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Basically, Cisco just supports HDLC, PPP, and Frame Relay on its serial interfaces, and you
can see this with the
encapsulation ?
command from any serial interface (your output may
vary depending on the IOS version you are running):
Corp#
config t
Corp(config)#
int s0/0/0
Corp(config-if)#
encapsulation ?
atm-dxi ATM-DXI encapsulation
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
smds Switched Megabit Data Service (SMDS)
x25 X.25
8.1 Describe different methods for connecting to a WAN
353
Understand that if I had other types of interfaces on my router, I would have other encapsulation
options, like ISDN or ADSL. And remember, you can’t configure Ethernet or Token
Ring encapsulation on a serial interface.
Next, I’m going to define the most prominently known WAN protocols used today: Frame
Relay, ISDN, LAPB, LAPD, HDLC, PPP, PPPoE, Cable, DSL, MPLS, and ATM. Just so you know,
the only WAN protocols you'll usually find configured on a serial interface are HDLC, PPP, and
Frame Relay, but who said we’re stuck with using only serial interfaces for wide area connections?
Frame Relay
A packet-switched technology that made its debut in the early 1990s,
Frame
Relay
is a high-performance Data Link and Physical layer specification. It’s pretty much a successor
to X.25, except that much of the technology in X.25 used to compensate for physical
errors (noisy lines) has been eliminated. An upside to Frame Relay is that it can be more costeffective
than point-to-point links, plus it typically runs at speeds of 64Kbps up to 45Mbps
(T3). Another Frame Relay benefit is that it provides features for dynamic bandwidth allocation
and congestion control.
ISDN
Integrated Services Digital Network (ISDN)
is a set of digital services that transmits
voice and data over existing phone lines. ISDN offers a cost-effective solution for remote users
who need a higher-speed connection than analog dial-up links can give them, and it’s also a good
choice to use as a backup link for other types of links like Frame Relay or T1 connections.
LAPB
Link Access Procedure, Balanced (LAPB)
was created to be a connection-oriented
protocol at the Data Link layer for use with X.25, but it can also be used as a simple data link
transport. A not-so-good characteristic of LAPB is that it tends to create a tremendous amount
of overhead due to its strict time-out and windowing techniques.
LAPD
Link Access Procedure, D-Channel (LAPD)
is used with ISDN at the Data Link layer
(layer 2) as a protocol for the D (signaling) channel. LAPD was derived from the Link Access
Procedure, Balanced (LAPB) protocol and is designed primarily to satisfy the signaling
requirements of ISDN basic access.
HDLC
High-Level Data-Link Control (HDLC)
was derived from Synchronous Data Link
Control (SDLC), which was created by IBM as a Data Link connection protocol. HDLC
works at the Data Link layer and creates very little overhead compared to LAPB.
It wasn’t intended to encapsulate multiple Network layer protocols across the same link—the
HDLC header doesn’t contain any identification about the type of protocol being carried
inside the HDLC encapsulation. Because of this, each vendor that uses HDLC has its own way
of identifying the Network layer protocol, meaning that each vendor’s HDLC is proprietary
with regard to its specific equipment.
PPP
Point-to-Point Protocol (PPP)
is a pretty famous, industry-standard protocol. Because
all multiprotocol versions of HDLC are proprietary, PPP can be used to create point-to-point
links between different vendors’ equipment. It uses a Network Control Protocol field in the
Data Link header to identify the Network layer protocol and allows authentication and multilink
connections to be run over asynchronous and synchronous links.
354
Chapter 8
Implement and verify WAN links
PPPoE
Point-to-Point Protocol over Ethernet
(PPoE) encapsulates PPP frames in Ethernet
frames and is usually used in conjunction with ADSL services. It gives you a lot of the familiar
PPP features like authentication, encryption, and compression, but there’s a downside—it has
a lower maximum transmission unit (MTU) than standard Ethernet does, and if your firewall
isn’t solidly configured, this little attribute can really give you some grief!
Still somewhat popular in the United States, PPPoE on Ethernet’s main feature is that it adds
a direct connection to Ethernet interfaces, while providing DSL support. It’s often used by
many hosts on a shared Ethernet interface for opening PPP sessions to various destinations via
at least one bridging modem.
In a modern HFC network, typically 500 to 2,000 active data subscribers are connected to a
certain cable network segment, all sharing the upstream and downstream bandwidth. (
Hybrid
fibre-coaxial
, or HFC, is a telecommunications industry term for a network that incorporates
both optical fiber and coaxial cable to create a broadband network.) The actual bandwidth for
Internet service over a cable TV (CATV) line can be up to about 27Mbps on the download
path to the subscriber, with about 2.5Mbps of bandwidth on the upload path. Typically, users
get an access speed from 256Kbps to 6Mbps. This data rate varies greatly throughout the U.S.
DSL
Digital subscriber line
(DSL) is a technology used by traditional telephone companies to
deliver advanced services (high-speed data and sometimes video) over twisted-pair copper telephone
wires. It typically has a lower data-carrying capacity than HFC networks, and data speeds
can be range limited by line lengths and quality. DSL is not a complete end-to-end solution but
rather a Physical layer transmission technology like dial-up, cable, or wireless. DSL connections
are deployed in the last mile of a local telephone network—the local loop. The connection is set
up between a pair of modems on either end of a copper wire that is run between the customer
premises equipment (CPE) and the
Digital Subscriber Line Access Multiplexer
(DSLAM). A
DSLAM is the device located at the provider’s central office (CO) and concentrates connections
from multiple DSL subscribers.
MPLS
MultiProtocol Label Switching (MPLS)
is a data-carrying mechanism that emulates
some properties of a circuit-switched network over a packet-switched network. MPLS is a
switching mechanism that imposes labels (numbers) to packets and then uses those labels to
forward packets. The labels are assigned on the edge MPLS of the network, and forwarding
inside the MPLS network is done solely based on labels. Labels usually correspond to a path
to layer 3 destination addresses (equal to IP destination-based routing). MPLS was designed to
support forwarding of protocols other than TCP/IP. Because of this, label switching within the
network is performed the same regardless of the layer 3 protocol. In larger networks, the result
of MPLS labeling is that only the edge routers perform a routing lookup. All the core routers
forward packets based on the labels, which makes forwarding the packets through the service
provider’s network faster. (Most companies are replacing their Frame Relay networks with
MPLS today).
ATM
Asynchronous Transfer Mode
(ATM) was created for time-sensitive traffic, providing
simultaneous transmission of voice, video, and data. ATM uses cells that are a fixed 53 bytes
long instead of packets. It also can use isochronous clocking (external clocking) to help the
data move faster. Typically, if you are running Frame Relay today, you will be running Frame
Relay over ATM.
8.2 Configure and verify a basic WAN serial connection
355
Exam Objectives
Remember the default serial encapsulation on Cisco routers.
Cisco routers use a proprietary
High-Level Data Link Control (HDLC) encapsulation on all their serial links by default.
Remember what encapsulation you can use if you need basic dial-up, authentication and multiple
Network layer protocols.
PPP is a non-vendor-specific circuit-switching protocol, and PPP can
be used to create point-to-point links. It uses a Network Control Protocol field in the Data Link
header to identify the Network layer protocol and allows authentication and multilink connections
to be run over asynchronous and synchronous links.
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