TDC 361 Basic Communications Systems Class 4

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TDC 361Basic Communications Systems Class 4

• Greg Brewster
• DePaul University

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Todays Class Topics

• Local Area Networks
• What is a LAN?
• LAN Topologies
• Ethernet LANs
• Token Ring and FDDI LANs
• LAN Interconnection
• Bridges
• Switches
• Routers

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Introduction A local area network is a
communication network that interconnects a
variety of data communicating devices within a
small geographic area and broadcasts data at high
data transfer rates with very low error
rates. Since the local area network first
appeared in the 1970s, its use has become
widespread in commercial and academic
environments.
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What is a LAN?

• Key Elements
• High Communications Speed
• Very Low Error Rate
• Limited Geographic Boundaries
• Simple Cabling System
• Provides resource sharing (files, printers,
  disks, applications, etc.)

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LAN Components

• Workstations (PCs, etc.)
• Network Interface Card (NIC) or Network Adapter
  Card provides LAN interface.
• Interconnecting Cable
• Typically twisted copper wire or optical fiber.
• Network Operating System (NOS) Software

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Advantages of Local Area Networks Ability to
share hardware and software resources. Individual
workstation might survive network
failure. Component and system evolution are
possible. Support for heterogeneous forms of
hardware and software. Access to other LANs and
WANs (Figure 7-1). Private ownership. Secure
transfers at high speeds with low error rates.
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Basic Network Topologies Local area networks are
interconnected using one of four basic
configurations 1. Bus/tree 2. Star-wired bus 3.
Dual Ring 4. Star-wired ring 5. Wireless
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Bus/Tree Topology Baseband signals are
bidirectional and more outward in both directions
from the workstation transmitting. Broadband
signals are usually uni-directional and transmit
in only one direction. Because of this, special
wiring considerations are necessary. Buses can be
split and joined, creating trees, but this is not
commonly done.
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Bus Topology

• Used in early LANs (1970s, 1980s)
• All workstations and servers connected to same
  physical cable.
• Each transmission was broadcast to all other
  devices on LAN.
• Problems
• One cable problem downs the network
• Hard to locate problems when they occur

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Bus Topology Example10Base5 Ethernet
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Star-wired Bus Topology Logically operates as a
bus, but physically looks like a star. Star
design is based on hub. All workstations attach
to hub. Hub takes incoming signal and immediately
broadcasts it out all connected links. Hubs can
be interconnected to extend size of network. This
is THE most common LAN topology used today, in
the form of 10BASE-T Ethernet.
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Star Topology

• Advantages
• Hub can isolate any network problems that occur
• Intelligent (managed) hub can collect network
  performance statistics

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Ring Topology

• Devices connect sequentially in a ring.
• Each device transmits to the next.
• Problems
• One cable problem downs the network
• One workstation problem downs the network
• Used today only with dual-ring systems that can
  survive cable cuts

