Future LANs IEEE 802.12, ATM

• 100VG-AnyLAN (IEEE 802.12)
• Asynchronous Transfer Mode (ATM)

100VG-AnyLAN (IEEE 802.12)

The IEEE 802.12 standard ratified in 1995 is based on the Hewlett Packard high-speed (100 Mbps) 100VG-AnyLAN technology [LIN96] [NOR96] . Traditional Ethernet or Token Ring operation is not adhered to, instead a new MAC technique called Demand Priority is proposed. Only one node at a time is allowed to transmit, with nodes polled in a round-robin fashion for requests to send packets of data. This removes the chance of collisions, such as are prevalent on CSMA/CD networks, making network performance more deterministic than Ethernet networks, but without the token-processing overhead inherent in Token Ring networks.

Prioritisation is supported by the 100VG-AnyLAN system, making it suitable for time-critical traffic (frequently found in multimedia applications). 100VG networks can be connected to, and will support traffic from, Token Ring as well as Ethernet networks, with the aid of fairly simple bridges. The main problem with 802.12 is that no network operating systems or applications support the Demand Priority features at the present time, somewhat limiting its usefulness at the moment.

Asynchronous Transfer Mode (ATM)

ATM is not a LAN standard, but a network transport protocol, which, by the arrangement of data into fixed length cells (currently 53 bytes), can accommodate the disparate bandwidth, latency and bit-rate requirements of both LAN and multimedia traffic (voice, video and data). Depending on the type of information being passed, it will be 'packetised' in one of four ways ready for transmission, based on an ATM Adaptation Layer (AAL) format. Each AAL type will co-exist with the other, giving each the conditions it needs to traverse the network.

So far, however, the AAL types for voice and video traffic have not been standardised by the standards bodies. Vendors that claim to offer voice and video over ATM do it in a proprietary way by pretending that it is actually LAN data, and therefore it may be a number of years before true multimedia support is available. The ATM Forum is a non-profit international industry consortium which aims to accelerate ATM acceptance and interoperability.

For data, where the standards are more advanced, two different and incompatible LAN emulation techniques are proposed. Multi-Protocol over ATM (MPOA) is a relatively new standardization effort in the ATM Forum that will specify how existing and future network-layer protocols will exploit certain features of ATM networks, including quality of service (QoS) and connections between 'Virtual LANs' (VLANs) - a section of the network which is built upon an emulated LAN seqment.

LAN Emulation (LANE) is the process whereby an ATM network emulates a significant part of the MAC protocol of an existing IEEE LAN, specifically Ethernet and Token Ring. LANE allows existing higher-layer protocols and applications to be used transparently over an ATM network that interconnects Ethernet and Token Ring LANs.

ATM is really designed for voice and WANs, though the schemes outlined above enable it to be used for LANs. However it may be some time (if ever) before ATM technology is fully embraced for LANs by network managers and users as it is still very new (and not fully standardized). For the moment it is used for high-speed WANs and backbones.