- The 2 most popular data link (L2) protocols that are being used over point-to-point serial links:
HDLC High-Level Data Link Control. It was derived from Synchronous Data Link Control (SDLC) created by IBM. The HDLC specification was not intended to encapsulate (or support) multiple network layer protocols across the same link. It does not include a Protocol Type field and hence unable to identify the encapsulated network layer packets. Vendors that support HDLC must define their own proprietary HDLC headers that contain the Protocol Type field to identify the network layer protocols.
Due to this reason, HDLC implementations are proprietary – Cisco routers will not work in connections to other vendor routers. It is the default encapsulation type used on Cisco synchronous serial interfaces. HDLC does not support error recovery.
PPP Point-to-Point Protocol. It is an industry-standard protocol that provides an open standard for creating point-to-point links between different vendor equipments.
It uses a standardized Protocol Type field – Network Control Protocol (NCP) field in the header to identify the encapsulated network layer packets. It supports authentication, multilink connections, and can operates over synchronous and asynchronous links. PPP uses HDLC to encapsulate and transmit packets over point-to-point links. PPP supports error recovery but is disabled by default.
Note: HDLC was developed by the International Organization for Standardization (ISO).
- HDLC does not need the arbitration feature as in Ethernet. Because on point-to-point serial links, routers can send over the 4 wires (2 twisted-pair wires) circuit at any time, and the signal (eventually data) will arrived at the other end of the link.
- Both HDLC and PPP frames include an address field, but it is not being used and not really needed over point-to-point links.
- Do not confuse error detection with error recovery! Almost all data link layer protocols including HDLC and PPP perform error detection with the FCS field in their trailers. Frames that fail the
FCS check will be noticed and discarded (error detection). Error recovery is normally being performed by higher-layer protocols, eg: TCP.
Point-to-Point Protocol (PPP)
- When PPP was being developed, it includes many additional features that never been seen in other WAN data link protocols up to that time.
- The PPP specification includes the following 2 subprotocols or sublayers:
i) Link Control Protocol (LCP). Responsible for establishing, configuring, testing, maintaining, and terminating PPP connections. PPP was designed to work with several network layer protocols, hence each PPP link has a LCP and a NCP for the corresponding L3protocol encapsulated and transmitted over the PPP link.
ii) Network Control Protocols (NCPs). Responsible for encapsulating network layer protocols and assigning network layer addresses to the remote end over PPP links after the link establishment and authentication phases. The negotiated network layer protocols can then be transmitted across the PPP connection.
Ex: IP Control Protocol (IPCP) for IP, IPX Control Protocol (IPXCP) for IPX, and AppleTalk Control Protocol (ATCP) for AppleTalk. A router configured for PPP encapsulation will try to identify the network layer protocol and use the appropriate
- PPP establishes a connection in 3 phases – link establishment (LCP negotiation), authentication (optional), and network layer protocol negotiation (NCP negotiation). Both ends must agree on the same parameters to establish a link, and authenticate each other using the authentication protocol and option negotiated during the LCP negotiation. Authentication must be success prior to NCP negotiation.
Figure 21-6: Point-to-Point Protocol Stack - PPP also uses LCP to negotiate the following functions or features:
Function LCP Feature Description
Error Detection
Link Quality Monitoring (LQM)
Monitors link quality. PPP LCP at the receiving end sends messages that describe the number of correctly received packets and bytes to the other end. The sending end calculates the percent of loss. A link can be terminated after a configured error rate is exceeded.
Looped Link Detection
Magic number Endpoints send PPP LCP messages that include a magic number, which is different for each endpoint. A looped link is detected when an endpoint receives an LCP message with its own magic number instead of the magic number of the other peer. PPP can be configured to terminate detected looped links.
Multilink Multilink PPP Allows connections of multiple parallel channels between 2 routers, and perform load balancing across the multiple links.
Commonly used in ISDN links.
Authentication Password Authentication Protocol (PAP) and Challenge Handshake Authen. Protocol (CHAP)
PPP initiates an authentication process to verify the identity of the device on the other end of the serial link. Normally used in dial-up links (eg: modems, ISDN, and DSL).
Callback PPP Callback The calling router (client) first contacts the remote router (server) and authenticates itself.
After the authentication process, the remote router terminates the connection and then reinitiates a connection to the calling router.
Normally used to minimize the long distance dialing cost on the client side.
Compression Stacker and Predictor Compresses data at source and reproduces data at destination. Predictor uses more memory than Stacker; while Stacker is more CPU
- Comparisons between synchronous and asynchronous links:
Synchronous Links Asynchronous Links
Have identical frequencies. Each individual character is encapsulated in control bits – the Start / Stop bits, which designate the beginning and ending of each character.
Send digital signals without timing. They agree on the same speed, but there is no checking or adjustment of the rates if they are slightly different.
Send frame continuously. Send idle frames called Receiver Ready when there is no data transmission over the link. The purpose is to maintain the clock synchronization.
No bits are sent during idle times. Only 1 byte is sent per transfer.
Allow more throughputs (due to clock synchronization).
Require less expensive hardware (do not require clock synchronization).
Normally used for links between routers. Normally used for dial-up links (modems).
Isochronous transmission allows asynchronous data transfer over a synchronous link. It requires constant bit rate for reliable transport.
- Below list some other common WAN data link protocols:
Protocol Error
Yes No Developed by IBM. Supports
multipoint links. It assumes an IBM SNA header after the SDLC header. Replaced by HDLC. but is restricted to be used only in point-to-point links.
[1] Cisco’s implementation of LAPB and HDLC includes a proprietary Protocol Type field.
- PPP and ATM can operate on synchronous and asynchronous links; while HDLC and Frame Relay can only operate on synchronous links.
- PPP, HDLC, and LAPB are the data link layer protocols (or encapsulations) that can be used on ISDN B channels. PPP uses HDLC to encapsulate and transmit packets over point-to-point links.