Configuring Serial Encapsulation

Configuring Serial Encapsulation

You can use serial point-to-point connections to connect your LAN to your service provider WAN. You are likely to have serial point-to-point connections within your network, between your network and a service provider, or both. You need to know how to configure the serial ports for such connections.

Circuit-switched WANs used to be the most common method of connecting remote sites. Because of the bandwidth requirements of modern applications, circuit-switching technology has been relegated to backup solutions and very small home offices. Because of this, this lesson provides only an overview of circuit-switching technology.

One of the most common types of WAN connections is the point-to-point connection. A point-to-point connection is also referred to as a serial connection or leased-line connection, 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.
Understanding how point-to-point communication links function to provide access to a WAN is important to an overall understanding of how WANs function.

Frame Relay and ATM are packet-switching technologies used to connect sites. Because of their complexity, only an overview is provided in this lesson. More in-depth coverage of these packet-switching technologies is provided in Interconnecting Cisco Network Devices, Part 2 (ICND2).

This lesson describes the protocols that encapsulate both data link layer and network layer information over serial links and how to configure those links.

Circuit-Switched Communication Links

Switched circuits allow connections to be initiated when transmission is needed and terminated when the transmission is complete. Figure 5-28 shows examples of circuitswitched WAN connections.

Figure 5-28 Circuit-Switched Communications
Configuring Serial Encapsulationfig5.28

In circuit switching, a dedicated path is established, maintained, and terminated through a carrier network for each communication session. Only the access path is a dedicated physical circuit; the network uses some form of multiplexing technology within the cloud.

Circuit switching operates much like a normal dialup telephone call and is used extensively in telephone company networks. Circuit switching establishes a dedicated physical connection for voice or data between a sender and receiver. Before communication can start, it is necessary to establish the connection by setting the switches through a dialup activity. Whereas point-to-point communication links can accommodate only two sites on a single connection, circuit switching allows multiple sites to connect to the switched network of a carrier and communicate with each other.

An example of a circuit-switched connection is a public switched telephone network (PSTN).

Public Switched Telephone Network

The most common type of circuit-switched WAN communications is the PSTN (also referred to as the plain old telephone service [POTS]).

When intermittent, low-volume data transfers are needed, asynchronous modems and analog dialed telephone lines provide low capacity, on-demand, dedicated switched connections. Traditional telephony uses a copper cable, called the local loop, to connect the telephone handset in the subscriber premises to the telephone network. The signal on the local loop during a call is a continuously varying electronic signal that is a translation of the subscriber voice.

The local loop is not suitable for direct transport of binary computer data, but a modem can send computer data through the voice telephone network. The modem modulates the binary data into an analog signal at the source and, at the destination, demodulates the analog signal to binary data.

The physical characteristics of the local loop and the connection of the local loop to the PSTN limit the rate of the signal. The upper limit is around 53 kbps.

For small businesses, the PSTN can be adequate for the exchange of sales figures, prices, routine reports, and e-mail. Using automatic dialup at night or on weekends for large file transfers and data backup can take advantage of lower off-peak tariffs (line charges). Tariffs are based on the distance between the endpoints, the time of day, and the duration of the call.

Using PSTN has a number of advantages, including the following:

  • Simplicity: Other than a modem, no additional equipment is required, and analog modems are easy to configure.
  • Availability: Because a public telephone network is available virtually everywhere, it is easy to locate a telephone service provider, and the maintenance of the telephone system is very high quality, with few instances in which lines are not available.
  • Cost: The cost associated with the implementation of a PSTN connection link for a WAN is relatively low, consisting primarily of line charges and modems.
    Using PSTN also has some disadvantages, including the following:
  • Low data rates: Because the telephone system was designed to transmit voice data, the transmission rate for large data files is noticeably slow.
  • Relatively long connection setup time: Because the connection to the PSTN requires a dialup activity, the time required to connect through the WAN is very slow compared to other connection types.

Point-to-Point Communication Links

A point-to-point (or serial) communication link provides a single, established WAN communications path from the customer premises through a carrier network, such as a telephone company, to a remote network. Figure 5-29 shows an example of using a leased line to connect two corporate offices.

Figure 5-29 Leased Line
Configuring Serial Encapsulationfig5.29

A point-to-point (or serial) line can connect two geographically distant sites, such as a corporate office in New York and a regional office in London. Point-to-point lines are usually leased from a carrier and are therefore often called leased lines. For a point-to-point line, the carrier dedicates fixed transport capacity and facility hardware to the line leased by the customer. The carrier, however, still uses multiplexing technologies within the network.

