Today I am going to talk about serial interfaces. Serial interfaces are a point-to-point connection that is mostly used for WAN links or connections. By WAN connections, I mean T1/E1 circuits, DSL (digital subscriber lines), or leased lines. This WAN connection is most commonly from an ISP to a customer. Serial interfaces function very differently from regular parallel interfaces being how bits are transmitted. Parallel interfaces transmit multiple bits simultaneously while serial interfaces transmit data one bit at a time in sequential order.
DCE/DTE- An important factor of serial interfaces are DCE (Data Communications Equipment) and DTE (Data Terminal Equipment). These are important because without clocking data will not be able to be read properly and cause a lot of misalignments. The DCE is the network device that is responsible for setting the clock rate and clock timing to dictate the speed and timing at which data bits will be sent and received. The clock rate is measured in bits per second, so if the command “clock rate 64000” is inputted into a DCE CLI then data will be transmitted and received at the rate of 64000 bits per second. Common devices that are to act as a DCE are the DSU/CSU (Channel Service Unit/Data Service Unit) or ISP modems. The device that is responsible for adhering to the clock rate is the DTE. DTE is usually the customer’s router, and is responsible for generating and processing data. It is the source or destination of data. The DTE is reliant on the clock rate that it receives from the DCE and is crucial for bits being properly interpreted. Without a specific clock rate set by the DCE, the DTE can read the bits at the wrong bits per second, essentially misinterpreting the data as not having a clock rate can lead to bit misalignment.
HDLC– One other caveat of serial interfaces is that they do not use the same encapsulation protocols as a regular ethernet interface. Instead of an ethernet data link layer encapsulation (ethernet II or 802.3), serial interfaces use either HDLC (High Level Data Link Control) or PPP (Point-to-Point). HDLC is responsible for encapsulating layer 3 packets and also provides framing and error detection. HDLC uses the same FCS as the data link layer, using the same polynomial division that is the CRC. Unlike the open standard HDLC, Cisco’s HDLC supports multiple network layer protocols (not in the same packet) such as IPv4, IPv6 or IPX.
PPP- PPP performs the same way but it adds authentication such as PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol) which HDLC does not offer. PAP sends passwords in plain text, while CHAP uses hashing and periodic challenges for better security. PPP also supports multilink PPP which essentially works the same way as etherchannel by combining multiple physical links and bundling them into one logical link. Another feature of PPP is link quality monitoring where PPP evaluates the health of a serial connection by tracking error rates. PPP is vendor-neutral, and supports multiple network layer protocols such as IPv4, IPv6, and IPX.
OSPF- When it comes to OSPF, the type of encapsulation being used is important to be able to tell which kind of connection is being used. Whenever HDLC or PPP is being used, OSPF detects that and treats the segment as a point-to-point network versus when the serial interface is using frame relay encapsulation which will be a non-broadcast multi-access.
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