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This chapter is from the book

This chapter is from the book

Serial Point-to-Point Overview (3.1)

This section gives an overview of point-to-point serial communications. A basic understanding of point-to-point serial communications is essential to understanding protocols that are used over these types of serial links. HDLC encapsulation and configuration is discussed later in this section.

Serial Communications (3.1.1)

The earliest form of computer communications involved serial links between mainframe computers. Serial communications is still a widely used method of connecting two networks usually over long distances.

Serial and Parallel Ports (

One of the most common types of WAN connections is the point-to-point connection. As shown in Figure 3-1, point-to-point connections are used to connect LANs to service provider WANs, and to connect LAN segments within an enterprise network.

Figure 3-1

Figure 3-1 Serial Point-to-Point Communications

A LAN-to-WAN point-to-point connection is also referred to as a serial connection or leased line connection. This is because the lines are leased from a carrier (usually a telephone company) and are dedicated for use by the company leasing the lines. Companies pay for a continuous connection between two remote sites, and the line is continuously active and available. Leased lines are a frequently used type of WAN access, and they are generally priced based on the bandwidth required and the distance between the two connected points.

Understanding how point-to-point serial communication across a leased line works is important to an overall understanding of how WANs function.

Communications across a serial connection is a method of data transmission in which the bits are transmitted sequentially over a single channel. This is equivalent to a pipe only wide enough to fit one ball at a time. Multiple balls can go into the pipe, but only one at a time, and they only have one exit point, the other end of the pipe. A serial port is bidirectional, and often referred to as a bidirectional port or a communications port.

This is in contrast to parallel communications in which bits can be transmitted simultaneously over multiple wires. As shown in Figure 3-2, a parallel connection theoretically transfers data eight times faster than a serial connection. Based on this theory, a parallel connection sends a byte (eight bits) in the time that a serial connection sends a single bit. However, parallel communications do have issues with crosstalk across wires, especially as the wire length increases. Clock skew is also an issue with parallel communications. Clock skew occurs when data across the various wires does not arrive at the same time, creating synchronization issues. Finally, most parallel communications support only one-direction, outbound-only communication from the hard drive.

Figure 3-2

Figure 3-2 Serial and Parallel Communications

At one time, most PCs included both serial and parallel ports. Parallel ports were used to connect printers, computers, and other devices that required relatively high bandwidth. Parallel ports were also used between interior components. For external communications, a serial bus was primarily used for signal conversion. Because of their bidirectional ability, serial communications are considerably less expensive to implement. Serial communications use fewer wires, cheaper cables, and fewer connector pins.

On most PCs, parallel ports and RS-232 serial ports have been replaced by the higher speed serial Universal Serial Bus (USB) interfaces. However, for long-distance communication, many WANs use still serial transmission.

Serial Communication (

Figure 3-3 shows a simple representation of a serial communication across a WAN. 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.

Figure 3-3

Figure 3-3 Serial Communication Process

There are many different serial communication standards, each one using a different signaling method. There are three important 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. These types of peripheral devices for computers have been replaced by new and faster standards such as USB but many network devices use RJ-45 connectors that conform to the original RS-232 standard.
  • 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 Mbps. 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 T3 Plus Networking to address the need for high-speed communication over WAN links.

Point-to-Point Communication Links (

When permanent dedicated connections are required, a point-to-point link is used to provide a single, pre-established WAN communications path from the customer premises, through the provider network, to a remote destination, as shown in Figure 3-4.

Figure 3-4

Figure 3-4 Point-to-Point Communication Links

A point-to-point link can connect two geographically distant sites, such as a corporate office in New York and a regional office in London. For a point-to-point line, the carrier dedicates specific resources for a line that is leased by the customer (leased line).

Point-to-point links are usually more expensive than shared services. The cost of leased line solutions can become significant when used to connect many sites over increasing distances. However, there are times when the benefits outweigh the cost of the leased line. The dedicated capacity removes latency or jitter between the endpoints. Constant availability is essential for some applications such as VoIP or video over IP.

Time-Division Multiplexing (

With a leased line, despite the fact that customers are paying for dedicated services, and dedicated bandwidth is provided to the customer, the carrier still uses multiplexing technologies within the network. Multiplexing refers to a scheme that allows multiple logical signals to share a single physical channel. Two common types of multiplexing are time-division multiplexing (TDM) and statistical time-division multiplexing (STDM).


