03 - fundamentals of wans

03 - Fundamentals of WANs

By Muhammad Asghar Khan

Reference: CCENT/CCNA ICND1 Official Exam Certification Guide By Wendell Odom

www.asghars.blogspot.com2

Agenda Wide Area Networks Point-to-Point WANs Physical Layer (L1)

WAN Cabling Standards WAN Speeds WAN Link in a LAB

Data Link Layer (L2) WAN Protocols

HDLC Frame Relay ATM

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Wide Area Networks WANs interconnect host devices over a broad area

(i.e. any network that links across metropolitan, regional, or national boundaries)

Enterprise networks use WANs to connect remote sites

LANs & WANs both implement the same OSI Layer 1 and Layer 2 functions, but with different mechanisms and details

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Point-to-Point WANs A point-to-point WAN link acts like an Ethernet

trunk between two Ethernet switches Example LAN in two buildings

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Point-to-Point WANs Imagine that the buildings are 1000 miles apart

instead of right next to each other, you will face two problems: Ethernet does not support cabling to run for 1000 miles Even if Ethernet support 1000 mile trunk, you don’t

have the right-of-way needed to bury a cable b/w buildings

To create such long links/circuits the physical cabling is owned, installed and managed by a company/service provider usually called telco or telephone company

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Point-to-Point WANs The circuit or point-to-point WAN link provided by

the service provider is called a leased line/circuit For point-to-point WAN link the service provider

will install the circuit Figure below shows the key concepts and terms

relating WAN circuits

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Point-to-Point WANs Routers connect to a device called Channel Service Unit/

Data Service Unit (CSU/DSU), the cable length could be up to 50 feet long

The much longer four-wire cable from telco plugs into the CSU/DSU while on other side ends up in the WAN switch

The term demarcation point (demarc) is a point at which the telco’s responsibility is on one side and customer’s responsibility is on the other

The term customer premises equipment (CPE) refers to devices that are at the customer site from telco’s perspective e.g. CSU/DSU and Routers

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WAN Cabling Standards As several types of physical layer implementations

for LANs exist, various kinds of serial and router connections can also be used in a WAN environment

Serial connections support dedicated leased lines that run the Point-to-Point Protocol (PPP) or Frame Relay, speed of these connections ranges up to E1 (2.048 Mbps)

Serial connection uses one-at-a-time transmission contrasts with parallel data transmission, which transmits several bits at a time

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WAN Cabling Standards Figure shows a typical connection with some serial

cabling options

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WAN Cabling Standards Router is termed as DTE (Data Terminal

Equipment), a device that receive clocking CSU/DSU is termed as DCE (Data Communications

Equipment), a device that provides clocking To install a new point-to-point leased line b/w two

routers A network engineer contacts service provider and

orders the circuit and speed of circuit Purchases two CSU/DSUs, install & configure one at

each site

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WAN Cabling Standards Purchases and install routers, and connects serial

cables from each router to the respective CSU/DSU At the end the telco installs the new line into the

customer premises and the line is connected to the CSU/DSU

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WAN Speeds

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WAN Link in a LAB You can create a serial link in a lab without using

CSU/DSU, to do so one router must supply clocking You must have two routers, a DTE serial cable for

one router and a DCE serial cable for the other

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WAN Link in a LAB The router with DCE cable can be configured to

provide clocking (clock rate command) This type of connection b/w two routers is called a

back-to-back serial link

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WAN Protocols Data link layer protocols define how data is

encapsulated for transmission to remote sites and the mechanisms for transferring the resulting frames

The most common WAN data link protocols are: HDLC PPP Frame Relay ATM

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High-Level Data Link Control Protocol (HDLC) HDLC uses the services of a physical layer, and

provides a reliable communications path between the transmitter and receiver (i.e. with acknowledged data transfer)

HDLC discards the frame if errors occurred Additionally, HDLC needs to identify the type of

packets inside the HDLC frame Figure on next slide shows the HDLC framing

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High-Level Data Link Control Protocol (HDLC) The beginning of an HDLC frame is marked by flag

characters Control indicates whether the frame is a Control or

Data frame

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High-Level Data Link Control Protocol (HDLC) Type field indicates the type packet held inside the

frame FCS field in the HDLC trailer is used for error

detection

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Point-to-Point Protocol (PPP) PPP is used for communication between two computers

using a serial interface or point-to-point link Comparing the basics, PPP behaves much like HDLC The framing looks identical to the Cisco proprietary HDLC

framing The address field is there but it does not matter It discard errored frames that don’t pass the FCS check Type field is part of the standard for PPP, any vendor that

conforms to the PPP standard can communicate So, when connecting a Cisco router to another vendor’s

router over a point-to-point serial link, PPP is the protocol of choice

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Frame Relay Frame Relay is a high-performance WAN protocol

that operates at the physical and data link layers Frame Relay is an example of a packet-switched

technology Packet-switched networks enable end stations to

dynamically share the network medium and the available bandwidth

Company can connect a large number of routers to the packet-switching service, and each router can send packets to all other routers; like devices connected to Ethernet switch

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Frame Relay Frame Relay has benefit of scaling over PPP WANs In PPP WANs only a pair of routers can be

connected Figure below shows the components of Frame

Relay

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Frame Relay Access Links are leased line b/w each router and a

nearby Frame Relay switch Access Links run at the same speed and signaling

standards as that of PPP leased lines; however, instead of extending from one router to other, each leased line runs from one router to a Frame Relay switch

Frame Relay defines its own data-link header and trailer

Frame Relay header holds an address field called a Data-Link Connection Identifier (DLCI)

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Frame Relay Frame Relay header and trailer are defined by a

protocol; Link Access Procedure-Frame (LAPF) Figure below depicts the end-to-end connectivity

associated with virtual circuit (VC)

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Frame Relay The logical path that frame travels b/w each pair of

routers is called a Frame Relay VC Typically service provider pre-configures all the

required details of a VC; these VCs are called permanent VC (PVC)

Figure below

shows typical

Frame Relay

network with

three sites

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Asynchronous Transfer Mode (ATM) ATM is a high-speed WAN protocol designed to support

both voice and data communications ATM operates at the data link layer ATM is a switching technology that organizes digital data

into 53-byte cell Each cell consists of a 5-byte header and a payload of 48

bytes of information ATM also uses VCs ATM connections has two levels:

Virtual Path Connections Virtual Channel Connections

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03 - fundamentals of wans

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