The High-Speed Serial
Interface (HSSI) is a DTE/DCE
interface developed by Cisco Systems and T3plus
Networking to address the need for high-speed communication over WAN
links. The HSSI specification is available to any organization wanting
to implement HSSI.
HSSI is now in the American National
Standards Institute (ANSI)
Electronic Industries Association (EIA)/TIA TR30.2 committee for formal
standardization. It has recently moved into the International
Telecommunication Union Telecommunication Standardization Sector (ITU-T)
(formerly the Consultative Committee for International Telegraph and
Telephone [CCITT]) and the
International Organization for Standardization (ISO),
and is expected to be standardized by these bodies.
HSSI defines both the
electrical and the physical DTE/DCE interfaces. It
therefore corresponds to the physical layer of the OSI reference model.
HSSI technical characteristics are summarized in Table 11-1.
Table 11-1: HSSI
technical characteristics.
| Characteristic |
Value |
|
Maximum signaling rate
|
52 Mbps
|
|
Maximum cable length
|
50 feet
|
|
Number of connector pins
|
50
|
|
Interface
|
DTE-DCE
|
|
Electrical technology
|
Differential ECL
|
|
Typical power consumption
|
610 mW
|
|
Topology
|
Point-to-point
|
|
Cable type
|
Shielded twisted-pair
wire
|
The maximum signaling rate
of HSSI is 52 Mbps. At this rate, HSSI can handle the T3 speeds
(45 Mbps) of many of today's fast WAN technologies, as well as the Office
Channel -1 (OC-1) speeds (52 Mbps) of the synchronous
digital hierarchy (SDH). In addition, HSSI easily can provide
high-speed connectivity between LANs, such as Token
Ring and Ethernet.
The use of differential emitter-coupled
logic (ECL) helps HSSI achieve high data rates and low noise
levels. ECL has been used in Cray interfaces for years and is specified
by the ANSI High-Performance
Parallel Interface (HIPPI) communications standard for
supercomputer LAN communications. ECL is off-the-shelf technology that
permits excellent retiming on the receiver, resulting in reliable timing
margins.
HSSI uses a subminiature, FCC-approved
50-pin connector that is smaller than its V.35 counterpart. To reduce
the need for male-male and female-female adapters, HSSI cable connectors
are specified as male. The HSSI cable uses the same number of pins and
wires as the Small
Computer Systems Interface 2 (SCSI-2) cable, but the HSSI
electrical specification is tighter.
The flexibility of the HSSI clock and
data-signaling protocol makes user (or vendor) bandwidth
allocation possible. The DCE controls the clock by changing its speed or
by deleting clock pulses. In this way, the DCE can allocate bandwidth
between applications. A PBX,
for example, may require a particular amount of bandwidth, a router
another amount, and a channel extender a third amount. Bandwidth
allocation is key to making T3 and other broadband services affordable
and popular.
HSSI assumes a peer-to-peer intelligence
in the DCE
and DTE. The control protocol is simplified, with just two control
signals required ("DTE available" and "DCE
available"). Both signals must be asserted before the data circuit
can be is valid. The DCE and DTE are expected to be able to manage the
networks behind their interfaces. Reducing the number of control signals
improves circuit reliability by reducing the number of circuits that can
fail.
HSSI provides four loopback
tests, which are illustrated in Figure 11-1. The first provides a
local cable test as the signal loops back after it reaches the DTE port.
The second test reaches the line port of the local DCE. The third test
reaches the line port of the remote DCE. Finally, the fourth
test is a DCE-initiated test of the DTE's DCE port.
Figure 11-1: HSSI supports four loopback tests.
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