User Manual

ADCP-XX-XXX • November 2000 • Section 3: WMTS Functional Description
Page 3-6
 2000, ADC Telecommunications, Inc.
latency incurred by interleaving. Typical interleaving depth provides 95 microsecond burst protection
at the cost of 4 milliseconds of latency. Four milliseconds of latency is insignificant when using
standard Internet data services such as web browsing, e-mail and file transfer. However, in near real-
time Constant Bit Rate (CBR) services, like IP telephony that have tight end-to-end latency,
performance may be impacted. The variable depth interleave, enables the system engineer to trade
between how much error protection is required and how much latency is tolerated by the services, to
be delivered. The depth of the interleave is also dynamically controlled by the WMTS, based on the
RF channel conditions.
4.6 Downstream RF Channels
The RF channel transmission characteristics of the wireless network in the downstream direction,
assumed for the purposes of minimal operating capability, are described in the table below. This
assumes a nominal analog carrier level of 6 MHz channel bandwidth. All conditions are concurrently
presented.
Table 4-1. Assumed MMDS Downstream RF Channel Transmission Characteristics
PARAMETER VALUES
Frequency range 2.5 GHz to 2.686 GHz
RF channel spacing (design bandwidth) 6 MHz
Transit delay from the headend to the most
distant customer
Less than 0.8 msec (typically much less)
Carrier-to-noise ratio in a 6-MHz band 23.5 dB @ 64QAM
18 dB @ 16QAM
15 dB @ QPSK
Amplitude ripple 0.5 dB within the design bandwidth
Group delay ripple in the spectrum 75 ns within the design bandwidth
Burst noise Not longer than 25 µsec at 10 Hz average rate
Seasonal and diurnal signal level variation +8 dB
4.7 Transmission Convergence Protocol
A transmission convergence protocol (MPEG-2) allows multiple types of services to share the same
RF carrier. For DOCSIS, the TC layer is MPEG-2, a protocol used with increasing frequency in
networks. Using MPEG-2 means that other types of information, also encapsulated within MPEG-2
(e.g., voice and video), can all be sent on the same RF carrier as WMU data. MPEG-2 provides a
mechanism to identify individual packets within the MPEG-2 stream, in the same way that WMU or a
set top box identifies which packets to decode. This mechanism called the PID (Packet Identifier) is
present in all MPEG-2 frames. DOCSIS has declared the value Ox1FFE to be the well known PID for
all the traffic on the channel. WMUs only operates with MPEG packets with this PID. In addition,
MPEG-2 provides a frame structure that facilitates channel lock. MPEG-2 frames start every 188 bytes