Bell Laboratories originally invented TDM to maximize the amount of voice traffic carried over a medium. Before multiplexing, each telephone call required its own physical link. This was an expensive and unscalable solution. TDM divides the bandwidth of a single link into separate time slots. TDM transmits two or more channels (data stream) over the same link by allocating a different time slot for the transmission of each channel. In effect, the channels take turns using the link.

TDM is a physical layer concept. It has no regard for the nature of the information that is multiplexed on to the output channel. TDM is independent of the Layer 2 protocol that has been used by the input channels.

TDM can be explained by an analogy to highway traffic. To transport traffic from four roads to another city, all traffic can be sent on one lane if the roads are equally serviced and the traffic is synchronized. If each of the four roads puts a car on to the main highway every four seconds, the highway gets a car at the rate of one each second. As long as the speed of all the cars is synchronized, there is no collision. At the destination, the reverse happens and the cars are taken off the highway and fed to the local roads by the same synchronous mechanism.

This is the principle used in synchronous TDM when sending data over a link. TDM increases the capacity of the transmission link by dividing transmission time into smaller, equal intervals so that the link carries the bits from multiple input sources.

In Figure 3-5, a multiplexer (MUX) at the transmitter accepts three separate signals. The MUX breaks each signal into segments. The MUX puts each segment into a single channel by inserting each segment into a time slot.

Figure 3-5

Figure 3-5 Time-Division Multiplexing

A MUX at the receiving end reassembles the TDM stream into the three separate data streams based only on the timing of the arrival of each bit. A technique called bit interleaving keeps track of the number and sequence of the bits from each specific transmission so that they can be quickly and efficiently reassembled into their original form upon receipt. Byte interleaving performs the same functions, but because there are eight bits in each byte, the process needs a bigger or longer time slot.

The operations of TDM are summarized as follows:

  • TDM shares available transmission time on a medium by assigning a time slot to users.
  • The MUX accepts input from attached devices in an alternating sequence (round-robin) and transmits the data in a recurrent pattern.
  • T1/E1 and ISDN telephone lines are common examples of synchronous TDM.

Statistical Time-Division Multiplexing (

In another analogy, compare TDM to a train with 32 railroad cars. Each car is owned by a different freight company, and every day the train leaves with the 32 cars attached. If one of the companies has cargo to send, the car is loaded. If the company has nothing to send, the car remains empty, but stays on the train. Shipping empty containers is not very efficient. TDM shares this inefficiency when traffic is intermittent, because the time slot is still allocated even when the channel has no data to transmit.


STDM was developed to overcome this inefficiency. As shown in Figure 3-6, STDM uses a variable time slot length allowing channels to compete for any free slot space. It employs a buffer memory that temporarily stores the data during periods of peak traffic. STDM does not waste high-speed line time with inactive channels using this scheme. STDM requires each transmission to carry identification information or a channel identifier.

Figure 3-6

Figure 3-6 Statistical Time-Division Multiplexing

TDM Examples – Sonet and SDM (

On a larger scale, the telecommunications industry uses the Synchronous Optical Networking (SONET) or Synchronous Digital Hierarchy (SDH) standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely related standards that specify interface parameters, rates, framing formats, multiplexing methods, and management for synchronous TDM over fiber.

Figure 3-7 displays SONET, which is an example of STDM. SONET/SDH takes n bit streams, multiplexes them, and optically modulates the signals. It then sends the signals out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) ´ n. Thus, traffic arriving at the SONET multiplexer from four places at 2.5 Gbps goes out as a single stream at 4 ´ 2.5 Gbps, or 10 Gbps. This principle is illustrated in the figure, which shows an increase in the bit rate by a factor of four in time slot T.

Figure 3-7

Figure 3-7 TDM Example: SONET

Demarcation Point (

Prior to deregulation in North America and other countries, telephone companies owned the local loop, including the wiring and equipment on the premises of the customers. The local loop refers to the line from the premises of a telephone subscriber to the telephone company central office. Deregulation forced telephone companies to unbundle their local loop infrastructure to allow other suppliers to provide equipment and services. This led to a need to delineate which part of the network the telephone company owned and which part the customer owned. This point of delineation is the demarcation point, or demarc. The demarcation point marks the point where your network interfaces with a network that is owned by another organization. In telephone terminology, this is the interface between customer premises equipment (CPE) and network service provider equipment. The demarcation point is the point in the network where the responsibility of the service provider ends, as shown in Figure 3-8.

