Transcript Local area Network

CS3505:
Loca Area Networks
CSMA/CD : IEEE 802.3
and
LAN Internetworks
CSMA/CD

basic LAN architecture and protocol

most widespread LAN; estimate: more that 50% of all
LANs on the Internet... implemented by many different
companies

developed by Robert Metcalfe, XEROX PARC, early
1970s..... led to founding of “3COM” company, (Santa
Clara). [later Metcalfe sold his company for $400M)
CSMA / CD : basic protocol
 broadcast
medium
 first version used bus
when the MAC receives a packet to transmit:
1. sense carrier (listen);
if no signal detected
then begin Xmitting message, & continue sensing;
if collision detected
then Xmit jam, stop Xmitting, wait, then goto (1);
when end-of-packet Xmitted, END.
else {carrier is busy} go to (1)
CSMA / CD : basic protocol
 the
“wait random time” is precisely defined:
binary exponential backoff
 physical
encoding: digital signals, manchester
encoding. Broadcast medium.
 because
short
collisions are detected, wasted time is
CSMA / CD : backoff algorithm
binary exponential backoff (BACKOFF)
1
slot = 51.2 s
 time
a
following collision measured in slots
random slot between 1 and 1023 is chosen, and
the packet is retransmitted then
CSMA / CD : backoff algorithm
 example:
happens?
 what
 why
suppose 2 stations collide. What
is Prob [another collision]?
measure time in 51.2 s slots?
CSMA / CD : frame format
length: 64 to 1518 bytes
preamble : 7 bytes; SOF : 1 byte;
DA,SA : 2/6 bytes; length : 2 bytes;
data : 0-1500; pad : 0-46; FCS : 4
CSMA / CD : topologies, media , etc.
 transmission
media
 coaxial
cable (decreasing)
 twisted pair (Cat 5, widely used)
 fiber (less common but increasing)
 topologies
 bus
- original design; used for many years
 star with dumb or smart hub; now usual
 data
 10
rates
Mbps
 100 Mbps
 Gbps where needed available
CSMA / CD : network components
 medium
(coax, tp, fiber)
 transceivers
 drop
 NIC
cable; station to coax
(MAC protocol logic)
 repeaters
(needed to extend coax)
 test
equipment
 hub
(multiport repeater) : for star configuration
 bridges
(to connect to other LANs)
CSMA / CD : physical layout
coax. cable, physical(and logical) bus CSMA/CD
CSMA / CD : physical layout
star/hub configuration. physical star, logical bus
2-twisted pair connections; hub is a repeater
CSMA / CD : some specifics
 prop
speed 0.77 c on coax, 0.59 c on t.p.
 at most 4 repeaters between 2 stations; so at most
5 cable segments
 500 m /segment max, or 2000 with repeaters
(coax);
 drop cables 25 m max
 max 100 stations per segment on coax
 at least 2.5 m between adjacent receivers on coax
 at most 1024 stations per ethernet
CSMA / CD : standard s
IEEE 802.3 : several physical configurations:
 10BASE5
: baseband coaxial cable; original
 10BASE2
: thin coaxial cable; cheaper alternative
 10BASE-T
: twisted pair, hub configuration
 10BROAD36
 10BASE-F
: uses broadband coax (TV cable)
: fiber
 100BASE-X
: fiber OR twisted pair
NOTE: all use the same frame format and basic
MAC protocol
CSMA / CD : practical considerations
 you
have 3 PCs in your house; what will it take
(equipment/dollars) to connect them together ?
 design
or plan a network for a small business
with 30-40 machines (PCs, Macs, etc), which is
located in a single building. (eqmt, dollars)
CS3505:
Bridges / LAN internetworks
Bridges : connecting LANs together
 why
do we need to connect LANs
 what
is a bridge?
 types
of bridges
 routing
in LAN internetworks
 comparison:
bridges, routers, repeaters
 connecting
similar LANs
 connecting
dissimilar LANs
why LANs need to be connected
1. connect 2 existing LANs (CS, math)
2. LAN too big; split it, but stay connected
-- too many stations or traffic for one LAN
3. connect geographically separate LANs.
-- 2 offices in different towns
4. reduce collisions
--increase efficiency
5. security
--help restrict traffic to one LAN
bridge : what is it?
 low
level “switch” that connects two or more
LANs. “low level” => “MAC layer”
 transparent
: there is no change in the LANs or in
the protocols of the networks
 able
to do simple routing
 retains
 faster
the simplicity and flexibility of LANs
than “software” switches (routers)
 reasonable
cost; cheaper than routers
bridge or switch? terminology
 1st
bridges sold in 1984, were 2-ports
 early 1990s, multiport bridges appeared; were
called “switches” by marketing vendors;
technically no difference between a “switch”
(layer 2) and a bridge
 multiple LANs connected by high port density
bridges commonly called “switched LANs” actually an internet of LANs
 “switching hub”, “LAN switch” - other terms for
a multiport bridge
bridges & LAN connectors :types
 local
bridge
 remote
 same
(2 half bridges)
LAN, different LAN
 two
port, multiport
 hub
(not a bridge)
 repeater
(not a bridge)
 router(not
a bridge)
bridge : basic function
suppose a bridge B connects networks X & Y.
Then B :
1. reads all packets on X and Y, noting the
destination, source addresses (DA, SA)
2. each packet on Y with DA on X is copied and
transmitted on X.
3. each packet on X with DA on Y is copied,
transmitted on Y.
 the
bridge operates on X, Y using the MAC
protocol of those LANs.
bridge connecting 2 ethernets
notes on bridges
 all
stations have unique MAC addresses
 bridge must “know” which LAN station is on
 multi-port bridges - similar; extends to multiple
LANs
 no change or adjustment in NIC needed; bridge
completely transparent
 bridge operates on each LAN using the MAC
protocol
 remote (half bridge) - may use another protocol
between the 2 half bridges, while using MAC on
each LAN
LAN internet
half - bridge, connecting 2 LANs
2
halves communicate through some other
protocol, e.g., PPP, HDLC.
bridges - routing
 how
do bridges “know” which packets to
forward, and in which direction?
2 basic techniques :
1. fixed routing - the information is loaded
manually into the bridge (typing it in, etc.). This
info is then stored in a routing table.
2. dynamic routing: “learning bridges” the bridge “learns” where the stations are by
watching the traffic on its ports
bridges - routing

