Networks and Communication Lecture 4: Datalink LAN

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Networks and CommunicationLecture 4 Datalink -
LAN

• Peter Steenkiste
• School of Computer Science
• Carnegie Mellon University
• ECOM, Summer 2000

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MAC/Logical Link Control

• How do we transfer packets between two hosts
  connected to the same network?
• Switches connected by point-to-point links --
  store-and-forward.
• Used in WAN, LAN, and for home connections
• Discussed on Monday (for WAN)
• Multiple access networks -- broadcast based
• Multiple hosts are sharing the same transmission
  medium
• Used primarily in LANs and wireless
• Need to control access to the medium
• Contention based networks, e.g. Ethernet
• Token based, e.g. FFDI
• Reservation based, e.g. DQDB

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Todays Lecture

• LAN technologies Ethernet.
• Fast LAN technologies.
• Bridging and layer 2 switching.
• Connectivity to the home.
• Wireless.
• ATM.

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IEEE 802 NetworksMAC in the LAN and MAN

• Common addressing at MAC level 48 bit IEEE
  address.
• 802.3 (Ethernet)
• 100Base-T Fast Ethernet
• Gigabit Ethernet
• 802.12 (100Base-VG Ethernet)
• 802.5 (Token ring)
• FDDI
• 802.6 (Distributed queue dual bus)

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802.3 Ethernet
Broadcast technology
host
host
host
host
host
host
host
host
Hub

• Carrier-sense multiple access with collision
  detection (CSMA/CD).
• 10Mbps cable rate.
• Maximum diameter 2.5km.
• Minimum frame 64 bytes.
• Thick or thin coax 10Base-T unshielded twisted
  pair in star configuration using hub.

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Carrier-Sense Multiple Access with Collision
Detection

• Multiple access multiple nodes sharing the same
  wire.
• Carrier-sense check whether somebody else
  sending before starting to send.
• Collision detection detect whether a collision
  happened and retry.
• Collisions are possible because of propagation
  delay
• Exponential back off to reduce the chance of
  another collision

host
host
host
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Fiber Distributed Data Interface (FDDI)

• One token holder may send, with a time limit.
• known upper bound on delay.
• Optical version of 802.5 token ring, but multiple
  packets may travel in train token released at
  end of frame.
• 100 Mbps, 100km.
• Optional dual ring for fault tolerance.
• CDDI FDDI over unshielded twisted pair, shorter
  range

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802.6 Distributed Queue Dual Bus (DQDB)
host
host
host
host
head A
head B

• Slot reservation scheme.
• Cell-based transmission at 1.5, 45, 155 Mbps.
• Up to 100km.
• MAN technology that never caught on.

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Fiber Channel

• Technology of the future for a long time
• Initially defined as a device interconnect (like
  HIPPI)
• Later pushed as a network technology
• Supports everything (802, ATM, HIPPI, SCSI...)
• Is becoming more widely used as an interconnect
  for devices.
• E.g. arrays of storage servers

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How Do We Go Faster?

• How about FDDI?
• Too complex
• How about ATM?
• Too expensive and complicated
• How about a faster Ethernet?
• It is simple
• It inter-operates with a large installed base
• It is Ethernet
• Fast Ethernet and Gigabit Ethernet

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802.3u Fast Ethernet

• Apply original CSMA/CD medium access protocol at
  100Mbps
• Must change either minimum frame or maximum
  diameter change diameter
• Requires
• 2 UTP5 pairs (4B5B) or
• 4 UTP3 pairs (8B6T) or
• 1 fiber pair
• No more shared wire connectivity.
• Hubs and switches only
• lt100 host interface, 100 hub port, 400
  switch port (1998)

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802.3z Gigabit Ethernet

• Same frame format and size as Ethernet.
• This is what makes it Ethernet
• Full duplex point-to-point links in the backbone
  are likely the most common use.
• Added flow control to deal with congestion
• Alternative is half-duplex shared-medium access.
• Cannot cut the diameter any more (set to 200m)
• Raise the min frame time (256 bytes), but not
  frame size
• Choice of a range of fiber and copper
  transmission media.
• Defining jumbo frames for higher efficiency.

