AAL 3/4 Convergence Sublayer (CS)

AAL 3 was split into 2 sub-layers:

• SSCS - Service specific convergence sub-layer.
• CPCS - Common part convergence sub-layer.

• Non-assured transfer of SSCS-PDU's.
• An SSCS-PDU may be as big as 65,535 bytes.
• Errored CPCS-PDU's can be discarded or delivered to SSCS. It may also be possible to detect errors.
• Can provide buffer allocation size. (How much space does the receiver need to allocate to reassemble a CPCS-PDU?)

SSCS Provides:

• End to end assured connection (reliable data transfer analogous to TCP).

AAL 5

• Originally proposed by IBM.
• AAL 3/4 SAR overhead is 4 bytes/44 bytes = 9% overhead.
• AAL 3/4 uses 10 bits for CRC, 4 bits for sequence number, but existing applications use TCP (or some other transport protocol) which already does this.
• AAL service access point (SAP) should look like a standard LAN interface such as Ethernet.

But data only needs to know where packets begin. Other services will be provided above and only if necessary.

• So AAL 5 has no SAR header!
• Uses third bit of ATM payload type indicator (PTI) field for End of Message (EOM) indication
  • Third bit of PTI = 1 for EOM.

AAL 5 Convergence Sub-layer

CPCS:
• Similar to AAL 3/4 but
  • No buffer allocation notification.
  • 32 bit CRC.
  • 16 bit length indicator.
  • Padding to make packets multiples of 48 bytes

SSCS:

• May be NULL or like AAL 3/4 SSCS.
• Provides full connection oriented services.

N.B. AAL 5 is now used for signaling and network management.

Some basic terms:

1. Admission Control - Given a connection request, does the network have the resources to grant it and still satisfy the quality of service requirements of existing connections?
2. Congestion Control - "Congestion" means too much traffic at some point(s) in the network. Congestion control prevents it or reduces it. Routing, priority, flow control and admission control can all help, but at different time scales.
3. Flow Control - Regulating the amount of traffic or flow on a connection or a group of connections. Could be hop by hop (Data Link Layer) or end to end (Transport Layer).
   • Why do flow control?
     • So slow receiver isn't swamped by faster sender.
     • Congestion control.
     • Traffic shaping and policing of flows in the network, i.e. ensuring compliance with traffic specifications.
4. Quality of Service Guarantees - The delivery of traffic consistent with pre-specified constraints.

Recall from CCN Class: Basic Congestion Control Mechanisms

2 Broad Classes:

Closed Loop

• Choke packet - too much flow, send back packet to source to slow down sender.
• Window based flow control, e.g. Go Back N, TCP.
• Pre-allocation of buffers with hop by hop flow control. In ATM this is called credit-based congestion control.

Open Loop

• Fixed allocation - Synchronous Transfer Mode (STM).
• Discarding.
  • Used with a re-transmission scheme (must use a re-transmission scheme for no-loss traffic).
  • Used with priority scheme (some loss is allowed).
  • Used with error-correction coding.