Chapter 6 Medium Access Control Protocols and Local Area Networks

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Chapter 6 Medium Access Control Protocols and
Local Area Networks

• Part I Medium Access Control
• Part II Local Area Networks

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Chapter 6Medium Access Control Protocols and
Local Area Networks

• Multiple Access Communications

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Networking (Network access lay

• Point to point communication not usually
  practical
• Devices are too far apart
• Large set of devices would need impractical
  number of connections
• Solution is a communications network
• Wide Area Network (WAN)
• Large geographical area
• Local Area Network (LAN)
• Smaller scope
• Building or small campus
• Usually owned by same organization

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LAN Topologies
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NetworkingConfiguration
Typical configuration
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Multiple Access Communications

• Shared media basis for broadcast networks
• Inexpensive radio over air copper or coaxial
  cable
• M users communicate by broadcasting into medium
• Key issue How to share the medium?

Wire Radio Fiber
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Random Access
Multitapped Bus
Transmit when ready
Transmissions can occur need retransmission
strategy
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Chapter 6Medium Access Control Protocols and
Local Area Networks

• Random Access

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ALOHA

• Wireless link to provide data transfer between
  main campus remote campuses of University of
  Hawaii
• Simplest solution just do it
• A station transmits whenever it has data to
  transmit
• If more than one frames are transmitted, they
  interfere with each other (collide) and are lost
• If ACK not received within timeout, then a
  station picks random backoff time (to avoid
  repeated collision)
• Station retransmits frame after backoff time

First transmission
Retransmission
Backoff period B
t
t0
t0X
t0-X
t0X2tprop? B
t0X2tprop
Vulnerable period
Time-out
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Slotted ALOHA

• Time is slotted in X seconds slots
• Stations synchronized to frame times
• Stations transmit frames in first slot after
  frame arrival
• Backoff intervals in multiples of slots

Backoff period B
t
(k1)X
t0 X2tprop
kX
t0 X2tprop B
Time-out
Vulnerableperiod
Only frames that arrive during prior X seconds
collide

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Carrier Sensing Multiple Access (CSMA)

• A station senses the channel before it starts
  transmission
• If busy, either wait or schedule backoff
  (different options)
• If idle, start transmission
• Vulnerable period is reduced to tprop (due to
  channel capture effect)
• When collisions occur they involve entire frame
  transmission times
• If tprop gtX (or if agt1), no gain compared to
  ALOHA or slotted ALOHA

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CSMA Options

• Transmitter behavior when busy channel is sensed
• 1-persistent CSMA (most greedy)
• Start transmission as soon as the channel becomes
  idle
• Low delay and low efficiency
• Non-persistent CSMA (least greedy)
• Wait a backoff period, then sense carrier again
• High delay and high efficiency
• p-persistent CSMA (adjustable greedy)
• Wait till channel becomes idle, transmit with
  prob. p or wait one mini-slot time re-sense
  with probability 1-p
• Delay and efficiency can be balanced

Sensing
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CSMA with Collision Detection (CSMA/CD)

• Monitor for collisions abort transmission
• Stations with frames to send, first do carrier
  sensing
• After beginning transmissions, stations continue
  listening to the medium to detect collisions
• If collisions detected, all stations involved
  stop transmission, reschedule random backoff
  times, and try again at scheduled times
• In CSMA collisions result in wastage of X seconds
  spent transmitting an entire frame
• CSMA-CD reduces wastage to time to detect
  collision and abort transmission

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CSMA-CD Application Ethernet

• First Ethernet LAN standard used CSMA-CD
• 1-persistent Carrier Sensing
• R 10 Mbps
• tprop 51.2 microseconds
• 512 bits 64 byte slot
• accommodates 2.5 km 4 repeaters
• Truncated Binary Exponential Backoff
• After nth collision, select backoff from 0, 1,,
  2k 1, where kmin(n, 10)

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Chapter 6Medium Access Control Protocols and
Local Area Networks

• Part II Local Area Networks
• Overview of LANs
• Ethernet
• Token Ring and FDDI
• 802.11 Wireless LAN
• LAN Bridges

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Chapter 6Medium Access Control Protocols and
Local Area Networks

• Overview of LANs

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What is a LAN?

