.sh "NETWORK INTERFACES" Networks can be categorized as \fIlocal area networks (LANs)\fP or \fIlong haul networks\fP according to their geographical limitations. The most widely publicized local area network is the Ethernet. An example of a long haul network is the DARPA Internet which spans many continents and includes devices such as communication satellites for connecting disjoint \fIsub-networks\fP. .LP Among local area networks there are several competing modulation schemes. The Ethernet and several other networks uses \fIbaseband\fP modulation techniques, while newer technologies, such as \fIbroadband\fP, are available from other vendors. Some of the major differences between baseband and broadband technologies are maximum station separation, cable bandwidth, and, currently, per station connection cost. At this time, the least expensive, and most readily available local area networking hardware use baseband modulation. However, given the limitations inherent in baseband modulation schemes, companies are placing more work into developing low cost parts for use in broadband networks. .LP Aside from the question of baseband versus broadband, selection of medium is an issue. Coax cable is commonly used but types of coax vary. Broadband networks normally use the same standard 75 ohm coaxial cable used for CATV, while baseband uses 50 ohm cable. This implies that upgrading a network from baseband to broadband requires expensive installation of a new cable unless one thinks ahead, or your site already has installed cabling for in-house CATV use. Further, the best medium in terms of signal loss and noise immunity is fiber optic cable. However, due to problems such as tapping the cable, few vendors have selected this technology. If you plan to consider broadband at some time in the future, while at the outset using baseband, it is well worth the cost of the extra cable to run parallel 50 and 75 ohm coax. .LP In looking at network controllers, we will consider only the available local area networking hardware; our experience with long haul networks is limited to the Internet and so is of minimal interest. .LP There are at least four vendors with existing or announced Ethernet controllers, and with the soon to be available \*(lqEthernet chips\*(rq more vendors may announce products. It is unlikely, however, that the Ethernet chips will significantly influence the current prices as the price of an Ethernet controller has already been driven down by the market competition. While the influx of new technology may not lower controller prices, it is sure to improve their performance and reliability. .LP We currently use 10Mb/s UNIBUS Ethernet controllers from both Interlan and 3Com. The two controllers have almost identical throughput characteristics with 4.2BSD, but neither have proven entirely satisfactory. The 3Com controller is the less expensive of the two. Its design is optimal for small PDP-11s and LSI-11s where the processor is resident on the same bus with the controller. The design employs 16 or 32 Kbytes of dual-ported RAM which is directly addressable as UNIBUS (or Q-bus) memory. While this is effective for machines such as the PDP-11 or LSI-11 where no penalty is required when accessing the on-board memory, with a VAX any memory access must be arbitrated by the intervening UNIBUS adaptor. The result of this is that accesses to the on-board memory are heavily constrained by the characteristics of the UNIBUS adaptor. .LP In accessing memory through a UNIBUS adaptor, all accesses must be performed on even byte boundaries and be no more than two bytes at a time. Consequently, one must either be very careful about the coding of a network interface driver, or the contents of any on-board memory must be copied into main memory before manipulating it. Due to the architecture of the networking subsystem included in 4.2BSD and the lack of control over the code generated by the VAX C compiler, constraining memory fetches was infeasible and the second alternative was taken. This implies that data must be block copied in to and out of the on-board memory a word at a time. The VAX \fImovc3\fP instruction is not usable in the UNIBUS address space, making this an expensive operation. .LP A second problem with the 3Com controller is that it lacks an on-board timer for implementing a backoff algorithm when accessing the Ethernet. This implies the host must perform a timing loop when backing off from a congested Ethernet. When an Ethernet is heavily congested this may prove to be very costly as no other processing may take place while the host timing loop is executing. .LP A third problem with the 3Com controller is that it does not allow a host to receive its own broadcast packets. This implies that broadcast packets must be captured in software. We consider this a serious deficiency as it prevents hardware testing without an auxiliary echo server. .LP The second Ethernet controller we have used is made by Interlan. This controller provides DMA access, as well as several desirable features such as on-board retransmissions. Unfortunately, while the DMA interface should be expected to provide higher throughput than the shared memory approach, using the Interlan interface we have been able to attain only comparable transfer rates to those measured with the 3Com interface. In addition, the controller consumes a significant amount of of +5 volt power. While broadcast packets are retrieved by the interface, the Ethernet CRC calculation is not performed. .LP We know of two other Ethernet controllers, one from ACC and one from DEC. We have two ACC controllers for evaluation, but have yet to gain any experience with them. The ACC controller is based on the UMC-Z80 and provides a DMA host interface. The DEC Ethernet controller was announced at the last DECUS meeting, but as of yet we know of none in customer hands. .LP To summarize the Ethernet controller situation, it appears the best strategy to follow is to wait for Ethernet chips to become widely available so the vendors can reengineer their existing controllers with minimal cost. If you require Ethernet access from your VAX now, you may wish to follow our approach: select the lowest priced product and treat it as \*(lqdisposable\*(rq in the expectation that something better will eventually be available. .LP Other than Ethernet, the Proteon proNET 10 Mb/s ring network is also popular. This device is also known as the Version II lni ring network and is in heavy use at LBL and MIT with good results. The Proteon proNET outperforms both the 3Com and Interlan controllers mentioned above in throughput benchmarks run with the 4.2BSD networking support. Further, the ring design eliminates the standard complaints about ring architectures by use of a star-shaped ring configuration. The star-shaped ring allows easy addition and deletion of nodes without splicing drilling or taping. Also, any node can fail without bringing down the ring because it is bypassed at the star-shaped ring's passive wire center. The major concern with a ring network is that it is incompatible with the de facto standard Ethernet. Cost per station is slightly higher than the Ethernet, but startup costs are lower (unless you use a fiber optic wire center). Proteon has announced they are working on an 80 Mb/s controller which should make the network even more attractive.