Switching Techniques Lecture12,wireless
- 2. Switching Terms ο Switching Nodes: ο Intermediate switching device that moves data ο Not concerned with content of data ο Stations: ο End devices that wish to communicate ο Each station is connected to a switching node ο Communications Network: ο A collection of switching nodes 2
- 4. Observations of Figure ο Some nodes connect only to other nodes (e.g., 5 and 7) ο Some nodes connect to one or more stations ο Node-station links usually dedicated point-to- point links ο Node-node links usually multiplexed links ο Frequency-division multiplexing (FDM) ο Time-division multiplexing (TDM) ο Not a direct link between every node pair 4
- 5. Techniques Used in Switched Networks ο Circuit switching ο Dedicated communications path between two stations ο E.g., public telephone network ο Packet switching ο Message is broken into a series of packets ο Each node determines next leg of transmission for each packet 5
- 6. Phases of Circuit Switching ο Circuit establishment ο An end to end circuit is established through switching nodes ο Information Transfer ο Information transmitted through the network ο Data may be analog voice, digitized voice, or binary data ο Circuit disconnect ο Circuit is terminated ο Each node deallocates dedicated resources 6
- 7. Characteristics of Circuit Switching ο Can be inefficient ο Channel capacity dedicated for duration of connection ο Utilization not 100% ο Delay prior to signal transfer for establishment ο Once established, network is transparent to users ο Information transmitted at fixed data rate with only propagation delay ο Propagation delay = d/s 7
- 8. Components of Public Telecommunications Network ο Subscribers - devices that attach to the network; mostly telephones ο Subscriber line - link between subscriber and network ο Also called subscriber loop or local loop ο Exchanges - switching centers in the network ο A switching centers that support subscribers is an end office ο Trunks - branches between exchanges 8
- 9. Example Connection Over a Public Circuit-Switching Network 9
- 11. How Packet Switching Works ο Data is transmitted in blocks, called packets ο Before sending, the message is broken into a series of packets ο Typical packet length is 1000 octets (bytes) ο Packets consists of a portion of data plus a packet header that includes control information ο At each node en route, packet is received, stored briefly and passed to the next node 11
- 13. Packet Switching Advantages ο Line efficiency is greater ο Many packets over time can dynamically share the same node to node link ο Packet-switching networks can carry out data-rate conversion ο Two stations with different data rates can exchange information ο Unlike circuit-switching networks that block calls when traffic is heavy, packet-switching still accepts packets, but with increased delivery delay ο Priorities can be used 13
- 14. Disadvantages of Packet Switching ο Each packet switching node introduces a delay ο Overall packet delay can vary substantially ο This is referred to as jitter ο Caused by differing packet sizes, routes taken and varying delay in the switches ο Each packet requires overhead information ο Includes destination and sequencing information ο Reduces communication capacity ο More processing required at each node 14
- 15. Packet Switching Networks - Datagram ο Each packet treated independently, without reference to previous packets ο Each node chooses next node on packetβs path ο Packets donβt necessarily follow same route and may arrive out of sequence ο Exit node restores packets to original order ο Responsibility of exit node or destination to detect loss of packet and how to recover 15
- 17. Packet Switching Networks β Datagram ο Advantages: ο Call setup phase is avoided ο Because itβs more primitive, itβs more flexible ο Datagram delivery is more reliable 17
- 18. Packet Switching Networks β Virtual Circuit ο Preplanned route established before packets sent ο All packets between source and destination follow this route ο Routing decision not required by nodes for each packet ο Emulates a circuit in a circuit switching network but is not a dedicated path ο Packets still buffered at each node and queued for output over a line 18
- 20. Packet Switching Networks β Virtual Circuit ο Advantages: ο Packets arrive in original order ο Packets arrive correctly ο Packets transmitted more rapidly without routing decisions made at each node 20
- 22. Assignment ο In the assignment , you people are going to submit: Where to use circuit switching? Where to use Packet switching? 22
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Editor's Notes
- #8: propagation delay is equal toΒ d / sΒ whereΒ dΒ is the distance andΒ sΒ is theΒ wave propagation speed
- #15: Jitter is any deviation in, or displacement of, the signal pulses in a high-frequencyΒ digital signal. overhead is any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to perform a specific task