Here are the three major characteristics of WANs:
• WANs generally connect devices that are separated by a broader geographical area than can be served by a LAN.
• WANs use the services of carriers, such as telephone companies, cable companies, satellite systems, and network providers.
• WANs use serial connections of various types to provide access to bandwidth over large geographic areas.
LAN technologies provide both speed and cost-efficiency for the transmission of data in organizations over relatively small geographic areas.
let us look at an example of a fictitious company called Span Engineering, and watch how its network requirements change as the company grows from a small local business into a global enterprise.
Campus (Multiple LANs)
Five years later, Span Engineering has grown rapidly. As the owners had hoped, the company was contracted to design and implement a full-sized waste conversion facility soon after the successful implementation of their first pilot plant.
Branch (WAN)
Another five years later, Span Engineering has been so successful with its patented process that demand for its services has skyrocketed, and new projects are now being built in other cities. To manage those projects, the company has opened small branch offices closer to the project sites.
Distributed (Global)
Span Engineering has now been in business for 20 years and has grown to thousands of employees distributed in offices worldwide. The cost of the network and its related services is now a significant expense.
Enterprise Architecture
To help prevent this situation, Cisco has developed a recommended architecture called the Cisco Enterprise Architecture that has relevance to the different stages of growth of a business.
This architecture is designed to provide network planners with a roadmap for network growth as the business moves through different stages.
Enterprise Campus Architecture
A campus network is a building or group of buildings connected into one enterprise network that consists of many LANs.
Enterprise Edge Architecture
This module offers connectivity to voice, video, and data services outside the enterprise. This module enables the enterprise to use Internet and partner resources, and provide resources for its customers.
The Enterprise WAN and Metropolitan-Area Network (MAN) Architecture, which the technologies covered later in this course are relevant to, are considered part of this module.
Enterprise Branch Architecture
This module allows businesses to extend the applications and services found at the campus to thousands of remote locations and users or to a small group of branches. Much of this course focuses on the technologies that are often implemented in this module.
Enterprise Data Center Architecture
Data centers are responsible for managing and maintaining the many data systems that are vital to modern business operations. Employees, partners, and customers rely on data and resources in the data center to effectively create, collaborate, and interact.
Enterprise Teleworker Architecture
Many businesses today offer a flexible work environment to their employees, allowing them to telecommute from home offices. To telecommute is to leverage the network resources of the enterprise from home.
WANs and the OSI Model
As described in relation to the OSI reference model, WAN operations focus primarily on Layer 1 and Layer 2. WAN access standards typically describe both Physical layer delivery methods and Data Link layer requirements, including physical addressing, flow control, and encapsulation. WAN access standards are defined and managed by a number of recognized authorities, including the International Organization for Standardization (ISO), the Telecommunication Industry Association (TIA), and the Electronic Industries Alliance (EIA).
The Physical layer (OSI Layer 1) protocols describe how to provide electrical, mechanical, operational, and functional connections to the services of a communications service provider.
The Data Link layer (OSI Layer 2) protocols define how data is encapsulated for transmission toward a remote location and the mechanisms for transferring the resulting frames. A variety of different technologies are used, such as Frame Relay and ATM. Some of these protocols use the same basic framing mechanism, High-Level Data Link Control (HDLC), an ISO standard, or one of its subsets or variants.
commonly used to describe physical WAN connections, including:
Customer Premises Equipment (CPE)-The devices and inside wiring located at the premises of the subscriber and connected with a telecommunication channel of a carrier. The subscriber either owns the CPE or leases the CPE from the service provider. A subscriber, in this context, is a company that arranges for WAN services from a service provider or carrier.
Data Communications Equipment (DCE)-Also called data circuit-terminating equipment, the DCE consists of devices that put data on the local loop. The DCE primarily provides an interface to connect subscribers to a communication link on the WAN cloud.
Data Terminal Equipment (DTE)-The customer devices that pass the data from a customer network or host computer for transmission over the WAN. The DTE connects to the local loop through the DCE.
Demarcation Point-A point established in a building or complex to separate customer equipment from service provider equipment. Physically, the demarcation point is the cabling junction box, located on the customer premises, that connects the CPE wiring to the local loop. It is usually placed for easy access by a technician. The demarcation point is the place access by a technician.
Local Loop-The copper or fiber telephone cable that connects the CPE at the subscriber site to the CO of the service provider. The local loop is also sometimes called the “last-mile.”
Central Office (CO)-A local service provider facility or building where local telephone cables link to long-haul, all-digital, fiber-optic communications lines through a system of switches and other equipment.
WAN Devices
Modem-Modulates an analog carrier signal to encode digital information, and also demodulates the carrier signal to decode the transmitted information.