• Local area means
• Private ownership
• freedom from regulatory constraints of WANs
• Short distance (1km) between computers
• low cost
• very high-speed, relatively error-free
  communication
• complex error control unnecessary
• Machines are constantly moved
• Keeping track of location of computers a chore
• Simply give each machine a unique address
• Broadcast all messages to all machines in the LAN
• Need a medium access control protocol

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Typical LAN Structure
Refer to sl.25

• Transmission Medium
• Network Interface Card (NIC)
• Unique MAC physical address

Ethernet Processor
ROM
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Chapter 6Medium Access Control Protocols and
Local Area Networks

• Ethernet

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IEEE 802.3 MAC Ethernet

• MAC Protocol
• CSMA/CD
• Slot Time is the critical system parameter
• upper bound on time to detect collision
• upper bound on time to acquire channel
• upper bound on length of frame segment generated
  by collision
• quantum for retransmission scheduling
• maxround-trip propagation, MAC jam time
• Truncated binary exponential backoff
• for retransmission n 0 lt r lt 2k, where
  kmin(n,10)
• Give up after 16 retransmissions

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IEEE 802.3 MAC Frame
802.3 MAC Frame
7
1
6
6
2
4
Destination address
Source address
Information
FCS
Pad
Preamble
Length
SD
Synch
Start frame
64 - 1518 bytes

• Every frame transmission begins from scratch
• Preamble helps receivers synchronize their clocks
  to transmitter clock
• 7 bytes of 10101010 generate a square wave
• Start frame byte changes to 10101011
• Receivers look for change in 10 pattern

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IEEE 802.3 MAC Frame
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IEEE 802.3 MAC Frame

• Length bytes in information field
• Max frame 1518 bytes, excluding preamble SD
• Max information 1500 bytes 05DC
• Pad ensures min frame of 64 bytes
• FCS CCITT-32 CRC, covers addresses, length,
  information, pad fields
• NIC discards frames with improper lengths or
  failed CRC

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DIX Ethernet II Frame Structure

• DIX Digital, Intel, Xerox joint Ethernet
  specification
• Type Field to identify protocol of PDU in
  information field, e.g. IP, ARP
• Framing How does receiver know frame length?
• physical layer signal, byte count, FCS

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Ethernet Hubs Switches
Twisted Pair Cheap Easy to work
with Reliable Star-topology CSMA-CD
Twisted Pair Cheap Bridging increases
scalability Separate collision domains Full
duplex operation
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Fast Ethernet
Table 6.4 IEEE 802.3 100 Mbps Ethernet medium
alternatives

• To preserve compatibility with 10 Mbps Ethernet
• Same frame format, same interfaces, same
  protocols
• Hub topology only with twisted pair fiber
• Bus topology coaxial cable abandoned
• Category 3 twisted pair (ordinary telephone
  grade) requires 4 pairs
• Category 5 twisted pair requires 2 pairs (most
  popular)
• Most prevalent LAN today

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Gigabit Ethernet
Table 6.3 IEEE 802.3 1 Gbps Fast Ethernet medium
alternatives

• Slot time increased to 512 bytes
• Small frames need to be extended to 512 B
• Frame bursting to allow stations to transmit
  burst of short frames
• Frame structure preserved but CSMA-CD essentially
  abandoned
• Extensive deployment in backbone of enterprise
  data networks and in server farms

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10 Gigabit Ethernet
Table 6.5 IEEE 802.3 10 Gbps Ethernet medium
alternatives

• Frame structure preserved
• CSMA-CD protocol officially abandoned
• LAN PHY for local network applications
• WAN PHY for wide area interconnection using SONET
  OC-192c
• Extensive deployment in metro networks
  anticipated

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Typical Ethernet Deployment
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Chapter 7Packet-Switching Networks

• Network Services
• Packet Network Topology
• Datagrams and Virtual Circuits
• Routing in Packet Networks
• Shortest Path Routing
• \

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Chapter 7 Packet-Switching Networks

• Network Services

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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?

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Packet Switching

• Transfer of information as payload in data
  packets
• Packets undergo random delays possible loss
• Different applications impose differing
  requirements on the transfer of information

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Network Service

• Network layer can offer a variety of services to
  transport layer
• Connection-oriented service or connectionless
  service
• Best-effort or delay/loss guarantees

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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

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Chapter 7Packet-Switching Networks

• Packet Network Topology

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LAN Concentration
Switch / Router

• LAN Hubs and switches in the access network also
  aggregate packet streams that flows into switches
  and routers

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Campus Network
Servers have redundant connectivity to backbone
Organization Servers
To Internet or wide area network
s
s
Gateway
Backbone
R
R
R
S
S
S
R
Departmental Server
R
R
s
s
s
High-speed campus backbone net connects dept
routers
Only outgoing packets leave LAN through router
s
s
s
s
s
s
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Connecting to Internet Service Provider
Internet service provider
Border routers
Campus Network
Border routers
Interdomain level
Autonomous system or domain
Intradomain level
s
LAN
network administered by single organization
s
s
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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

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Chapter 7Packet-Switching Networks

• Datagrams and Virtual Circuits

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The Switching Function

• Dynamic interconnection of inputs to outputs
• Enables dynamic sharing of transmission resource
• Two fundamental approaches
• Connectionless
• Connection-Oriented Call setup control,
  Connection control

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Packet Switching Network

• Packet switching network
• Transfers packets between users
• Transmission lines packet switches (routers)
• Origin in message switching
• Two modes of operation
• Connectionless
• Virtual Circuit

