Lecture 3: Link Layer

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Lecture 3 Link Layer

• Prev. summary
• Networks
• Protocols
• Layers
• 4 layers of IP protocol stack

Application
Transport
Network
Link

• Todays lecture
• Link Layer

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IP Where do we stand?

• A network can be defined recursively as...

• two or more nodes connected by a link, or

• two or more networks connected by two or more
  nodes

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Switched Networks
telecommunication networks
circuit-switched packet-switched
FDM TDM networks datagram with
VCs networks
e.g., ATM, X.25
e.g., the Internet ( TCP,UDP )
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Switching Strategies

• Circuit Switching carry bit streams
• reserve a path (bandwidth, buffers) bw 2 hosts
• e.g., original telephone networks

• Packet Switching (e.g. Internet)
• no path is reserved no guarantees, best effort
• store and forward

Host2
Host1
R
Host3
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Circuit Switching

• resources are reserved for the complete session

limited resources (buffers, bandwidth) need to
support multiple simultaneous sessions
host A
host B
FDM (Frequency Division Multiplexing)
TDM (Time Division Multiplexing)

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Packet Switching

• resources are not reserved
• similar to postal service
• store-and-forward delays

host B
host A
host C
Statistical Multiplexing (NOT like TDM)
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Packet-Switching Delays - I

• nodal processing delay
• check bit errors
• determine output link
• queuing delay
• time waiting at output link for transmission
• depends on congestion level of router

• four sources of delay at each hop
• nodal processing
• queuing
• transmission
• propagation

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Packet-Switching Delays - II

• Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

• Propagation delay
• s propagation speed in medium (2x108 m/sec)
• d length of physical link
• propagation delay d/s

Note s and R are very different quantities!
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Store and Forward Delays Example
hostA
hostB
S
B bps
B bps
F bits total data
consider transmission delay only ( all other
delays are negligible
h
h
F bits total data, 2 packets each packet
F/2h bits
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Store and Forward Delays Example
hostA
hostB
S
B bps
B bps
t_time (h F/2) / B
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Store and Forward Delays Example
hostA
hostB
S
B bps
B bps
t_time 2 (h F/2) / B
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Store and Forward Delays Example
hostA
hostB
S
B bps
B bps
t_time 3 (h F/2) / B
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Switching Summary
circuit switching packet switching

• better resource sharing, more efficient
• less costly to initiate

• real time services
• links underutilized during silent periods
• requires reservation, complex start-up

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Link Layer Services

• Framing, link access
• encapsulate datagram into frame, adding header,
  trailer
• implement channel access if shared medium,
• physical addresses used in frame headers to
  identify source, dest
• different from IP address!
• Flow Control
• pacing between sender and receivers
• Error Detection
• errors caused by signal attenuation, noise.
• receiver detects presence of errors
• signals sender for retransmission or drops frame
• Error Correction
• receiver identifies and corrects bit error(s)
  without resorting to retransmission

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Ethernet

• dominant LAN technology
• cheap 5 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10, 100, 1000 Mbps
• 10-base T (Twisted Pair)
• 100-base T
• Gigabit Ethernet

Ethernet uses CSMA/CD
Metcalfes Etheret sketch
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Ethernet Frame Structure

• Sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• Preamble
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011
• used to synchronize receiver, sender clock rates

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Ethernet Frame Structure

• Addresses 6 bytes, frame is received by all
  adapters on a LAN and dropped if address does not
  match
• Type indicates the higher layer protocol, mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

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IEEE 802.3 Frame Structure
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IEEE 802.11 Wireless LAN

• wireless LANs untethered (often mobile)
  networking
• IEEE 802.11 standard
• MAC protocol
• unlicensed frequency spectrum 900Mhz, 2.4Ghz

• Basic Service Set (BSS) (a.k.a. cell) contains
• wireless hosts
• access point (AP) base station
• BSSs combined to form distribution system (DS)

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IEEE 802.11 MAC Protocol CSMA/CA

• 802.11 CSMA sender
• - if sense channel idle for DIFS sec.
• then transmit entire frame (no collision
  detection)
• -if sense channel busy then binary backoff
• 802.11 CSMA receiver
• if received OK
• return ACK after SIFS

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IEEE 802.11 MAC Protocol

• 802.11 CSMA Protocol others
• NAV Network Allocation Vector
• 802.11 frame has transmission time field
• others (hearing data) defer access for NAV time
  units

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Hidden Terminal effect

• hidden terminals A, C cannot hear each other
• obstacles, signal attenuation
• collisions at B
• goal avoid collisions at B
• CSMA/CA CSMA with Collision Avoidance

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Collision Avoidance RTS-CTS exchange

• CSMA/CA explicit channel reservation
• sender send short RTS request to send
• receiver reply with short CTS clear to send
• CTS reserves channel for sender, notifying
  (possibly hidden) stations
• avoid hidden station collisions

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Collision Avoidance RTS-CTS exchange

• RTS and CTS short
• collisions less likely, of shorter duration
• end result similar to collision detection
• IEEE 802.11 alows
• CSMA
• CSMA/CA reservations
• polling from AP

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Encapsulation
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Ad Hoc Networks

• Ad hoc network IEEE 802.11 stations can
  dynamically form network without AP
• Applications
• laptop meeting in conference room, car
• interconnection of personal devices
• battlefield
• IETF MANET (Mobile Ad hoc Networks) working
  group

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Point to Point Data Link Control

• one sender, one receiver, one link easier than
  broadcast link
• no Media Access Control
• no need for explicit MAC addressing
• e.g., dialup link, ISDN line
• popular point-to-point DLC protocols
• PPP (point-to-point protocol)
• HDLC High level data link control (Data link
  used to be considered high layer in protocol
  stack!

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PPP RFC 1557

• packet framing encapsulation of network-layer
  datagram in data link frame
• carry network layer data of any network layer
  protocol (not just IP) at same time
• ability to demultiplex upwards
• bit transparency must carry any bit pattern in
  the data field
• error detection (no correction)
• connection liveness detect, signal link failure
  to network layer
• network layer address negotiation endpoint can
  learn/configure each others network address

Error recovery, flow control, data re-ordering
all relegated to higher layers!
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PPP Data Frame

• Flag delimiter (framing)
• Address does nothing (only one option)
• Control does nothing in the future possible
  multiple control fields
• Protocol upper layer protocol to which frame
  delivered (eg, PPP-LCP, IP, IPCP, etc)
• info upper layer data being carried
• check cyclic redundancy check for error
  detection

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MTU

• Maximum transmission unit (MTU) is a
  characteristic of the link layer.
• Ethernet 1500 bytes
• FDDI 4352 bytes
• Point-to-point (low delay) 296 bytes
• Path MTU
• Smallest MTU in the path between a source and a
  destination

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Determining MTU

• For Ethernet and IEEE 802.3 there is a maximum
  limit on the size of frame.
• Over 11455 bytes 32bits CRC is not adequate
• Current standard is 1492 bits
• For point-to-point links
• Logical limit to provide adequate response time
  for interactive use

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Determining MTU Serial Line

• Consider a line speed of 9.6Kbps
• Asynchronous communication with 8 bit data and 1
  start and 1 stop bit
• If we have a 1024 byte packet, it will take 1066
  ms to transmit
• A small telnet packet has to wait on the average
  533 ms
• Reducing MTU to 256 bytes means that the maximum
  transmission time is 266ms and hence the average
  wait time is 133ms.
• Having very small MTU results in high overhead.