Making a case for flexible media Ethernet PHYs

As packet data networks evolve, several other interfaces have emerged to handle the growing demand in bandwidth. 100BASE-TX copper media is second to 10BASE-T in terms of network presence and can handle up to 10 times more packet data traffic than its predecessor. 100BASE-FX fiber was also conceived for applications spanning distances longer than a copper PHY could attain. After the year 2000, Ethernet PHY silicon vendors evolved to Gigabit per second speeds by introducing 1000BASE-T PHYs for copper media and 1000BASE-X PHYs for fiber media. Even with this change, most of the existing legacy packet data systems support only 10BASE-T, 100BASE-TX, and/or 100BASE-FX interfaces and are still being utilized in today's Ethernet networks.

The challenge for Ethernet equipment suppliers
Many equipment suppliers who are building products to handle packet data today must contend with connecting to the growing number of new Ethernet media interfaces as well as maintain connectivity with 10BASE-T, 100BASE-TX, and 100BASE-FX legacy products. In order to provide some level of flexibility for their end consumers, these suppliers have been coming up with creative methods utilizing existing Ethernet PHY technology. Some equipment vendors designing a system will provision for example both a 10/100/1000BASE-T copper PHY and a 1000BASE-X fiber PHY in the same chassis. Others build modular chassis systems allowing their end customer to purchase various daughter modules compatible to their chassis system that can be swapped and configured depending on what diversity of network interfaces are needed. This approach, however, adds complexity by requiring additional hardware that both the supplier and the consumer have to contend with as well as an increase in support costs for both product ordering and customer installation.

Handling copper Ethernet networks
Even with the growing deployment of 1000BASE-T Gigabit Ethernet, 10BASE-T and 100BASE-TX copper PHYs remain the top two networks interfaces actively used in network hardware. Therefore, it is expected that Gigabit copper PHY vendors must create silicon able to support all three copper speeds: 10BASE-T, 100BASE-TX, and 1000BASE-T (See Figure 1). What is convenient is the predominant use of PHY-to-PHY link auto-negotiation and a parallel-detection scheme defined by clause 28 of the IEEE 802.3 specification. Auto-negotiation is essentially active media interface selection performed by two copper Ethernet PHYs communicating with each other across the cable to determine the highest supported speed and duplex setting between the two PHYs. When auto-negotiation fails, parallel detection is employed by a clause 28 capable PHY to attempt to link to a legacy 10BASE-T or 100BASE-TX PHY that does not have auto-negotiation capability.

Handling Ethernet fiber networks
An alternate media to copper is fiber optic cabling. Unlike copper networks, Ethernet fiber networks do not have a defined specification to auto-negotiate or detect and react to the various fiber link bandwidth speeds. There is 1000BASE-X auto-negotiation defined by clause 37 of IEEE 802.3, however this is mainly for duplex discovery and remote fault indication for 1000BASE-X only. It does not include 100BASE-FX advertisement as part of the auto-negotiation. Therefore, an equipment supplier usually dedicates each Small Form-factor Pluggable (SFP) port to one fiber speed such as 1000BASE-X or 100BASE-FX only. This means that a single SFP slot cannot process difference types of SFPs that are plugged into them. When multiple fiber speed support is required, the various fiber speed modes are generally handled by swapping out different fiber-based daughter modules into a dedicated opening slot within the chassis system. Recently, there have been developments in systems where a fiber media PHY can support both 1000BASE-X and 100BASE-FX, but a supplier must have this support implemented in their hardware via custom MAC ASICs and the user must manually switch software configuration settings in order to recognize the change in fiber SFPs from 1000BASE-X to 100BASE-FX and vice-versa.

Plugging copper into fiber SFP ports
As 1000BASE-X fiber has increased in production, several vendors have been offering chassis systems that only support SFPs slots (no copper RJ45 interfaces). These vendors and their customers have been looking for an option to be able to provide at least one or more copper ports on these boxes, without having to dedicate an entire copper PHY and RJ45 jack on their printed circuit board. Over time a 10/100/1000BASE-T copper SFP was developed. These particular copper SFPs (also known as triple-speed copper SFPs) can be plugged into a newer multi-port chassis originally designed for fiber only applications, giving them the ability to support 10/100/1000BASE-T copper by plugging in one or more of these SFP modules. While not particularly the most cost effective method as it requires the purchase of this SFP and the loss of the SFP port that could have been used for fiber, there are real market demands to have this option so that copper systems such as a data server or a LAN switch could be plugged into this chassis box.

Dual media Gigabit Ethernet PHYs
Within the past few years the copper and the fiber packet systems have started to converge. Also, the growth of systems with 24- to 48-copper ports plus 2- to 4-fiber uplink ports has been on the rise in today's data network systems. To accommodate this market trend, dual media Gigabit Ethernet PHYs were developed with the capability to support both 10/100/1000BASE-T copper and 1000BASE-X fiber (See Figure 2). These dual media PHYs help reduce the chip count and Bill-of-Material (BOM) cost by eliminating the need for an on-board discrete 1000BASE-X PHY. Another advantage is if a silicon vendor offered a copper only media PHY and dual media Gigabit Ethernet PHY that are pin-to-pin and software register compatible, an equipment vendor could utilize the same printed circuit board and software, thus allowing them to create two SKUs for their product portfolio with minimal effort. The first SKU would support 10/100/1000BASE-T only and the second would have the dual-media PHY and SFP cage populated to offer 1000BASE-X fiber capability.

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