Data Link Layer

1
Data Link Layer

• Two sublayer
• Medium access sublayer (MAC)
• Logical link control (LLC)

2
Data Link Layer

• LANs -- Referred to as
• Multiaccess channels
• Random access channels
• LANs Characterized by
• Data rate of at least several Mbps
• Low error rates
• A diameter of not more than a few kilometers
• Complete ownership by a single organization

3
Networks

• Two types
• Point-to-Point
• e.g., WANs
• Broadcast
• e.g., Packet radio,
• Satellite
• LANs
• Note In between LANs and WANs are MANs.

4
Channel Allocation in LANs and MANs

• Static
• e.g., FDM
• Dynamic
• e.g., Slotted time,
• Carrier sense
• Note MANs use LANs Technology.

5
The 3 popular types of LCNs
6
Basic packet radio architecture
Central controller
Central Resources
(a) Centralized
(b) Distributed
7
ALOHA

• A medium access control technique for multiple
  access transmission media.
• Pure ALOHA -- a station transmits whenever it has
  data to send. Unacknowledged transmissions are
  repeated.
• Notation for analysis
• S Throughput of the network
• G Total rate of data presented
• I Total Rate of data generated by the stations
  (input load)
• D Average delay between the time a packet is
  ready for transmission and the completion of
  successful transmission.

8
ALOHA (cont.)

• Assumptions
• 1. All packets are of constant length
• 2. The channel is noise-free
• 3. Packets do not queue at individual stations
  (i.e., IS)
• 4. G is Poisson distributed

9
ALOHANET BroadcastChannel Multiplexing
Data packet from user node
f1 channel
Generate ACK
ACK queue
1
f2 channel
Data packet queue
Data packet to user nodes
2
10
ALOHA Protocols
Collides with the start of the shaded frame
Collides with the end of the shaded frame
t
t0
t0 t
t0 2t
t0 3t
Time
Vulnerable
Vulnerable period for the shaded frame
11
Pure ALOHA (cont.)

• G S ( of retransmitted packets per unit
  time)
• Now, express rate of retransmission as G
  Pr(individual packet suffers a collision)
• For a Poisson process with rate l, The Pr of
  transmission in a period of time t is 1 - e-lt.
  Thus the Pr of transmission during the
  vulnerable period is 1 - e-2G. Therefore
• G S G(1 - e-2G)
• So ALOHA S G e-2G

12
Pure ALOHA (cont.)

• Note If we differentiate S Ge-2G with respect
  to G and set it equal to 0, we find the max
  occurs at G 0.5 and that S 1 / 2e 0.18.
  So, the maximum thru put is only 18 of capacity.
• ALOHANET uses a data rate of 9600bps Þ max total
  throughput (sum of data arriving from all user
  nodes) is only 0.18 9600 1728bps.

13
Slotted ALOHA

• Channel is organized into uniform slots whose
  size equals the packet transmission time.
  Transmission is permitted only to begin at a slot
  boundary.
• Note Since the vulnerable period is now reduced
  in half, the Pr of transmission during this
  period is 1 - e-G thus we have
• S-ALOHA S Ge-G
• Now, differentiating with respect to G, we have
  the max possible value for S is 1 / e 0.37 or
  37.

14
Slotted ALOHA (cont.)
Throughput versus offered traffic for ALOHA system
15
Delay (approx)

• Time interval from when a user is ready to
  transmit a packet until when it is successfully
  received by the central node. Simply the sum of
  queuing delay, propagation delay, and
  transmission time.
• Note ALOHA has queueing delay 0.
• So, we need to view queueing time in the context
  of above definition for delay.

16
Delay (approx) (cont.)

• Expected of transmissions per packet º G / S Þ
• Expected of retransmissions per packet º G / S
  -1
• G / S - 1 e2G - 1
• so D (e2G - 1) d a 1,
• where d is the average delay for one
  retransmission
• ALOHA
• D (e2G - 1)(1 2a w (K1)/2) a 1
• Note Assume no collision for w

17
IEEE 802 Standards For LANs

• Include ì CSMA/CD
• í Token bus
• î Token ring
• Standards parts

• 802.1 -- Introduction to set of standards and
  define the interface primitives
• 802.2 -- Describes upper part of data link layer
  which uses LLC protocol
• 802.3 - 802.5 -- Describe the three LAN standards

18
IEEE Standards For LANs
(a) Position of the transiver and interface (b)
Connecting two cable segments with a repeater
19
Cable topology
A
B
C
D
Tap
(a) Linear
(a) Spine
20
Cable topology (cont.)
A
B
C
D
E
F
Selective repeater
(c) Tree
(d) Segmented
21
Carrier Sense Multiple Access (CSMA)
22
CSMA/CD Physical Layer

• Current Standard
• Baseband coaxial cable (50W)
• 500 M segments, 100 Taps/segment
• Maximum 4 repeaters in path
• 10 Mbps
• Similar to Ethernet

23

• For Baseband CSMA/CD, packet length should be at
  least twice the propagation delay (a
  0.5)

24

• For Broadband CSMA/CD, packet length should be at
  least quadruple the propagation delay (a
  0.5)

