somebody call the WANbulance...

WAN types

Leased line- dedicated, always on circuit between two endpoints. More expensive than packet switched

Dial/circuit switched- dedicated bandwidth per call duration, cheaper than leased especially when constant connectivity is not essential.  A good back up for packet switched or leased

Packet switched- virtual circuits between two points, contracted traffic rates.  Consists of a leased line from the site to provider network and usually cheaper than leased

DCE is always the clock rate provider

HDLC and PPP provision data delivery across a single serial point to point link

HDLC and  PPP can use Synchronous serial; PPP can also use Asynchronous.

Synchronous WAN datalink protocols are frame oriented.

HDLC and PPP define idle frames, sending back and forth signal transitions to maintain clock synchronization. 

Synchronous protocols allow more throughput than do asynchronous, but async uses less expensive hardware because transitions do not need to be monitored to adjust clock rate.  Between routers, synchronous is usually preferred.  A PC through a modem to an ISP uses Async. 

Both PPP and HDLC perform error detection using an FCS in the frame trailer.  Frames with errors are discarded.  Error recovery may be performed by the datalink layer protocol, a higher layer protocol or not at all.  Error recovery results in the retransmission of  errored or lost frames.  Again error detection is just that, and errored frames are discarded.

HDLC has a proprietary protocol type field; PPP has a standardized protocol type field.  This field defines the type of packet encapsulated in the frame. 

HDLC only supports synchronous without error correction, while PPP supports async and sync, and error correction is supported but not on by default.

Encap PPP will place the interface in PPP mode.  No encap PPP will place the interface in HDLC and remove all traces of PPP, including other config commands.  Encap hdlc also accomplishes this.

PPP has more features than HDLC, and is therefore preferred. To wit:

PPP has at it’s heart LCP (link control protocol)  LCP provides:

   Error detection in the form of Link Quality Monitoring.  LQM determines the quality of a link based on its errored percentage.  It will take a link down based on a configuration determined amount of loss.

    Looped link detection using Magic Number.  Magic number messages are passed among routers.  Receipt of a magic number means the link is looped.  A config setting determines whether or not a link should be shut (in redundant routes)

    Multilink using Multilink PPP.  Fragmented packets are load balanced across multiple links.

    Authentication using PAP or CHAP.  PAP and CHAP can exchange identities on each of the links. (security) PAP is clear message, CHAP uses MD5 hashing. The username is the opposite routers hostname, and vice versa, the passwords are the same. Until there is a match on both sides, the link will come down.