WAN CISCO Internetworking


	Internetworking Technologies Instructor: Prabul CCNA

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8. Wide Area Networks (WANs)

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WANs

A Wide Area Network (WAN) is used to interconnect Local: Area Networks (LANs) that are separated by a large; geographical distance.
A Wide Area Network predominately operates at the OSI: 1Physical and 2Data link layers.
The WAN provides a data path between routers and the: LANs that each router supports.
A WAN subscriber must know how to interface to the: WAN providers service.
Mapping associates the Layer 3 logical address with the: Layer 2 WAN address.
There are many types of WAN services that are available: to the WAN subscriber.

WANs are designed to:

Operate over large geographic areas
Allow access over serial interfaces operating at lower speeds
Provide full-time and part-time connectivity
Connect devices seperated over wide, even global areas

WAN devices:

Routers - offer many services including internetworking and WAN interface ports
WAN Bandwidth Switchs - connect to WAN bandwidth for voice, data, and video communication
Modems (CSU/DSU) (TA/NT1) - interface voice-grade services which include:
- Channel Service Units/Digital Service Units (CSU/DSU) that interface T1/E1 services
- Terminal Adapters/Network Termination 1 (TA/NT1) that interface Integrated Services Digital Network (ISDN) services.
Communication Servers - concentrate dialin and dial-out user communication

Regional Bell Operating Company (RBOC) - Local or regional telephone company that owns and operates telephone lines and switches in one of seven U.S. regions. The RBOCs were created by the divestiture of AT&T. Also called Bell Operating Company (BOC).

Post, Telephone, and Telegraph (PTT) - Government agency that provides telephone services. PTTs exist in most areas outside North America and provide both local and long-distance telephone services.

Modem (Modulator-demodulator) - Device that converts digital and analog signals. At the source, a modem converts digital signals to a form suitable for transmission over analog communication facilities. At the destination, the analog signals are returned to their digital form. Modems allow data to be transmitted over voice-grade telephone lines.

Channel service unit (CSU) - Digital interface device that connects end-user equipment to the local digital telephone loop. Often referred to together with DSU, as CSU/DSU.

Data service unit (DSU) - Device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. The DSU is also responsible for such functions as signal timing. Often referred to together with CSU, as CSU/DSU.

T1 - Digital WAN carrier facility. T1 transmits DS-1-formatted data at 1.544 Mbps through the telephone-switching network, using AMI or B8ZS coding. Compare with E1.

Terminal adapter (TA) - Device used to connect ISDN BRI connections to existing interfaces such as EIA/TIA-232. Essentially, an ISDN modem.

Communication Server - Communications processor that connects asynchronous devices to a LAN or WAN through network and terminal emulation software. Performs only asynchronous routing of IP and IPX.

NOTE: WANs use the OSI layered approach to encapsulation just like LAN's but are mainly focused on the (1)Physical and (2)Data-Link layers.

WAN Data-link protocols describe how frames are carried between systems on a single data path:

point-to-point
multi-point
multi-access switch services

WAN Standards Organizations:

ITU-T (International Telecommunication Union Telecommunication standardization sector). formerly CCITT (Committee for International Telegraph and Telephone) - An international organization that develops communication standards.
ISO (International Organization for Standardization) - International organization that is responsible for a wide range of standards, including those relevant to networking. ISO developed the OSI reference model, a popular networking reference model.
IETF (Internet Engineering Task Force) - Task force consisting of over 80 working groups responsible for developing Internet standards. The IETF operates under the auspices of ISOC.
EIA (Electronic Industries Association) - Group that specifies electrical transmission standards. The EIA and TIA have developed numerous well-known communications standards, including EIA/TIA-232 and EIA/TIA-449.

WAN standards typically describe both 'physical layer delivery methods' and 'data link layer requirements' including addressing and flow control encapsulation.

WAN Physical Layer - describes the interface between:

data terminal equipment (DTE - attached device) and the
data circuit-terminating equipment (DCE - service provider)

Services offered to the DTE are made available through a modem CSU/DSU.

DTE (Data Terminal Equipment) - Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes such devices as computers, protocol translators, and multiplexers.

DCE (Data Circuit-terminating Equipment) - ITU-T defination or (Data Communications Equipment - EIA defination) - The devices and connections of a communications network that comprise the network end of the user-to-network interface. The DCE provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE.

