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Quiz #5
What is the difference between DCF
and PCF?
1
Note 7: Local Area Networks
LAN Bridges
2
Hubs, Bridges & Routers
• Hub: Active central element in a star topology
–
–
–
–
Twisted Pair: inexpensive, easy to insall
Simple repeater in Ethernet LANs
“Intelligent hub”: fault isolation, net configuration, statistics
Requirements that arise:
User community grows, need to interconnect hubs
Hubs are for different types of LANs
?
Hub
Two Twisted
Pairs
Two Twisted
Pairs
Station
Hub
Station
Station
Station
Station
Station
3
Hubs, Bridges, Routers & Gateways
• Interconnecting Hubs
– At the physical layer

Repeater
– At the MAC or data link layer

Higher
Scalability
Bridges
– At the network layer

Router
– At even higher layers

Gateway
?
Hub
Hub
Two Twisted
Pairs
Station
Two Twisted
Pairs
Station
Station
Station
Station
Station
4
General Bridge Issues
Network
Network
LLC
LLC
MAC
802.3
802.3
802.5
802.5
MAC
PHY
802.3
802.3
802.5
802.5
PHY
802.3
CSMA/CD
802.5
Token Ring
• Operation at data link level implies capability to work
with multiple network layers
• However, must deal with
– Difference in MAC formats
– Difference in data rates; buffering; timers
– Difference in maximum frame length
5
Bridges of Same Type
Network
Network
Bridge
LLC
LLC
MAC
MAC
MAC
MAC
Physical
Physical
Physical
Physical
• Common case involves LANs of same type
• Bridging is done at MAC level
6
Transparent Bridges
• Interconnection of IEEE LANs with
complete transparency
• Use table lookup, and
– discard frame, if source & destination
in same LAN
– forward frame, if source & destination
in different LAN
– use flooding, if destination unknown
• Use backward learning to build table
– observe source address of arriving
frames
– handle topology changes by removing
old entries
S1
S2
S3
LAN1
Bridge
LAN2
S4
S6
S5
7
S1
S2
S3
LAN1
LAN2
LAN3
B1
Port 1
S5
S4
B2
Port 2
Address Port
Port 1
Port 2
Address Port
8
S1→S5
S1
S2
S3
S1 to S5
S1 to S5
S1 to S5
LAN1
S1 to S5
LAN2
LAN3
B1
B2
Port 1
Port 2
Address Port
S1
S5
S4
1
Port 1
Port 2
Address Port
S1
1
9
S3→S2
S1
S2
S3
S3S2
S3S2
S3S2
S3S2
S3S2
LAN1
LAN2
LAN3
B1
B2
Port 1
Port 2
Address Port
S1
S3
S5
S4
1
2
Port 1
Port 2
Address Port
S1
S3
1
1
10
S4S3
S1
S2
S3
S4
B1
Port 1
S4S3
Port 2
Address Port
S1
S3
S4
1
2
2
S3
S4S3
S4S3
LAN1
S5
S4
LAN2
LAN3
B2
Port 1
Port 2
Address Port
S1
S3
S4
1
1
2
11
S2S1
S1
S2
S3
S5
S4
S2S1
LAN1
LAN2
S2S1
LAN3
B1
B2
Port 1
Port 2
Address Port
S1
S3
S4
S2
1
2
2
1
Port 1
Port 2
Address Port
S1
S3
S4
1
1
2
12
Adaptive Learning
• In a static network, tables eventually store all
addresses & learning stops
• In practice, stations are added & moved all the
time
– Introduce timer (minutes) to age each entry & force it
to be relearned periodically
– If frame arrives on port that differs from frame
address & port in table, update immediately
13
Avoiding Loops
LAN1
(1)
(1)
B1
B2
(2)
B3
LAN2
B4
LAN3
B5
LAN4
14
Spanning Tree Algorithm
1. Select a root bridge among all the bridges.
• root bridge = the lowest bridge ID.
2. Determine the root port for each bridge except the root
bridge
• root port = port with the least-cost path to the root bridge
3. Select a designated bridge for each LAN
• designated bridge = bridge has least-cost path from the LAN
to the root bridge.
