Definition Of Serial Port In Networking

serial port


In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in paralle

An asynchronous port on the computer used to connect a serial device to the computer and capable of transmitting one bit at a time. Serial ports are usually identified on IBM compatible computers as COM (communications) ports. For example, a mousemight connect to COM1 and a modem to COM2.

What is serial port used for?

A socket that connects to a serial interface (one bit following another over one line). Serial ports are widely used by sensors for data acquisition, and they were standard on early computers for connecting a modem, mouse and other peripherals. Old Macs used the serial port for printers.

Applications of Serial Port

Various applications and devices which use the port are as follows:

  • For managing and configuring networking equipment such as routers, switches, etc.
  • Present in barcode scanners.
  • LED and LCD text displays.
  • Present in test and measuring equipment.
  • Updating firmware on various consumer devices.
  • LCD and plasma use it to control screen functions by external computers.
  • Present in CNC controllers

Parallel Port

In contrast to the serial port, parallel ports perform a parallel communication interface. Moreover, it can transfer multiple bits at a time. Besides they also connect devices like printers and scanners. We can also call them printer port. Furthermore, they contain 25 pins, and the speed of data travel is 150-kilo bits per second.

Difference Between Serial Port and Parallel Port

Serial PortParallel Port
It performs serial transmission.It performs parallel transmission.
The transmission speed is lesser than the parallel port.The transmission speed is higher than the serial port.
The number of wires is less.The number of wires is more as compared to the serial port.
It transmits a single stream of data which means bit by bit transmission.It can transmit multiple streams of data which means multiple bits transmission.
Serial ports involve male ports.Parallel ports involve female ports.
Devices using serial ports are mainly modems, connecting devices, controllers, security cameras, etc.Devices using parallel ports are mainly printers, hard drives, zip-drives, CD-ROM drives, etc.

What is the function of serial port?

In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel

In computer networking, a serial port enables external modems to connect to a PC or network router via a serial cable. The term “serial” signifies that data sent in one direction always travels over a single wire within the cable.

Standards For Serial Ports

The prevailing standard for traditional serial port communications historically has been RS-232. These serial ports and cables are the same used for PC keyboards and other computer peripheral devices. Serial ports and cables for RS-232 PCs generally feature 9-pin DE-9 connectors, although 25-pin DB-25 and other variations exist on specialized hardware. The alternative RS-422 standard applied to many Macintosh computers.

Both of these standards are now obsolete in favor of USB or Firewire standard ports and serial communication

There are two methods in serial communication viz. synchronous and asychronous. In synchronous communication, block of data (or characters) are transferred at a time. In asynchronous communication, single byte (or bit) is transferred at a time. There are special ICs used for this communication such as UART and USART. The microcontroller IC such as 8051 consists of in-built UART chip. PISO and SIPO shift registers are used to convert serial data into parallel data and vice versa.


Configuring the Serial Interface

The Cisco 800M Series Integrated Services Router (ISR) provides serial WAN connectivity to remote sites using Cisco High-Level Data Link Control (HDLC), Point-to-Point Protocol (PPP), or Frame Relay encapsulation through the pluggable, serial WAN interface module. The Cisco 800M Series ISR supports both synchronous and asynchronous modes of communication.

Features Supported by Serial Module

The Cisco 800M Series ISR has 2 WAN slots that can host single-port serial module or multi-mode 3G module and supports the following combinations.

  • Serial module in slot 0 and 3G module in slot 1
  • 3G module in slot 0 and serial module in slot 1
  • Serial module in slot 0 and serial module in slot 1

The features supported by the single-port serial module on Cisco 800M Series ISR is given as follows:

  • Supports the following encapsulations :

blank HDLC

blank PPP

blank Frame Relay

blank Serial Line Internet Protocol (SLIP)

  • Supports the following serial protocols

blank EIA-232

blank EIA-449

blank EIA-530

blank EIA-530A

blank V.35

blank X.21

  • Supports synchronous speed of up to 8 Mbps
  • Supports asynchronous speed of up to 115.2 kbps
  • Supports network clock synchronization

Cisco 800M Series ISRs use Cisco smart serial connectors. Information about the cables supported by Cisco 800M Series ISRs are provided in Table 4-1 .

