• With a serial connection, information is sent across one wire, one data bit at a time. The 9-pin serial connector on most PCs uses two loops of wire, one in each direction, for data communication, plus additional wires to control the flow of information. In any given direction, data is still flowing over a single wire.
• A parallel connection sends the bits over more wires simultaneously. In the case of the 25-pin parallel port on your PC, there are eight data-carrying wires to carry 8 bits simultaneously. Because there are eight wires to carry the data, the parallel link theoretically transfers data eight times faster than a serial connection. So based on this theory, a parallel connection sends a byte in the time a serial connection sends a bit.
In a parallel connection, it is wrong to assume that the 8 bits leaving the sender at the same time arrive at the receiver at the same time. Rather, some of the bits get there later than others. This is known as clock skew.
Interface
At higher frequencies,crosstalk causes bytes to be dropped.
Parallel wires are physically bundled in a parallel cable, and signals can imprint themselves on each other. The possibility of crosstalk across the wires requires more processing, especially at higher frequencies.
There are three key serial communication standards affecting LAN-to-WAN connections:
RS-232 – Most serial ports on personal computers conform to the RS-232C or newer RS-422 and RS-423 standards. Both 9-pin and 25-pin connectors are used. A serial port is a general-purpose interface that can be used for almost any type of device, including modems, mice, and printers. Many network devices use RJ-45 connectors that also conform to the RS-232 standard. The figure shows an example of an RS-232 connector.
V.35 – Typically used for modem-to-multiplexer communication, this ITU standard for high-speed, synchronous data exchange combines the bandwidth of several telephone circuits. In the U.S., V.35 is the interface standard used by most routers and DSUs that connect to T1 carriers. V.35 cables are high-speed serial assemblies designed to support higher data rates and connectivity between DTEs and DCEs over digital lines. There is more on DTEs and DCEs later in this section.
HSSI – A High-Speed Serial Interface (HSSI) supports transmission rates up to 52 Mb/s. Engineers use HSSI to connect routers on LANs with WANs over high-speed lines such as T3 lines. Engineers also use HSSI to provide high-speed connectivity between LANs, using Token Ring or Ethernet. HSSI is a DTE/DCE interface developed by Cisco Systems and T3plus Networking to address the need for high-speed communication over WAN links.
Time Division Multiplexing
Bell Laboratories invented time-division multiplexing (TDM) to maximize the amount of voice traffic carried over a medium. Before multiplexing, each telephone call required its own physical link. This was an expensive and unscalable solution.
Statistical time-division multiplexing (STDM
Statistical time-division multiplexing (STDM) was developed to overcome this inefficiency. STDM uses a variable time slot length allowing channels to compete for any free slot space. It employs a buffer memory that temporarily stores the data during periods of peak traffic. STDM does not waste high-speed line time with inactive channels using this scheme.
TDM Examples – ISDN and SONET
On a larger scale, the telecommunications industry uses the SONET or SDH standard for optical transport of TDM data.
Demarcation Point
The demarcation point marks the point where your network interfaces with the network owned by another organization. In telephone terminology, this is the interface between customer-premises equipment (CPE) and network service provider equipment. The demarcation point is the point in the network where the responsibility of the service provider ends.
DTE-DCE
Serial connection has a DTE device at one end of the connection and a DCE device at the other end. The connection between the two DCE devices is the WAN service provider transmission network. In this case:
• The CPE, which is generally a router, is the DTE. The DTE could also be a terminal, computer, printer, or fax machine if they connect directly to the service provider network.
• The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to the WAN service provider transmission link. This signal is received at the remote DCE, which decodes the signal back into a sequence of bits. The remote DCE then signals this sequence to the remote DTE.
The more common WAN protocols and where they are used is shown in the figure, following are short descriptions:
• HDLC – The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices. HDLC is now the basis for synchronous PPP used by many servers to connect to a WAN, most commonly the Internet.
• PPP – Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several Network layer protocols, such as IP and IPX. PPP also has built-in security mechanisms such as PAP and CHAP. Most of this chapter deals with PPP.
• Serial Line Internet Protocol (SLIP) – A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.
• X.25/Link Access Procedure, Balanced (LAPB) – ITU-T standard that defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a Data Link layer protocol. X.25 is a predecessor to Frame Relay.
• Frame Relay – Industry standard, switched, Data Link layer protocol that handles multiple virtual circuits. Frame Relay is a next generation protocol after X.25. Frame Relay eliminates some of the time-consuming processes (such as error correction and flow control) employed in X.25. The next chapter is devoted to Frame Relay.