WAN Physical Layer interface standards:

EIA/TIA-232 - Common physical layer interface standard, developed by EIA and TIA, that supports unbalanced circuits at signal speeds of up to 64 kbps. Closely resembles the V.24 specification. Formerly known as RS-232.
EIA/TIA-449 - Popular physical layer interface developed by EIA and TIA. Essentially, a faster (up to 2 Mbps) version of EIA/TIA-232 capable of longer cable runs. Formerly called RS-449.
V.24 - ITU-T standard for a physical layer interface between DTE and DCE. V.24 is essentially the same as the EIA/TIA-232 standard.
V.35 - ITU-T standard describing a synchronous, physical layer protocol used for communications between a network access device and a packet network. V.35 is most commonly used in the United States and in Europe, and is recommended for speeds up to 48 Kbps.
X.21 - ITU-T standard for serial communications over synchronous digital lines. The X.21 protocol is used primarily in Europe and Japan.
G.703 - ITU-T electrical and mechanical specifications for connections between telephone company equipment and DTE using BNC connectors and operating at E1 data rates.
EIA-530 - Refers to two electrical implementations of EIA/TIA-449: RS-422 (for balanced transmission) and RS-423 (for unbalanced transmission).

Data Link Layer - WAN Protocols:

HDLC - High-Level Data Link Control
Frame Relay - Simplified version of HDLC framing
PPP - Point-to-Point Protocol
ISDN - Integrated Service Digital Network

Data Link Layer WAN Protocols support both connectionless and connection-oriented higher layer protocols.

HDLC (High-Level Data Link Control) - Bit-oriented synchronous data link layer protocol developed by ISO. Derived from SDLC, HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums. Supports both point-to-point and multi-point configurations.

Frame Relay - Industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement. By using a simplified framing with no error correction mechanisms over high-quality digital facilities, Frame Relay can transmit data very rapidly, compared to these other WAN protocols. Frame Relay uses Layer 2 identifiers and Permanent Virtual Circuits (PVCs).

PPP (Point-to-Point Protocol) - A successor to SLIP, PPP provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP contains a 'protocol field' to identify the network-layer protocol.

ISDN (Integrated Services Digital Network) - Communication protocol, offered by telephone companies, that permits telephone networks to carry data, voice, and other source traffic.

Overview of Wide Area Services

Must subscribe to an outside WAN provider to use network resources
Basic telephone service is the most commonly used WAN service
Telephone service and data service are connected from the building Point of Presence (POP) to the WAN provider's Central Office (CO).

WAN provider services into three main types:

Call setup service sets up and clears calls between telephone users (SS7) (Also called signaling, call setup)
Time-division multiplexing (TDM) - Information from many sources has bandwidth allocation on a single media.
X.25 or Frame Relay service - Information contained in packets or frames shares nondedicated bandwidth with other WAN subscribers' frames.

X.25 packet switching uses Layer 3 routing with sender and receiver addressing contained in the packet. X.25 can use Switched Virtual Circuits (SVCs), with some initial delay for call setup, or Permanent Virtual Circuits (PVCs) which avoid delays for call setup.

Frame Relay uses Layer 2 identifiers and Permanent Virtual Circuits (PVCs).

POP (Point of Presence) - is the point of interconnection between the communication facilities provided by the telephone company and the building's main distribution facility.

CO (Central office) - Local telephone company office to which all local loops in a given area connect and in which circuit switching of subscriber lines occurs.

SS7 (Signaling System number 7) - Also called signaling, Call Setup uses a separate telephone channel not used for other traffic. Standard CS system used with BISDN and ISDN. Developed by Bellcore.

TDM (Time-division multiplexing) - Technique in which information from multiple channels can be allocated bandwidth on a single wire based on preassigned time slots. Bandwidth is allocated to each channel regardless of whether the station has data to transmit.

X.25 - ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in PDNs (Public Data Networks). X.25 specifies LAPB, a data link layer protocol, and PLP, a network layer protocol. Frame Relay has to some degree superseded X.25.

Interfacing WAN Service Providers:

Provider gives connection requirements to subscriber
CPE (Customer premises equipment) - Terminating equipment, such as terminals, telephones, and modems, supplied by the telephone company, installed at customer sites, and connected to the telephone company network.
Demarc (demarcation) - The point at which the CPE ends and the local loop portion of the service begins. Often occurs at the Point of Presence (POP) of a building.
Local loop (or last-mile) - Cabling (usually copper wiring) that extends from the demarc into the WAN service provider’s central office.
Central office (CO) switch - A switching facility that provides the nearest point of presence for the provider’s WAN service.
Toll network - The collective switches and facilities (called trunks) inside the WAN provider's cloud.

Subscriber to Provider Interface:

DTE/DCE - The point where responsibility passes
DTE - End of user's device on the WAN link
- Typically, DTE is the router
DCE - End of the WAN provider's side of the communications facility
- typically, Modem, CSU/DSU, TA/NT1
- Device used to convert the user data from the DTE into a form acceptable to the WAN service's facility
- Primarily provides an interface for the DTE into the communication link in the WAN cloud

The WAN path between the DTEs is called:

Link - Network communications channel consisting of a circuit or transmission path and all related equipment between a sender and a receiver. Most often used to refer to a WAN connection. Sometimes referred to as a line or a transmission link.
Circuit - Communications path between two or more points.
Channel - A communication path. Multiple channels can be multiplexed over a single cable in certain environments.

