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Beginner

PPP, HDLC leased line and packet switching

Hi all

This is my first post so pls be gentle!

I am doing home study for my ccent.

I am a little confused about where PPP and HDLC fit into the types of WAN technologies.

Is HDLC and PPP  a technology utilised for providing leased line connections, or is it a packet switching technology or can it be used for both?

I know that they use a serial connection on the router but am now quite sure how the big picture fits together!

Many thanks for any comments/suggestions

Ian

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Hall of Fame Cisco Employee

PPP, HDLC leased line and packet switching

Hello Ian,

Welcome to Cisco Support Community

PPP and HDLC are both Layer2 protocols - they define how two directly connected devices should send data over their interfaces to each other. PPP and HDLC can be used on top of several technologies - dial-up, serial interfaces, leased lines, ISDN, even DSL. All these technologies provide a communication medium through which data can be sent between the routers. However, you can not just put IP packets on this "wire". At least, you need to check whether the data have been carried over correctly, and you have to identify the type of data you have sent (IPv4 packet, IPv6 packet, CDP packet, etc.) as the link may be used to carry many different protocols simultaneously. This is where the PPP and HDLC come in. Using checksums, they verify whether the data has been received correctly. By an embedded Type/Protocol field in their header, HDLC and PPP identify the exact type of packets they carry. And there are some additional functions these protocols provide.

Neither PPP nor HDLC in fact define whether the underlying network technology is circuit-switched or packet-switched. You can use both these protocols on dialup or ISDN which are circuit-switched technologies, or you can use PPP over Frame Relay which would be a packet-switched network. However, principially, both PPP and HDLC could be described as packet-switching-oriented protocols. However, neither of these protocols is limited to a particular physical layer so that's why they are basically fine with both circuit and packet switching.

Feel welcome to ask further!

Best regards,

Peter

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4 REPLIES 4
Hall of Fame Cisco Employee

PPP, HDLC leased line and packet switching

Hello Ian,

Welcome to Cisco Support Community

PPP and HDLC are both Layer2 protocols - they define how two directly connected devices should send data over their interfaces to each other. PPP and HDLC can be used on top of several technologies - dial-up, serial interfaces, leased lines, ISDN, even DSL. All these technologies provide a communication medium through which data can be sent between the routers. However, you can not just put IP packets on this "wire". At least, you need to check whether the data have been carried over correctly, and you have to identify the type of data you have sent (IPv4 packet, IPv6 packet, CDP packet, etc.) as the link may be used to carry many different protocols simultaneously. This is where the PPP and HDLC come in. Using checksums, they verify whether the data has been received correctly. By an embedded Type/Protocol field in their header, HDLC and PPP identify the exact type of packets they carry. And there are some additional functions these protocols provide.

Neither PPP nor HDLC in fact define whether the underlying network technology is circuit-switched or packet-switched. You can use both these protocols on dialup or ISDN which are circuit-switched technologies, or you can use PPP over Frame Relay which would be a packet-switched network. However, principially, both PPP and HDLC could be described as packet-switching-oriented protocols. However, neither of these protocols is limited to a particular physical layer so that's why they are basically fine with both circuit and packet switching.

Feel welcome to ask further!

Best regards,

Peter

View solution in original post

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Beginner

Re: PPP, HDLC leased line and packet switching

Peter, your example is mixing between PPP over other L2 technologies and the physical line. Maybe you meant to express yourself in a way that wasn't too clear IMHO.

Beginner

PPP, HDLC leased line and packet switching

Thanks very much for your reply!

Thats helped to clarify my thoughts.

Ive got my exam on Thursday!!

Hall of Fame Cisco Employee

PPP, HDLC leased line and packet switching

Hey Ian,

You are welcome. Good luck with your exam! Let us know how you did on the exam but I sincerely hope you'll pass with flying colors.

Take care!

Best regards,

Peter

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