Network Delivery Mechanisms - A Practical Guide to Video and Audio Compression - page 334

Table 19-1 Typical Connection Speeds for Networks

Connection Speed

Analog modems 9.6, 14.4, 28.8, 33.6, 56 Kbps

ISDN (1 channel) 64 Kbps

ISDN (2 channel) 128 Kbps

T1 1.544 Mbps

ISDN-32 2.048 Mbps

ADSL 8 Mbps down,1 Mbps up

Ethernet LAN -10Base-T 10 Mbps

ATM25 25.6 Mbps

Ethernet LAN -100Base-T 100 Mbps

19.3 Dial-Up Telephone Services

Internet connections have used dial-up modems for many years. Back in the 1970s the con-
nection could only manage 1200 baud. Modems supporting 56K baud and broadband con-
nections running at better than 500 Kbps are now becoming commonplace. Note that
broadband is measured in bits per second (bps) and modems are described in baud rates.
The baud rate is a measure of physical bits per second, but they are not all usable bits
for our payload. Figure 19-1 shows how the bit pattern in a single frame uses some bits to
manage the data flow.
The 7-bit character requires a start bit to synchronize the delivery. The format has a
parity bit for error correction and in this case it has 2 stop bits to allow characters to be sent
continuously. Parity might be on or off, the stop bits might vary, and the data size might
change. So the frame is 11 bits long in this case but the payload is only 7. For a baud rate
of 9600, the usable transfer speed would only be 6109 bps.

Start
bit

Parity
bit

Stop
bits

Data bits

Frame length

Figure 19-1 Serial bit streams in telephony.