20-03-2014, 03:02 PM
Point-to-Point Communication
Point-to-Point .PPT (Size: 443 KB / Downloads: 94)
Terminal-Host Communication
Components
Terminal
Host (locus of processing)
Transmission line (here, phone line and modem)
Telephone line acts as a point-to-point link
VT100 Terminals
VT100 Emulation Only Needs Software
Communications program
Terminal emulation software
Most Hosts Support VT100 Terminals
Lowest Common Denominator
Slow: maximum speed of 19 kbps, usually slower
Uses inefficient asynchronous ASCII transmission, discussed later
No graphics or even multiple fonts: plain text only
No color
Terminal Emulation Software
File Transfer
Transfer whole files with error correction
Upload: from PC to host
Download: from host to PC
Terminal emulation program and host file transfer program must support the same file transfer protocol standard
Kermit
XMODEM, YMODEM
IBM 3270 Terminals
Digital and Binary
Digital Transmission
Can have multiple voltage levels, say 4
Change to one at start of each bit cycle
If 4, changes can represent 2 bits each:
00, 01, 10, 11
[b]Baud Rate and Bit Rate[/b]
Baud Rate
Number of times line changes per second
Let baud rate be 4 (4 changes per second)
Let bits per line change be 2
Bit rate = 8 bits per second
Bit rate = x2 Baud rate in this example
Another Look at Compression
With 4:1 Compression, a V.34 Modem Can Receive Data at 115.2 kbps from the PC
However the ~30 kbps limit of the phone system is not exceeded. Still transmit at 33.6 kbps.
56 kbps Modems
What they can do
Send at 33.6 kbps (V.92 with PCM upstream can go up to 48 kbps)
Receive at 56 kbps (V92 with V.44 compression can go up to 120 kbps)
Problems
past: competing standards from Rockwell, U.S. Robotics (V.90 ended them)
present: ISPs must support V.92 (all support V.90)
Users and ISPs
Users V.90 analog modem or V.92
ISPs V.90 digital modem or V.92