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 DSU/CSU. The purpose of the DSU/CSU is to provide a clocked 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. Most T1 or E1 TDM interfaces on current routers include approved DSU/CSU capabilities.

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


Bandwidth refers to the rate at which data is transferred over the communication link. The underlying carrier technology depends on the bandwidth available. A difference exists between bandwidth points in the North American (T-carrier) specification and the European (E-carrier) system. Both of these systems are based on the plesiochronous digital hierarchy (PDH) supported in their networks. Optical networks use a different bandwidth hierarchy, which again differs between North America and Europe. In the United States, the Optical Carrier (OC) defines the bandwidth points, and in Europe, the Synchronous Digital Hierarchy (SDH) defines the bandwidth points.

In North America, the bandwidth is usually expressed as a digital service level number (DS0, DS1, and so forth) that technically refers to the rate and format of the signal. The most fundamental line speed is 64 kbps, or DS0, 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 43.736 Mbps. Figure 5-30 illustrates the different levels of bandwidth.

Figure 5-30 WAN Bandwidth
Configuring Serial Encapsulationfig5.30

NOTE E1 (2.048 Mbps) and E3 (34.368 Mbps) are European standards similar to T1 and T3, but they possess different bandwidths and frame structures.

To configure a serial interface, follow these steps:
Step 1 Enter global configuration mode (configure terminal command).
Step 2 When in global configuration mode, enter the interface configuration mode. In this example, it is the interface serial 0/0 command.
Step 3 If a DCE cable is attached, use the clock rate bps interface configuration command to configure the clock rate for the hardware connections on serial interfaces, such as network interface modules (NIM) and interface processors, to an acceptable bit rate. Be sure to enter the complete clock speed. For example, a clock rate of 64000 cannot be abbreviated to 64.

On serial links, one side of the link acts as the DCE, and the other side of the link acts as the DTE. By default, Cisco routers are DTE devices, but can be configured as DCE devices. In a “back-to-back” router configuration in which a modem is not used, one of the interfaces must be configured as the DCE to provide a clocking signal. You must specify the clock rate for each DCE interface that is configured in this type of environment. Clock rates in bits per second are as follows: 1200, 2400, 4800, 9600, 19200, 38400, 56000, 64000, 72000, 125000, 148000, 500000, 800000, 1000000, 1300000, 2000000, and 4000000.

Step 4 Enter the specified bandwidth for the interface. The bandwidth command overrides the default bandwidth that is displayed in the show interfaces command and is used by some routing protocols, such as the Enhanced Interior Gateway Routing Protocol (EIGRP), for routing metric calculations. The router also uses the bandwidth for other types of calculations, such as those required for the Resource Reservation Protocol (RSVP). The default bandwidth for serial lines is T1 speed (1.544 Mbps). The bandwidth entered has no effect on the actual speed of the line.

NOTE The attached serial cable determines the DTE or DCE mode of the Cisco router. Choose the cable to match the network requirement.

The show controller command displays information about the physical interface itself. This command is useful with serial interfaces to determine the type of cable connected without the need to physically inspect the cable itself.

The information displayed is determined when the router initially starts and represents only the type of cable that was attached when the router was started. If the cable type is changed after startup, the show controller command display doesn’t show the cable type of the new cable.

Point-to-Point Communication Considerations

Point-to-point links have been the traditional connection of choice. The advantages to this type of WAN access include the following:

  • Simplicity: Point-to-point communication links require minimal expertise to install and maintain.
  • Quality: Point-to-point communication links usually offer a high quality of service, provided that they have adequate bandwidth. The dedicated capacity gives no latency or jitter between the endpoints.
  • Availability: Constant availability is essential for some applications, such as electronic commerce, and point-to-point communication links provide permanent, dedicated capacity that is always available.

This type of WAN access also has some disadvantages, including the following:

  • Cost: Point-to-point links are generally the most expensive type of WAN access, and this cost can become significant when they connect many sites. In addition, each endpoint requires an interface on the router, which increases equipment costs.
  • Limited flexibility: WAN traffic is often variable, and leased lines have a fixed capacity, resulting in the bandwidth of the line seldom being exactly what is needed. Any changes to the leased line generally require a site visit by the ISP or carrier personnel to adjust capacity.

High-Level Data Link Control Protocol

The High-Level Data Link Control (HDLC) protocol is one of two major data-link protocols commonly used with point-to-point WAN connections.