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Internetworking Options
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data link
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physical
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repeater
bridge (e.g. 802 MAC)
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. . .
network
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router
gateway
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Building Larger LANsBridges

• Dumb bridge repeat packet on every outgoing
  link.
• Learning bridge
• keep track of the source address of every
  packet on every link
• use this information to learn what segment
  hosts are on
• periodically flush old information
• Spanning tree bridge more complex topologies

host
host
host
host
host
host
Bridge
host
host
host
host
host
host
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What Makes a LAN a LAN?

• Broadcast nodes can send messages that can be
  heard by all nodes on the network.
• Almost essential for network administration
• A very useful features of (the original) Ethernet
• Can also be used for applications, e.g. video
  conferencing
• Problem broadcast fundamentally does not scale.
• Overhead increases linearly with the number of
  hosts

host
host
host
host
host
host
Bridge
host
host
host
host
host
host
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Ethernet Switches

• Bridges make it possible to increase LAN
  capacity.
• Packets are no longer broadcasted - they are only
  forwarded on selected links
• Adds a switching flavor to the broadcast LAN
• Ethernet switch is a special case of a bridge
  each bridge port is connected to a single host.
• Simplifies the protocol and hardware used (only
  two stations on the link)
• Can make the link full duplex (really simple
  protocol!)
• Can have different port speeds

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Example LAN Configuration

• 10 or 100 Mbit/second connectivity to the desk
  top using switch or hubs in wiring closets.
• 100 or 1000 Mbit/second switch fabric between
  wiring closets or floors.
• Network manager can manage capacity in two ways
  link speeds and hub/bridge/switch tradeoff.

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Connectivity to the Home

• Modems.
• ISDN.
• xDSL.
• Cable modem.

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Modems

• Modem offers a bit stream.
• Aggressive signal processing has dramatically
  increased the available throughput - beats the
  Nyquist limit!
• SLIP Serial Line IP.
• Protocol to sent IP packets with minimum framing
• Lacks authentication, error detection, non-IP
  support, ..
• PPP Point-to-Point Packets.
• Better framing, error control, and testing
  support
• Can negotiate choice of higher layer protocols,
  IP address
• Can support unreliable and reliable transmission

Modem Bank
Home PC
Telephone Network
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Integrated Services Digital Network (ISDN)

• ISDN integrates voice and data services.
• Provides a set of bit pipes that can be used for
  voice, data, signaling.
• Implemented by using time multiplexing
• Basic rate ISDN offers to 64Kbs data bit pipes
  and one 16 Kbs signaling channel.

ISDN Exchange
Home PC
Telephone Network
Public Branch Exchange
Phone
LAN
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Digital Subscriber Line

• Squeeze more bandwidth out of the telephone line
  using advanced signal processing.
• Asymmetric digital subscriber line (ADSL).
• More download bandwidth, e.g. video on demand
  or web surfing
• Example T1 incoming path, 64 Kbs outgoing path
• (Symmetric) digital subscriber line (DSL).
• Same bandwidth both ways, e.g. 768 Kbs

ADSL Network Unit
Home PC
Telephone Network
ADSL Subscription Unit
Phone
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Cable Modem

• Use cable infrastructure for data service.
• Inherently has more bandwidth
• The last mile is a shared infrastructure that was
  designed for broadcasting.
• Meaning the bandwidth is shared by users
• See Figure 2.23 in the book for topology
  difference
• Example 27 Mbs shared incoming path 768 Kbs
  common outgoing path

Fiber Infrastructure
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Comparison

• Modems use worst case technology.
• Has to fit within a voice channel so encoding
  suboptimal
• ISDN can be more aggressive.
• But it is old technology by now
• DSL is highly optimized for the transmission
  medium.
• Although there are some constraints on distance
• Cablemodem uses a transmission medium that has
  inherently a higher bandwidth, but the network
  architecture will limit throughput.
• Designed for broadcasting, not for point-point
  connections

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Wireless Access Networks

• Use a basestation that functions as an edge
  router in the wired infrastructure.
• IEEE 802.11 wireless networks.
• Are becoming very popular in LANs
• Cellular digital packet data (CDPD).
• Shares spectrum with cellular telephone network
• Datagrams are sent over idle channels
• Speeds of 10s of Kbit/second of shared bandwidth
• Low-altitude satellites.
• Only at 750 km, in circular (polar) orbit
• Can be viewed a cellular with cells moving
• Eg, www.iridium.com, www.teledesic.com

