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: - Link Access Procedure,
Balanced (LAPB) for X.25
- Link Access Procedure on
the D channel (LAPD) for ISDN
- Link Access Procedure
for Modems (LAPM) and PPP for modems
- Link Access
Procedure for Frame Relay (LAPF) for Frame Relay
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: - Information frames (I-frames) – Carry
the data to be transmitted for the station. Additional flow and
error control - data may be piggybacked on an information
frame.
- Supervisory frames (S-frames) – Provide
request/response mechanisms when piggybacking is not
used.
- Unnumbered frames (U-frames) – Provide
supplemental link control functions, such as connection setup.
The code field identifies the U-frame type.
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: - Serial x is down, line protocol is
down
- Serial x is up, line protocol is down
- Serial x is up, line protocol is up (looped)
- Serial x is up, line protocol is down (disabled)
- Serial x is administratively down, line protocol is
down
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: - debug serial interface –
Verifies whether HDLC keepalive packets are incrementing. If
they are not, a possible timing problem exists on the interface
card or in the network.
- debug arp – Indicates
whether the router is sending information about or learning
about routers (with ARP packets) on the other side of the WAN
cloud. Use this command when some nodes on a TCP/IP network are
responding, but others are not.
- debug frame-relay
lmi – Obtains Local Management Interface (LMI) information
which is useful for determining whether a Frame Relay switch
and a router are sending and receiving LMI packets.
- debug frame-relay events – Determines whether
exchanges are occurring between a router and a Frame Relay
switch.
- debug ppp negotiation – Shows
Point-to-Point Protocol (PPP) packets transmitted during PPP
startup where PPP options are negotiated.
- debug
ppp packet – Shows PPP packets being sent and received.
This command displays low-level packet dumps.
- debug
ppp – Shows PPP errors, such as illegal or malformed
frames, associated with PPP connection negotiation and
operation.
- debug ppp authentication – Shows
PPP Challenge Handshake Authentication Protocol (CHAP) and
Password Authentication Protocol (PAP) packet exchanges.
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