Lecture notes - University of St. Thomas

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Oct 26, 2013 (3 years and 9 months ago)

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CSIS 625

1

CSIS 625 Week 6

LANs & Switching



Copyright 2001
-

Dan Oelke


For use by students of CSIS 625 for purposes of this class only.

CSIS 625

2

Overview


LANs


More on Ethernet from last week


Lesser used LAN technologies


Token Bus, Token Ring, FDDI, Others


Newer fields


Wireless Ethernet, Resilient Packet Rings, Others


Switching


Circuit Switching


Space


Time division


Packet Switching


Datagram


Virtual circuit

CSIS 625

3

Ethernet terminology


Collision Domain


the group of nodes that are using
CSMA/CD between them.


Bridge


a device which connects 2 or more networks
at layer 2.


Switch = Bridge
-

works at L2 (data link) layer


Switch is newer name
-

faster & done in HW.


Bridges are older and often done in SW


Allows use of full
-
duplex links


Hub = Repeater
-

works at physical layer


Regenerates signal


Must be a half
-
duplex link connected to these


Baseband


use of digital signals


Broadband


use of RF modulated signals

CSIS 625

4

Ethernet Physical Layer


10BaseT Ethernet
-

2 pair Cat 3


Uses Manchester encoding


This results in 10 to 20 million transitions per
second on the line


Spectrum is in the 5
-
10Mhz range


100BaseFx Ethernet


Uses 4b5b
-
NRZI on fiber


This increases the bits sent to 125Mbps


100BaseT Ethernet
-

2 pair Cat 5


Uses 4b5b MLT
-
3 on twisted pair

CSIS 625

5

4b5b
-

Why


4b5b table on next page


only 16 of the 32 5b symbols needed for data.


Plus a few for control


Ensures that transitions still present on line for clock
recovery


No more than 3 zeros in a row


More efficient than Manchester


MLT
-
3
-

Multi Level Transmit


Transition on a 1, no transition on a 0


Goes +1, 0,
-
1, 0, +1, 0,
-
1, 0, etc.


Reduces bandwidth to 31.25Mhz

CSIS 625

6

4b5b table

Symbol

Meaning

4b Code

5b Code

0

Data 0

0000

01110

1

Data 1

0001

01001

2

Data 2

0010

10100

3

Data 3

0011

10101

4

Data 4

0100

01010

5

Data 5

0101

01011

6

Data 6

0110

01110

7

Data 7

0111

01111

Symbol

Meaning

4b Code

5b Code

8

Data 8

1000

10010

9

Data 9

1001

10011

A

Data A

1010

10110

B

Data B

1011

10111

C

Data C

1100

11010

D

Data D

1101

11011

E

Data E

1110

11100

F

Data F

1111

11101

Symbol

Meaning

5b Code

I

Idle

11111

J

SSD

11000

K

SSD

10001

T

ESD

01101

R

ESD

00111

H

Error

00100

V

Invalid

00000

V

Invalid

00001

Symbol

Meaning

5b Code

V

Invalid

00010

V

Invalid

00011

V

Invalid

00101

V

Invalid

00110

V

Invalid

01000

V

Invalid

01100

V

Invalid

10000

V

Invalid

11001

CSIS 625

7

Ethernet Physical Layer cont.


1000BaseX Ethernet
-


Uses 8b10b NRZ @ 1.25Gbaud


8b10b Ensures 50% ones density


1000BaseT Ethernet
-

4 pair Cat 5


Uses PAM5 (Pulse Amplitude Modulation)


Provides 2 bits, plus extra symbol for FEC, special
codes, transition density.


Each pair is used in both directions with echo
cancellation


The use of PAM5 is a 6dB hit, but made up for
with FEC.

CSIS 625

8

Ethernet Physical layer


10G Ethernet


Use of PAM5 on Fiber?


Use of SONET framing?


Wait and see


All 100Base & 1000Base


Send idle codes when line is not in use.


Keeps clocks in sync.


Lets connections know when cable is broken


10BaseX


Had link pulse to keep nodes aware of connection
status, but that isn’t used now that everything is point to
point.