Figure 3-8

Figure 3-8 Demarcation Point

The differences in demarcation points can best be shown using ISDN. In the United States, a service provider provides the local loop into the customer premises, and the customer provides the active equipment such as the channel service unit / data service unit (CSU/DSU) on which the local loop is terminated. This termination often occurs in a telecommunications closet, and the customer is responsible for maintaining, replacing, or repairing the equipment. In other countries, the network terminating unit (NTU) is provided and managed by the service provider. This allows the service provider to actively manage and troubleshoot the local loop with the demarcation point occurring after the NTU. The customer connects a CPE device, such as a router or Frame Relay access device, to the NTU using a V.35 or RS-232 serial interface.

A router serial port is required for each leased line connection. If the underlying network is based on the T-carrier or E-carrier technologies, the leased line connects to the network of the carrier through a CSU/DSU. The purpose of the CSU/DSU is to provide a clocking signal to the customer equipment interface from the DSU and terminate the channelized transport media of the carrier on the CSU. The CSU also provides diagnostic functions such as a loopback test.

As shown in Figure 3-9, most T1 or E1 TDM interfaces on current routers include CSU/DSU capabilities. A separate CSU/DSU is not required because this functionality is embedded in the interface. IOS commands are used to configure the CSU/DSU operations.

Figure 3-9

Figure 3-9 T1/E1 with Embedded CSU/DSU


From the point of view of connecting to the WAN, a serial connection has a data terminal equipment (DTE) device at one end of the connection and a data circuit-terminating equipment or data communications equipment (DCE) device at the other end. The connection between the two DCE devices is the WAN service provider transmission network, as shown in Figure 3-10. In this example

  • The CPE, which is generally a router, is the DTE. The DTE could also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network.
  • The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to the WAN service provider transmission link. This signal is received at the remote DCE, which decodes the signal back into a sequence of bits. The remote DCE then signals this sequence to the remote DTE.

    Figure 3-10

    Figure 3-10 Serial DCE and DTE WAN Connections

The Electronics Industry Association (EIA) and the International Telecommunication Union Telecommunications Standardization Sector (ITU-T) have been most active in the development of standards that allow DTEs to communicate with DCEs.

Serial Cables (

Originally, the concept of DCEs and DTEs was based on two types of equipment: terminal equipment that generated or received data, and the communication equipment that only relayed data. In the development of the RS-232 standard, there were reasons why 25-pin RS-232 connectors on these two types of equipment must be wired differently. These reasons are no longer significant, but there are two different types of cables remaining: one for connecting a DTE to a DCE, and another for connecting two DTEs directly to each other.

The DTE/DCE interface for a particular standard defines the following specifications:

  • Mechanical/physical: Number of pins and connector type
  • Electrical: Defines voltage levels for 0 and 1
  • Functional: Specifies the functions that are performed by assigning meanings to each of the signaling lines in the interface
  • Procedural: Specifies the sequence of events for transmitting data

The original RS-232 standard only defined the connection of DTEs with DCEs, which were modems. However, to connect two DTEs, such as two computers or two routers in a lab, a special cable called a null modem eliminates the need for a DCE. In other words, the two devices can be connected without a modem. A null modem is a communication method to directly connect two DTEs using an RS-232 serial cable. With a null modem connection, the transmit (Tx) and receive (Rx) lines are cross-linked, as shown in Figure 3-11.

Figure 3-11

Figure 3-11 Null Modem to Connect Two DTEs

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, as shown in Figure 3-12. 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, as shown in Figure 3-13.

Figure 3-12

Figure 3-12 DB-60 Router Connection

Figure 3-13

Figure 3-13 WAN Serial Connection Options

To support higher port densities in a smaller form factor, Cisco has introduced a Smart Serial cable, as shown in Figure 3-14. The router interface end of the Smart Serial cable is a 26-pin connector that is significantly more compact than the DB-60 connector.

Figure 3-14

Figure 3-14 Smart Serial Connector

When using a null modem, synchronous connections require a clock signal. An external device can generate the signal, or one of the DTEs can generate the clock signal. When a DTE and DCE are connected, the serial port on a router is the DTE end of the connection, by default, and the clock signal is typically provided by a CSU/DSU, or similar DCE device. However, when using a null modem cable in a router-to-router connection, one of the serial interfaces must be configured as the DCE end to provide the clock signal for the connection, as shown in Figure 3-15.