• designated port connects the LAN and the designated bridge
4. All root ports and all designated ports are placed into a
“forwarding” state. These are the only ports that are
allowed to forward frames. The other ports are placed
into a “blocking” state.
15
LAN1
(1)
(1)
B1
B2
(1)
(2)
(2)
LAN2
B3
(3)
(2)
(1)
B4
(2)
LAN3
(1)
B5
(2)
LAN4
16
LAN1
(1)
(1)
B1
Bridge 1 selected as root bridge
B2
(1)
(2)
(2)
LAN2
B3
(3)
(2)
(1)
B4
(2)
LAN3
(1)
B5
(2)
LAN4
17
LAN1
(1)
R (1)
B1
B2
(2)
(2)
LAN2
R
(1)
B3
R (1)
Root port selected for every
bridge except root bridge
(3)
(2)
B4
(2)
LAN3
R (1)
B5
(2)
LAN4
18
LAN1
D (1)
R (1)
B1
B2
(2)
D (2)
LAN2
R
(1)
B3
R (1)
Select designated bridge
for each LAN
D (2)
(3)
D
B4
(2)
LAN3
R (1)
B5
(2)
LAN4
19
LAN1
D (1)
R (1)
B1
B2
(2)
D (2)
LAN2
R
(1)
B3
R (1)
All root ports & designated
ports put in forwarding state
D (2)
(3)
D
B4
(2)
LAN3
R (1)
B5
(2)
LAN4
20
Source Routing Bridges
• To interconnect IEEE 802.5 token rings
• Each source station determines route to
destination
• Routing information inserted in frame
Routing
control
2 bytes
Route 1
Route 2
designator designator
2 bytes
2 bytes
Destination Source
Routing
address
address information
Route m
designator
2 bytes
Data
FCS
21
Route Discovery
• To discover route to a destination each station
broadcasts a single-route broadcast frame
• Frame visits every LAN once & eventually
reaches destination
• Destination sends all-routes broadcast frame
which generates all routes back to source
• Source collects routes & picks best
22
Detailed Route Discovery
•
•
•
•
•
Bridges must be configured to
form a spanning tree
Source sends single-route frame
without route designator field
Bridges in first LAN add incoming
LAN #, its bridge #, outgoing LAN
# into frame & forwards frame
Each subsequent bridge attaches
its bridge # and outgoing LAN #
Eventually, one single-route frame
arrives at destination
•
•
•
•
•
When destination receives singleroute broadcast frame it responds
with all-routes broadcast frame
with no route designator field
Bridge at first hop inserts
incoming LAN #, its bridge #, and
outgoing LAN # and forwards to
outgoing LAN
Subsequent bridges insert their
bridge # and outgoing LAN # and
forward
Before forwarding bridge checks
to see if outgoing LAN already in
designator field
Source eventually receives all
routes to destination station
23
Find routes from S1 to S3
LAN 2
S1
B4
LAN 4
B1
LAN 1
S2
B5
B3
B7
B2
S3
B6
LAN 3
LAN1
B1
B3
LAN3
B4
LAN4
LAN 5
B6
LAN5
LAN2
24
LAN 2
S1
LAN 4
B4
B1
S2
LAN 1
B3
B5
LAN 3
B6
B7
B2
B6
LAN3
S3
B2
LAN1
B1
LAN2
B3
LAN2
B1
B4
LAN1
LAN4
LAN4
B4
LAN2
B5
LAN5
B7
B1
B4
B7
LAN 5
B4
B2
B5
B7
B1
B3
LAN4
B5
B7
LAN1
B2
B2
LAN3
B2
B5
B6
B1
B1
B4
LAN1
B3
B5
B6
B1
LAN2
LAN1
B3
B4
B2
LAN2
LAN4
B5
LAN1
B3
LAN3
B3
LAN3
B2
B3
B6
LAN1
LAN2
25
Virtual LAN
VLAN 1
S3
VLAN 2
S6
VLAN 3
S9
Floor n + 1
Physical
S2
S5
S8
partition
Floor n
1 2 3 4 5 6
or
7
8
switch
9
Bridge
S1
S4
S7
Floor n – 1
Logical partition
26
Per-Port VLANs
VLAN 1
S3
VLAN 2
S6
VLAN 3
S9
Floor n + 1
S2
S5
S8
Floor n
1 2 3 4 5 6
Bridge
7
or
8
switch
9
S1
S4
S7
Floor n – 1
Logical partition
Bridge only forwards frames to outgoing ports associated with same VLAN
27
Tagged VLANs
• More flexible than Port-based VLANs
• Insert VLAN tag after source MAC address in
each frame
– VLAN protocol ID + tag
• VLAN-aware bridge forwards frames to outgoing
ports according to VLAN ID
• VLAN ID can be associated with a port statically
through configuration or dynamically through
bridge learning
• IEEE 802.1q
28
ECE 683
Computer Network Design & Analysis
Note 8: Packet Switching Networks
(Network Layer Protocols)
29
Outline
• Network Services and Internal Network
Operation
• Packet Network Topology
• Datagrams and Virtual Circuits
• Routing in Packet Networks
• Shortest Path Routing
30
Network Layer
• Network Layer: the most complex layer
– Requires the coordinated actions of multiple,
geographically distributed network elements
(switches & routers)
– Must be able to deal with very large scales
 Billions
of users (people & communicating devices)
– Biggest Challenges
 Addressing:
where should information be directed to?