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Dual Ring ExampleFiber Distributed Data
Interface (FDDI)
Self healed after Link Failure
Self-healing
FDDI dual-
link failure
attached
adapters
GOLDMAN LAN
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Star-wired Ring Topology Logically operates as a
ring but physically appears as a star. Star
topology is based on MAU (multi-station access
unit) which functions similarly to a hub. Where a
hub immediately broadcasts all incoming signals
onto all connected links, the MAU passes the
signal around in a ring fashion. Like hubs, MAUs
can be interconnected to increase network size.
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Wireless Topology Not really a specific topology
since a workstation in a wireless LAN can be
anywhere as long as it is within transmitting
distance to an access point. Newer IEEE 802.11
and 802.11b standard defines various forms of
wireless LAN connections. Speeds up to 11 Mbps
with 802.11b standard. Workstations reside within
a basic service set, while multiple basic service
sets create an extended service set.
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Wireless Topology Two basic components necessary
the client radio, usually a PC card with an
integrated antenna, and the access point (AP),
which is an Ethernet port plus a transceiver. The
AP acts as a bridge between the wired and
wireless networks and can perform basic routing
functions. Workstations with client radio cards
reside within a basic service set, while multiple
basic service sets create an extended service set.
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Wireless Topology With directional antennae
designed for point-to-point transmission, 802.11b
can work for more than 10 miles. With an
omnidirectional antenna on a typical AP, range
may drop to as little as 100 feet. Distance is
inversely proportional to transmission speed - as
speed goes up, distance goes down.
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Wireless Topology In actual tests, 11 Mbps
802.11b devices managed 5.5 Mbps (from a July
2000 test by Network Computing). To provide
security, most systems use Wired Equivalent
Privacy (WEP), which provides either 40- or
128-bit key protection. Management of roaming
clients not defined well by 802.11b. What will
Bluetooths impact be on 802.11b?
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Medium Access Control Protocols How does a
workstation get its data onto the LAN medium? A
medium access control protocol is the software
that allows workstations to take turns at
transmitting data. Three basic categories 1.
Contention-based protocols 2. Round robin
protocols 3. Reservation protocols
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Contention-Based Protocols Most common example
is carrier sense multiple access with collision
detection (CSMA/CD), used in Ethernet If no one
is transmitting, a workstation can transmit. If
someone else is transmitting, the workstation
backs off and waits.
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Contention-Based Protocols If two workstations
transmit at the same time, a collision occurs.
When the two workstations hear the collision,
they stop transmitting immediately. Each
workstation backs off a random amount of time and
tries again. Hopefully, both workstations do not
try again at the exact same time. CSMA/CD is an
example of a non-deterministic protocol.
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Round Robin Protocols Each workstation takes a
turn transmitting and the turn is passed around
the network from workstation to workstation. Most
common example is token ring LAN in which a
software token is passed from workstation to
workstation. Token ring is an example of a
deterministic protocol. Token ring more complex
than CSMA/CD. What happens if token is lost?
Duplicated? Hogged? Token ring LANs are losing
the battle with CSMA/CD LANs.
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Reservation Protocols Workstation places a
reservation with central server. Workstation
cannot transmit until reservation comes up. Under
light loads, this acts similar to CSMA/CD. Under
heavy loads, this acts similar to token
ring. Powerful access method but again losing out
to CSMA/CD. Most common example of reservation
protocol is demand priority protocol.
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Medium Access Control Sublayer To better support
local area networks, the data link layer of the
OSI model was broken into two sublayers 1.
Logical link control sublayer 2. Medium access
control sublayer Medium access control sublayer
defines the frame layout and is more closely tied
to a specific medium at the physical layer. Thus,
when people refer to LANs they often refer to its
MAC sublayer name, such as 10BaseT.
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IEEE 802 Frame Formats The IEEE 802 suite of
protocols defines the frame formats for CSMA/CD
(IEEE 802.3) and token ring (IEEE 802.5). Each
frame format describes how the data package is
formed. Note how the two frames are different.
If a CSMA/CD network connects to a token ring
network, the frames have to be converted from one
to another.
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Local Area Network Systems Ethernet or
CSMA/CD Most common form of LAN today. Star-wired
bus is most common topology but bus topology also
available. Ethernet comes in many forms depending
upon medium used and transmission speed and
technology.
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Ethernet Originally, CSMA/CD was 10 Mbps. Then
100 Mbps was introduced. Most NICs sold today
are 10/100 Mbps. Then 1000 Mbps (1 Gbps) was
introduced. 10 Gbps is in the process of being
standardized.
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Ethernet Enhancements Transmission can be full
duplex (separate transmit and receive), and
collisions can be eliminated if Ethernet switches
are used rather than standard Ethernet hubs.
(Switches explained further later) Data
prioritization is possible using 802.1p protocol.
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Local Area Network Systems IBM Token
Ring Deterministic LAN offered at speeds of 4, 16
and 100 Mbps. Very good throughput under heavy
loads. More expensive components than
CSMA/CD. Losing ground quickly to CSMA/CD. May
be extinct soon.
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Local Area Network Systems FDDI (Fiber
Distributed Data Interface) Based on the token
ring design using 100 Mbps fiber
connections. Allows for two concentric rings -
inner ring can support data travel in opposite
direction or work as backup. Token is attached to
the outgoing packet, rather than waiting for the
outgoing packet to circle the entire ring.
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Local Area Network Systems 100VG-AnyLAN Determinis
tic LAN based on demand priority access
method. Similar to hub topology (star
design). Two levels of priority - normal and
high. Supports a wide-variety of media
types. Losing ground quickly to CSMA/CD. Will be
extinct soon.
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• LANs In Action A Small Office Solution
• What type of system will interconnect twenty
  workstations to a central server, which offers
• Electronic mail
• A database that contains all customer
  information
• Internet access
• High quality printer access

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Data Communications and Computer Networks A
Business Users Approach

• Chapter 8
• Local Area Networks - Internetworking

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Introduction Many times it is necessary to
connect a local area network to another local
area network or to a wide area network. Local
area network to local area network connections
are usually performed with a bridge or
switch. Local area network to wide area network
connections are usually performed with a router.
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Why Interconnect? To separate / connect one
corporate division with another. To connect two
LANs using different protocols. To connect a LAN
to the Internet. To break a LAN into segments to
relieve traffic congestion. To provide a security
wall between two different groups of users.
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Bridges / Switches A bridge or switch can be used
to connect two similar LANs, such as two CSMA/CD
LANs. Bridges connect to hubs. Switches can
connect to PCs, hubs, or other switches. Each
bridge or switch examines the destination address
in a frame and either forwards this frame onto
the next LAN or drops the frame.
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Switched Ethernet
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Transparent Bridges A transparent bridge does not
need programming but observes all traffic and
builds routing tables from this observation. This
observation is called backward learning. Ethernet
switches are examples of Transparent Bridges
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Ethernet Switches