Product NumberCable TypeLengthConnector Type
CAB-SS-V35MTV.35 DTE10 ft (3m)Male
CAB-SS-V35FCV.35 DCE10 ft (3m)Female
CAB-SS-232MTEIA/TIA-232 DTE10 ft (3m)Male
CAB-SS-232FCEIA/TIA-232 DTE10 ft (3m)Female
CAB-SS-449MTEIA/TIA-449 DTE10 ft (3m)Male
CAB-SS-449FCEIA/TIA-449 DTE10 ft (3m)Female
CAB-SS-X21MTX.21 DTE10 ft (3m)Male
CAB-SS-X21FCX.21 DTE10 ft (3m)Female
CAB-SS-530MTEIA/TIA-530 DTE10 ft (3m)Male
CAB-SS-530AMTEIA/TIA-232 DTE10 ft (3m)Male

Table 4-2 describes the LEDs on the Cisco 800M series serial WAN module.

LED NameColor/ StatusDescription
CONNGreenIndicates the interface status and shows that line protocol is up.
OFFShows that the line protocol is down.
LOOP BACKGreenIndicates that the hardware loopback status is configured on the serial interface.
OFFIndicates that loopback is not configured.

Information About Configuring Serial Interfaces

To configure serial interfaces, you should understand the following concept:

Cisco HDLC Encapsulation

Cisco High-Level Data Link Controller (HDLC) is the Cisco proprietary protocol for sending data over synchronous serial links. Cisco HDLC also provides a simple control protocol called Serial Line Address Resolution Protocol (SLARP) to maintain serial link keepalives. Cisco HDLC is the default for data encapsulation at Layer 2 (data link) of the Open System Interconnection (OSI) stack for efficient packet delineation and error control.

Noteblank Cisco HDLC is the default encapsulation type for the serial interfaces.

When the encapsulation on a serial interface is changed from HDLC to any other encapsulation type, the configured serial subinterfaces on the main interface inherit the newly changed encapsulation and they do not get deleted.

Cisco HDLC uses keepalives to monitor the link state, as described in the “Keepalive Timer” section.

PPP Encapsulation

PPP is a standard protocol used to send data over synchronous serial links. PPP also provides a Link Control Protocol (LCP) for negotiating properties of the link. LCP uses echo requests and responses to monitor the continuing availability of the link.note

Noteblank When an interface is configured with PPP encapsulation, a link is declared down and full LCP negotiation is re-initiated after five echo request (ECHOREQ) packets are sent without receiving an echo response (ECHOREP).

PPP provides the following Network Control Protocols (NCPs) for negotiating properties of data protocols that will run on the link:

  • IP Control Protocol (IPCP) to negotiate IP properties
  • Multiprotocol Label Switching control processor (MPLSCP) to negotiate MPLS properties
  • Cisco Discovery Protocol control processor (CDPCP) to negotiate CDP properties
  • IPv6CP to negotiate IP Version 6 (IPv6) properties
  • Open Systems Interconnection control processor (OSICP) to negotiate OSI properties

PPP uses keepalives to monitor the link state, as described in the “Keepalive Timer” section.

PPP supports the following authentication protocols, which require a remote device to prove its identity before allowing data traffic to flow over a connection:

  • Challenge Handshake Authentication Protocol (CHAP)—CHAP authentication sends a challenge message to the remote device. The remote device encrypts the challenge value with a shared secret and returns the encrypted value and its name to the local router in a response message. The local router attempts to match the remote device’s name with an associated secret stored in the local username or remote security server database; it uses the stored secret to encrypt the original challenge and verify that the encrypted values match.
  • Microsoft Challenge Handshake Authentication Protocol (MS-CHAP)—MS-CHAP is the Microsoft version of CHAP. Like the standard version of CHAP, MS-CHAP is used for PPP authentication; in this case, authentication occurs between a personal computer using Microsoft Windows and a Cisco router or access server acting as a network access server.
  • Password Authentication Protocol (PAP)—PAP authentication requires the remote device to send a name and a password, which are checked against a matching entry in the local username database or in the remote security server database.