Figure 3-15

Figure 3-15 Smart Serial Connections in the Lab

Serial Bandwidth (

Bandwidth refers to the rate at which data is transferred over the communication link. The underlying carrier technology depends on the bandwidth available. There is a difference in bandwidth points between the North American (T-carrier) specification and the European (E-carrier) system. Optical networks also use a different bandwidth hierarchy, which again differs between North America and Europe. In the United States, Optical Carrier (OC) defines the bandwidth points.

In North America, the bandwidth is usually expressed as a DS (digital signal level) number (DS0, DS1, etc.), which refers to the rate and format of the signal. The most fundamental line speed is 64 Kbps, or DS-0, which is the bandwidth required for an uncompressed, digitized phone call. Serial connection bandwidths can be incrementally increased to accommodate the need for faster transmission. For example, 24 DS0s can be bundled to get a DS1 line (also called a T1 line) with a speed of 1.544 Mbps. Also, 28 DS1s can be bundled to get a DS3 line (also called a T3 line) with a speed of 44.736 Mbps. Leased lines are available in different capacities and are generally priced based on the bandwidth required and the distance between the two connected points.

OC transmission rates are a set of standardized specifications for the transmission of digital signals carried on SONET fiber-optic networks. The designation uses OC, followed by an integer value representing the base transmission rate of 51.84 Mbps. For example, OC-1 has a transmission capacity of 51.84 Mbps, whereas an OC-3 transmission medium would be three times 51.84 Mbps, or 155.52 Mbps.

Table 3-1 lists the most common line types and the associated bit rate capacity of each.

Table 3-1 Carrier Transmission Rates

Line Type

Bit Rate Capacity


56 Kbps


64 Kbps


1.544 Mbps


2.048 Mbps


2.048 Mbps


34.064 Mbps


44.736 Mbps


51.84 Mbps


155.54 Mbps


466.56 Mbps


622.08 Mbps


933.12 Mbps


1.244 Gbps


1.866 Gbps


2.488 Gbps


4.976 Gbps


9.954 Gbps


39.813 Gbps

HDLC Encapsulation (3.1.2)

HDLC is a synchronous data link layer protocol developed by the International Organization for Standardization (ISO). Although HDLC can be used for point-to-multipoint connections, the most common usage of HDLC is for point-to-point serial communications.

WAN Encapsulation Protocols (

On each WAN connection, data is encapsulated into frames before crossing the WAN link. To ensure that the correct protocol is used, the appropriate Layer 2 encapsulation type must be configured. The choice of protocol depends on the WAN technology and the communicating equipment. Figure 3-16 displays the more common WAN protocols and where they are used. The following are short descriptions of each type of WAN protocol:

  • HDLC: The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices. HDLC is now the basis for synchronous PPP used by many servers to connect to a WAN, most commonly the Internet.
  • PPP: Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several network layer protocols, such as IPv4 and IPv6. PPP uses the HDLC encapsulation protocol, but also has built-in security mechanisms such as PAP and CHAP.
  • Serial Line Internet Protocol (SLIP): A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.
  • X.25/Link Access Procedure, Balanced (LAPB): An ITU-T standard that defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data link layer protocol. X.25 is a predecessor to Frame Relay.
  • Frame Relay: An industry standard, switched, data link layer protocol that handles multiple virtual circuits. Frame Relay is a next-generation protocol after X.25. Frame Relay eliminates some of the time-consuming processes (such as error correction and flow control) employed in X.25.
  • ATM: The international standard for cell relay in which devices send multiple service types, such as voice, video, or data, in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware; thereby, reducing transit delays. ATM takes advantage of high-speed transmission media such as E3, SONET, and T3.
Figure 3-16

Figure 3-16 WAN Encapsulation Protocols

HDLC Encapsulation (

HDLC is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The current standard for HDLC is ISO 13239. HDLC was developed from the Synchronous Data Link Control (SDLC) standard proposed in the 1970s. HDLC provides both connection-oriented and connectionless service.

