Chapter 3: Point-to-Point Connections Part 2

This part will cover: Demarcation points, DTE-DCEs, Serial cables, Smart Serial Cables, Serial bandwidth, and Overview of WAN encapsulation protocols.

 

Demarcation Point

Prior to deregulation, telephone companies owned the local loop, wiring, and equipment at the customers location. Deregulation forced companies to unbundle local loop infrastructure and allow other suppliers to provide equipment and services. Demarcation Points became necessary to establish who (customer or provider) owned and were responsible for which hardware.

The picture shows demarcation points in the United States and Internationally. In the US the CSU/DSU is the subscribers responsibility. This hardware may be provided by the service provider.

©2014 Cisco Press, Connecting Networks Companion Guide

 

DTE-DCE

A DTE is usually a CPE router or could be anything directly connecting to the service provider network. A DCE is Usually a modem or CSU/DSU. Chapter 2 part 2 Covered DTE-DCE.

 

Serial Cables

DCEs and DTEs were based on two types of equipment; Terminal equipment that sent and received data and the equipment that relayed data. The RS-232 standard was developed with different wiring on both ends to support the original concept. The reasons are no longer significant. Two cables remain; A DTE to DCE cable and a DTE to DTE cable. The original RS-232 only defined DCE/DTE connections.

 

DTE/DCE interface standards define four specifications:

• Mechanical/physical – number of pins and connector type
• Electrical – Defines voltage levels for 0/1
• Functional – Specifies the function that are preformed by assigning meanings to each signaling line in the interface
• Procedural- Specifies the sequence of events for transmitting data

DCE/DTE cables are shield serial cables. The router end may be a DB-60 connector.

Picture of a DB60 port.

©2014 Cisco Press, Connecting Networks Companion Guide

Null modem cables were developed for DTE to DTE connections. They are used for connecting devices without a modem using a RS-232 connection. The transmit (Tx) and receive (Rx) lines are cross linked. A clock signal is still required. It can come from an external device or by configuring one of the DTE devices as a DCE.

This picture shows the pin-out of the Null modem cable. 

©2014 Cisco Press, Connecting Networks Companion Guide

Smart Serial Cable support higher port densities in a smaller form factor. They were made by Cisco and use a 26 pin connector that is more compact than DB-60.

A smart Serial Cable and a Wic-2t card showing the interface

©2014 Cisco Press, Connecting Networks Companion Guide

WAN providers or the CSU/DSU usually dictates the cable type.


Cisco supports EIA/TIA-232, 449, 530, V.35, and x.21 serial standards.

More examples of physical serial ports

©2014 Cisco Press, Connecting Networks Companion Guide

Serial Bandwidth

• Bandwidth is expressed as a digital signal level (DSx) number (DS1, DS2, DS3, ect)
• DS0 is the fundamental speed of 64kbps and is the bandwidth required for an uncompressed digital phone call.
• The speeds can be incrementally increased for faster transmission
• 24 DS0s can be bundled to get DS1, or T1 speeds. 28 DS0s bundled is a DS3 or T3 Line speeds.
• OC rates are transmission standards of sonet fiber networks. OC1 is the base rate of 51.84mbps. OC3 is three times faster than OC1 or 155.52mbps.

See also: Chapter 2 Part 4

 

Wan Encapsulation Protocols

Packets are encapsulated, by protocols, into frames for transmission over the WAN. The choice of protocol depends on the WAN technology.

 

HDLC

• Default point-to-point protocol on Cisco devices
• Used as basis for synchronous PPP

 

PPP

• Used for router to router and host to host connections
• Can be synchronous or asynchronous
• Works with IPv4/IPv6
• Built off of HDLC
• Built in security of PAP/CHAP

 

Serial Line Internet Protocol (SLIP)

• Standard protocol for Point-to-Point connections using TCP/IP
• Mostly replaced by PPP
• X.25/Link Access Procedure, Balanced (LAPB)
• ITU-T Standard for Connections between a DTE and DCE for remote terminal access and computer communications in public networks
• LAPB is a data link layer protocol
• X.25 is a predecessor to frame relay

 

Frame Relay

• Industry standard that handles multiple Virtual Circuits (VCs) over a switched network.
• Is a Data Link layer protocol
• Replaces X.25
• Eliminates time-consuming error correction and flow control from X.25

 

ATM

• International standard for cell relay
• Handles Multiple data types (Voice/Data/Video)
• Sends data in fixed-length 53byte cells
• Allows processing on hardware that reduces transit delays
• Uses High-speed media (E3/Sonet/T3)

This chapter will be continued in Part 3.