Transmission
Figure 2-1 shows a VoIP transmission scenario.
Figure 2-1 Passing Voice Packets
CCNP ONT
[ 254 ] CCNP ONT Quick Reference
IP
Phone
Analog
Phone
CCM
Router
WAN Router/
Voice
Gateway
PSTN
2) Packets
1) Sound
3) Analog
4) Sound
101011010... IP
101011010... IP 101011010... IP 101011010... IP 101011010... IP
Voice is passed over an IP network by packetization. Example 2-1 shows an
IP phone communicating with an older analog phone, but any combination
of the two is supported. The numbered list below matches the steps involved
in taking sound and converting it packets and then back to sound:
1. Incoming sounds are grouped into slices of sound (typically 20 ms),
sampled, and digitized.
2. Each slice of sound is fitted with headers (data link, IP, User Datagram
Proocol [UDP], and Reliable Transport Protocol [RTP]) and transmitted
across the IP network.
3. Because the analog phone doesn’t understand packets, a gateway (in
this case, it is housed in a router) translates the stream of packets into
an analog electrical signal.
4. The analog phone receives an analog electrical signal and sends it to a
speaker, where the recording is restored to audio.
Cisco routers are commonly deployed as gateways. Three types of analog
connections are supported:
■ Foreign Exchange Station (FXS)—FXS ports connect analog phones.
FXS ports supply line voltage.
■ Foreign Exchange Office (FXO)—FXO ports connect to a Private
Branch Exchange (PBX) or to the Public Switched Telephone Network
(PSTN). FXO ports receive line voltage.
■ E&M—E&M (which is alternately said to stand for Ear and Mouth or
Earth and Magneto) interfaces supply advanced signaling to a PBX
using a separate set of wires.
Three digital phone ports are supported:
1. ISDN—ISDN interfaces support advanced Q.931 signaling.
2. T1/E1 CCS (Common Channel Signaling)—T1/E1 CCS uses a
channel for signaling. ISDN PRI uses CCS.
3. T1/E1 CAS (Channel Associated Signaling)—Robs bits from the
analog waveform for signaling and is not as full-featured.
Although Figure 2-1 focused on the flow of voice records, signaling is
equally important to understand. Signaling is what tells the system which
phone to ring and when the line is hung up. Phone companies, in particular,
are interested in this (and might write it $ignaling) because signaling is used
in billing. Figure 2-2 shows the types of signaling that are expected.
Signaling plays several important roles:
■ Information about the receiver is obtained.
■ Capacity is checked before starting; otherwise, call quality suffers.
■ Call quality is monitored so that adjustments may be made to maintain
call quality.
■ Connect and disconnect times are kept for billing.
In Figure 2-2, a Call Manager is shown receiving the signaling. A Call
Manager allows centralized call control, which provides oversight of the call
and records of connections and quality. Voice trunking may be accomplished
without such supervision (called distributed call control), but care must be
taken to not overburden links and quality must be manually maintained.