for fixed routing, many topologies possible

dynamic routing - the internet must be configured as a
tree; this simplifies routing

tree : LANs and bridges are the nodes, and the links
between them are the edges, and
 LANs
can be connected only to bridges, not (directly)
to other LANs

if a cycle exists, the bridges will detect it and remove one
from the active network, so that a tree structure is
maintained
bridges - dynamic routing
 bridge
has a routing table, 3 fields :
[ dest.address, next port # , time]
when bridge receives a packet [DA,SA] on port X:
1. if SA found in table, reset timer,
else add [SA, port#, time ] to table.
2. if [DA] found in table send packet out on next
port indicated; else send packet on all ports
except X.
bridge dynamic routing
 timer:
typical value : 300 seconds (why have the
timer? is this a good default value?)
 given
the tree structure, bridges will learn a
station’s direction (explain how?)
 MAC
addresses could be divided into (network,
station) parts. If so, tables can be made smaller,
but same algorithm used.
bridges, routers, repeaters, hubs

repeaters : simply connect 1 cable to another, repeat the
bits. No routing decisions or filtering.

hubs : serve to extend the ethernet. No routing or filtering
of messages.

bridges - connect LANs together at the MAC layer; filter
and rout messages at the MAC layer.

routers -. Layer 3/3.5 (internet). Software, IP protocol..
Usually more expensive. Discussed in CS4550 ....