CSIS 625

9

Ethernet MLT
-
3 & PAM
-
5


Eye diagrams for MLT
-
3 from 100BaseT
and PAM
-
5 from 1000BaseT


100Base
-
T2 also uses PAM5

CSIS 625

10

Collision Domain


10BaseX


512 bit times => about 2km


100BaseX


512 bit times => about 200m


1000BaseX


512 byte times => about 200m


Has to extend short frames to 512 byte times


Didn’t want to expand minimum frame size because
of mixed environments.


Some methods exist to send multiple frames so the
bandwidth isn’t wasted

CSIS 625

11

More collision stuff


With xBaseT standards, the hub sends the
incoming signal to everyone but the sender.


The Sender knows a collision occurred if it
receives anything while transmitting.


With Switched Ethernet, the collision
domain doesn’t matter much.


With full duplex collisions do not exist


CSIS 625

12

VLANs


VLAN
-

Virtual LAN


This is not the same as VPN


Virtual Private Network


VLANs are a configuration on some switches that
group multiple ports together as one LAN or
broadcast domain.


Different VLANs must be bridged using a router


Often this router functionality is in the same box.


VLANs can span switches, by adding a field to the
Ethernet frame that has a VLAN number in it.


All switches must be configured with the same set of
numbers

CSIS 625

13

Token Bus


802.4


Since CDMA/CD leads to unknown amount
of delay before a packet is transmitted


Token passing architectures were
developed.


Token bus uses coaxial cable with
broadband (RF) modulation.


1, 5, & 10 Mbps possible


Token Bus allows for 4 priorities of traffic
at each node.


Frame format slightly different from 802.3

CSIS 625

14

Token bus
-

Token Passing


Each node gets a Token


Node has the right to transmit for some time period.


When done, Node transmits the Token to the next node


Periodically, a node solicits bids from new nodes
wanting to join the ring.


If one responds


it is inserted into the ring and placed in order
after the solicitor


If two respond


a collision occurs and a


To leave the ring, a node tells it’s predecessor who it’s
successor is


If transmission failure in token passing


Retry of sending token


Then, sending broadcast to find out who’s next and giving
them token

CSIS 625

15

Token ring


802.5


Cabling of each node having two connections


one to each of it’s neighbors.


Shielded twisted pair


1, 4 or 16Mbps using differential Manchester


Typically wired in a star shaped ring


All spokes plug into MAU that has relays that allow
isolation of failed spokes.


MAU


Multi
-
Station Access Unit


Each bit arriving at an interface is copied into a 1
-
bit buffer and then copied out onto the ring again.


Each interface creates a 1
-
bit delay


Each node can prioritize traffic

CSIS 625

16

Token Ring


Token passing


A token circulates the ring when it is idle.


Station wanting to transmit grabs the token and
transmits data frame.


When done with the frame, and the frame has
come back around


node transmits a token again.


As frames go by


there is a priority field that a
node may modify if it has higher priority traffic


Other nodes that have lower priority traffic will then
pass on the token until the requesting node gets it.

CSIS 625

17

FDDI
-

Fiber Distributed Data Interface


100Mbps over Fiber optic lines


2000m
max


Uses a token passing architecture similar to
Token ring


Typically is not set up with a star
-
shaped
ring

CSIS 625

18

Token vs. CSMA/CD


Token passing architectures allow for
prioritization of traffic and guarantees that this
traffic will get through in a fixed amount of time.


Even if heavily loaded


high priority traffic gets
through


CSMA/CD architecture allows for lower latencies
when the LAN is lightly loaded


A node doesn’t have to wait for a token.


CSMA/CD implementations tend to be simpler
(and therefore cheaper)

CSIS 625

19

ATM LANs


They’re dead Jim


ATM allows for prioritization of traffic, mixing
very time critical traffic with non
-
time critical
traffic


Protocol elegance has been overpowered by cheap
silicon.


Has nice idea of being able to use same protocols
from WANs, MANs, and LANs


voice, video
and data.


In MANs/WANs ATM is over SONET links


OC
-
3, OC
-
12, OC
-
48 (155Mbps, 622Mbps, 2.4Gbps)


In LANs, ATM is over twisted pair


25Mbps, or 51Mbps

CSIS 625

20

Fibre Channel


A high speed protocol over fiber optics that
is tailored to use for computer interfaces.


