V.35 is an ITU-T Physical Layer standard for synchronous, serial, DTE-to-DCE connections. Although the standard is technically designed for transmission at 48 kbps or less, it is commonly used for 56 and 64 kbps connections and is capable of operating as high as 2 Mbps fairly easily.
In 1989, the CCITT recommended the interface be designated obsolete. It was replaced with the V.10/V.11 standard. Apparently, that recommendation was not well received because the V.35 interface remains widely used. It has been modified to use specifications from V.11.
One of the major criticisms of V.35 is the cumbersome connector it implements, known as a Winchester connector, depicted to the right. Other high-speed interfaces have smaller, more modular connectors. There are also companies that have implemented smaller, proprietary, "V.35" connectors. The use the V.35 control signals and electrical specifications, but implement a smaller connector.
Technical V.35 Specifications
V.35 (and other high speed interfaces) achieve their high data rates and high noise immunity by implementing differential signaling on the clock and data leads. Many lower speed interfaces read the voltage of a lead with reference to a common ground cable (e.g., EIA-232-E, V.24). In V.35, however, signals are conveyed as voltage differences between two specified wires. By twisting those two wires around each other, noise tends to affect both wires equally. That effectively cancels out the noise because the receiver is comparing these two cables to read the signal. Other high-speed interfaces that use this technique include EIA-530, EIA-449, and the twisted pair Ethernet standards (e.g., 10BASE-T, 100BASE-T, 1000BASE-T).
|C||RTS||Request To Send||Unbalanced|
|D||CTS||Clear To Send||Unbalanced|
|E||DSR||Data Set Ready||Unbalanced|
|F||RLSD||Received Line Signal Detect||Unbalanced|
|H||DTR||Data Terminal Ready||Unbalanced|
|P||SDA||Send Data (A)||Differential|
|R||RDA||Receive Data (A)||Differential|
|S||SDB||Send Data (B)||Differential|
|T||RDB||Receive Data (B)||Differential|
|U||TCEA||Transmit Clock Ext (A)||Differential|
|V||RCA||Receive Clock (A)||Differential|
|W||TCEB||Transmit Clock Ext (B)||Differential|
|X||RCB||Receive Clock (B)||Differential|
|Y||TCA||Transmit Clock (A)||Differential|
|AA||TCB||Transmit Clock (B)||Differential|
The wires that implement differential signaling are traditionally labeled "A" and "B". An "A" Wire always pairs with another "A" wire, and a "B" wire pairs with a "B" wire. These lead designations are depicted in the graphic to the left and detailed in the table to the right.
V.35 Distance Limitations
It is difficult to make a precise statement about distance limitations because the standard speaks of signal strength requirements at the receiver. Some general observations may be made, however. As with all Physical Layer technologies, as the distance increases, the maximum possible speed decreases, and vice-versa. At 48kbps, the maximum cable length is approximately 400 meters. At 2 Mbps, the maximum cable length decreases to approximately 15 meters.
V.35 interfaces remain common in many parts of the world largely due to its adoption by telephone companies and equipment vendors. At 56 and 64 kbps, it is ideally suited for the connection between a router and a DSU for connecting to a DS-0 circuit. With speeds up to 2 Mbps, it is also suitable for router-to-DSU connection when the carrier service is DS-1/T-1 or E-1, or any fractional rate of those two circuit types. That means it will be part of any service that uses this technology, including private lines, frame relay, ATM, and Internet access, as depicted in the graphic to the left.
|<mp3>http://podcast.hill-vt.com/podsnacks/2008q3/v.35.mp3%7Cdownload</mp3> | V.35|