2.1.2 WAN devices / 2.1.3 WAN Standards

2.1.2 WAN devices
WANs are groups of LANs connected together with communications links from a service provider. Because the communications links cannot plug directly into the LAN, it is necessary to identify the various pieces of interfacing equipment.
LAN-based computers with data to transmit send data to a router that contains both LAN and WAN interfaces. The router will use the Layer 3 address information to deliver the data on the appropriate WAN interface. Routers are active and intelligent network devices and therefore can participate in network management. Routers manage networks by providing dynamic control over resources and supporting the tasks and goals for networks. Some of these goals are connectivity, reliable performance, management control, and flexibility.
The communications link needs signals in an appropriate format. For digital lines, a channel service unit (CSU) and a data service unit (DSU) are required. The two are often combined into a single piece of equipment, called the CSU/DSU. The CSU/DSU may also be built into the interface card in the router.
A modem is needed if the local loop is analog rather than digital. Modems transmit data over voice-grade telephone lines by modulating and demodulating the signal. The digital signals are superimposed on an analog voice signal that is modulated for transmission. The modulated signal can be heard as a series of whistles by turning on the internal modem speaker. At the receiving end the analog signals are returned to their digital form, or demodulated.
When ISDN is used as the communications link, all equipment attached to the ISDN bus must be ISDN-compatible. Compatibility is generally built into the computer interface for direct dial connections, or the router interface for LAN to WAN connections. Older equipment without an ISDN interface requires an ISDN terminal adapter (TA) for ISDN compatibility.
Communication servers concentrate dial-in user communication and remote access to a LAN. They may have a mixture of analog and digital (ISDN) interfaces and support hundreds of simultaneous users.
WAN Standards
2.1.3
WANs use the OSI reference model, but focus mainly on Layer 1 and Layer 2. WAN standards typically describe both physical layer delivery methods and data link layer requirements, including physical addressing, flow control, and encapsulation. WAN standards are defined and managed by a number of recognized authorities.
The physical layer protocols describe how to provide electrical, mechanical, operational, and functional connections to the services provided by a communications service provider. Some of the common physical layer standards are listed in Figure , and their connectors illustrated in Figure .
The data link layer protocols define how data is encapsulated for transmission to remote sites, and the mechanisms for transferring the resulting frames. A variety of different technologies are used, such as ISDN, Frame Relay or Asynchronous Transfer Mode (ATM). These protocols use the same basic framing mechanism, high-level data link control (HDLC), an ISO standard, or one of its sub-sets or variants.

2.1 WAN Technologies Overview / 2.1.1 WAN Technology

2.1 WAN Technologies Overview 
2.1.1 WAN Technology
A WAN is a data communications network that operates beyond the geographic scope of a LAN. One primary difference between a WAN and a LAN is that a company or organization must subscribe to an outside WAN service provider in order to use WAN carrier network services. A WAN uses data links provided by carrier services to access the Internet and connect the locations of an organization to each other, to locations of other organizations, to external services, and to remote users. WANs generally carry a variety of traffic types, such as voice, data, and video. Telephone and data services are the most commonly used WAN services.
Devices on the subscriber premises are called customer premises equipment (CPE). The subscriber owns the CPE or leases the CPE from the service provider. A copper or fiber cable connects the CPE to the service provider’s nearest exchange or central office (CO). This cabling is often called the local loop, or "last-mile". A dialed call is connected locally to other local loops, or non-locally through a trunk to a primary center. It then goes to a sectional center and on to a regional or international carrier center as the call travels to its destination.
In order for the local loop to carry data, a device such as a modem is needed to prepare the data for transmission. Devices that put data on the local loop are called data circuit-terminating equipment, or data communications equipment (DCE). The customer devices that pass the data to the DCE are called data terminal equipment (DTE). The DCE primarily provides an interface for the DTE into the communication link on the WAN cloud. The DTE/DCE interface uses various physical layer protocols, such as High-Speed Serial Interface (HSSI) and V.35. These protocols establish the codes and electrical parameters the devices use to communicate with each other.
WAN links are provided at various speeds measured in bits per second (bps), kilobits per second (kbps or 1000 bps), megabits per second (Mbps or 1000 kbps) or gigabits per second (Gbps or 1000 Mbps). The bps values are generally full duplex. This means that an E1 line can carry 2 Mbps, or a T1 can carry 1.5 Mbps, in each direction simultaneously.