Using WAN services with routers:

Use Switched or relayed services. (Frame Relay, ISDN)
Use Protocols that connect peer devices. (HDLC or PPP encapsulation)
Use DDR as a trigger for the router to make a WAN call

DDR (Dial-on-demand routing) - Technique whereby a Cisco router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adaptor or modem.

WAN Frame Encapsulation formats (Layer 2 Protocol)

Dedicated Point-to-Point:
- Cisco HDLC, PPP, LAPB
Packet Switched:
- Frame Relay, IETF
Circuit Switched:
- ISDN D channel LAPD
- ISDN B channel PPP, X.25, Frame Relay

WAN Frame Format Summary:
(Formats assume framing on dedicated WAN facilities)

LCP (Link Control Protocol)
[Code][Identifier][Length][Data]
PPP
[Flag][Address][Control][Protocol][LCP][FCS][Flag]
Cisco HDLC
[Flag][Address][Control][Proprietary][Data][FCS][Flag]
LAPB (derived from HDLC)
[Flag][Address][Control][ Data ][FCS][Flag]

LAPB (Link Access Procedure, Balanced) is derived from HDLC. HDLC is the popular ISO-standard bit-oriented data-link protocol that encapsulates data on synchronous serial data links. Frame Relay also uses a variation of HDLC.

HDLC (Highlevel Data Link Control) - does not inherently support multiple LAN protocols on a single link because it does not have a standard way to indicate which protocol it is carrying. The Cisco HDLC frame uses a proprietary type field that acts as a protocol field, which makes it possible for multiple network-layer protocols to share the same serial link.

PPP (Point-to-Point Protocol) - frame has a protocol field. The protocol field identifies the network layer protocol encapsulated in the information field of the frame.

LCP (Link Control Protocol) - used by PPP provides a method of establishing, configuring, maintaining, and terminating the point-to-point connection.

Serial Line Encapsulation methods:

All the encapsulations share a common frame format.
The frame has the following fields:
- Flag - Indicates the beginning of the frame and is set to a hexadecimal pattern of 7F
- Address - A one- or two-byte field to address the end station in multidrop environments
- Control - Indicates whether the frame is an information, supervisory, or unnumbered type frame; also contains specific function codes
- Data - The encapsulated data
  - FCS - The frame check sequence
  - Flag - The trailing 7E flag identifier
Encapsulation types:
- Cisco HDLC
  - Default Cisco serial line encapsulation
  - Supports autoinstall
  - Proprietary (uses a 2-byte type code)
  - No windowing or flow control
  - Only point-to-point connections (no multipoint)
  - address field is always set to all ones
- PPP
  - Standard (RFC) serial line encapsulation
  - Protocol type specified
  - Additional cababilities
  - Link Control Protocol (link quality)
    - Authentication -
      - PAP - Password Authentication Protocol
      - CHAP - Challenge Handshake Authentication Protocol
- LAPB
  - Bit-oriented protocol derived from HDLC
  - Balanced protocol in the X.25 protocol stack

PAP (Password Authentication Protocol) - Authentication protocol that allows PPP peers to authenticate one another. The remote router attempting to connect to the local router is required to send an authentication request. Unlike CHAP, PAP passes the password and host name or username in clear text (unencrypted). PAP does not itself prevent unauthorized access, but merely identifies the remote end. The router or access server then determines if that user is allowed access. PAP is supported only on PPP lines.

CHAP (Challenge Handshake Authentication Protocol) - Security feature supported on lines using PPP encapsulation that prevents unauthorized access. CHAP does not itself prevent unauthorized access, it merely identifies the remote end. The router or access server then determines whether that user is allowed access.

LAPB (Link Access Procedure, Balanced) - Data link layer protocol in the X.25 protocol stack. LAPB is a bit-oriented protocol derived from HDLC.

WAN Link Options:

Dedicated Lines
- Leased lines
- Fractional T1/E1
  - T1/E1
  - T3/E3
Switched
- Circuit Switched
  - Basic Telephone Service
  - Switched 56
  - ISDN
- Packet/Cell Switched
  - X.25
  - Frame Relay
  - ATM
  - SMDS

Dedicated line - Communications line that is indefinitely reserved for transmissions, rather than switched as transmission is required.

Leased from WAN service provider, full-time service
Point-to-point serial links
Connections use the router’s synchronous serial ports
Transmission speeds of up to T3 (44.736 Mbps)
Most widely used id T1 (1.54 Mbps)
Fractional T1 in increments of 64 kbps
Often carry data and voice, occasionally video
Core WAN connectivity
LAN to LAN connectivity
Ideal for high-volume environments with steady-rate traffic patterns

Leased line - Transmission line reserved by a communications carrier for the private use of a customer. A leased line is a type of Dedicated line.