• ATM – The international standard for cell relay in which devices send multiple service types (such as voice, video, or data) in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware, thereby reducing transit delays. ATM takes advantages of high-speed transmission media such as E3, SONET, and T3.
HDLC Frame Types
• Flag – The flag field initiates and terminates error checking. The frame always starts and ends with an 8-bit flag field.
What is PPP?
cisco HDLC can only work with other Cisco devices. However, when you need to connect to a non-Cisco router, you should use PPP encapsulation.
PPP encapsulation has been carefully designed to retain compatibility with most commonly used supporting hardware. PPP encapsulates data frames for transmission over Layer 2 physical links.
• The link quality management feature monitors the quality of the link. If too many errors are detected, PPP takes the link down.
• PPP supports PAP and CHAP authentication. This feature is explained and practiced in a later section.
PPP contains three main components:
• HDLC protocol for encapsulating datagrams over point-to-point links.
• Extensible Link Control Protocol (LCP) to establish, configure, and test the data link connection.
• Family of Network Control Protocols (NCPs) for establishing and configuring different Network layer protocols. PPP allows the simultaneous use of multiple Network layer protocols. Some of the more common NCPs are Internet Protocol Control Protocol, Appletalk Control Protocol, Novell IPX Control Protocol, Cisco Systems Control Protocol, SNA Control Protocol, and Compression Control Protocol.
The LCP sets up the PPP connection and its parameters, the NCPs handle higher layer protocol configurations, and the LCP terminates the PPP connection.
The LCP provides automatic configuration of the interfaces at each end, including:
• Handling varying limits on packet size
• Detecting common misconfiguration errors
• Terminating the link
• Determining when a link is functioning properly or when it is failing
PPP also uses the LCP to agree automatically on encapsulation formats (authentication, compression, error detection) as soon as the link is established.
NCPs include functional fields containing standardized codes (PPP protocol field numbers shown in the figure) to indicate the Network layer protocol that PPP encapsulates. Each NCP manages the specific needs required by its respective Network layer protocols. The various NCP components encapsulate and negotiate options for multiple Network layer protocols.
IPCP negotiates two options:
• Compression – Allows devices to negotiate an algorithm to compress TCP and IP headers and save bandwidth. Van Jacobson TCP/IP header compression reduces the size of the TCP/IP headers to as few as 3 bytes. This can be a significant improvement on slow serial lines, particularly for interactive traffic.
• IP-Address – Allows the initiating device to specify an IP address to use for routing IP over the PPP link, or to request an IP address for the responder. Dialup network links commonly use the IP address option.
PPP may include the following LCP options:
• Authentication – Peer routers exchange authentication messages. Two authentication choices are Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). Authentication is explained in the next section.
• Compression – Increases the effective throughput on PPP connections by reducing the amount of data in the frame that must travel across the link. The protocol decompresses the frame at its destination. Two compression protocols available in Cisco routers are Stacker and Predictor.
• Error detection – Identifies fault conditions. The Quality and Magic Number options help ensure a reliable, loop-free data link. The Magic Number field helps in detecting links that are in a looped-back condition. Until the Magic-Number Configuration Option has been successfully negotiated, the Magic-Number must be transmitted as zero. Magic numbers are generated randomly at each end of the connection.
• Multilink – Cisco IOS Release 11.1 and later supports multilink PPP. This alternative provides load balancing over the router interfaces that PPP uses. Multilink PPP (also referred to as MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple physical WAN links while providing packet fragmentation and reassembly, proper sequencing, multivendor interoperability, and load balancing on inbound and outbound traffic. Multilink is not covered in this course.
• PPP Callback – To enhance security, Cisco IOS Release 11.1 and later offers callback over PPP. With this LCP option, a Cisco router can act as a callback client or a callback server. The client makes the initial call, requests that the server call it back, and terminates its initial call. The callback router answers the initial call and makes the return call to the client based on its configuration statements. The command is ppp callback [accept | request].
You can use the debug ppp error command to display protocol errors and error statistics associated with PPP connection negotiation and operation.
PAP Authentication Protocol
PAP is a very basic two-way process. There is no encryption-the username and password are sent in plain text. If it is accepted, the connection is allowed. CHAP is more secure than PAP. It involves a three-way exchange of a shared secret.
Challenge Handshake Authentication Protocol (CHAP)
Unlike PAP, which only authenticates once, CHAP conducts periodic challenges to make sure that the remote node still has a valid password value.
