Previous | Table of Contents | Next |
Gilbert Held
100VG-AnyLAN is an increasingly attractive transport mechanism for upgrading LANs because it is affordable, available, and easily interfaces with Token Ring and Ethernet.
Although ATM networking has received a considerable amount of attention, its use as a transport mechanism for local area networks at an economical price may require a wait until the turn of the century. In the interim, one of three successors to the Ethernet 100VG-AnyLAN appears to provide network users with a high-speed local area networking capability to support emerging multimedia applications and alleviate existing network bottlenecks. 100VG-AnyLAN is designed from the ground up to interconnect with essentially any type of LAN, such as Ethernet, Token Ring, FDDI, and even ATM, when it becomes available for use. An understanding of the operational capability of 100VG-AnyLAN can assist network managers in developing a LAN upgrade strategy. This chapter provides network managers and users with information required to determine how 100VG-AnyLAN can be used in future networking plans and how its functionality and operational capability can satisfy networking requirements.
The IEEE 802 Committee was originally limited to developing standards for operating rates up to 20M b/s, and the ANSI was tasked with developing standards exceeding that data rate. In 1992, the IEEE Obtained an additional level of responsibility and requested proposals for Fast Ethernet, designed to raise the Ethernet operating rate from 10M b/s to 100M b/s. This request resulted in two initial proposals.
One proposal, now referred to as 100BaseT, was developed by a consortium that included Synoptics Communications, Inc., 3Com Corp., and Ungermann-Bass, Inc. This proposal retained the CSMA/CD access proposal, but it does not support prioritization or the multiplexing of time-sensitive traffic. As a result, it is difficult to support a large number of concurrent LAN sessions that include multimedia applications. In addition, the CSMA/CD access protocol cannot distinguish between different types of network traffic, which makes connection with other CSMA/CD networks difficult.
A second 100M-b/s proposal was developed by AT&T Microelectronics and Hewlett-Packard Co. This proposal, referred to as 100VG-AnyLAN, replaced the CSMA/CD access protocol by a demand-priority scheme that supports Ethernet, Token Ring, FDDI, and other types of LANs. In addition, 100VG-AnyLAN can transport data at 100M b/s for distances up to 100 meters using category 3 unshielded twisted-pair cable, a capability that enables it to support a significant installed base of wiring.
Although the IEEE was supposed to select only one Fast Ethernet proposal, it has not done so to date. 100VG-AnyLAN has become the IEEE 802.12 standard, while the CSMA/CD proposal has become an addendum to the IEEEs 802.3 Ethernet standard, 802.3e.
A third Fast Ethernet proposal asynchronous Ethernet was developed by National semiconductor. It also supports the CSMA/CD access protocol. However, this LAN technique only supports an operating rate of 16M b/s and results in multiplexed 64K-b/s ISDN channels being carried within the bandwidth. This proposal has not yet received as much interest as the other proposals.
100VG-AnyLAN was designed as a hub-centric network architecture. A central hub, known as a level 1 or root hub, functions as an inverted tree base in establishing a 100VG-AnyLAN network. From this hub, other hubs or nodes form a star topology, fanning out underneath the root hub as illustrated in Exhibit 1. All hubs located in the same network segment must be configured to support the same frame format IEEE 802.3 Ethernet or IEEE 802.5 Token Ring. Through the attachment of a bridge or router to a hub port, the 100VG-AnyLAN network can be extended to interconnect with other Ethernet or Token Ring networks, FDDI, and ATM-based networks, or a WAN transmission facility.
Exhibit 1. 100VG-AnyLan Topology.
Each hub in a 100VG-AnyLAN network has one uplink port (labeled up in Exhibit 1) and n downlink ports (labeled 1 through N). The uplink port on each hub is reserved for connecting lower-level hubs to an upper-level hub, whereas the downlink ports are used to connect an upper-level hub to workstations, bridges, routers, and other network devices, including lower-level hubs. Up to three levels of cascading can be used on a 100VG-AnyLAN network.
Each hub port can be configured to operate in one of two modes normal or monitor. Ports configured to operate in their normal mode forward only those packets specifically addressed to the attached node. In comparison, ports configured to operate in the monitor mode forward every packet received by the hub.
Devices connected to nodes gain access to a 100VG-AnyLAN network through the use of a centrally controlled access method, referred to as demand priority. Under the demand-priority access method, a node issues a request referred to as a demand to the hub it is connected to, thus transmitting a packet onto the 100VG-AnyLAN network. Each request includes a priority label assigned by the upper-layer application. The priority label is either normal, for normal data packets, or high, for packets carrying time-critical multimedia information. Naturally, high-priority requests are granted access to the network prior to normal-priority requests.
The level 1, or root, hub continuously scans its ports using a round-robin sequence for requests. Lower-level hubs connected as nodes also perform a round-robin scanning process and forward node requests to the root hub. The root hub determines which nodes are requesting permission to transmit a packet, as well as the priority level associated with the packet. Each hub maintains a separate list for both normal- and high-priority requests. Normal-priority requests are serviced in their port order until a higher-priority request is received. Upon receipt of a higher-priority request, the hub will complete any packet transmission in progress and then service the high-priority packet before servicing the normal-priority list.
To prevent a long sequence of high-priority requests from abnormally delaying low-priority requests, the hub also monitors node request-to-send response times. If the delay exceeds a predefined time, the hub automatically raises the normal-priority level to a high-priority level.
Exhibit 2. 100VG-AnyLAN Hub Round Robin Scanning.
Exhibit 2 illustrates an example of the 100VG-AnyLAN hub round-robin scanning process. Assuming all ports initially have normal-priority requests pending, the packet service order begins at the level 1 hubs first port (1-1). Next, the level 1 hubs second port is serviced (1-2). When the third port is examined, it inserts the round-robin sequence generated by the level 2 hub. That is, it inserts the packet order sequence 2-1, 2-2, 2-3, ...2-N. This sequence is then followed by adding the remaining level 1 hub ports. Thus, the inserted packet order sequence is followed by the sequence 1-4, ...1-N. If at time t equals 0, nodes 2-1 and 1-4 generate high-priority requests, then the packet service order at the level 1 hub would be revised, becoming 2-1, 1-4, 1-1, 1-2, 2-2, ...2-N, 1-5, ...1-N.
As 100VG-AnyLAN was developed to comply with IEEE network modes, its design resulted in the separation of network functions into sublayers. Exhibit 3 illustrates the relationship of the IEEE 802.12 DTE reference model to the well-known ISO/OSI model.
Similar to other IEEE LAN standards, 802.12 subdivides the International Standards Organization data link layer into two sublayers LLC and MAC. Information is transmitted over the LLC sublayer in either IEEE 802.3 or 802.5 frame formats, whereas the MAC sublayer uses the demand-priority mechanism to access the network. The 802.12 model differs from the Ethernet and Token Ring models in its subdivision of the physical layer into four sublayers.
Previous | Table of Contents | Next |