The major difference between HDLC and PPP is that HDLC is the bit oriented protocol, while PPP is the character-oriented protocol. The HDLC and PPP are the crucial data link layer protocols used in WAN (wide area network) where the HDLC can also be implemented with PPP for the efficient results.
HDLC describes the encapsulation technique employed on the data in the synchronous serial data link. On the other hand, the PPP protocol deals with the encapsulation of the data transported in the point-to-point links and it could be synchronous or asynchronous.
Content: HDLC Vs PPP
|Basis for comparison||HDLC||PPP|
|Expands to||High-level Data Link Layer Protocol||Point-to-Point Protocol|
|Type of protocols||Bit-oriented protocol||Byte oriented protocol|
|Used in||Only synchronous media||Synchronous as well as asynchronous media|
|Authentication||No provision of authentication||Provides authentication|
|Dynamic addressing||Does not offer dynamic addressing.||Dynamic addressing is used.|
|Implemented in||Point-to-point and multipoint configurations.||Only point-to-point configurations.|
|Compatibility with other protocols||Can not be operated with non-Cisco devices.||Interoperable with non-Cisco devices also.|
Definition of HDLC
HDLC (High-level Data Link Control) is a WAN protocol intended to perform the encapsulation of the data in the data link layer. The encapsulation of the data means to change the format of the of the data. SDLC is the predecessor of the HDLC which stands for the Synchronous Data Link Control protocol. Both SDLC and HDLC protocol are developed by IBM and submitted to the ANSI and ISO for the acceptance as the international standards.
The HDLC protocol follows the bit-oriented concept and uses bit stuffing for achieving data transparency. Here bit oriented approach signifies that the single bit is used to present the control information. The frame structure of HDLC contains the address, control, data, checksum and flag fields. The default encapsulation protocol in the Cisco devices is the HDLC. The Cisco proprietary HDLC only works when the devices in both of the ends of the link are of cisco. Standard HDLC can have different devices in the ends.
Frame format for the bit-oriented protocols
- Address field – It is used to describe the terminal.
- Control field – The bits in the control field is intended for the sequence number and acknowledgements.
- Data field – This field is used to hold the information.
- Checksum field -In this field, the bits are reserved for the performing the cyclic redundancy code.
HDLC Commands and Requests
The HDLC uses a group of commands and responses for its working. There are three types of frames information, supervisory and unnumbered.
- Information transfer format (I-Frame) – It transports the numbered frames in a sequential manner, which contain the information field.
- Supervisory format (S-Frame) – The supervisory frames conduct the managerial functions such as acknowledgement, information transfer status, polling and error recovery. The commands and requests included in this are RECEIVE READY, RECEIVE NOT READY, REJECT, etcetera.
- Unnumbered format (U-Frame) – It basically extends the data link control functions. There several commands and requests fall under this category such as RESET, TEST, FRAME REJECT, REQUEST DISCONNECT, etcetera.
Definition of PPP
PPP (Point-to-Point Protocol) is also a WAN protocol, but there are several enhancements made in the PPP protocol after HDLC. Priorly, the PPP protocol is not proprietary, which means that it can be used with two different type of devices without committing changes over the format of the data. All of the links collaboratively treated as single, independent IP network which is having its own frame format, hardware addressing method, and data link protocol. A point-to-point connection is obtained without assigning multiple IP addresses to the tangible wires, and it just needs the IP network number.
There are several features of PPP, which are discussed below.
- To clearly identify the start and end of the frame, the framing method is used on the asynchronous data. It is also beneficial in the detection of the errors.
- A link control protocol is used for enabling the network lines, testing them, terminate them when no longer used. This link control protocol is basically helpful in handling the synchronous and asynchronous circuits, and byte and bit-oriented encodings.
- It can select the NCP (Network Control Protocol) for each supported network layer.
Frame format for the PPP
The PPP frame contains two flag fields, a protocol filed to determine the type of packet residing in the payload, and a payload field which can variate. However, the rest of the fields are the same as the HDLC protocol.
Working of the PPP protocol
- The User first calls the internet service provider’s router to establish the physical connection; the request goes through the modem.
- After the generation of the reply by the router’s modem, the physical connection is established.
- User device sends a set of LCP packets in the payload field of one or more PPP frames sequence wise.
- The PPP parameters are chosen according to the packets and their responses.
- After selecting PPP parameters, the NCP packets are delivered for the configuration of the network layer.
- Then the IP addresses are dynamically assigned with the help of the NCP to the newly attached devices for the duration of its login by the provider.
- Now the device is an internet host and is able to send and receive the IP packets.
- At, the NCP ends the network layer connection and spares the IP addresses.
Key Differences Between HDLC and PPP
- The HDLC is a bit-oriented protocol while PPP is byte-oriented as well as bit oriented because it can be sent over the dial-up modem lines and also true bit-oriented HDLC.
- Only synchronous media can be used in HDLC. In contrast, PPP can work with synchronous and asynchronous media.
- No link authentication is provided in HDLC, whereas it is provided in PPP.
- PPP can dynamically assign and frees up the IP address according to the use. As against, this not the case in HDLC.
- Interoperability between the non-cisco devices in HDLC is not achievable. However, this limitation of HDLC is eliminated from the PPP protocol.
Between HDLC and PPP, the PPP protocol performs better than HDLC because there is no compulsion of using the Cisco devices in both the ends, which means these are interoperable. PPP also supports multiple protocols and support authentication.
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