Transcript Week 5 Link Layer & Local Area Networking

Link Layer & Local Area
Networking
Data Link Layer
 The data link layer is level two of the
seven-level OSI model. It responds to
service requests from the network layer
and issues service requests to the
physical layer.
 The data link layer is the layer of the
model which ensures that data is
transferred correctly between adjacent
network nodes in a wide area network.
Data Link Layer
 The data link layer provides the functional
and procedural means to transfer data
between network entities and to detect
and possibly correct errors that may occur
in the Physical layer.
 Examples of data link protocols are
Ethernet for LANs and PPP.
Data Link Layer
 The data link is all about getting
information from one place to a selection
of other places. At this layer one does not
need to be able to go everywhere, just
able to go somewhere else. For instance,
one needs to know at least one other
person, but not necessarily know Fred
Jones of Ohio, USA.
Data Link Layer
 This layer is made up of two components.
The first component is Logical Link
Control. This component determines
where one frame of data ends and the
next one starts. In a snail-mail network,
each letter is one frame of data, and you
can tell where it begins and ends because
it is inside an envelope.
Data Link Layer
 The second component is Media Access
Control. This component determines who
is allowed to access the media at any one
time. There are generally two forms of
media access control: distributed and
centralized.
 Often implemented in a network card
driver.
Data Link Layer
 The Data Link layer deals with issues on a
single segment of the network, the
Network layer deals with issues spanning
networks.
 There are three types of internetwork
connection devices: routers, brouters,
and CSU/DSU.
Some Basics… and a little review
 A LAN is limited by:
 Maximum distance because of signal degradation
 Maximum distance because of collision detection
 Maximum number of stations per LAN
 LANs can be interconnected with:
 Hubs
 Bridges
 Switches
More Basics
 Hubs haven’t been discussed yet because
they operate on the physical layer.
 Hubs are not intelligent, they just forward
traffic.
 Each connected LAN is a segment.
 Collisions are propagated by hubs.
More Basics
 Benefits are price, and you can connect
multiple segments.
 Limitations: single collision domain
(forwards collisions), restrictions on the
number of nodes for the domain, can’t
connect different LAN types on most hubs
(Gig with 100BT)
 A hub ONLY repeats.
More Basics..
 A switch also concentrates connections,
but it can also operate on Layers 2 and 3
and forward, segment, and provision.
 Switches can filter and forward.
Critical Statement…
A switch is always a hub, but a hub
is not necessarily a switch!
Switches vs. Hubs
 Follow the green & red arrows on the next
slide. It depicts why switches are faster
than hubs. Switches only forward packets
to the intended recipient.
Routers
 Use network address information to move
data through the best path to its
destination.
 To connect different networks, routers
must be aware of network addresses.
 Routers can determine network addresses
so they can be "intelligent" about routing
data through a complex internetwork
involving multiple networks.
Brouters
 A hybrid of routers and bridges, Brouters
function like routers, relaying data transmissions
between networks, but when they encounter a
data unit that uses a protocol with which they
are unfamiliar, they work like a bridge and
forward the data to the next segment by using a
physical address. You can use Brouters for
networks on which there is mixed-protocol traffic
and for networks that use protocols that do not
support routing.
CSU/DSU
 Channel Service Unit/Digital Service Unit
 The units are designed to shield network users
from electric voltages as well as electrical
interference. They also ready data for
transmission by adhering to any network rules.
 A CSU/DSU operates like a modem; however, it
is a digital-to-digital device rather than digital-toanalog. It readies digital signals and guarantees
that they have sufficient strength and the proper
format to be transmitted over digital WAN links.
Examples
 Examples of Data Link Layer operation
are:
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Ethernet
Token Ring
Point to Point Protocol (PPP)
LocalTalk
EtherTalk
Frame Relay
Logical WAN Network Example
WAN Point to Point
Frame Relay
 Frame relay is an efficient data transmission
technique used to send digital information
quickly and cheaply to one or many destinations
from one point.
 It can be used for voice, data, LAN and WAN
traffic.
 Each frame relay end user gets a private line to
a frame relay node. The frame relay network
handles the transmission to its other end users
over a path which is always changing and is
invisible to the end users.
Frame Relay
 Frame relay is a packet-switched network,
commonly used at the data link layer.
 Generally the concept of permanent virtual
circuits (PVCs) is used to form logical endto-end links mapped over a physical
network.