Module 2: WAN Technologies / Overview

Module 2: WAN Technologies/Overview
As the enterprise grows beyond a single location, it is necessary to interconnect the LANs in the various branches to form a wide-area network (WAN). This module examines some of the options available for these interconnections, the hardware needed to implement them, and the terminology used to discuss them.
There are many options currently available today for implementing WAN solutions. They differ in technology, speed, and cost. Familiarity with these technologies is an important part of network design and evaluation.
If all data traffic in an enterprise is within a single building, a LAN meets the needs of the organization. Buildings can be interconnected with high-speed data links to form a campus LAN if data must flow between buildings on a single campus. However, a WAN is needed to carry data if it must be transferred between geographically separate locations. Individual remote access to the LAN and connection of the LAN to the Internet are separate study topics, and will not be considered here.
Most students will not have the opportunity to design a new WAN, but many will be involved in designing additions and upgrades to existing WANs, and will be able to apply the techniques learned in this module.
Students completing this module should be able to:

• Differentiate between a LAN and WAN
• Identify the devices used in a WAN
• List WAN standards
• Describe WAN encapsulation
• Classify the various WAN link options
• Differentiate between packet-switched and circuit-switched WAN technologies
• Compare and contrast current WAN technologies
• Describe equipment involved in the implementation of various WAN services
• Recommend a WAN service to an organization based on its needs
• Describe DSL and cable modem connectivity basics
• Describe a methodical procedure for designing WANs
• Compare and contrast WAN topologies
• Compare and contrast WAN design models

Recommend a WAN design to an organization based on its needs 

Summary Module 1

Summary Module 1

Before students begin Module 2, they must be able to explain the concepts of NAT, PAT, and DHCP.
Online assessment options include the end-of-module online quiz in the curriculum and the online Module 1 exam. From memory, students should be able to fill in Drag and Drop assessments for Basic NAT Operation and NAT with Overload. Students should also be able to complete a Checkbox activity for the advantages and disadvantages of NAT.
A comprehension of the following key points should have been achieved:

• Private addresses are for private, internal use and should never be routed by a public Internet router.
• NAT alters the IP header of a packet so that the destination address, the source address, or both addresses are replaced with different addresses.
• PAT uses unique source port numbers on the inside global IP address to distinguish between translations.
• NAT translations can occur dynamically or statically and can be used for a variety of uses.
• NAT and PAT may be configured for static translation, dynamic translation, and overloading.
• The process for verifying NAT and PAT configuration includes the clear and show commands.
• The debug ip nat command is used for troubleshooting NAT and PAT configuration.
• NAT has advantages and disadvantages.
• DHCP works in a client/server mode. This enables clients to obtain IP configurations from a DHCP server.
• BOOTP is the predecessor of DHCP and shares some operational characteristics with DHCP. However, BOOTP is not dynamic.
• A DHCP server manages pools of IP addresses and associated parameters. Each pool is dedicated to an individual logical IP subnet.
• The DHCP client configuration process has four steps.
• A DCHP server is usually configured to assign more than IP addresses.
• The show ip dhcp binding command is used to verify DHCP operation.
• The debug ip dhcp server events command is used to troubleshoot DHCP.

When a DHCP server and a client are not on the same segment and are separated by a router, the ip helper-address command is used to relay broadcast requests. 