HDLC specifies an encapsulation method for data on synchronous serial data links using frame character and checksum. HDLC supports both point-to-point and multipoint configurations and includes a means for authentication. However, HDLC might not be compatible between devices from different vendors because of the way each vendor might have chosen to implement it.

A Cisco implementation of HDLC exists; it is the default encapsulation for serial lines. Cisco HDLC is streamlined. It has no windowing or flow control, and only point-to-point connections are allowed. The Cisco HDLC implementation includes proprietary extensions in the data field, as shown in Figure 5-31; the extensions allowed multiprotocol support at a time before PPP was specified. Because of the modification, the Cisco HDLC implementation does not interoperate with other HDLC implementations. HDLC encapsulations vary; however, PPP should be used when interoperability is required. Figure 5-31 shows the differences between HDLC and Cisco HDLC.

Configuring HDLC Encapsulation

By default, Cisco devices use the Cisco HDLC serial encapsulation method on synchronous serial lines. However, if the serial interface is configured with another encapsulation protocol and you want to change the encapsulation back to HDLC, enter the interface configuration mode of the interface that you want to change. Use the encapsulation hdlc interface configuration command to specify HDLC encapsulation on the interface:

Cisco HDLC is a PPP that can be used on leased lines between two Cisco devices. When you are communicating with a device from another vendor, synchronous PPP is a better option.

Figure 5-31 HDLC Versus Cisco HDLC
Configuring Serial Encapsulationfig5.31

Point-to-Point Protocol

PPP originally emerged as an encapsulation protocol for transporting IP traffic over pointto-point links. PPP also established a standard for the assignment and management of IP addresses, asynchronous (start and stop bit) and bit-oriented synchronous encapsulation, network protocol multiplexing, link configuration, link quality testing, error detection, and option negotiation for such capabilities as network layer address negotiation and datacompression negotiation.

PPP provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits. An example of an asynchronous connection is a dialup connection.
An example of a synchronous connection is a leased line. Figure 5-32 illustrates using PPP instead of HDLC over a leased line.

Figure 5-32 PPP

Configuring Serial Encapsulationfig5.32

PPP provides a standard method for transporting multiprotocol datagrams (packets) over point-to-point links. PPP comprises these three main components:

  • A method for encapsulating multiprotocol datagrams
  • A link control protocol (LCP) for establishing, configuring, and testing the data-link connection
  • A family of Network Control Programs (NCP) for establishing and configuring different network layer protocols

PPP provides that an LCP be sufficiently versatile and portable to a wide variety of environments. The LCP is used to automatically determine the encapsulation format option, handle varying limits on sizes of packets, and detect a loopback link and terminate the link. Other optional facilities provided are authentication of the identity of its peer on the link and determination of when a link is functioning properly or failing.

The authentication phase of a PPP session is optional. After the link has been established and the authentication protocol chosen, the peer can be authenticated. If the authentication option is used, authentication takes place before the network layer protocol configuration phase begins.

The authentication options require that the calling side of the link enter authentication information to help ensure that the user has permission from the network administrator to make the call. Peer routers exchange authentication messages. Figure 5-33 shows the basic PPP frame.

Figure 5-33 PPP Frame

Configuring Serial Encapsulationfig5.33

PPP Layered Architecture

Developers designed PPP to make the connection for point-to-point links. PPP, described in RFCs 1661 and 1332, encapsulates network layer protocol information over point-topoint links. RFC 1661 is updated by RFC 2153, “PPP Vendor Extensions.”

You can configure PPP on the following types of physical interfaces:

  • Asynchronous serial
  • Synchronous serial
  • Basic Rate Interface (BRI)
  • High-Speed Serial Interface (HSSI)

PPP uses its NCP component to encapsulate and negotiate options for multiple network layer protocols.

PPP uses another of its major components, the LCP, to negotiate and set up control options on the WAN data link.

To enable PPP encapsulation, enter interface configuration mode. Use the encapsulation ppp interface configuration command to specify PPP encapsulation on the interface:

NOTE Additional configuration steps are required to enable PPP on an asynchronous serial interface. These steps are not taught in this course. For information about configuring PPP on an asynchronous serial interface, see the CCNP ISCW Official Exam Certification Guide book.

Example: PPP Configuration
Figure 5-34 shows a typical example of a PPP configuration.

Figure 5-34 PPP Configuration
Configuring Serial Encapsulationfig5.34

After configuring a serial interface, use the show interface serial command to verify the changes:

NOTE Notice in this example that the line is up and the bandwidth is set to 1544 kbps.

