Ex4 Chapter 1 - Introduction to WANs

A WAN is a data communications network that operates beyond the geographic scope of a LAN.

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.

Business Networks:
 - Small Office (Single LAN)
 - Campus (Multiple LANs)
 - Branch (WAN)
 - Distributed (Global)

The Hierarchical Network Model

 - Access layer - Grants user access to network devices. In a network campus, the access layer generally incorporates switched LAN devices with ports that provide connectivity to workstations and servers. In the WAN environment, it may provide teleworkers or remote sites access to the corporate network across WAN technology.
 - Distribution layer - Aggregates the wiring closets, using switches to segment workgroups and isolate network problems in a campus environment. Similarly, the distribution layer aggregates WAN connections at the edge of the campus and provides policy-based connectivity.
 - Core layer (also referred to as the backbone) - A high-speed backbone that is designed to switch packets as fast as possible. Because the core is critical for connectivity, it must provide a high level of availability and adapt to changes very quickly. It also provides scalability and fast convergence.

WAN Terminology
- 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 where the responsibility for the connection changes from the user to the service provider. This is very important because when problems arise, it is necessary to determine whether the user or the service provider is responsible for troubleshooting or repair.
 - 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

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WAN connectionDTEDCE DCE Location
Leased Line
CSU/DSUCustomer
PSTN (analog)
Dial up modemCustomer
ISDN (digital)
TA /NT1Customer
X.25
CSU/DSUCustomer
ATM
ATM switchService Provider
Frame Relay
Frame Relay modemService Provider
DSL
DSL modemCustomer
Cable HFC
Cable modemCustomer

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).
Access server - Concentrates dial-in and dial-out user communications.
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.
 
Router - Provides internetworking and WAN access interface ports that are used to connect to the service provider network.
Core router - A router that resides within the middle or backbone of the WAN rather than at its periphery.

WAN Physical Layer Standards
EIA/TIA-232 (aka RS-232) -This protocol allows signal speeds of up to 64 kb/s on a 25-pin D-connector over short distances. 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.



Data Link Protocols
 - HDLC
 - PPP
 - Frame Relay
 - ATM

Another Data Link layer protocol is the Multiprotocol Label Switching (MPLS) protocol. MPLS is increasingly being deployed by service providers to provide an economical solution to carry circuit-switched as well as packet-switched network traffic. It can operate over any existing infrastructure, such as IP, Frame Relay, ATM, or Ethernet. It sits between Layer 2 and Layer 3 and is sometimes referred to as a Layer 2.5 protocol.

WAN switching concepts

Circuit Switching
network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate.
Packet Switching:
    -
Connectionless systems, such as the Internet, carry full addressing information in each 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).

Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks.

Virtual Circuits
Packet-switched networks may establish routes through the switches for particular end-to-end connections. These routes are called virtual circuits:
 - 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;
- 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.  SVCs are used in situations in which data transmission between devices is intermittent, largely to save costs.

X.25 - Older low-capacity WAN technology with a maximum speed of 48 kbps - 2mbps(however, public networks are usually low capacity with speeds rarely exceeding above 64 kb/s. ), typically used in dialup mode with point-of-sale card readers to validate transactions on a central computer.For these applications, the low bandwidth and high latency are not a concern, and the low cost makes X.25 affordable.
Frame Relay has replaced X.25 at many service provider locations.

Frame Relay - Layer 2 WAN protocol that typically offers data rates of 4 Mb/s or higher. It provides permanent, shared, medium-bandwidth connectivity using virtual circuits capable of carrying both voice and data traffic.VCs are uniquely identified by a DLCI, which ensures bidirectional communication from one DTE device to another.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. Usualy uses PVC.

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. ATM was designed to be extremely scalable and can support link speeds of T1/E1 to OC-12 (622 Mb/s) and higher.

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.
Benefits of VPN include the following:
Cost savings
Security
Scalability
Compatibility with broadband technology

Metro Ethernet
 By extending Ethernet to the metropolitan area, companies can provide their remote offices with reliable access to applications and data on the corporate headquarters LAN.
Benefits of Metro Ethernet include:
Reduced expenses and administration
Easy integration with existing networks
Enhanced business productivity

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