 Routing: what path should be used to get information there?
31
Packet Switching
t1
t0
Network
• Transfer of information as payload in data packets
• Packets undergo random delays & possible loss
• Different applications impose differing requirements on
the transfer of information
32
Network Service
Messages
Messages
Segments
Transport
layer
Transport
layer
Network
service
Network
service
End
system
α
Network
layer
Network
layer
Network
layer
Network
layer
Data link
layer
Data link
layer
Data link
layer
Data link
layer
Physical
layer
Physical
layer
Physical
layer
Physical
layer
End
system
β
• Network layer can offer a variety of services to transport layer
• Connection-oriented service or connectionless service
• Best-effort or delay/loss guarantees
33
Network Service vs. Operation
Network Service
• Connectionless
– Datagram Transfer
• Connection-Oriented
– Reliable and possibly
constant bit rate transfer
Internal Network Operation
• Connectionless
– IP
• Connection-Oriented
– ATM
Various combinations are possible
• Connection-oriented service over Connectionless operation
• Connectionless service over Connection-Oriented operation
• Context & requirements determine what makes sense
34
Complexity at the Edge or in the Core?
C
12
3
21
End system
α
4 3 21
End system
β
12
3
21
Medium
A
12
3
B
2
1
Network
1
Physical layer entity
2
Data link layer entity
3
Network layer entity
21
123 4
3
Network layer entity
4
Transport layer entity
35
The End-to-End Argument
for System Design
• An end-to-end function is best implemented at a higher
level than at a lower level
– End-to-end service requires all intermediate components to work
properly
– Higher-level better positioned to ensure correct operation
• Example: stream transfer service
– Establishing an explicit connection for each stream across
network requires all network elements (NEs) to be aware of
connection; All NEs have to be involved in re-establishment of
connections in case of network fault
– In connectionless network operation, NEs do not deal with each
explicit connection and hence are much simpler in design
36
Network Layer Functions
Essential
• Routing: mechanisms for determining the set
of best paths for routing packets requires the
collaboration of network elements
• Forwarding: transfer of packets from NE inputs
to outputs
• Priority & Scheduling: determining order of
packet transmission in each NE
Optional: congestion control, segmentation &
reassembly, security
37
Note 8: Packet Switching Networks
Packet Network Topology
38
End-to-End Packet Network
• Packet networks very different from telephone networks
• Individual packet streams are highly bursty
– Statistical multiplexing is used to concentrate streams
• User demand can undergo dramatic change
– Peer-to-peer applications stimulated huge growth in traffic
volumes
• Internet structure highly decentralized
– Paths traversed by packets can go through many networks
controlled by different organizations
– No single entity responsible for end-to-end service
39
Access Multiplexing
Access
MUX
To
packet
network
• Packet traffic from users multiplexed at access to network into
aggregated streams
• DSL traffic multiplexed at DSL Access Mux
• Cable modem traffic multiplexed at Cable Modem Termination
System
40
Oversubscription
• Access Multiplexer
•••
•••
r
r
r
Nr
–
–
–
–
–
nc
Nc