Frame Relay
 Datalink Connection Identifiers or DLCIs
are a locally significant numeric value to
represent each end point. Multiple PVCs
can be mapped to the same physical end
point. It is often provisioned with a
Committed Information Rate (CIR) and a
burstable component sometimes known as
Extended Information Rate (EIR).
Frame Relay
 Frame relay was designed to make more
efficient use of existing physical resources,
thereby allowing the over-provisioning of
data services to telco customers, as most
clients are unlikely to be utilizing a data
service 100 percent of the time. Frame
relay has acquired a bad reputation in
some markets because of excessive
bandwidth overbooking by telcos.
Frame Relay Service
 There are ways to help improve service,
such as using Quality of Service (QoS)
and setting Service Level Agreements
(SLAs).
Frame Relay and VPNs
 Frame Relay Networks can support VPNs
 A Virtual Private Network, or VPN, is a private
network usually used within a company, or by
several different companies or organizations,
communicating over a public network.
 VPN message traffic is carried on public
networking infrastructure (ie, the Internet) using
standard (possibly insecure) protocols.
VPN
 VPNs use cryptographic tunneling
protocols to provide the necessary
confidentiality, sender authentication and
message integrity to achieve the privacy
intended.
 When properly chosen, implemented, and
used, such techniques can indeed provide
secure communications over insecure
networks.
Frame Relay
Multiple Subnet and Local Frame
Relay WAN to Remote Site
802.11/Wi-Fi
 IEEE 802.11 or Wi-Fi is a set of WLAN
standards developed by working group 11 of
IEEE802.
 The 802.11 family currently includes three
separate protocols that focus on encoding (a, b,
g); other standards in the family (c-f, h-j, n) are
service enhancement and extensions, or
corrections to previous specifications. 802.11b
was the first widely accepted wireless
networking standard, followed by 802.11a and
802.11g.
Wi-Fi Standards
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IEEE 802.11 - The original 2 Mbps, 2.4 GHz standard
IEEE 802.11a - 54 Mbps, 5 GHz standard 1999
IEEE 802.11b – To support 5.5 & 11 Mbps (1999)
IEEE 802.11d - New countries
IEEE 802.11e - Enhancements: QoS & packet bursting
IEEE 802.11f – Inter-Access Point Protocol (IAPP)
IEEE 802.11g - 54 Mbps, 2.4 GHz backward compatible w/ b
(2003)
IEEE 802.11h - 5 GHz spectrum, European compatibility
IEEE 802.11i - Enhanced security
IEEE 802.11j - Extensions for Japan
IEEE 802.11n - Higher throughput improvements
BlueTooth
 Bluetooth, a micro-chip, provides a way to
connect and exchange information between
devices like PDA's, mobile phones, laptops,
PCs, printers and digital cameras via a secure,
low-cost, globally available short range radio
frequency.
 Bluetooth lets these devices talk to each other
when they come in range, even if they're not in
the same room, as long as they are within 10
meters of each other.
BlueTooth vs. Wireless
 Bluetooth should not be compared to WiFi, a faster protocol requiring more
expensive hardware that covers greater
distances and uses the same frequency
range.
 While Bluetooth is a cable replacement
creating personal area networking
between different devices, Wi-Fi is a cable
replacement for local area network
access. They serve different purposes.
ATM
 Asynchronous Transfer Mode is a cell relay
network protocol which encodes data traffic into
small fixed sized (53 byte) cells instead of
variable sized packets as in packet-switched
networks (such as IP or Ethernet)
 ATM was intended to provide a single unified
networking standard that could support both
synchronous channel networking and packetbased networking (IP, Frame) while supporting
multiple levels of QoS for packet traffic.
ATM
 ATM sought to resolve the conflict between
circuit-switched networks and packet-switched
networks by mapping both bit streams and
packet-streams onto a stream of small fixed-size
'cells' tagged with VC identifiers.
 The cells are typically sent on demand within a
synchronous time-slot pattern in a synchronous
bit-stream: what is asynchronous here is the
sending of the cells, not the low-level bit stream
that carries them.
Use of ATM
 Numerous telcos have implemented wide-area
ATM networks, and many ADSL implementations
use ATM.
 ATM has failed to gain wide use as a LAN
technology, and its great complexity has held
back its full deployment as the single integrating
network technology in the way that its inventors
originally intended.
 Still good for burstable transmissions (video)