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Chapter 5: LAN Equipment Operations
FDDI and ATM Connections
FDDI and ATM ports are incorporated into some intelligent switching hubs to provide a high-speed data transfer capability between the switch and an attached device. Although many users employ high-speed ports for use with servers, such ports can also be used to develop a hierarchical network of conventional hubs, switching hubs, and high-speed backbone hubs. An example of the creation of a hub hierarchy is shown in Figure 5.25.
In the hub hierarchy illustrated in Figure 5.25, a FDDI hub is used to form a network consisting of three middle-tier switching hubs. The switching hubs in turn support port switching or port and segment switching, with the connection of conventional hubs forming the lower tier in the three-tier hub hierarchy. Through the use of one FDDI port on each switching hub access is obtained to an FDDI ring constructed through the use of an FDDI hub. Assuming each switching hub supports two different types of high-speed ports, the second port might be a 100BASE-T port used to provide access at 100 Mbps between the switch and an attached server. Thus, this configuration allows users attached to a conventional hub to access servers on that hub, access a server connected to the switching hub their conventional hub is connected to, or access a server connected to a different switching hub.
Although the FDDI operating rate is fixed at 100 Mbps, there are two ATM LAN operating rates you must consider. One rate is 25 Mbps, and the other rate is approximately 155 Mbps. Since the lower rate may provide only a marginal improvement over full-duplex 10BASE-T and full-duplex 16-Mbps Token Ring operations, it might be advisable to ignore that technology.
Flow control represents the orderly regulation of the transmission of information between devices, a necessity whenever there is a positive speed differential between the source and destination of information. Although some vendors include buffer storage to act as a temporary holding area until transmission ceases or the amount of data flow is reduced, no amount of buffer storage can guarantee data integrity. Thus, you can expect to lose frames if a switching hub does not provide flow control to regulate the flow of traffic between ports that have different operating rates.
The effect of frame loss depends upon the overall traffic supported by the hub. If you have several servers connected to highspeed ports and short queries result in lengthy transmissions, the loss of frames is compensated for by server retransmissions. Under certain situations the additional retransmissions lock out responses to other workstation queries and can actually result in a level of performance being less than that obtainable through the use of a conventional hub. For this reason the use of highspeed ports should normally be accompanied by the use of flow control, either the use of backpressure in an Ethernet environment or a server software module which works in tandem with the switch in both Ethernet and Token Ring environments.
Backpressure Backpressure is a term used to represent the generation of a false collision signal. Since a collision signal causes an Ethernet workstation or server to delay further transmission based upon an exponential backoff algorithm, it provides a mechanism for implementing flow control. That is, once buffer storage in the hub reaches a predefined level of occupancy, the switch will generate a collision signal. As the transmitting device delays further transmission, the hub's destination port has the opportunity to empty the contents of its buffer, precluding data loss.
Since backpressure requires the use of a second wire pair, it is mutually exclusive with full-duplex transmission. If you require full-duplex transmission on a high-speed port and want to preclude the loss of frames via flow control, you must turn to the use of a server software module.
Server Software Module When this book was prepared, switching hub vendors indicated they were developing software to operate under NetWare and Windows NT that would regulate the flow of data between hub ports and servers. Once such products are available they will support full-duplex transmission as well as preclude frame loss, something currently lacking in the switching hub marketplace.
Token Ring Switching
Until now we have primarily focused our attention on Ethernet switches, using Ethernet segments to illustrate intelligent switch operations. Token Ring switches, in many ways, are similar to Ethernet switches; however, there are certain key differences between the two.
Due to the Token Ring protocol, stations gain access to a ring by acquiring a free token. A port-switching Token Ring switch forms a ring between the port and an attached station or network node. This type of Token Ring switch is also known as a Dedicated Token Ring and allows that station to obtain the full bandwidth of the ring instead of having to share it with other network users.
Bridging Support Figure 5.26 illustrates a four-port Dedicated Token Ring (DTR) switch. In effect, the switch consists of four separate rings and will use bridging to provide a mechanism to transmit data from one ring to another. Concerning the method of bridging supported, unlike Ethernet, which is restricted to transparent bridging, Token Ring switches can also support source routing and source routing transparent bridging.
Switches supporting source routing operate at the data link layer of the OSI Reference Model. Such switches use source routing to make forwarding decisions on each frame it receives. That decision occurs relatively quickly, as the switch can use information in the routing information field to make its forwarding decisions. In comparison, when transparent bridging is used, forwarding decisions are based upon the switch first reading MAC address information in a frame and comparing that address to the tables of addresses it maintains to initiate its switching operation.
In examining Figure 5.26, note that the data transfer unit (DTU) provides the data highway which enables the transfer of information from one ring to another. To illustrate the connection of a ring to a switch port, small rings will be shown for each connection between a switch port and a workstation throughout the remainder of this book, with solid lines used to illustrate Ethernet switch connections....