An understanding of the following key points should have been achieved:

• Private addresses are for private, internal use and should never be routed by a public Internet router.
• NAT alters the IP header of a packet so that the destination address, the source address, or both addresses are replaced with different addresses.
• PAT uses unique source port numbers on the inside global IP address to distinguish between translations.
• NAT translations can occur dynamically or statically and can be used for a variety of uses.
• NAT and PAT may be configured for static translation, dynamic translation, and overloading.
• The process for verifying NAT and PAT configuration include the clear and show commands.
• The debug ip nat command is used for troubleshooting NAT and PAT configuration.
• NAT has advantages and disadvantages.
• DHCP works in a client/server mode, enabling clients to obtain IP configurations from a DHCP server.
• BOOTP is the predecessor of DHCP and shares some operational characteristics with DHCP, but BOOTP is not dynamic.
• A DHCP server manages pools of IP addresses and associated parameters. Each pool is dedicated to an individual logical IP subnet.
• The DHCP client configuration process has four steps.
• Usually, a DCHP server is configured to assign more than IP addresses.
• The show ip dhcp binding command is used to verify DHCP operation.
• The debug ip dhcp server events command is used for troubleshooting DHCP.

When a DHCP server and a client are not on the same segment and are separated by a router, the ip helper-address command is used to relay broadcast requests. 

Verifying the Catalyst switch default configuration

Verifying the Catalyst switch default configuration 

Switch name	Command	Explanation
SydneySwitch#	show version	Displays the configuration of the system hardware, software version, names, and sources of configuration files and boot images
SydneySwitch#	show running-configuration	Displays the current active configuration file of the switch
SydneySwitch#	show interfaces	Displays the statistics for all interfaces configured on the switch
SydneySwitch#	show ip	Displays the IP address, subnet mask, and default gateway

1.2.8 DHCP Relay

1.2.8 DHCP Relay

DHCP clients use IP broadcasts to find the DHCP server on the segment. What happens when the server and the client are not on the same segment and are separated by a router? Routers do not forward these broadcasts.
DHCP is not the only critical service that uses broadcasts. Cisco routers and other devices may use broadcasts to locate TFTP servers. Some clients may need to broadcast to locate a TACACS server. A TACACS server is a security server. Typically, in a complex hierarchical network, clients do not reside on the same subnet as key servers. Such remote clients will broadcast to locate these servers. However, routers, by default, will not forward client broadcasts beyond their subnet.
Because some clients are useless without services such as DHCP, one of two choices must be implemented. The administrator will need to place servers on all subnets or use the Cisco IOS helper address feature. Running services such as DHCP or DNS on several computers creates overhead and administrative difficulties making the first option inefficient. When possible, administrators should use the ip helper-address command to relay broadcast requests for these key UDP services.
By using the helper address feature, a router can be configured to accept a broadcast request for a UDP service and then forward it as a unicast to a specific IP address. By default, the ip helper-address command forwards the following eight UDP services:

• Time
• TACACS
• DNS
• BOOTP/DHCP Server
• BOOTP/DHCP Client
• TFTP
• NetBIOS Name Service
• NetBIOS datagram Service

In the particular case of DHCP, a client broadcasts a DHCPDISCOVER packet on its local segment. This packet is picked up by the gateway. If a helper-address is configured, the DHCP packet is forwarded to the specified address. Before forwarding the packet, the router fills in the GIADDR field of the packet with the IP address of the router for that segment. This address will then be the gateway address for the DHCP client, when it gets the IP address. 
The DHCP server receives the discover packet. The server uses the GIADDR field to index into the list of address pools in order to find one which has the gateway address set to the value in GIADDR. This pool is then used to supply the client with its IP address.

1.2.6 Verifying DHCP operation / 1.2.7 Troubleshooting DHCP

1.2.6 Verifying DHCP operation
To verify the operation of DHCP, the command show ip dhcp binding can be used. This displays a list of all bindings created by the DHCP service.
To verify that messages are being received or sent by the router, use the command show ip dhcp server statistics. This will display count information regarding the number of DHCP messages that have been sent and received. 
1.2.7Troubleshooting DHCP
To troubleshoot the operation of the DHCP server, the command debug ip dhcp server events can be used. This command will show that the server periodically checks to see if any leases have expired. Also displayed are the processes of addresses being returned and addresses being allocated.