- An optical, digital physical layer.
- Initially organized into the T hierarchy (T-1, 2, 3)
- T-1 = 1.544 mbits/sec (24*64 bits/sec); T-2 = 4*T-1; T-3 = 7*T-2
- The other hierarchy is the E (E-1 = 32*64 kbit/sec; E-2 = 4*E-1; E-3 =
4*E-2)
- Optical lines became faster, cheaper, more reliable.
- Re-align the global hierarchy, exploit new capabilities, and provide
new services.
- The re-alignment lead to SONET in the U.S. and SDH in Europe.
- Motivations behind ATM:
- High-speed transport.
- Low-latency transport.
- Real-time (or bounded) transport.
- Mixed-media transport, both synchronized (multi-media) and
unsynchronized).
- ATM objectives:
- Traffic diversity.
- Guaranteed delay bounds.
- Multi-cast operation.
- Interoperability with LANs, MANs, and WANs.
- Expected applications include
- Medical imaging (massive data, reliable transport).
- Supercomputer visualization (massive data, interaction).
- Distributed computing (network configurability).
- ATM takes a minimalist approach to network nodes.
- Speed and diversity requirements make it so.
- ATM is a packet-switched technology.
- The packets are called cells of 5 header + 48 payload bytes.
- ATM is also a (virtual) circuit-switched technology.
- The need for guarantees makes it so.
- A virtual channel (VC) goes from end-point to end-point.
- A virtual path is a bundle of virtual channels.
- Cell switching.
- Fast and simple.
- Preserve the independence of virtual channels in a virtual path.
- Statistical multiplexing.
- Path parameters (peak, average rates; maximum burst size).
- The reference model.
- Physical; ATM; ATM adaptation layer (AAL).
- Physical layer: MDS and transmission convergence.
- ATM: packing and unpacking cells, multiplexing.
- AAL: interface to higher-level protocols.
- Physical layer.
- Transmission and reception.
- Header error detection.
- Statistics and performance monitoring.
- Cell transmission.
- Synchronously via SONET.
- Asynchronously.
- Synchronous Optical Network (SONET).
- Framing and interleaving structure.
- Both intra- and inter-frame interleaving.
- ATM layer and cell management.
- Cell multiplexing and demultiplexing.
- Rate management for synchronous transmission.
- Idle and data cell interleaving within the SONET frame.
- QoS enforcement.
- Cell markings for flow control (admit or not admit), quality of service
(drop or no drop), and payload type (network or user).
- ATM cell header, five bytes.
- General flow control, four bits.
- Virtual channel and path identifiers, 8 + 16 = 24 bits.
- Payload type (network or user), two bits.
- Cell loss probability, two bits.
- Header check sum, eight bits.
- Asynchronous transmission in SONET.
- ATM cells don't have fixed position within a SONET frame.
- Idle cells take up the slack when no ATM cells are available.
- The SONET header contains pointers to the ATM cells.
- The AAL links higher-level protocols to ATM.
- AAL services are classified in three ways:
- End-to-end timing (delay sensitive or not).
- Required bit-rate (fixed or variable).
- Connection oriented or not.
- There are four service classes.
- Class A provides a synchronous, constant bit rate,
connection-oriented service.
- Useful for uncompressed and real-time media traffic.
- Class B provides a synchronous, variable bit rate,
connection-oriented service.
- Useful for compressed and real-time media traffic.
- Class C provides an asynchronous, variable bit rate,
connection-oriented data service.
- Class D provides an asynchronous, variable bit rate,
connection-less data service.
- AAL 1 through 4 provide service classes A through D respectively.
- Classes C and D were combined into AAL 3/4.
- But - oops - AAL 3/4 proved hard to implement, promoting the
introduction of AAL 5, a simpler, less constrained version of AAL
3/4.
This page last modified on 14 November 2004.