5 Comparison Analysis of ATM, Fibre Channel, Gigabit Ethernet

Figure Comp-1 illustrates a communication technology tree, which categorizes the various data communication standards. The diagram gives an overall view of where the technologies discussed in this paper relate to each other as well as other data communication standards. The classification of Fibre Channel is somewhat arbitrary since it also supports LAN environments.

Figure Comp-1.

Data Communications Technology Tree (note 4)

5.1 Bandwidth

Typical Ethernet networks have throughput of approximately 70% of the wire speed. Early simulations of Gigabit Ethernet show improvement over 100 Mbps Ethernet by an order of magnitude. The Gigabit Alliance claims that with full-duplex operation, close to the theoretical limit of 2 Gbps will be achieved [24].

Currently the fastest ATM products available run at the OC-48 rate of 2.48 Gbps. Fore Systems offers a OC-48c switch module on their ASX-4000 switch.

The fastest Fibre Channel products available now run at 800 Mbps (1.062 Gbaud). ANSI has approved the 2.134 Gbaud and 4.25 Gbaud standards in 1995, however the optic technology is still immature. The 2 Gbps products are becoming available now, but the 4 Gbps products will be a few more years until available.

If higher layer protocols are employed, the actual data throughput will be significantly less and latencies higher than the raw speeds given in Table Comp-1. The difference is primarily due to protocol overhead in moving data from the application to the wire, and the efficiency of the host network adapter.

In particular, TCP/IP traffic over a wide area, T3 (45 Mbps) ATM link may be as low as 2 Mbps [25]. Actual performance is dependent on the system in use. Therefore each system should undergo benchmarking tasks to obtain realistic performance statistics

Table Comp-1

Network Comparison Chart

5.2 Latency

Because Fibre Channel can also support dedicated channels, it can provide low delay links for large data transfers. Comparatively, ATM has much higher latencies, on the order of hundreds of microseconds. Note, the measured latency is highly dependent on the size of the transfer. A University of Minnesota study reported similar latencies (500 usec) for ATM and Fibre Channel when transferring only 4 Kbytes [26]. Latencies for Gigabit Ethernet were unavailable at the time of this writing.

5.3 Ability to Scale

5.4 Error Detection/Recovery

ATM provides single bit error correction over the cell header, and only error detection over the payload. It is up to the higher layer protocols to handle error correction for multiple bit errors or payload errors. Fibre Channel provides error correction as an option. Applications may request Class 2 service, which provides guaranteed delivery. For Ethernet, error detection is defined in the CSMA/CD algorithm. The sending station re-transmits upon detecting a collision.

5.5 Multicasting/Broadcasting

Traditional IP multicasting may also be done over ATM using LAN Emulation (LANE). The IETF has published several RFCs on multicast support over ATM using Classical IP. Fibre Channel supports multicasting within its switched topology. The FC-3 Fibre Channel layer provides the capability to send a single transmission to all or a subset of N_ports. The FC Class 3 service (unconfirmed datagram service) is used for multicasting. Broadcast is inherently supported in Ethernet, since it is a shared medium.

5.6 Traffic Management

The ATM Forum published the Traffic Management version 4.0 specification April 1996, which defined many of the necessary parameters and algorithms for traffic management. These include the QoS parameters for each class of service, traffic contract specification and conformance, and functions such as connection admission control.

The FCA has not addressed traffic management in as much detail. Fibre Channel simply returns a "Busy" frame when congestion is encountered, providing a sliding window flow control mechanism.

Ethernet does not provide any traffic management services.

NOTES:

4. Original diagram from "Networks and their Architectures", C. Smythe, Electronics & Communication Engineering Journal, Feb. 1991 pp. 18-28. The original version was modified to include channel technologies.