HDLC uses synchronous serial transmission to provide error-free communication between two points. HDLC defines a Layer 2 framing structure that allows for flow control and error control through the use of acknowledgments. Each frame has the same format, whether it is a data frame or a control frame.

When frames are transmitted over synchronous or asynchronous links, those links have no mechanism to mark the beginning or end of frames. For this reason, HDLC uses a frame delimiter, or flag, to mark the beginning and the end of each frame.

Cisco has developed an extension to the HLDC protocol to solve the inability to provide multiprotocol support. Although Cisco HLDC (also referred to as cHDLC) is proprietary, Cisco has allowed many other network equipment vendors to implement it. Cisco HDLC frames contain a field for identifying the network protocol being encapsulated. Figure 3-17 compares standard HLDC to Cisco HLDC.

Figure 3-17

Figure 3-17 Standard and Cisco HLDC Frame Format

HDLC Frame Types (

HDLC defines three types of frames, each with a different control field format.


The Flag field initiates and terminates error checking. The frame always starts and ends with an 8-bit Flag field. The bit pattern is 01111110. Because there is a likelihood that this pattern occurs in the actual data, the sending HDLC system always inserts a 0 bit after every five consecutive 1s in the data field, so in practice the flag sequence can only occur at the frame ends. The receiving system strips out the inserted bits. When frames are transmitted consecutively, the end flag of the first frame is used as the start flag of the next frame.


The Address field contains the HDLC address of the secondary station. This address can contain a specific address, a group address, or a broadcast address. A primary address is either a communication source or a destination, which eliminates the need to include the address of the primary.


The Control field, shown in Figure 3-18, uses three different formats, depending on the type of HDLC frame used:

  • Information (I) frame: I-frames carry upper layer information and some control information. This frame sends and receives sequence numbers, and the poll final (P/F) bit performs flow and error control. 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. A primary station uses the P/F bit to tell the secondary whether it requires an immediate response. A secondary station uses the P/F bit to tell the primary whether the current frame is the last in its current response.
  • Supervisory (S) frame: S-frames provide control information. An S-frame can request and suspend transmission, report on status, and acknowledge receipt of I-frames. S-frames do not have an information field.
  • Unnumbered (U) frame: U-frames support control purposes and are not sequenced. Depending on the function of the U-frame, its Control field is 1 or 2 bytes. Some U-frames have an Information field.

    Figure 3-18

    Figure 3-18 HDLC Frame Types


Only used in Cisco HDLC. This field specifies the protocol type encapsulated within the frame (e.g., 0×0800 for IP).


The Data field contains a path information unit (PIU) or exchange identification (XID) information.

Frame Check Sequence (FCS)

The FCS precedes the ending flag delimiter and is usually a cyclic redundancy check (CRC) calculation remainder. The CRC calculation is redone in the receiver. If the result differs from the value in the original frame, an error is assumed.

Configuring HDLC Encapsulation (

Cisco HDLC is the default encapsulation method used by Cisco devices on synchronous serial lines.

Use Cisco HDLC as a point-to-point protocol on leased lines between two Cisco devices. If connecting non-Cisco devices, use synchronous PPP.

If the default encapsulation method has been changed, use the encapsulation hdlc command in privileged EXEC mode to reenable HDLC.

There are two steps to re-enable HDLC encapsulation:

Step 1. Enter the interface configuration mode of the serial interface.

Step 2. Enter the encapsulation hdlc command to specify the encapsulation protocol on the interface.

The following shows an example of HDLC reenabled on a serial interface:

R2(config)# interface s0/0/0

R2(config-if)# encapsulation hdlc

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, as highlighted in Example 3-1. Serial 0/0/0 is up, line protocol is up indicates that the line is up and functioning; encapsulation HDLC indicates that the default serial encapsulation (HDLC) is enabled.

Example 3-1 Displaying Serial Interface Information

R1# show interface serial 0/0/0

Serial0/0/0 is up, line protocol is up

  Hardware is GT96K Serial

  Internet address is

  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation HDLC, loopback not set

  Keepalive set (10 sec)

  CRC checking enabled

  <Output omitted for brevity>

The show interfaces serial command returns one of six possible states:

  • Serial x is up, line protocol is up.
  • 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.

Of the six possible states, there are five problem states. Table 3-2 lists the five problem states, the issues associated with that state, and how to troubleshoot the issue.

Table 3-2 Troubleshooting a Serial Interface

Status Line Condition

Possible Problem


Serial x is up, line protocol is up.