A router port is required for each connection
Uses CSU/DSU
Service provider circuit for each remote site
Reasonable design options for the core WAN

Circuit switching - Switching system in which a dedicated physical circuit path must exist between sender and receiver for the duration of the "call".

Packet switching - Networking method in which nodes share bandwidth with each other by sending packets.

WAN Signaling standards and capacities:


[ Line Type ][ Signal Standard ][ Bit Rate Capacity ]
    56              DS0                56 kbps
    64              DS0                64 kbps
    T1              DS1             1.544 Mbps
    E1              ZM              2.048 Mbps
    J1              Y1              2.048 Mbps
    E3              M3             34.064 Mbps
    T3              DS3            44.736 Mbps

T1/T3 used primarily in North America and parts of Asia
E1/E3 used in most of the rest of the world
J1 used primarily in Japan

Multiple Paths in the Core WAN:

Minimize number of hops for high traffic
Equal metric paths for load balancing
Use when too expensive to design a full-mesh WAN core
Creates a partial mesh
Multiple-hop paths can be put between sites that generate less traffic

1. Packet/Cell Switched Connections:

Frame Relay
- Simpler and faster than X.25
- PVC only, SVC has been specified
- Access is at 56 kbps, 64 kbps, or 1.544 Mbps
- Typically operates up to T1
- Very streamlined (little error checking)
- Has none of the reliability features/complexity of X.25
  - Uses:
    - LAN-to-LAN connectivity
    - Remote access
  - Logical DTE (router) to logical DCE (Frame Relay switch)
  - Virtual circuits are end-to-end

VC (Virtual circuit) - Logical circuit created to ensure reliable communication between two network devices. A virtual circuit is defined by a VPI/VCI pair, and can be either permanent virtual circuit (PVC) or switched virtual circuit (SVC). Virtual circuits are used in Frame Relay and X.25. In ATM, a virtual circuit is called a virtual channel.

PVC (Permanent Virtual Circuit) - Virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits must exist all the time. Called a permanent virtual connection in ATM terminology.

SVC (Switched Virtual Circuit) - Virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic. Called a switched virtual connection in ATM terminology.

FRAD (Frame Relay Access Device) - Any network device that provides a connection between a LAN and a Frame Relay WAN.

Frame Relay Service and Circuits:

Data-link connection (DLCI)
Committed information rate (CIR)
Access rate is 56 kbps, 64 kbps, or 1.544 Mbps
One physical access interface
One logical hop to any point
Implemented mostly as a carrier-provided service
Can also be used for private networks

DLCI (Data-link Connection Identifier) - (a local identifier between the DTE and the DCE) Value that specifies a PVC or SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the LMI extended specification, DLCIs are globally significant (DLCIs specify individual end devices).

LMI (Local Management Interface) - Set of enhancements to the basic Frame Relay specification. LMI includes support for a keepalive mechanism, which verifies that data is flowing; a multicast mechanism, which provides the network server with its local DLCI and the multicast DLCI; global addressing, which gives DLCIs global rather than local significance in Frame Relay networks; and a status mechanism, which provides an on-going status report on the DLCIs known to the switch. Known as LMT in ANSI terminology.

CIR (Committed Information Rate) - The rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics.

2. Circuit-Switched Connections

Connections on demand
Relatively low bandwidth
Uses:
- Remote users
- Mobile users
- Backup lines
Basic Telephone Service limited to 28.8 kbps without compression
ISDN service limited to 64 or 128 kbps
Dial-on-Demand Routing (DDR)
- connection made only when traffic dictates a need
- when traffic patterns are low-volume or periodic
- when you need a backup connection for 'redundancy' or 'load sharing'

ISDN Overview:

Developed by the telephone companies with the intention of creating a totally digital network
End-to-end digital network for data, fax, voice, and video
Most common data link encapsulation is PPP
ISDN devices include the following:
- Terminal Equipment 1 (TE1) - Designates a device that is compatible with the ISDN network
- Terminal Equipment 2 (TE2) - Designates a device that is not compatible with ISDN and requires a Terminal Adapter
- Terminal Adapter (TA) - Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network
- Network Termination Type 1 (NT1) - Connects 4-wire ISDN subscriber wiring (ST) to the conventional 2-wire local loop facility (U)
- Network Termination Type 2 (NT2) - Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating.

ISDN Services:

BRI - Basic Rate Interface (3 channels 2B+D)
- Two 64 kbps (bearer) B channels
- One 16 kbps (signaling) D channel
PRI - Primary Rate Interface (T1/E1)
- 23 (T1) or 30 (E1) B channels
- One 64 kbps (signaling) D channel

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