All long-haul communications and most computer networks use serial connections, because the cost of cable and synchronization difficulties make parallel connections impractical. The most significant advantage is simpler wiring. Also, serial cables can be longer than parallel cables, because there is much less interaction (crosstalk) among the conductors in the cable. In this chapter, we will confine our consideration of serial communications to those connecting LANs to WANs.
The figure is a simple representation of a serial communication. Data is encapsulated by the communications protocol used by the sending router. The encapsulated frame is sent on a physical medium to the WAN. There are various ways to traverse the WAN, but the receiving router uses the same communications protocol to de-encapsulate the frame when it arrives.
There are many different serial communication standards, each one using a different signaling method. There are three key serial communication standards affecting LAN-to-WAN connections:
RS-232 - Most serial ports on personal computers conform to the RS-232C or newer RS-422 and RS-423 standards. Both 9-pin and 25-pin connectors are used. A serial port is a general-purpose interface that can be used for almost any type of device, including modems, mice, and printers. Many network devices use RJ-45 connectors that also conform to the RS-232 standard. The figure shows an example of an RS-232 connector.
V.35 - Typically used for modem-to-multiplexer communication, this ITU standard for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits. In the U.S., V.35 is the interface standard used by most routers and DSUs that connect to T1 carriers. V.35 cables are high-speed serial assemblies designed to support higher data rates and connectivity between DTEs and DCEs over digital lines. There is more on DTEs and DCEs later in this section.
HSSI - A High-Speed Serial Interface (HSSI) supports transmission rates up to 52 Mb/s. Engineers use HSSI to connect routers on LANs with WANs over high-speed lines such as T3 lines. Engineers also use HSSI to provide high-speed connectivity between LANs, using Token Ring or Ethernet. HSSI is a DTE/DCE interface developed by Cisco Systems and T3plus Networking to address the need for high-speed communication over WAN links.
As well as using different signaling methods, each of these standards uses different types of cables and connectors. Each standard plays a different role in a LAN-to-WAN topology. While this course does not examine the details of V.35 and HSSI pinning schemes, a quick look at a 9-pin RS-232 connector used to connect a PC to a modem helps illustrate the concept. A later topic looks at V.35 and HSSI cables.
Pin 1 - Data Carrier Detect (DCD) indicates that the carrier for the transmit data is ON.
Pin 2 - The receive pin (RXD) carries data from the serial device to the computer.
Pin 3 - The transmit pin (TxD) carries data from the computer to the serial device.
Pin 4 - Data Terminal Ready (DTR) indicates to the modem that the computer is ready to transmit.
Pin 5 - Ground
Pin 6 - Data Set Ready (DSR) is similar to DTR. It indicates that the Dataset is ON.
Pin 7 - The RTS pin requests clearance to send data to a modem
Pin 8 - The serial device uses the Clear to Send (CTS) pin to acknowledge the RTS signal of the computer. In most situations, RTS and CTS are constantly ON throughout the communication session.
Pin 9 - An auto answer modem uses the Ring Indicator (RI) to signal receipt of a telephone ring signal.
The DCD and RI pins are only available in connections to a modem. These two lines are used rarely because most modems transmit status information to a PC when a carrier signal is detected (when a connection is made to another modem) or when the modem receives a ring signal from the telephone line.