Serial Encapsulation Configuration Verification

You need to verify the encapsulation types when configuring WAN connectivity. If the encapsulation is not consistent on each end of a point-to-point link, then the communication between the sites fails.

Use the show interface command to verify proper configuration. The following example illustrates a PPP configuration. When HDLC is configured, “Encapsulation HDLC” should be reflected in the output of the show interface command. When PPP is configured, you can also use this command to check LCP and NCP states.

Frame Relay

Frame Relay is a packet-switching protocol that grew in its popularity by being much more cost-effective and thereby replaced older technologies such as X.25 and leased lines. Figure 5-35 illustrates where the Frame Relay protocol operates.

Figure 5-35 Frame Relay
Configuring Serial Encapsulationfig5.35

With increasing demand for higher bandwidth and lower latency packet switching, service providers introduced Frame Relay. Frame Relay provides both permanent virtual circuit (PVC) and switched virtual circuit (SVC) service using shared medium-bandwidth connectivity that carries both voice and data traffic. Available data rates are commonly up to 4 Mbps, with some providers offering even higher rates. In addition, Frame Relay is a much simpler protocol that works at the data link layer rather than at the network layer.

Frame Relay implements no error or flow control. The simplified handling of frames leads to reduced latency, and measures taken to avoid frame buildup at intermediate switches help reduce jitter.

Most Frame Relay connections are PVCs rather than SVCs. The connection to the network edge is often a leased line, but dialup connections are available from some providers using ISDN or xDSL lines.

Frame Relay is ideal for connecting enterprise LANs, because a router on the LAN needs only a single WAN interface, even when multiple virtual circuits (VC) are used. The dedicated line to the Frame Relay network edge allows cost-effective connections between widely scattered LANs.

Frame Relay operates over virtual circuits, which are logical connections created to enable communication between two remote devices across a network. VCs provide a bidirectional communications path from one DTE device to another. A data-link connection identifier

(DLCI) within the Frame Relay address header uniquely identifies a virtual circuit. The DLCI is specific only to the router where it is configured. A VC can pass through any number of intermediate DCE devices located within the network. Numerous VCs can be multiplexed into a single physical circuit for access to and transmission across the network. Figure 5-36 illustrates a VC through the Frame Relay cloud.

Figure 5-36 Frame Relay Virtual Circuit

Configuring Serial Encapsulationfig5.36

ATM and Cell Switching

ATM is a type of cell-switching connection technology that is capable of transferring voice, video, and data through private and public networks. ATM is used primarily in enterprise LAN backbones or WAN links. Figure 5-37 depicts an ATM connection.

Figure 5-37 ATM Connectivity
Configuring Serial Encapsulationfig5.37

Service providers saw a need for a permanent shared network technology offering low latency and jitter with high bandwidth. Their solution was to leverage the same technology used in their own core network: ATM. ATM has data rates beyond 155 Mbps. Topology diagrams for ATM WANs look similar to other shared technologies, such as X.25 and Frame Relay.

ATM is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The 53-byte ATM cell contains a 5-byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted.

The 53-byte ATM cell is less efficient than the larger frames and packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload. When the cell is carrying segmented network layer packets, the overhead is higher because the ATM 48-byte data payload might not map very well to other packet sizes (64-byte IP packets, for example). A typical ATM line needs almost 20 percent greater bandwidth than Frame Relay to carry the same volume of network layer data.

Like Frame Relay, ATM is implemented using VCs that can be either PVC or SVC. With ATM, the data is divided into small 53-byte cells before it is transmitted. The ATM cell header contains a field called the virtual path identifier/virtual channel identifier (VPI/ VCI) that indicates to which VC an ATM cell belongs. At the physical layer, ATM can run over a variety of physical media, including fiber optics using Synchronous Digital Hierarchy (SONET)/Synchronous Digital Hierarchy (SDH) framing and coaxial cable using DS3.

An ATM network includes ATM switches, which are responsible for cell forwarding. The ATM switch receives the incoming cell from an ATM endpoint or another ATM switch. The ATM switch then uses the incoming VPI/VCI to map to the outgoing interface and new VPI/VCI to be used on the next link toward its destination. The ATM cell-switching process is extremely fast and can be programmed in hardware.

An ATM VC is a logical connection created between two ATM endpoints across an ATM network. ATM VCs fall into the two categories of PVC and SVC. VCs provide a bidirectional communications path from one ATM endpoint to another. The VPI/VCI within the ATM cell header uniquely identifies the VCs.

A VC can pass through any number of intermediate ATM switches in the ATM network. Numerous VCs can be multiplexed into a single physical circuit for transmission across the network.

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