configured as DTE, which is the default setting, external clocking is required from the CSU/DSU or other DCE device. The cable for the DTE to DCE connection is a shielded serial transition cable. The router end of the shielded serial transition cable may be a DB-60 connector, which connects to the DB-60 port on a serial WAN interface card. The other end of the serial transition cable is available with the connector appropriate for the standard that is to be used. The WAN provider or the CSU/DSU usually dictates this cable type. Cisco devices support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530 serial standards. To support higher densities in a smaller form factor, Cisco has introduced a smart serial cable. The serial end of the smart serial cable is a 26-pin connector significantly more compact than the DB-60 connector. Interactive Media Activity PhotoZoom: DCE/DTE Cable In this PhotoZoom, the student will view DCE and DTE cable. Web Links High-Speed Serial Interface (HSSI) http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ hssi.htm
Content 3.1 Serial Point-to-Point Links 3.1.5 HDLC encapsulation Initially, serial communications were based on character-oriented protocols. Bit-oriented protocols were more efficient but they were also proprietary. In 1979, the ISO agreed on HDLC as a standard bit-oriented data link layer protocol that encapsulates data on synchronous serial data links. This standardization led to other committees adopting it and extending the protocol. Since 1981, ITU-T has developed a series of HDLC derivative protocols. The following examples of derivative protocols are called link access protocols: HDLC uses synchronous serial transmission providing error-free communication between two points. HDLC defines a Layer 2 framing structure that allows for flow control and error control using acknowledgments and a windowing scheme. Each frame has the same format, whether it is a data frame or a control frame. Standard HDLC does not inherently support multiple protocols on a single link, as it does not have a way to indicate which protocol is being carried. Cisco offers a proprietary version of HDLC. The Cisco HDLC frame uses a proprietary ‘type’ field that acts as a protocol field. This field enables multiple network layer protocols to share the same serial link. HDLC is the default Layer 2 protocol for Cisco router serial interfaces. HDLC defines the following three types of frame, each with a different control field format: The first one or two bits of the control field serve to identify the frame type. In the control field of an Information (I) frame, the send-sequence number refers to the number of the frame to be sent next. The receive-sequence number provides the number of the frame to be received next. Both sender and receiver maintain send and receive sequence numbers. Web Links Configuring Synchronous Serial Ports http://www.cisco.com/en/US/products/sw/ iosswrel/ps1828/ products_configuration_ guide_ chapter09186a0080087283.html
Content 3.1 Serial Point-to-Point Links 3.1.6 Configuring HDLC encapsulation The default encapsulation method used by Cisco devices on synchronous serial lines is Cisco HDLC. If the serial interface is configured with another encapsulation protocol, and the encapsulation must be changed back to HDLC, enter the interface configuration mode of the serial interface. Then enter the encapsulation hdlc command to specify the encapsulation protocol on the interface. Cisco HDLC is a point-to-point protocol that can be used on leased lines between two Cisco devices. When communicating with a non-Cisco device, synchronous PPP is a more viable option. Web Links Configuring Synchronous Serial Ports http://www.cisco.com/en/US/products/sw/ iosswrel/ps1828/ products_configuration_ guide_ chapter09186a0080087283.html
Content 3.1 Serial Point-to-Point Links 3.1.7 Troubleshooting a serial interface The output of the show interfaces serial command displays information specific to serial interfaces. When HDLC is configured, “Encapsulation HDLC” should be reflected in the output. When PPP is configured, "Encapsulation PPP" should be seen in the output. Five possible problem states can be identified in the interface status line of the show interfaces serial display: The show controllers command is another important diagnostic tool when troubleshooting serial lines. The show controllers output indicates the state of the interface channels and whether a cable is attached to the interface. In Figure , serial interface 0/0 has a V.35 DTE cable attached. The command syntax varies, depending on platform. For serial interfaces on Cisco 7000 series routers, use the show controllers cbus command.If the electrical interface output is shown as UNKNOWN, instead of V.35, EIA/TIA-449, or some other electrical interface type, an improperly connected cable is the likely problem. A problem with the internal wiring of the card is also possible. If the electrical interface is unknown, the corresponding display for the show interfaces serial <X> command will show that the interface and line protocol are down. Following are some debug commands that are useful when troubleshooting serial and WAN problems: Caution: Debugging output is assigned high priority in the CPU process and can render the system unusable. For this reason, debug commands should only be used to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. It is good practice to use debug commands during periods of low network traffic and when the fewest users are online. Debugging during