CSU/DSU-Digital lines, such as T1 or T3 carrier lines, require a channel service unit (CSU) and a data service unit (DSU). The two are often combined into a single piece of equipment, called the CSU/DSU. The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the T-carrier line frames into frames that the LAN can interpret and vice versa.
Access server-Concentrates dial-in and dial-out user communications. An access server may have a mixture of analog and digital interfaces and support hundreds of simultaneous users.
WAN switch-A multiport internetworking device used in carrier networks. These devices typically switch traffic such as Frame Relay, ATM, or X.25, and operate at the Data Link layer of the OSI reference model. Public switched telephone network (PSTN) switches may also be used within the cloud for circuit-switched connections like Integrated Services Digital Network (ISDN) or analog dialup.
Router-Provides internetworking and WAN access interface ports that are used to connect to the service provider network. These interfaces may be serial connections or other WAN interfaces.
Core router-A router that resides within the middle or backbone of the WAN rather than at its periphery. To fulfill this role, a router must be able to support multiple telecommunications interfaces of the highest speed in use in the WAN core, and it must be able to forward IP packets at full speed on all of those interfaces.
WAN Physical Layer Standards
WAN Physical layer protocols describe how to provide electrical, mechanical, operational, and functional connections for WAN services.
The DTE/DCE interface uses various Physical layer protocols, including:
EIA/TIA-232-This protocol allows signal speeds of up to 64 kb/s on a 25-pin D-connector over short distances. It was formerly known as RS-232. The ITU-T V.24 specification is effectively the same.
EIA/TIA-449/530-This protocol is a faster (up to 2 Mb/s) version of EIA/TIA-232. It uses a 36-pin D-connector and is capable of longer cable runs. There are several versions. This standard is also known as RS422 and RS-423.
EIA/TIA-612/613-This standard describes the High-Speed Serial Interface (HSSI) protocol, which provides access to services up to 52 Mb/s on a 60-pin D-connector.
V.35-This is the ITU-T standard for synchronous communications between a network access device and a packet network. Originally specified to support data rates of 48 kb/s, it now supports speeds of up to 2.048 Mb/s using a 34-pin rectangular connector.
X.21-This protocol is an ITU-T standard for synchronous digital communications. It uses a 15-pin D-connector.
The most common WAN data-link protocols are:
HDLC
PPP
Frame Relay
ATM
WAN Encapsulation
Data from the Network layer is passed to the Data Link layer for delivery on a physical link, which is normally point-to-point on a WAN connection.
The internal path taken by the circuit between exchanges is shared by a number of conversations. Time-division multiplexing (TDM) gives each conversation a share of the connection in turn. TDM assures that a fixed capacity connection is made available to the subscriber.
PSTN and ISDN are two types of circuit-switching technology that may be used to implement a WAN in an enterprise setting.
Packet Switching
In contrast to circuit switching, packet switching splits traffic data into packets that are routed over a shared network. Packet-switching networks do not require a circuit to be established, and they allow many pairs of nodes to communicate over the same channel.
There are two approaches to this link determination, connectionless or connection-oriented.
Connectionless systems, such as the Internet, carry full addressing information in each packet. Each switch must evaluate the address to determine where to send the packet.
Connection-oriented systems predetermine the route for a packet, and each packet only has to carry an identifier. In the case of Frame Relay, these are called Data Link Connection Identifiers (DLCIs). The switch determines the onward route by looking up the identifier in tables held in memory.
Virtual Circuits
Packet-switched networks may establish routes through the switches for particular end-to-end connections. These routes are called virtual circuits.
Two types of VCs exist:
• Permanent Virtual Circuit (PVC)-A permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with establishing and terminating VCs, but they increase costs because of constant virtual circuit availability.
• Switched Virtual Circuit (SVC)-A VC that is dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination. The establishment phase involves creating the VC between the source and destination devices. Data transfer involves transmitting data between the devices over the VC, and the circuit termination phase involves tearing down the VC between the source and destination devices.
Switched communication links can be either circuit switched or packet switched.
Circuit-switched communication links-Circuit switching dynamically establishes a dedicated virtual connection for voice or data between a sender and a receiver. Before communication can start, it is necessary to establish the connection through the network of the service provider. Examples of circuit-switched communication links are analog dialup (PSTN) and ISDN.
Packet-switched communication links-Many WAN users do not make efficient use of the fixed bandwidth that is available with dedicated, switched, or permanent circuits because the data flow fluctuates. Communications providers have data networks available to more appropriately service these users. In packet-switched networks, the data is transmitted in labeled frames, cells, or packets. Packet-switched communication links include Frame Relay, ATM, X.25, and Metro Ethernet.