Use the ppp authentication command in interface configuration mode to enable CHAP, MS-CHAP, and PAP on a serial interface.

Noteblank Enabling or disabling PPP authentication does not effect the local router’s willingness to authenticate itself to the remote device.

Multilink PPP

Multilink Point-to-Point Protocol (MLPPP) is supported on the Cisco 800M Series ISR serial interface. MLPPP provides a method for combining multiple physical links into one logical link. The implementation of MLPPP combines multiple PPP serial interfaces into one multilink interface. MLPPP performs the fragmenting, reassembling, and sequencing of datagrams across multiple PPP links.

MLPPP provides the same features that are supported on PPP Serial interfaces with the exception of QoS. It also provides the following additional features:

  • Fragment sizes of 128, 256, and 512 bytes
  • Long sequence numbers (24-bit)
  • Lost fragment detection timeout period of 80 ms
  • Minimum-active-links configuration option
  • LCP echo request/reply support over multilink interface
  • Full T1 and E1 framed and unframed links

Keepalive Timer

Cisco keepalives are useful for monitoring the link state. Periodic keepalives are sent to and received from the peer at a frequency determined by the value of the keepalive timer. If an acceptable keepalive response is not received from the peer, the link makes the transition to the down state. As soon as an acceptable keepalive response is obtained from the peer or if keepalives are disabled, the link makes the transition to the up state.

Noteblank The keepalive command applies to serial interfaces using HDLC or PPP encapsulation. It does not apply to serial interfaces using Frame Relay encapsulation.

For each encapsulation type, a certain number of keepalives ignored by a peer triggers the serial interface to transition to the down state. For HDLC encapsulation, three ignored keepalives causes the interface to be brought down. For PPP encapsulation, five ignored keepalives causes the interface to be brought down. ECHOREQ packets are sent out only when LCP negotiation is complete (for example, when LCP is open).

Use the keepalive command in interface configuration mode to set the frequency at which LCP sends ECHOREQ packets to its peer. To restore the system to the default keepalive interval of 10 seconds, use the keepalive command with the no keyword. To disable keepalives, use the keepalive disable command. For both PPP and Cisco HDLC, a keepalive of 0 disables keepalives and is reported in the show running-config command output as keepalive disable.

When LCP is running on the peer and receives an ECHOREQ packet, it responds with an ECHOREP packet, regardless of whether keepalives are enabled on the peer.

Keepalives are independent between the two peers. One peer end can have keepalives enabled; the other end can have them disabled. Even if keepalives are disabled locally, LCP still responds with ECHOREP packets to the ECHOREQ packets it receives. Similarly, LCP also works if the period of keepalives at each end is different.

Frame Relay Encapsulation

When Frame Relay encapsulation is enabled on a serial interface, the interface configuration is hierarchical and comprises the following elements:

  • The serial main interface comprises the physical interface and port. If you are not using the serial interface to support Cisco HDLC and PPP encapsulated connections, then you must configure subinterfaces with permanent virtual circuits (PVCs) under the serial main interface. Frame Relay connections are supported on PVCs only.
  • Serial subinterfaces are configured under the serial main interface. A serial subinterface does not actively carry traffic until you configure a PVC under the serial subinterface. Layer 3 configuration typically takes place on the subinterface.
  • When the encapsulation on a serial interface is changed from HDLC to any other encapsulation type, the configured serial subinterfaces on the main interface inherit the newly changed encapsulation and they do not get deleted.
  • Point-to-point PVCs are configured under a serial subinterface. You cannot configure a PVC directly under a main interface. A single point-to-point PVC is allowed per subinterface. PVCs use a predefined circuit path and fail if the path is interrupted. PVCs remain active until the circuit is removed from either configuration. Connections on the serial PVC support Frame Relay encapsulation only.

