficient due to existence of the first phase (it will exist even if there’s no data transfer)
• need for covering bandwidth allocation
• important amount of cabling
• no buffers in switches for transmission equalization

Question 2

Message Switching

Answer 2

• data transfer using messages (independent data units with diff. lengths, but similar structures)
• types: control and data
• need for addressing (source and destination)
• communications nodes are not physical switches, but computing systems (with memory and processing units)
• main idea: message store & forward
• not more dedicated communications path; established in an optimum way by nodes (using routing tables)
• advantages:
• improvement in efficiency (path multiplexing)
• introduces message priority
• equilibrated transmissions
• drawbacks:
• messages are too long, memory waste and difficult recovery

Question 3

Packet Switching

Answer 3

• combines the advantages of message and circuit switching
• packet has similar message structure, but lower length
• methods:
• use of datagrams - more speedy and flexible method
• use of virtual circuits - use of the three phases (connection request, data transfer, disconnect) for a logical connection activation and use of special control packets + embedding of control information
• logical connection may be implemented with more different physical connections
• advantages:
• line efficiency
• data rate conversion
• packets are accepted even when network is busy
• priorities can be used

Question 4

Circuit Switching vs Packet Switching

Answer 4

• circuit switching:
• designed for voice
• resources dedicated to a particular call
• much of the time, a data connection is idle
• data rate is fixed
• packet switching:
• basic operation
• data transmitted in small packets - longer messages split into series of packets and each packet contains a portion of user data plus some control info