ATM network basic

1
ATM network basic

• The fundamental data unit is a packet with a
  fixed size called ATM cell.
• ATM allows integrated service transmission so it
  is needed that some data type (i.e. voice and
  video) will be delayed as less as possible.
• The ATM cell consts of 53 bytes the firts 5 are
  ATM header while the 6th refers to AAL (ATM
  Adaptation Layer) header. the remainig 47 are the
  payload.

2
The synchronization problem

• The cells within the ATM switches will be
  affected by a random delay due to the traffic
  load of the network.
• The inter-cell time frame (Tcell) in transmission
  is constant while it is not at the receiver.
• Considering real-time Constant Bit Rate (CBR)
  services it is needed to have, at receiver side,
  a proper buffer (Cell Delay Variation Smoothing
  Buffer) allowing to recreate the CBR stream
  transmitted
• The buffer lenght must be L int(Qmax/Tcell) 2
  where Qmax is the maximum cell delay within the
  network.
• Starting to read the buffer, with a frequency
  exactly equals to the one used in transmission,
  when the L-th cell arrives, overflow or underflow
  condition will never happen.

3
The synchronization problem
Stream CBR to be transmitted
Output cell from the transmitter
Received cell
Number of bits present within the Cell Delay
Variation Smoothing Buffer
Output stream from the buffer
Really the 2 user clocks have got only nominal
equality
Long term overflow or underflow possible
The transmitting clock must be recovered
4
The recovery system in a synchronous network