Noteblank The administrative state of a parent interface drives the state of the subinterface and its PVC. When the administrative state of a parent interface or subinterface changes, so does the administrative state of any child PVC configured under that parent interface or subinterface.

To configure Frame Relay encapsulation on serial interfaces, use the encapsulation (Frame Relay VC-bundle) command.

Frame Relay interfaces support two types of encapsulated frames:

  • Cisco (default)
  • IETF

Use the encap command inPVC configuration mode to configure Cisco or IETF encapsulation on a PVC. If the encapsulation type is not configured explicitly for a PVC, then that PVC inherits the encapsulation type from the main serial interface.

Noteblank Cisco encapsulation is required on serial main interfaces that are configured for MPLS. IETF encapsulation is not supported for MPLS.

Before you configure Frame Relay encapsulation on an interface, you must verify that all prior
Layer 3 configuration is removed from that interface. For example, you must ensure that there is no IP address configured directly under the main interface; otherwise, any Frame Relay configuration done under the main interface will not be viable.

LMI on Frame Relay Interfaces

The Local Management Interface (LMI) protocol monitors the addition, deletion, and status of PVCs. LMI also verifies the integrity of the link that forms a Frame Relay UNI interface. By default, cisco LMI is enabled on all PVCs.

If the LMI type is cisco (the default LMI type), the maximum number of PVCs that can be supported under a single interface is related to the MTU size of the main interface. Use the following formula to calculate the maximum number of PVCs supported on a card:

(MTU - 13)/8 = maximum number of PVCs

Noteblank The default setting of the mtu command for a serial interface is 1504 bytes. Therefore, the default numbers of PVCs supported on a serial interface configured with cisco LMI is 186.

How to Configure Serial Interfaces

This section contains the following tasks:

Configuring a Synchronous Serial Interface

To configure a synchronous serial interface, perform the tasks in the following sections. Each task in the list is identified as either required or optional.

Specifying a Synchronous Serial Interface

To specify a synchronous serial interface and enter interface configuration mode, use the following commands in global configuration mode.

Router(config)# interface serial wic/slot/port Router# interface serial 0/0/0 Specifies the serial interface and enters interface configuration mode.

Specifying Synchronous Serial Encapsulation

By default, synchronous serial lines use the High-Level Data Link Control (HDLC) serial encapsulation method, which provides the synchronous framing and error detection functions of HDLC without windowing or retransmission. The serial interfaces support the following serial encapsulation methods:

  • HDLC
  • Frame Relay
  • PPP

To define the encapsulation method, use the following command in interface configuration mode.


Router(config-if)# encapsulation { hdlc | frame-relay | ppp } Router(config-if)# encapsulation pppConfigures synchronous serial encapsulation.

Encapsulation methods are set according to the type of protocol or application you configure in the Cisco IOS software.

For configuration examples, see the “Configuration Examples” section.


Configuring Asynchronous Serial Interface

You can use the physical-layer async command to change the interface mode from the default synchronous mode to asynchronous mode


1.blank physical-layer async


Command or ActionPurpose
Step 1physical-layer async Router(config-if)# physical-layer asyncSpecifies the mode of a low-speed interface as either synchronous or asynchronous.

Benefits or advantages of Serial Interface

Following are the benefits or advantages of Serial Interface:
➨It uses less number of conducting wires, hence reduces cost of the interface.
➨It supports long distance data communication.
➨It uses less number of wires often only one, this leads to simple interface between transmitting and receiving devices or ICs. It is easy to implement.

Drawbacks or disadvantages of Serial Interface

Following are the disadvantages of Serial Interface:
➨It uses less number of lines for transmission between devices. Hence it supports slower speed of transmission.
➨It occupies overhead of about 20% other than useful information. This leads to wastage of bandwidth meant for data trasmission for useless stuff.

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