There are two types of ISDN interfaces:
Basic Rate Interface (BRI)-ISDN is intended for the home and small enterprise and provides two 64 kb/s B channels and a 16 kb/s D channel. The BRI D channel is designed for control and often underused, because it has only two B channels to control. Therefore, some providers allow the D channel to carry data at low bit rates, such as X.25 connections at 9.6 kb/s.
Primary Rate Interface (PRI)-ISDN is also available for larger installations. PRI delivers 23 B channels with 64 kb/s and one D channel with 64 kb/s in North America, for a total bit rate of up to 1.544 Mb/s. This includes some additional overhead for synchronization. In Europe, Australia, and other parts of the world, ISDN PRI provides 30 B channels and one D channel, for a total bit rate of up to 2.048 Mb/s, including synchronization overhead. In North America, PRI corresponds to a T1 connection. The rate of international PRI corresponds to an E1 or J1 connection.
X.25
X.25 is a legacy Network layer protocol that provides subscribers with a network address. Virtual circuits can be established through the network with call request packets to the target address. The resulting SVC is identified by a channel number.
Frame Relay
Although the network layout appears similar to X.25, Frame Relay differs from X.25 in several ways. Most importantly, it is a much simpler protocol that works at the Data Link layer rather than the Network layer. Frame Relay implements no error or flow control. The simplified handling of frames leads to reduced latency, and measures taken to avoid frame build-up at intermediate switches help reduce jitter. Frame Relay offers data rates up to 4 Mb/s, with some providers offering even higher rates.
ATM
Asynchronous Transfer Mode (ATM) technology is capable of transferring voice, video, and data through private and public networks. It is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The ATM cell contains a 5 byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted.
DSL
DSL technology is an always-on connection technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, and provides IP services to subscribers. A DSL modem converts an Ethernet signal from the user device to a DSL signal, which is transmitted to the central office.
Cable Modem
Coaxial cable is widely used in urban areas to distribute television signals. Network access is available from some cable television networks. This allows for greater bandwidth than the conventional telephone local loop.
Broadband Wireless
Wireless technology uses the unlicensed radio spectrum to send and receive data. The unlicensed spectrum is accessible to anyone who has a wireless router and wireless technology in the device they are using.
Municipal WiFi-Many cities have begun setting up municipal wireless networks. Some of these networks provide high-speed Internet access for free or for substantially less than the price of other broadband services. Others are for city use only, allowing police and fire departments and other city employees to do certain aspects of their jobs remotely.
WiMAX-Worldwide Interoperability for Microwave Access (WiMAX) is a new technology that is just beginning to come into use. It is described in the IEEE standard 802.16. WiMAX provides high-speed broadband service with wireless access and provides broad coverage like a cell phone network rather than through small WiFi hotspots.
Satellite Internet-Typically used by rural users where cable and DSL are not available. A satellite dish provides two-way (upload and download) data communications. The upload speed is about one-tenth of the 500 kb/s download speed. Cable and DSL have higher download speeds, but satellite systems are about 10 times faster than an analog modem.
VPN Technology
A VPN is an encrypted connection between private networks over a public network such as the Internet. Instead of using a dedicated Layer 2 connection such as a leased line, a VPN uses virtual connections called VPN tunnels, which are routed through the Internet from the private network of the company to the remote site or employee host.
There are two types of VPN access:
• Site-to-site VPNs-Site-to-site VPNs connect entire networks to each other, for example, they can connect a branch office network to a company headquarters network, as shown in the figure. Each site is equipped with a VPN gateway, such as a router, firewall, VPN concentrator, or security appliance. In the figure, a remote branch office uses a site-to-site-VPN to connect with the corporate head office.
• Remote-access VPNs-Remote-access VPNs enable individual hosts, such as telecommuters, mobile users, and extranet consumers, to access a company network securely over the Internet. Each host typically has VPN client software loaded or uses a web-based client.
Metro Ethernet
Metro Ethernet is a rapidly maturing networking technology that broadens Ethernet to the public networks run by telecommunications companies. IP-aware Ethernet switches enable service providers to offer enterprises converged voice, data, and video services such as IP telephony, video streaming, imaging, and data storage
Benefits of Metro Ethernet include:
• Reduced expenses and administration-Metro Ethernet provides a switched, high-bandwidth Layer 2 network capable of managing data, voice, and video all on the same infrastructure. This characteristic increases bandwidth and eliminates expensive conversions to ATM and Frame Relay. The technology enables businesses to inexpensively connect numerous sites in a metropolitan area to each other and to the Internet.
• Easy integration with existing networks-Metro Ethernet connects easily to existing Ethernet LANs, reducing installation costs and time.
• Enhanced business productivity-Metro Ethernet enables businesses to take advantage of productivity-enhancing IP applications that are difficult to implement on TDM or Frame Relay networks, such as hosted IP communications, VoIP, and streaming and broadcast video.