Looks similar to SCSI from a software
perspective.


Very common in Storage Area Networks
(SANs)


1.06 Gbps


Up to 10km over single mode fiber

CSIS 625

21

HIPPI


HIgh Performance Parallel
Interface


800 or 1600Mbps using 50 or 100 twisted
pairs.


Developed in late 80’s when LAN was
10Mbps ethernet


Used for supercomputer interfaces


Was relatively cheap to create as it used
parts from other technologies

CSIS 625

22

Ethernet In First Mile


A new IEEE group


Looking at ways to use Ethernet to connect
to your home.


Currently unsure if this will be point to
point, or using PON technology


PON


Passive Optical Network.


Use of optical splitter/combiners that don’t require
any electronics.


PON technology requires 2 “tricky” things


Ranging of nodes


Contention for the upstream bandwidth.

CSIS 625

23

802.11
-

Wireless Ethernet


Started out as 1 or 2Mbps using RF or infra
-
red links.


Recently added 5.5 and 11Mbps


RF uses frequencies around 2.4GHz


This is the same range used by microwave
ovens


Water absorbs this energy very well making
hard to use for long distances.

CSIS 625

24

802.11


For last mile


Since this is unlicensed spectrum, people have
started to use this with directional antennas for
Internet Access.


Unlicensed spectrum


RF bandwidth that you don’t
need to buy a license from the government to use


To overcome the problems with water absorption,
the link has to be carefully set up


Line of site links


No trees in the way


Typically will run at less than peak rates

CSIS 625

25

802.11


Additional wireless links


http://www.wlana.com/


http://www.wirelessethernet.org/


http://www.midcoast.net/wirelessfaq.html


http://www.stonebridgewireless.net/SBWireless%20Files/faqs_page2.htm

CSIS 625

26

RPR
-

Resilient Packet Rings


802.17


Another new technology


just starting out in the IEEE


Idea is to provide good protection that Sonet rings enjoy,
using packet technology (most likely gig
-
Ethernet)


Sonet Rings are circuit switch oriented, and reserve 50% of
the bandwidth on the ring for protection purposes.


A Sonet protection switch occurs within 60ms


Packet rings or meshs today rely on the routing protocols
to converge when a failure occurs


This will often take minutes


RPR goal is to protection switch as fast as Sonet

CSIS 625

27

Switching (generic)


A switched network has both end stations or nodes
and switching nodes.


Switching nodes may connect to


only other switching nodes
-
or
-


other switching nodes and end stations


Connections between switching nodes can handle
multiple circuits or sessions using TDM or FDM


Often not enough resources in the network for all
nodes to be communicating simultaneously.

CSIS 625

28

Switching (generic)


Switching nodes don’t have direct connections to
all other switching nodes


Usually there is more than one way to get between
any two switching nodes


This improves reliability


Circuit switching is common in the public
telephony networks


Packet switching is common in data networks.


Latency
-

the amount of time it takes a signal to
propagate from one node to another.


Jitter
-

the amount of variance in the latency

CSIS 625

29

Circuit Switching


Circuit Switching
-

A dedicated path is
established between two stations for
communication


Once a connection is established it appears
to the attached devices as if they are directly
connected.


Switching and transmission resources are
reserved for the exclusive use of the circuit
for the duration of the connection.

CSIS 625

30

Circuit Switching Networks


Communication over a dedicated path has 3
parts to it


Circuit establishment


Data Transfer


Circuit disconnect

CSIS 625

31

Circuit Lifespan


Circuit Establishment


Node has to signal to switching nodes who it wants to
talk to.


Switching node finds and establishes path through
network


Success or failure of circuit setup is communicated
back to originating node


Data transfer


typically full duplex communication of data


Circuit disconnect


Both ends informed of disconnect and resources in
network released for another user.

