Wide Area Networks

Characteristics of WAN

  • A WAN is a network in a large geographical location. Maybe across cities or countries.
  • Crosses the public ‘right of the way’.
  • Is usually owned by a service provider.
  • More expensive and complicated than a LAN.
  • X.25, Frame Relay, ISDN  and ATM are some WAN technologies.

 

 

Switching Technologies

A switched network goes through a switch instead of a router. This actually is the way most networks are headed, towards flat switches of VLAN instead of routers. Long distance transmission is typically done over a network of switched nodes.

 

Circuit  Switching

  • Originated in public telephone networks.
  • Well suited to the analog transmission of the voice signal.
  • A dedicated communication path between two stations.

 

Three Phases

  1. Establish
  2. Transfer
  3. Disconnect

 

  • Must have is switching capacity and channel capacity to establish a connection.
  • Mast intelligence to work out routing.
  • Once connected, transfer is transparent.
  • Develop for voice traffic Jerry cartoon( phone).
  • Inefficient- channel capacity dedicated for the duration of the connection. if no data,  capacity wasted.
  • Set up( connection) takes time.

 

P 49

 

Blocking or non-blocking circuit switching

1. Blocking

  • A  network may not be able to connect is all paths in use (most stations than the path).
  • Used on voice systems (short duration calls).

2. Non-blocking

  • Permits all stations to connect (in pairs)  at once ( at least as many parts as stations).
  • Used for some data connections.

 

Message switching

An alternative switching strategy is message switching.  When this form of switching is used, no physical path is established in advance between sender and receiver. Instead, when the sender has a block of data to be sent, it is a store in the first switching office (i.e., router)  and then warded it later, one hop at a time. Each block is received in its entirety, Inspected for errors, and then transmitted. A network using this technique is called that store-and-forward network.

 

Packet Switching

1. Data transmitted in small packets.

  • Longer messages are split into serious of packets.
  • Each packet contains a portion of user data plus some control info. Control info-group routing ( addressing)  info.

2. Packets are received, Stored briefly ( Buffered) and passed into the next node (store and forward).

 

The differences between circuit switching and packet switching are

 

Item Circuit Switched  Packet Switched
Call setup  Required Not needed
Dedicated physical path Yes No
Each packet follows the same route Yes No
Packets arrive in order Yes No
Is a switch crash fatal Yes No
Bandwidth available Fixed Dynamic
Time of possible congestion At setup time On every packet
Potentially Wasted bandwidth Yes No
Stored-and- forward transmission No Yes 
Transparency Yes No
Charging Per Minute Per-packet

 

Types of service in a packet switched network

Packets are handled in two ways

1. Datagram

  • Each packet is treated independently.
  • Perfect can take any practical route.
  • Packets made out of order.
  • No Sequencing is guaranteed.
  • The packet may go missing.
  • Up to receiver to re-order packets and recover from missing packets
  • A datagram service is similar to a mail service. Each packet is treated as a completely different entity and is routed independently.
  • Reliable delivery may or may not be guaranteed.

 

2. Virtual circuit

  • Preplanned route established Before any packets sent.
  • Appears to be similar to circuit switched in that all data is received in the order sent.
  • Call request and call accept packets to establish a connection (handshake).
  • No routing decisions required for each packet.
  • Each connection is associated with a particular virtual circuit identifier.
  • Provides error and flow control.
  • Clear requests to draw the circuit.
  • Not a dedicated path.

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