DTE-DCE

DTE-DCE Interface

Relevant terms and distinctions

1. direction of transmission

simplex : one way (one-way street)
half duplex : either way, but only one at a time (single line railroad)
full duplex : both ways concurrently (two-way street)

2. synchronization

asynchronous : data may appear at any time
synchronous : sender and receiver work in lock-step

3. serial/parallel

serial : one bit after the other
parallel : multiple bits at once
(hardware required to convert)

4. DTE / DCE

DTE : Data Terminal Equipment (e.g. computer)
DCE : Data Circuit-terminating Equipement (e.g. modem)
Info originates and terminates at DTE
DCE converts info to signals and vice versa

5. Interface

For purposes of this chapter: Connection between DTE and DCE.

Common Physical Interface Standards

Examples from two organizations: EIA, ITU-T
EIA: Electronic Industries Association
ITU-T : International Telecommunications Union - Telecom standards
Interface standards comprise:

Mechanical : wiring and connectors

Electrical : signals

Functional : protocol for using signals

EIA-232 Standard

aka RS-232 (Recommended Standard, from early 60s)

mechanical :

male DB-25 connector on DTE end (cable has female)
female DB-25 connector on DCE end (cable has male)
cable length limited to 50 ft. (15 m.)
20kbps limit

electrical:

digital signal, NRZ-L (NRZ) encoding
1 encoded as voltage in range -3v to -15v
0 encoded as voltage in range +3v to +15v
(wide range allows transmission on noisy line)

functional:

most of the 25 pins have defined functions
few of them are normally used, most for control
pin 2 for transmitted data (transmitted from DTE)
pin 3 for received data (transmitted from DCE)
can be used for half or full-duplex

Typical EIA-232 modem usage scenario (Handshakin')

Unless you have the handout (Halsall, Figure 2.32, p84), you may have difficulty following this scenario.

Connection

calling DTE: set DTR (DTE Ready, pin 20) on
calling DCE: set DSR (DCE Ready, pin 6) on
calling DTE sends calling DCE the dialing command
calling DCE dials the call

called DCE sets RI (ring indicator, pin 22) on (getting the ring)

called DTE sets RTS (request to send, pin 4) on

called DCE transmits carrier signal to calling DCE

called DCE sets CTS (clear to send, pin 5) on

calling DCE detects carrier and sets CD (carrier detect, pin 8) on

Invitation

called DTE transmits "invitation to send" on TxD (Transmit data, pin 2)

called DCE transmits message to calling DCE

calling DCE hands message DTE on RxD (Receive data, pin 3)

called DTE set RTS (request to send, pin 4) off

called DCE stops transmitting carrier signal to calling DCE

called DCE sets CTS (clear to send, pin 5) off

calling DCE detects carrier gone, and sets CD (carrier detect, pin 8) off

Data Transfer

calling DTE sets RTS (request to send, pin 4) on
calling DCE transmits carrier signal

called DCE sets CD (pin 8) on

calling DCE sets CTS (clear to send, pin 5) on
calling DTE hands message to DCE on TxD (pin 2)
calling DCE transmits message to called DCE

called DCE hands data message to DTE on RxD (pin 3)

repeat in either direction until all data transfers complete

Terminate

both DTE set RTS (request to send, pin 4) off
both DCE stop transmitting carrier
both DCE set CD (carrier detect, pin 8) off
both DCE set CTS (clear to send, pin 5) off
both DTE set DTR (DTE ready, pin 20) off
both DCE set DSR (DCE ready, pin 6) off

EIA-232 Null Modem

Method for directly connecting two DTE's via DTE-DCE port
Cannot use DTE-DCE cable. Why?
Need to cross the transmit and receive, reconfigure some control lines

EIA-449

Desire compatibility with EIA-232 plus higher xfer rates and cable lengths

Mechanical:

DB-37 plus DB-9

Functional:

DB-37 similar to BD-25 of EIA-232, except no secondary channel

Pin assignments classified as Category I (EIA-232 equiv) or Category II (additional)

DB-9 handles secondary channel (compatible with EIA-232 secondary)

Electrical:

RS-423 (unbalanced, NRZ-L, 0 is 2 to 6V, 1 is -2 to -6V)

RS-422 (balanced, -- see X.21 notes)

ITU-T X.21 Standard

Intended for use with digital public data networks such as X.25 and ISDN.
Fewer control pins, send control info over data lines (DCE must be able to generate and decode control messages)

Mechanical specs:

DB-15 connector
Uses RS-422A, cable length up to 1km
Max bps inverse of length (10Mbps @ 10m, 100kbps @ 1km)
Typical 64kbps, for digital transmissions

Electrical specs:

based on RS-422A, balanced mode

transmit two NRZ-L signals, one is complement (mirror image) of other
receiver subtracts (negate then add) second signal from first, rescale (cut in half, to restore original amplitude), use result
advantage: noise cancels out. (both signals get same noise component. Negation by receiver complements noise, addition wipes it out)

Example: signal 1 is +5v, signal 2 is -5v. No noise, receive +5 and -5. Negate signal 2 then add: 5+5 = +10v then rescale in half: +5v.
If both hit with +4v spike, received signals are +9v and -1v. Negate signal 2 then add: 9+1 = +10v then rescale in half: +5v. If unbalanced, received is +9v instead of +5v.