CSIS 625

32

Circuit Switching
-

pros & cons


Pathway in place even if data isn’t being sent


Not the best efficiency


There may be some delay in getting a circuit set
up, but once done the


latency is typically minimal


jitter is very small

CSIS 625

33

Space Division Switches


Space division switching uses space to
separate the paths of different circuits


Cross bar switches have n inputs and m
outputs and a switch at every intersection


needs n * m switches


Multistage switches


Use multiple smaller crossbar switches

CSIS 625

34

Cross bar switches


Simplest cross bar has n inputs and m
outputs

inputs

1

2

3

4

...

n

outputs

1 2 3 4 . . . n

CSIS 625

35

Folded Cross bar switch


A folded cross bar switch has inputs and outputs
wrapped around to allow full duplex any to any
connections.

CSIS 625

36

Multi stage switches


Combine multiple smaller crossbar switches


May be blocking or non
-
blocking based on
number of input lines and stages and size of
different stages


Also called Clos switches after Charles Clos of
Bell Laboratories who published analysis of these
type of switches


Usually has multiple paths increasing reliability of
equipment

CSIS 625

37

Three stage switch Example

N/n
x
N/n

N/n
x
N/n

N/n
x
N/n

n x k

N/n Arrays

N

inputs

n x k

n x k

...

...

k x n

N/n Arrays

N

outputs

k x n

k x n

...

CSIS 625

38

Multi
-
stage switches pros and cons


Allows for a far greater number of lines to
be serviced with a far smaller number of
cross connect switches


If blocking is allowable, even smaller
number of switches

CSIS 625

39

Time Division circuit switches


By using TDM techniques, switching can be
achieved


Time
-
slot Interchange (TSI) technique


Mux and demux on ends with the TSI in the
middle


The TSI buffers all inputs and then re
-
sends it
in a different output


TDM bus technique


Mux and demux are “smart” and can take any
timeslot to any I/O port

CSIS 625

40

Time Division pros and cons


With higher clocking speeds possible with
modern silicon, it is possible to build TDM
switches much larger and cheaper than
crossbar switches


The clock rates after the TDM operation
may get very high when high speed inputs
or high number of inputs are used.

CSIS 625

41

Circuit Switches
-

combinations


For very large circuit switches a
combination of TDM and crossbar switches
is often the best.


Multiple stage switches using different
techniques in different stages

CSIS 625

42

Packet Switching


Packet Switching
-

a packet of data is
transferred from one node to another


Packet switching is designed to be more
efficient for data than circuit switching.


Datagram Packet Switching
-

Each packet
of data is treated independently from all
others


Virtual Circuit Packet Switching
-

all
packets of data that are part of a session are
sent via a single route

CSIS 625

43

Packet switching


Packets allow for greater usage of trunks
between nodes


Packets may be queued and transmitted as
quickly as possible


May create more latency and much more jitter
than circuit switched systems


Unused Bandwidth in circuit switched networks
can be used.


On circuit switched networks, calls may be
blocked, but on packet switching the
packets are just delayed

CSIS 625

44

Packet switching
-

prioritization


In packet switched networks, some packets
can be given higher priority than others.


This allows for those packets to be sent
before lower priority ones reducing their
latency and jitter.


May allow for some packets to be sent
different (faster) paths than others.

CSIS 625

45

Datagram packet switching


Every packet is treated independently.


Every switching node must examine every
packets destination and decide on where to
send it next.


If any node disappears it takes with it a few
packets, but data keeps flowing.

CSIS 625

46

Virtual circuit packet switching


A path is set up that each packet between
two nodes always follows the same path


Makes switching node’s job easier because
it always knows where to send a packet to
next.


If any node disappears, that session must be
torn down and a new one created

CSIS 625

47

Virtual circuit types


Switched virtual circuit
-

a virtual circuit is
set up for each session independently


Requires a circuit setup before data flows


Adds to time required to transfer a message


Permanent virtual circuit


Requires provisioning of path when creating
network.


May mean that any node failure requires human
intervention.

CSIS 625

48

Mid
-
term


Everyone is allowed two 8½” x 11” sheet of
paper, single side, worth of notes.


Note sheet will be turned in with exam.


Name must be written on back side of note sheet.


No open books or notes other than the allowed
sheet of paper.


Exam will be similar to homework assignments.


Some Vocabulary


Some Short answer (short essay)


Any Questions?