N subscribers connected @ c bps to mux
Each subscriber active r/c of time
Mux has C=nc bps to network
Oversubscription rate: N/n
Find n so that at most 1% overflow probability
Feasible oversubscription rate increases with size
N
r/c
n
N/n
10
0.01
1
10
10 extremely lightly loaded users
10
0.05
3
3.3
10 very lightly loaded user
10
0.1
4
2.5
10 lightly loaded users
20
0.1
6
3.3
20 lightly loaded users
40
0.1
9
4.4
40 lightly loaded users
100
0.1
18
5.5
100 lightly loaded users
41
Home LANs
WiFi
Ethernet
Home
Router
To
packet
network
• Home Router
– LAN Access using Ethernet or WiFi (IEEE 802.11)
– Private IP addresses in Home (192.168.0.x) using Network
Address Translation (NAT)
– Single global IP address from ISP issued using Dynamic Host
Configuration Protocol (DHCP)
42
LAN Concentration
Switch
/ Router
     
     
• LAN Hubs and switches in the access network
also aggregate packet streams that flows into
switches and routers
43
Campus Network
Organization
Servers
To Internet or
wide area
network
s
Servers have
redundant
connectivity
to backbone
s
Gateway
Backbone
R
R
R
S
R
Departmental
Server
S
S
R
R
s
s
High-speed
Only
outgoing
campus leave
packets
backbone
net
LAN
through
connects dept
router
routers
s
s
s
s
s
s
s
44
Connecting to Internet Service Provider
Internet service provider
Border routers
Campus
Network
Border routers
Interdomain level
Autonomous
system or
domain
Intradomain level
s
LAN
s
s
network administered
by single organization
Internet Backbone
National Service Provider A
National Service Provider B
NAP
NAP
National Service Provider C
Private
peering
• Network Access Points: set up during original commercialization
of Internet to facilitate exchange of traffic
• Private Peering Points: two-party inter-ISP agreements to
exchange traffic
National Service Provider A
(a)
National Service Provider B
NAP
NAP
National Service Provider C
(b)
NAP
Private peering
RA
Route
RB
Server
LAN
RC
Key Role of Routing
How to get packet from here to there?
• Decentralized nature of Internet makes routing a
major challenge
– Interior gateway protocols (IGPs) are used to
determine routes within a domain
– Exterior gateway protocols (EGPs) are used to
determine routes across domains
– Routes must be consistent & produce stable flows
• Scalability required to accommodate growth
– Hierarchical structure of IP addresses essential to
keeping size of routing tables manageable
48
Note 8: Packet Switching Networks
Datagrams and Virtual Circuits
49
Packet Switching Network
User
Transmission
line
Network
Packet
switch
Packet switching network
• Transfers packets
between users
• Transmission lines +
packet switches (routers)
• Origin in message
switching
Two modes of operation:
• Connectionless
• Virtual Circuit
50
Message Switching
Message
Message
Message
Source
Message
Switches
• Message switching invented
for telegraphy
• Entire messages
multiplexed onto shared
lines, stored & forwarded
• Headers for source &
destination addresses
• Routing at message
switches
• Connectionless
Destination
51
Message Switching Delay
Source
T
t
Switch 1
t

Switch 2
t
t
Destination
Delay
Minimum delay = 3 + 3T over a path with two intermediate switches
Additional queueing delays possible at each link
52
Long Messages vs. Packets
1 Mbit
message
source
dest
BER=p=10-6
BER=10-6
How many bits need to be transmitted to deliver message?