• Ethernet Switch
• Receives Ethernet frame
• Looks up 6-byte Destination Address in a
  Forwarding Table
• Sends frame out only the port associated with the
  Destination Address

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Ethernet Switch
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Bridge / Switch Operations

• Each frame received by a bridge is stored,
  checked for errors and then re-sent as follows
• If the Destination address is broadcast
  (FFFFFFFFFFFF), the frame is sent out all
  ports except the one it arrived on.
• Else if the Source and Destination are both
  reachable from the same port of the bridge, the
  frame is dropped
• Else if the Source and Destination are reachable
  from different ports of the bridge, then the
  frame is re-sent out the destination port
• Each bridge keeps an internal Forwarding Table
  that associates addresses with ports.

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Building Forwarding Table

• Bridge/Switch Learning
• For each arriving data frame, switch examines
  source address and adds/updates entry in
  Forwarding Table containing
• Source Address (6-byte format)
• Port that this frame arrived on
• Current Time

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Switch Learning
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Switches vs. Hubs

• How is Switch different than a Hub?
• Broadcasting (and Security)
• Hub broadcasts every packet to every device
• Switch forwards packet out single port
• Matching Data Rates
• All ports on hub must run at same data rate
• Switch ports can run at different rates (can
  buffer incoming packet at one data rate and
  re-transmit at another data rate).

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Full-Duplex Ethernet

• Full-Duplex Ethernet allows a workstation to send
  and receive data simultaneously.
• Requirements
• Must have a full-duplex NIC card
• Must be connected to Ethernet switch

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Advantages of Switched Ethernet over Traditional
Ethernet

• Improved Bandwidth
• Ethernet switches dont waste time on collisions
• Switches allow multiple simultaneous data streams
• Improved Security
• Switches do not broadcast information to everyone
• Redundant inter-switch connections
• Switches can utilize backup connections if data
  cables fail

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Switched Hierarchy

• Switches allow network manager to put bandwidth
  where it is needed.
• Some users get 10 Mbps shared (hub)
• Some users get 10 Mbps dedicated (switch)
• Some users get 100 Mbps shared (hub)
• Some users get 100 Mbps dedicated (switch)

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Source-routing Bridges A source-routing bridge is
found with token ring networks. Source-routing
bridges do not learn from watching tables. When
a workstation wants to send a frame, it must know
the exact path of network / bridge / network /
bridge / network If a workstation does not know
the exact path, it sends out a discovery
frame. The discovery frame makes its way to the
final destination, then as it returns, it records
the path.
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Routers The device that connects a LAN to a WAN
or a WAN to a WAN. A router accepts an outgoing
packet, removes any LAN headers and trailers, and
encapsulates the necessary WAN headers and
trailers. Because a router has to make wide area
network routing decisions, the router has to dig
down into the network layer of the packet to
retrieve the network destination address.
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Routers Thus, routers are often called layer 3
devices. They operate at the third layer, or
OSI network layer, of the packet. Thus, each time
they receive a packet, they strip off the layer
2 header (such as Ethernet), and then create a
new layer 2 header for the next hop to the next
router (or destination). Routers often
incorporate firewall functions. An example of a
routers operation is shown on the next slide.
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Router Operations

• Routers modify layer 2 frame headers trailers
  so packet can travel end-to-end over many links

IP packets / PPP frames
Router modifies frame
IP packets / TR frames
IP packets / TR frames
Router modifies frame
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Switches vs. Routers

• How is Switch different than a Router?
• Addressing
• Switch uses layer 2 MAC addresses
• Router uses layer 3 IP address
• Speed security
• Switch forwards packets fast (50-100 nsec).
• Router takes more time (1-50 msec) and provides
  other security features

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LAN Internetworking In Action A Small Office
Revisited Recall the In Action example from
Chapter Seven. A small office with 20
workstations were connected to a server via
100BaseTX. One hub was kept in a closet near the
20 workstations while a second hub was near the
server.
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LAN Internetworking In Action A Small Office
Revisited Now Hannah wants to connect the LAN to
the Internet. She adds a router next to the
server and connects it to the hub. She connects
the router to a high-speed telephone line such as
a T1 service.
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LAN Internetworking In Action A Small Office
Revisited Now network usage is so high that
Hannah must consider segmenting the network. She
decides to install a database server near the 20
workstations and replace the hub with a
switch. Hopefully this will separate the database
users from the Internet users.
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