Transmission Line Parameters

Metallic interconnections reign supreme for the conveyance of both data and power within digital systems. This chapter addresses the parameters of all metallic interconnections. Although this chapter is oriented towards the use of copper media, the same theory with slight modifications to account for the difference in conductivity also applies to interconnections built from aluminum, steel , nickel, and other metals. It may also be applied to hydrocarbon-based conductors and polysilicon, although in those cases, due to the high resistance of the conductors, generally the RC mode of propagation will predominate.

The following chapters concentrate on four types of copper-based transmission media used for high-speed data transmission (Table 2.1). As explained in the chapters that follow, the performance of each cabling type varies as a function of its length, quality, and other factors. Nevertheless, certain key features noted in the table stand out in a gross comparison of their capabilities.

Unshielded 100- W twisted-pair (UTP) cabling is the best choice for high-volume, cost-sensitive LAN applications. UTP delivers an aggregate bandwidth (all four pairs) of 1 Gb/s at 100 meters , while meeting FCC and EN emissions requirements. The connector and cross-connect technologies for this style of cable are mature and available at low cost, in large measure because the ISO 11801 generic building wiring guidelines (see Chapter 7, "Generic Building-Cabling Standards") officially sanction UTP cabling. In high-volume applications, UTP is ideal.

Versions of UTP cabling are available with exterior shields (called screens). These configurations are popular in Europe.

In low-volume applications, manufacturers sometimes choose not to undertake the development of UTP transceivers because of the complexity and risk associated with the mixed-signal technology required to fully exploit the benefits of UTP. Low-volume applications have historically selected coaxial or 150- W shielded twisted-pair (150- W STP-A) cabling because it is easier to develop transceivers for those cables, even though the ultimate per-unit cost of the cabling and connectors are higher.

Table 2.1. Popular Transmission Media for Digital Applications

Cable type	Signals per cable	Best features	Worst features
UTP	4	Useful up to 250 Mb/s per pair; connectors are inexpensive; new types constantly being developed.	Significant mixed-signal technology is required for high-speed use.
150- W STP-A (IBM Type I)	2	Useful up to 1 Gb/s per pair; interfaces directly with differential transceivers.	Bulky and extremely difficult to install.
Coax	1	Useful beyond 1 Gb/s; interfaces directly with high-speed digital logic.	Not standardized for building wiring; difficult to install.
Pcb traces	1	Useful to 10 Gb/s and beyond.	High-frequency losses severely limit attainable distances.

One hundred-fifty- W STP-A is rated for single-pair operation up to 1 GHz. Designing a 150- W STP-A transceiver is a snap. The 150- W STP-A cable is well enough balanced and well enough shielded that it can directly accept signals from a high-speed differential driver and still pass FCC and EN emissions requirements. The primary disadvantages of 150- W STP-A are its extreme bulkiness and the difficulty of installation. Like UTP,150- W STP-A was originally sanctioned by the ISO 11801 generic building wiring guidelines, but support has been withdrawn in favor of newer category 5E, 6, and 7 twisted-pair wiring standards. Additional information about the use of UTP and 150- W STP-A cable styles appears in Chapter 7.

Coaxial cabling is the simplest means of interconnecting two systems and potentially delivers the highest bandwidth. Coaxial cabling is also generally the poorest performing in terms of radiated emissions. Above 100 MHz, data scrambling has been used to guarantee that ordinary cables will not radiate in excess of FCC or EN limits. [3] The primary disadvantages of coaxial cabling are the difficulty of installation, the lack of LAN industry standardization for the manufacture of coaxial cables, and the lack of standard tests for installation compliance. In the LAN market, coax is dead. In the television and audio/visual markets, however, coaxial cable is still very much in use.

[3] Unscrambled transmission systems radiate horribly because simple repetitive structures within the data stream, like the idle pattern, tend to concentrate all their radiated power at harmonics of the basic pattern repetition rate. These concentrated harmonics then leak from the coaxial cable, where they are easily detected by FCC or EN test antennas. In contrast, scrambled transmission systems spread their radiated power across a wide frequency range, limiting the peak radiation in any one radio-frequency band .

Printed-circuit board (pcb) traces, of course, are used for extremely high-frequency connections within a single pcb or on back-planes connecting one or more pcbs.

All metallic transmission media share the performance model described in Chapter 3.