• Approach 1: send 1 Mbit
message
• Probability message arrives
correctly
6 10 6
Pc  (1  10 )
e
10 610 6
 e 1  1 / 3
• On average it takes about 3
transmissions each hop
• Total # bits transmitted ≈ 6
Mbits (3 transmissions x 2
hops)
• Approach 2: send ten 100kbit packets
• Probability packet arrives
correctly
6
Pc  (1  10 6 )10  e 10 10  e 0.1  0.9
5
5
• On average it takes about
1/0.9=1.1 transmissions/hop
• Total # bits transmitted ≈ 2.2
Mbits (1.1 x 2)
53
Packet Switching - Datagram
• Messages broken into smaller
units (packets)
• Source & destination
addresses in packet header
• Connectionless, packets
routed independently
(datagram)
• Packet may arrive out of order
• Pipelining of packets across
network can reduce delay,
increase throughput
• Lower delay than message
switching, suitable for
interactive traffic
Packet 1
Packet 1
Packet 2
Packet 2
Packet 2
54
Packet Switching Delay
Assume three packets corresponding to one message
traverse the same path with two switches
t
1
2
3
t
1
2
3
t
1
2
3
t
Delay
Minimum Delay = 3τ + 5(T/3) (single path assumed)
Additional queueing delays possible at each link
Packet pipelining enables message to arrive sooner
55
Delay for k-Packet Message over L Hops
Source
Switch 1
Switch 2
t
1
2
3
t

1
2
3
t
1
Destination
2
3
t
L hops
3 hops
3 + 2(T/3) first bit received
L + (L-1)P first bit received
3 + 3(T/3) first bit released
L + LP first bit released
3 + 5 (T/3) last bit released
L + LP + (k-1)P last bit released
where T = k P
56
Routing Tables in Datagram Networks
Destination
address
Output
port
0785
7
1345
12
1566
6
2458
12
• Route determined by table
lookup
• Routing decision involves
finding next hop in route to
given destination
• Routing table has an entry for
each destination specifying
output port that leads to next
hop
• Size of table becomes
impractical for very large
number of destinations
57
Example: Internet Routing
• Internet protocol uses datagram packet
switching across networks
– Networks are treated as data links
• Hosts have two-part IP address:
– Network address + Host address
• Routers do table lookup on network address
– This reduces size of routing table
• In addition, network addresses are assigned so
that they can also be aggregated
– Discussed as CIDR (classless interdomain routing) in
Chapter 8
58
Packet Switching – Virtual Circuit
Packet
Packet
Packet
Packet
Virtual circuit
• Call set-up phase sets ups pointers in fixed path along
network
• All packets for a connection follow the same path
• Abbreviated header identifies connection on each link
• Packets queued for transmission
• Variable bit rates possible, negotiated during call set-up
• Delays variable, cannot be less than circuit switching
59
Connection Setup
Connect
request
Connect
confirm
SW
1
Connect
request
Connect
confirm
SW
2
…
SW
n
Connect
request
Connect
confirm
• Signaling messages propagate as a route is selected
• Signaling messages identify connection and setup tables in
switches
• Typically a connection is identified by a local tag, Virtual
Circuit Identifier (VCI)
• Each switch only needs to know how to relate an incoming
tag in one input to an outgoing tag in the corresponding
output
• Once tables are setup, packets can flow along path
60
Connection Setup Delay
t
Connect
request
CC
CR
CC
CR
Connect
confirm
1
2
3
1
2
Release
3
t
t
1
2
3
t
• Connection setup delay is incurred before any packet
can be transferred
• Delay is acceptable for sustained transfer of large
number of packets
• This delay may be unacceptably high if only a few
packets are being transferred
61
Virtual Circuit Forwarding Tables
Input
VCI
Output
port
Output
VCI
12
13
44
15
15
23
27
13
16
58
7
34
• Each input port of packet
switch has a forwarding table
• Lookup entry for VCI of
incoming packet
• Determine output port (next
hop) and insert VCI for next
link
• Very high speeds are possible
• Table can also include priority
or other information about how
packet should be treated
62
Cut-Through switching
Source
t
Switch 1
2
1
3
t
Switch 2
2
1
3
t
1
Destination
2
3
t
Minimum delay = 3 + T
• Some networks perform error checking on header only, so
packet can be forwarded as soon as header is received &
processed
• Delays reduced further with cut-through switching
63
Message vs. Packet Minimum Delay
• Message:
L + LT
= L  + (L – 1) T + T
• Packet
L  + L P + (k – 1) P = L  + (L – 1) P + T
• Cut-Through Packet (Immediate forwarding after
header)
= L+ T
Above neglect header processing delays
L-# of hops,  Propagation time, T - a message’s
transmission time, P=T/k, each packet’s transmission time 64