100Base-Tx

Q:I have several questions regarding 100Base-Tx. First, when converting from 10Base-T to 100Base-Tx, what would be the impact of splitting the Category 5 cable between dual 8-pin modular jacks? For example, could I use amp`s 557280-1 10Base-T Category 5 insert while still adhering to the T568A and T568B 1,2,3, and 6 configurations?

Q:I have several questions regarding 100Base-Tx. First, when converting from 10Base-T to 100Base-Tx, what would be the impact of splitting the Category 5 cable between dual 8-pin modular jacks? For example, could I use amp`s 557280-1 10Base-T Category 5 insert while still adhering to the T568A and T568B 1,2,3, and 6 configurations?

Second, when installing horizontal cabling for a 100Base-Tx network, can 110 blocks be used in the telecommunication closets, or is it better to go with Category 5-compliant patch panels?

Finally, does 100Base-Tx require the full use of all four pairs of a Category 5 cable? If so, why?

William Richey

Directorate of Information Management

Information Operation Div.

Fort Huachuca, AZ

A:Regarding your first question, the 557280-1 adapter insert from amp Inc. (Harrisburg, PA) is actually two individual 8-pin modular connectors. The 4-pair cable is terminated on a card-edge connector and the adapter insert is then plugged into the card-edge connector. The printed circuit board on the adapter insert routes two pairs to each 8-pin modular connector. In your case, the effect of splitting the 4-pair cable into two 2-pair links rests with the cable, not the connectors.

amp cautions users that "two 10Base-T or two 16-megabit-per-second Token Ring links may safely be combined on a single 4-pair Category 5 cable within certain design parameters. However, in order to allow transparent migration to 100-Mbit/sec or faster data rates, it is recommended that Mini-Dual 2 x 2-pair be used with dual Category 5 inserts." Was the cable manufactured to support multiple channels in the same sheath? This does not necessarily dictate separate sheaths--as is the case with the amp product--but it does raise the question: "Is the cable power-sum near-end crosstalk compliant with Category 5 in accordance with commercial building telecommunications cabling standard ansi/tia/eia-568a?"

Any pair carrying a signal within a cable will radiate energy. All other pairs in the cable act as antennae and pick up that radiated energy as crosstalk. If too much energy is transferred to an adjacent pair, it will interfere with the data that pair is transmitting, resulting in increased bit errors.

Telecommunications systems bulletin tsb-67 specifies the pair-to-pair met-hod of measuring crosstalk in 4-pair unshielded twisted-pair (utp) cables. The pair-to-pair method measures the signal coupling for each pair combination found in a cable. Crosstalk for a 4-pair cable is measured for a total of six pair combinations. The crosstalk value for a particular cable represents the worst measured crosstalk between any two pairs. The pair-to-pair method was developed under the assumption that only two pairs of a cable would be used at any given time for data transmission, which is not the case in a shared-sheath environment, in which two 100Base-Tx channels are present in the same cable.

The power-sum method of testing assumes that all pairs of a cable are active and determines the crosstalk influence from all pairs in that cable on the pair being measured. Sometimes, cable tested using the pair-to-pair method meets Category 5 requirements but, tested by the power-sum method, barely meets Category 4 requirements. To find out what kind of cable is installed at your facility, contact the cable manufacturer.

To answer your second question, choosing between Category 5 termination blocks or Category 5 patch panels is a design option, not a standards or application requirement. At The University of Texas at Austin, we use termination hardware; it is less expensive and generally requires less space in already overcrowded telecommunications closets.

Finally, 100Base-Tx is the ieee standard for baseband Ethernet at 100 Mbits/sec over two twisted pairs-- either 2-pair Category 5 utp or 2-pair shielded twisted-pair cabling. But don`t be too hasty--1000Base-T is coming soon to a distributor near you, and it will require all four pairs for signaling.

Donna Ballast is a communications analyst at The University of Texas at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, The University of Texas

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