An Introduction to Concepts and
ethernet: one ethernet is one "collision domain"
cabling rules ("4-repeater", etc.) allow growth of a
limited distance:~2500m or less, depending on cable
limited number of stations: 1024 (architecturally); 100
(practical, olden days); 20-30? (practical, today)
when you grow beyond these limits, build another
ethernet and connect the two together
the issue: expanding a local network
the issue: expanding a local network...
Intermediate stations repeat
Receiver copies data and
continues to repeat
Sender generates new token
one token ring is one "token path"
limited distance: ~2000m or so, depending on cabling
limited number of stations: 250 or fewer, depending on
why connect or split LANs?
why connect LANs?
to allow sharing of files, devices, etc.
why split LANs?
to provide physical security/isolation
to implement policies (user groups, etc)
to give greater average bandwidth per user
("segmentation" or "microsegmentation")
so, what are our options for interconnecting the
LAN segments we create?
the issue, restated: which LAN frames should be
forwarded from one segment to another?
a LAN frame on an ethernet:
SNA, IP, IPX, AppleTalk Address
Token-Ring, Ethernet Address
Also known as MAC address (Media
LAN interconnection technologies
at layer 1, connects two physical LAN segments
at layer 2, connected LANs look like a single logical
e.g., bridge forwards LAN broadcasts
forwards frames based on layer 2 info (e.g., MAC
thus, independent of higher layer protocols
easy to implement -- little or no configuration
bridges agree on a single path through the
path is called a "spanning tree"
all LAN traffic follows that single path
frames forwarded based on MAC address
parallel bridges may exist, but are inactive
source routing bridge
commonly used in token ring networks (not ethernet)
each ring is given a ring number (unique in the whole bridged
each bridge is given a bridge number (unique between same
pair of rings)
end stations discover routes via a broadcast process
bridges place path of broadcast in the frame (routing info
that same path (rings and bridges) is then used for other
frames forwarded based on routing info field in frame
for connection-oriented protocols, broadcast occurs
only when connection is established
parallel active paths are allowed
B via Rt 1!
B via Rt 2!
source routing bridge...
basically a fast, multiport, layer-two device
i.e., similar in function/capability to a bridge
fast, since functions often performed in hardware
low latency -- good for fast response time
easy implementation, low cost
each port connects to a separate LAN segment
shared or dedicated
dedicated ports may operate in full-duplex mode
router isolates logical subnetworks for more efficient
layer 2 traffic not typically forwarded unless addressed to
each subnetwork is given an identification--e.g., IP subnet;
IPX network number
end station sends traffic to router; router forwards
toward ultimate destination
router must understand the layer 3 protocol(s) it is to
routing protocols allow router to understand network
protocols (IP, IPX, NetBIOS, SNA, Appletalk, ...)
how do they work?
do they have a layer 3 structure (are they "routable?")
how often do they broadcast? how much traffic?
end user response time--delay/latency in the
configuration of router vs bridge/switch
network operations--e.g., moves/changes
network management, troubleshooting, etc.
example - distributed backbone with
example - distributed backbone with
easy to implement--little configuration
administration is easy
potential bridge congestion, depending on which
bridge management harder since bridges
example - collapsed backbone with
example -- collapsed backbone with
same as distributed bridge design, plus
centralized bridges/backbone hub are easier to
servers can be centralized while still physically
connected to floor LAN segments
same as distributed bridge design
riser cable considerations
fiber? copper? distance? port cost on device?
example - collapsed backbone router
example -- collapsed backbone router
more powerful device than bridge--faster, more
router limits broadcast traffic between subnets
more expensive device than bridge
operation, management much more complex than
user moves more complicated to handle--subnets
broadcast traffic not usually a problem in campus--
different from a WAN link
example -- collapsed backbone switch
same pros as bridged network -- low cost, easy
implementation and administration
avoids subnet issues with user moves
higher performance and lower latency than bridge or
servers can be attached to dedicated switch ports for
being deployed today as front end to router
Trend today is to use switching within a campus, and
routing for lower speed WAN links
what about campus backbone
generic picture: LANs (ethernet, token ring)
connected with some kind of high speed backbone
2 or 3 popular backbone technologies
the issues of interconnection devices are still the
same as before
latency; intelligence; administration; cost; etc.......
"big pipe" technologies
...i.e., a faster flavor of what you have today
e.g., fddi, fast ethernet
simplicity; scalability; faster speed to attached devices
sensitive to wiring installation quality
upgrades may be required to hub and all stations
some problems cannot be solved with more bandwidth
--- latency! (bigger pipe doesn't change the
interconnection device--still use switches or routers)
cell switching (ATM)
ATM: a layer 2 technology based on cell switching
low latency for high throughput
multiple traffic types in cells--mixed voice, data, multimedia
scalable from low to high speeds
25Mbps to ... 155Mbps? 622Mbps? 2.4Gbps?
individual links can be different speeds
Quality of Service (QoS) allows (will allow) applications
to specify the network service characteristics they need
LAN Emulation allows applications to use ATM without
mixed traffic (voice/video/data/multimedia)
high speed; scalable speed
very low latency
Quality of Service support
point to point technology allows broadcast
control (see IBM's MSS Server)
campus LAN interconnection
interconnection devices: bridge, switch, router
switches preferred today within campus
fast; low latency; easy implementation/administration
routers good for controlling use of low speed WAN links
campus backbone technologies
big pipes: fast ethernet, fddi
easy to deploy; faster speed to attached devices;
may or may not solve response time/performance
supports voice, video, data; gives true traffic control
for new applications; issues are cost, education
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