This is proper status line condition.

No action is required.

Serial x is down, line protocol is down.

The router is not sensing a carrier detect (CD) signal (that is, the CD is not active).

The line is down or is not connected on the far end.

Cabling is faulty or incorrect.

Hardware failure has occurred (CSU/DSU).

  1. Check the CD LEDs on the CSU/DSU to see whether the CD is active, or insert a breakout box on the line to check for the CD signal.
  2. Verify that the proper cable and interface are being used by looking at the hardware installation documentation.
  3. Insert a breakout box and check all control leads.
  4. Contact the leased line or other carrier service to see whether there is a problem.
  5. Swap faulty parts.
  6. If you suspect faulty router hardware, change the serial line to another port. If the connection comes up, the previously connected interface has a problem.

Serial x is up, line protocol is down (DCE mode).

The clock rate interface configuration command is missing.

The DTE device does not support or is not set up for SCTE mode (terminal timing).

The remote CSU or DSU has failed.

  1. Add the clockrate interface configuration command on the serial interface.


    clock rate bps

    Syntax Description:

    • bps: Desired clock rate in bits per second: 1200, 2400, 4800, 9600, 19200, 38400, 56000, 64000, 72000, 125000, 148000, 250000, 500000,800000, 1000000, 1300000, 2000000, 4000000, or 8000000.
  2. If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU or DSU.
  3. Verify that the correct cable is being used.
  4. If the line protocol is still down, there is a possible hardware failure or cabling problem. Insert a breakout box and observe leads.
  5. Replace faulty parts as necessary.

Serial x is up, line protocol is up (looped).

A loop exists in the circuit. The sequence number in the keepalive packet changes to a random number when a loop is initially detected. If the same random number is returned over the link, a loop exists.

  1. Use the show running-config privileged exec command to look for any loopback interface configuration command entries.
  2. If you find a loopback interface configuration command entry, use the no loopback interface configuration command to remove the loop.
  3. If you do not find the loopback interface configuration command, examine the CSU/DSU to determine whether they are configured in manual loopback mode. If they are, disable manual loopback.
  4. Reset the CSU/DSU, and inspect the line status. If the line protocol comes up, no other action is needed.
  5. If the CSU/DSU is not configured in manual loopback mode, contact the leased line or other carrier service for line troubleshooting assistance.

Serial x is up, line protocol is down (disabled) .

A high error rate has occurred due to a remote device problem.

A CSU or DSU hardware problem has occurred.

Router hardware (interface) is bad.

  1. Troubleshoot the line with a serial analyzer and breakout box. Look for toggling CTS and DSR signals.
  2. Loop CSU/DSU (DTE loop). If the problem continues, it is likely that there is a hardware problem. If the problem does not continue, it is likely that there is a telephone company problem.
  3. Swap out bad hardware, as required (CSU, DSU, switch, local or remote router).

Serial x is administratively down, line protocol is down.

The router configuration includes the shutdown interface configuration command.

A duplicate IP address exists.

  1. Check the router configuration for the shutdown command.
  2. Use the no shutdown interface configuration command to remove the shutdown command.
  3. Verify that there are no identical IP addresses using the show running-config privileged exec command or the show interfaces exec command.
  4. If there are duplicate addresses, resolve the conflict by changing one of the IP addresses.

The show controllers command is another important diagnostic tool when troubleshooting serial lines, as shown in Example 3-2. The output indicates the state of the interface channels and whether a cable is attached to the interface. In example 3-2, interface serial 0/0/0 has a V.35 DCE cable attached. The command syntax varies depending on the platform. Cisco 7000 series routers use a cBus controller card for connecting serial links. With these routers, use the show controllers cbus command.

If the electrical interface output displays as UNKNOWN instead of V.35, EIA/TIA-449, or some other electrical interface type, the likely problem is an improperly connected cable. 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 command shows that the interface and line protocol are down.

Example 3-2 Displaying Controller Hardware Information on a Serial Interface

R1# show controllers serial 0/0/0

Interface Serial0/0/0

Hardware is GT96K

DCE V.35, clock rate 64000

idb at 0x66855120, driver data structure at 0x6685C93C

wic_info 0x6685CF68

Physical Port 0, SCC Num 0

MPSC Registers:

<Output omitted for brevity>
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