Transcript Local area networks

Basic Communications
Systems
Class 4
Today’s Class Topics

Local Area Networks





What is a LAN?
LAN Topologies
Ethernet LANs
Token Ring and FDDI LANs
LAN Interconnection



Bridges
Switches
Routers
Data Communications and Computer Networks
Chapter 7
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.
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.)
LAN Components

Workstations (PCs, etc.)


Interconnecting Cable


Network Interface Card (NIC) or Network
Adapter Card provides LAN interface.
Typically twisted copper wire or optical
fiber.
Network Operating System (NOS)
Software
The Network Interface Card (NIC)
and Networking Software (NS) m us t
be com patible with each other and
with the com puter or device into
which they are ins talled.
Networking
Software
(NS)
Network Interf ace
Card (NIC)
NIC NS
NS
NS
Network Hub or
Wiring Center
NIC
Media
NIC
NIC NS
NIC NS
NIC
NS
NIC
Shared
Application
Serv er
GOLDMAN: LAN
FIG. 01-05
Shared Laser
Printer
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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
Bus Topology Example:
10Base5 Ethernet
Attachment
Unit Interface
(AUI Cable)
Terminating
Resister
Medium
Attachment
Unit (MAU)
Thick
Coaxial
Cable
Data Communications and Computer Networks
Chapter 7
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.
Star Topology

Advantages:


Hub can isolate any network problems that
occur
Intelligent (managed) hub can collect
network performance statistics
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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
Dual Ring Example:
Fiber Distributed Data Interface (FDDI)
Dual-Attached Workstations in Normal Operation
Dual-attached
counter
rotating rings
Self healed after Link Failure
Self-healing
FDDI dual- link failure
attached
adapters
GOLDMAN: LAN
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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 Bluetooth’s impact be on 802.11b?
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
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.
Data Communications and Computer Networks
Chapter 7
Local Area Network Systems
100VG-AnyLAN
Deterministic 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.
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
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
Data Communications and Computer Networks
Chapter 7
Data Communications and Computer Networks
Chapter 7
Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 8
Local Area Networks Internetworking
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8
Switched Ethernet
Twisted
Pair
Ethernet Switch
Data Communications and Computer Networks
Chapter 8
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
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
Ethernet Switch
MAC #11
MAC #12
E
D
MAC #13
C
B
MAC #14
A
10Base-T Switch
FORWARDING TABLE
Address
# 11
# 12
# 13
# 14
# 21
# 22
# 23
# 24
MAC #21
Port
D
C
B
A
E
E
E
E
MAC #22
UpdateTime
6:05.1441 PM
6:04.1523 PM
6:04.8722 PM
6:05.1422 PM
6:04.6623 PM
6:04.2355 PM
6:05.0233 PM
6:04.9722 PM
MAC #23
10Base-T Hub
MAC #24
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
(FF:FF:FF:FF:FF:FF), 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.
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
Switch Learning
MAC #11
Dest
MAC #12
MAC #13
MAC #14
10Base-T Hub
Src
13 11
FORWARDING TABLE
A
Transparent
Bridge
B
Dest
Address
# 11
# 23
Port
A
B
UpdateTime
6:05.1441 PM
6:04.4223 PM
Src
11 23
MAC #21
MAC #22
MAC #23
10Base-T Hub
MAC #24
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).
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
Advantages of Switched Ethernet
over Traditional Ethernet

Improved Bandwidth



Improved Security


Ethernet switches don’t waste time on collisions
Switches allow multiple simultaneous data streams
Switches do not broadcast information to everyone
Redundant inter-switch connections

Switches can utilize backup connections if data cables
fail
Data Communications and Computer Networks
Chapter 8
Data Communications and Computer Networks
Chapter 8
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)
POWER USERS
DeskTop
Switch
10Base-T Hub
Backbone
Switch
100 Mbps
Ethernet
links
File
Server
File
Server
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8
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 router’s operation is shown on the next slide.
Data Communications and Computer Networks
Chapter 8
Router Operations

Routers modify layer 2 frame headers & trailers
so packet can travel end-to-end over many
links
T1
Token
Ring
IP packets /
TR frames
C.O.
Router
Router
modifies
frame
CSU
T1
CSU
IP packets /
PPP frames
Router
Router
modifies
frame
Token
Ring
IP packets /
TR frames
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
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8
Data Communications and Computer Networks
Chapter 8
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.
Data Communications and Computer Networks
Chapter 8