Functional specs:

fewer pins than EIA-232
pin assignments reflect mech and elect specs
most pin assignments are in pairs (for balanced mode)

Modems

Modulator / Demodulator

See notes on Digital-Analog Encoding (ASK, FSK, PSK, QAM)

Hayes and Hayes-compatible

Refers to modem with command interpreter
AT (attention) commands: AT command [parm] command [parm] . . .
Used for dialing and for other functions

Bell Modem Standards

for POTS (Plain Old Telephone Service)
operate on 3000 Hz telephone channel
notice development trends
103 : FSK, 300 baud/bps (full duplex)
202 : FSK, 1200 baud/bps (half duplex)
212 : FSK 300 baud/bps (103-compatible) plus 4-PSK 600 baud/1200 bps
201 : 4-PSK 1200 baud/2400 bps
208 : 8-PSK 1600 baud/4800 bps
209 : 16-QAM 2400 baud/9600 bps (3? amplitudes, 12 phases)

ITU-T Modem Standards

The V series
Many are compatible with Bell (e.g. V.21 == Bell 103)
Selected standards:
V.22bis : 600 baud, 16-QAM for 2400bps plus 4-PSK for 1200 bps
V.32 : 32-QAM 2400 baud/9600 bps (trellis: each P and A combo represents 5 bits: 4 data bits plus one redundant bit)
V.33 : 128-QAM 2400 baud/14400 bps (trellis with 7 total, 6 data bits)
V.34 : 4096-QAM 2400 baud/28800 bps (12 bits/baud), 2-3 times higher thruput with compression

V.90: 56K modems

In Feb 1998, the ITU accepted K56flex as a new standard, now officially known as V.90.

x2 and K56flex are communication protocols that allow modems to communicate at speeds of up to 56K.

There are two different (and competing) 56K protocol available on the market, they are NOT compatible with each other: K56flex (supported by Lucent Technologies, Inc. and Rockwell International) and x2 (by U.S. Robotics).

Because of current FCC limitations, the maximum speed is 53K, not 56K.

56K is only possible in downstream (when you're DOWNLOADING data) transfer; upstream (when you are UPLOADING information) is still 33.6K maximum.

56K is only possible if you're calling an ISP. Two 56K modems calling each other will NOT be able to reach 56K due to the fact that 56K technology requires a digital connection (like an ISP) at one end.

Reason for assymetry? Here's the best explanation I can gather from consulting several sources: Upstream signal is analog from you to Central Office, where codec does A-D conversion into PCM encoded signal. The encoding will not be perfect due to signal degradation between your house and CO, plus error due to A-D quantization (recall assignment 1). Downstream signal is digital from source (ISP, which transmits digital version of audio wave) to Central Office, where codec does D-A conversion. Nothing is lost in the D-A conversion; the signal is precise going into the analog line and can therefore be transmitted more rapidly for given line.

Ability to achieve 56K depends on the quality of your phone line and many other factors (mostly beyond your control). See "Why you won't get 56K"

For more details on V.90, see http://www.v90.com. For K56flex, see http://www.lucent.com/micro/K56flex/k56faq.html

ADSL: Beyond modems

Assymetric Digital Subsriber Line (ADSL)

Allows downloads (from ISP to you) at over 1-8 Mbps, and uploads (from you to ISP) at typical 64-640 Kbps. That's why its called assymetric!

Uses existing copper wire. But you get the entire bandwidth, not just 4KHz. This is better than cable, where you have to share the bandwidth with others in your neighborhood.

Bandwidth is divided into 3 subbands: voice, upstream, downstream. So not only can you use the phone at the same time, but you get full-duplex digital communication to boot.

Higher frequencies are more sensitive to noise than lower frequencies, so maximum upload/download throughput will vary widely depending on distance from home to Central Office.

Works only if home is less than 3 miles from CO. Currently 80% of U.S. phones. If distance is greater, line contains "load coil" devices that boost the analog signal but also limit it to 4KHz bandwidth!

This appears to be the next hot new technology. Uses ordinary twisted pair, but requires special equipment and service. Coming to Springfield soon.

See www.adsl.com

Related Home Pages: notes | CSC 465 | Peter Sanderson | Computer Science | SMSU

Last reviewed: 14 February 2000

Peter Sanderson ( pete@csc.smsu.edu )