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Cells

• A cell corresponds to the area covered by a base
  station.
• Limiting the power avoids interference between
  cells.
• Frequency reuse
• Use of different frequencies between adjacent
  cells can reduce interference.
• Performance drops if there are too many users in
  the cell.
• Limited number of channels
• Mobile users require support for handoff.
• Base stations periodically send invite messages
• Mobile host selects base station

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The Hidden Transmitter Problem

• Because of their position, the transmitter and a
  receiver of a packet may not be able to hear the
  same set of stations.
• Example 2 can hear 1 and 3, but 1 and 3 cannot
  hear each other
• This can cause collisions.
• Node 1 starts to send a packet to node 2
• Node 3 wants to send to node 2 also
• Since node 3 cannot hear node 1, collision
  avoidance will not work and node 3 will start to
  send
• This causes a collision at node 2, the intended
  receiver, I.e. both packet transfers fail

1
2
3
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Wireless LANs 802.11

• Basically wireless Ethernet in each cell.
• Sender cannot detect collision, so collision
  avoidance only
• Positive ack after correct reception
• Communication goes through a basestation.
• Connected to the wired network
• Also supports other operating modes, e.g. point
  coordination function.
• Basestation controls the bandwidth
• Can send to specific stations and invite a
  station to transmit
• Contention period for new stations to request
  bandwidth
• Cells rarely cleanly separated.
• Transmitter consumes bandwidth in multiple cells
• 11 frequencies helps management

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RTS and CTS

• Before sending, the sender sends a Request To
  Send (RTS).
• Assuming it is available, the receiver replies
  with a Clear to Send (CTS).
• Stations that hear a RTS or CTS refrain from
  sending for the appropriate time period.
• RTS and CTS include the packet length
• Avoids collisions in many cases.

C
B
D
A
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A Complete Transmission

• Use of CTS warns all stations close to receiver
  that transmission will be in progress.
• Stations near sender can hear both RTS and Data
• Collisions involving CST and RTS frames are
  relatively inexpensive.
• Frames are short
• There should be no collisions involving Data or
  ACK frames
• DIFS Distributed Inter Frame Space
• SIFS Short Inter Frame Spacing

RTS
CTS
Data
ACK
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Satellite

• Satellite forwards data from an uplink channel to
  a downlink channel.
• Downlink channel is always broadcast
• Beams can be used to focus signal on specific
  areas - gives some frequency reuse
• long delay path about 270 msec
• Channel bandwidth can be distributed using
  frequency of time division multiplexing.
• FDM different users get different frequency
  range
• TDM users get different time slots
• Long delays complicate the management of the
  bandwidth
• e.g. cannot hear whether anybody else is sending

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ATM Motivation

• Develop a technology that can be used for voice,
  data, video, ..
• Flexible sharing of bandwidth
• Works well for voice also
• ATM features
• Small packets (cells) good real time behavior
• Fixed sized cells fast switching
• Virtual connection fast switching
• Support different service classes voice, data,
  ..
• Unifying technology LAN-WAN, absorb the Internet

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ATM Traffic Classes

• Support for traffic classes by using different
  adaptation layers (AAL)

• Class 1 voice
• Class 5 data
• Quite complicated!

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The ATM Cell
hdr
5 bytes
pld
48 bytes
(proportional)
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Virtual Circuit Switching
sw
P3
P7
sw
P1
P5
VCI5
VCI3
VCI27
VCI3
VCI16
P0
P12
sw
P6
P12
sw

• Signaling establishes mapping from (Portin,
  VCIin) to (Portout, VCIout) at each switch on
  path.
• Cells in a VC arrive in order.

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Virtual Paths

• Virtual path is a bundle of virtual circuits.
• VCs in a virtual paths follow the same route
• Benefits
• route and rerouting at the virtual path level,
  e.g. fast aggregate fault recovery
• fast connection set up
• bandwidth management

sw
P3
P7
sw
P1
P5
VPI5 VCI7
VPI3 VCI7
VPI27 VCI7
VPI3 VCI7
VPI16 VCI7
P0
P12
sw
P6
P12
sw
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ATM Forum Signaling

• Based on Q.921 standard
• Bidirectional point to point or unidirectional
  point to multipoint.
• source/destination (20 bytes each)
• QOS parameters for two streams of traffic
• AAL parameters (packet length, ..)
• network selection, compatibility checks, ...
• Functions
• Call setup and call clearing
• Multiparty call maintenance
• Status enquiry
• Signaling restart