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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 that message switching, suitable for
  interactive traffic

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Routing Tables in Datagram Networks

• 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

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Example Internet Routing

• Internet protocol uses datagram packet switching
  across networks
• Networks are treated as data links
• Hosts have two-port 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 in Chapter 8

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Packet Switching 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 queue for transmission
• Variable bit rates possible, negotiated during
  call set-up
• Delays variable, cannot be less than circuit
  switching

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Chapter 7Packet-Switching Networks

• Routing in Packet Networks

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Routing in Packet Networks

• Three possible (loopfree) routes from 1 to 6
• 1-3-6, 1-4-5-6, 1-2-5-6
• Which is best?
• Min delay? Min hop? Max bandwidth? Min cost?
  Max reliability?

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Creating the Routing Tables

• Need information on state of links
• Link up/down congested delay or other metrics
• Need to distribute link state information using a
  routing protocol
• What information is exchanged? How often?
• Exchange with neighbors Broadcast or flood
• Need to compute routes based on information
• Single metric multiple metrics
• Single route alternate routes

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Routing Algorithm Requirements

• Responsiveness to changes
• Topology or bandwidth changes, congestion
• Rapid convergence of routers to consistent set of
  routes
• Freedom from persistent loops
• Optimality
• Resource utilization, path length
• Robustness
• Continues working under high load, congestion,
  faults, equipment failures, incorrect
  implementations
• Simplicity
• Efficient software implementation, reasonable
  processing load

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Centralized vs Distributed Routing

• Centralized Routing
• All routes determined by a central node
• All state information sent to central node
• Problems adapting to frequent topology changes
• Does not scale
• Distributed Routing
• Routes determined by routers using distributed
  algorithm
• State information exchanged by routers
• Adapts to topology and other changes
• Better scalability

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Static vs Dynamic Routing

• Static Routing
• Set up manually, do not change requires
  administration
• Works when traffic predictable network is
  simple
• Used to override some routes set by dynamic
  algorithm
• Used to provide default router
• Dynamic Routing
• Adapt to changes in network conditions
• Automated
• Calculates routes based on received updated
  network state information

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Routing Tables in Datagram Packet Networks
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Non-Hierarchical Addresses and Routing

• No relationship between addresses routing
  proximity
• Routing tables require 16 entries each

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Hierarchical Addresses and Routing

• Prefix indicates network where host is attached
• Routing tables require 4 entries each

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Flat vs Hierarchical Routing

• Flat Routing
• All routers are peers
• Does not scale
• Hierarchical Routing
• Partitioning Domains, autonomous systems,
  areas...
• Some routers part of routing backbone
• Some routers only communicate within an area
• Efficient because it matches typical traffic flow
  patterns
• Scales

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Specialized Routing

• Flooding
• Useful in starting up network
• Useful in propagating information to all nodes
• Deflection Routing
• Fixed, preset routing procedure
• No route synthesis

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Chapter 7Packet-Switching Networks

• Shortest Path Routing

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Shortest Paths Routing

• Many possible paths connect any given source and
  to any given destination
• Routing involves the selection of the path to be
  used to accomplish a given transfer
• Typically it is possible to attach a cost or
  distance to a link connecting two nodes
• Routing can then be posed as a shortest path
  problem

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Routing Metrics

• Means for measuring desirability of a path
• Path Length sum of costs or distances
• Possible metrics
• Hop count rough measure of resources used
• Reliability link availability BER
• Delay sum of delays along path complex
  dynamic
• Bandwidth available capacity in a path
• Load Link router utilization along path
• Cost

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Shortest Path Approaches

• Distance Vector Protocols
• Neighbors exchange list of distances to
  destinations
• Best next-hop determined for each destination
• Ford-Fulkerson (distributed) shortest path
  algorithm
• Link State Protocols
• Link state information flooded to all routers
• Routers have complete topology information
• Shortest path ( hence next hop) calculated
• Dijkstra (centralized) shortest path algorithm

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Distance VectorDo you know the way to San Jose?
San Jose 294
San Jose 392
San Jose 596
San Jose 250
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Dijkstra Algorithm Finding shortest paths in
order
Find shortest paths from source s to all other
destinations
Closest node to s is 1 hop away
2nd closest node to s is 1 hop away from s or w
3rd closest node to s is 1 hop away from s, w,
or x
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Source Routing

• Source host selects path that is to be followed
  by a packet
• Strict sequence of nodes in path inserted into
  header
• Loose subsequence of nodes in path specified
• Intermediate switches read next-hop address and
  remove address
• Source host needs link state information or
  access to a route server
• Source routing allows the host to control the
  paths that its information traverses in the
  network
• Potentially the means for customers to select
  what service providers they use

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Example
3,6,B
6,B
1,3,6,B
1
3
B
6
A
4
B
Source host
2
Destination host
5