25
Comparison of the channel utilization versus
load for various random access protocols.
26
The 802.3 Frame Format
Byte 7 1 2 or 6 2 or 6 2
0 - 1500 0-46 4
Dest. address
Source address
Preamble
Data
Pad
Checksum
Start of frame delimiter
Length of Data field

• Destination address
• High-order bits (bit 47)
• 0 Þ ordinary addresses
• 1 Þ group addresses (multicast)

27
The 802.3 Frame Format (cont.)

• Destination address
• All 1 bits Þ broadcasting
• Note Such frame is propagated by all bridges
• Bit 46 designated for
• Local address, assigned by network adm.
• Global (address, assigned by IEEE) 7 1013
  global addresses.
• Data length and data Frame must be at least
  64 bits long from the destination address to the
  checksum.
• Pad Used to fill out the minimum size frame

28
IEEE STD 802.4 Token Bus

• Example GM (MAP)
• Logically, all stations are organized into a ring
• Note 802.4 MAC protocol is very complex,
  with each station having to maintain 10 different
  times and more than 2 dozen state variables. More
  than 200 pages.
• Token º A special control frame, and only the
  token holder is permitted to transmit frames.

29
IEEE STD 802.4 Token Bus (cont.)
A token bus
30
Token Bus MAC Sublayer Protocol

• Stations are inserted into ring in order of
  station address, from highest to lowest.
• Token passing is also done from high to low
  addresses.
• Four priority classes (0, 2, 4, 6) for traffic,
  with 0 the lowest and 6 the highest. When the
  token comes into the station, it passes to
  priority 6 substation, which may begin
  transmitting frames, if it has any. When it is
  done, (or when its timer expires), the token is
  passed to the priority 4 substations, etc.

31
Token Bus Priority Scheme
32
Ring Maintenance

• Frame
• control field Name Meaning
• 00000000 Claim_token Claim token during
  ring initialization
• 00000001 Solicit_successor_1 Allows stations to
  enter the ring
• 00000010 Solicit_successor_2 Allows stations to
  enter the ring
• 00000011 Who_follows Recover from lost
  token
• 00000100 Resolve_contention Used when multiple
  stations want to enter the ring
• 00001000 Token Pass the token
• 00001100 Set_successor Allows stations to
  leave the ring

The token bus control frames
33
Logical Ring Maintenance

• Adding a station
• Each station's interface must maintain address of
  predecessor and successor stations.
• Periodically, the token holder solicits bids from
  stations not currently in the ring and wish to
  join.
• Resolve contention -- token holder runs an
  arbitration algorithm when 2 or more stations bid
  to enter. All station interfaces maintain 2
  random bits which are used to delay all bids by
  0, 1, 2, or 3 slot times.

34
Logical Ring Maintenance (cont.)

• Deleting a station
• A station, X, with successor S, and predecessor
  P, leaves the ring by sending P a set_successor
  frame.
• Initialization
• Special case of adding new station. When first
  station comes on line, it notices that there is
  no traffic for a certain time period. Then it
  sends a claim_token frame, and later solicit bids
  from stations to join.

35
Failure (Stations)

• If a station tries to pass the token to a failed
  station, it listens to see if the station either
  transmits a frame or passes the token. If it does
  neither, the token is passed a second time. If
  that also fails, the station transmits a
  who_follows, specifying the address of its
  successor. If this fails, the station sends a
  solicit_successor_2 frame, etc.

36
Failure (Stations) (cont.)

• Token failure
• Use the ring initialization algorithm. Each
  station has a timer that is reset whenever a
  frame appears on the network. When timer hits a
  threshold value, the station issues a
  claim_token.
• Multiple tokens
• If a station holding the token notices a
  transmission from another station, it discards
  its token.

37
Sender looks for free token
Changes free token to busy token and appends data
Receiver copies data addressed to it
Sender generates free token upon receipt of
physical transmission header (from addressee)
38
Ring interface
Ring interface
1 bit delay
(a) A ring network (b) Listen mode
(c) Transmission mode
39
Station
Cable
Bypass relay
Connector
Wire center
Four stations connected via a wire center
40
Ring Maintenance (cont.)

• When the sending station drains the frame from
  the ring, it examines the A and C bits
• 1. A 0 and C 0 destination not present or
  not powered up.
• 2. A 1 and C 0 destination present but frame
  not accepted.
• 3. A 1 and C 1 destination present and frame
  copied.

41
Ring Maintenance

• Monitor station oversees the ring
• Every station has the capability of becoming the
  monitor
• Monitor station responsibility
• Lost token
• Ring breaks
• Cleaning up ring
• Orphan frame
• Garbled frame
• 802.4 committee interested in fractory issues,
  802.5 committee interested in office automation

42
IEEE token ring priority scheme
1. A is sending to B, D reserves at
higher level 2. A generates higher priority
token and remembers lower priority 3. D uses
higher priority token to send data to C 4. D
generates token at higher level 5. A sees
the high priority token and captures it. 6. A
generates token at the pre-empted, lower
priority level
1
4
2
5
3
6
43
Ring Maintenance (cont.)
Frame control field Name
Meaning
00000000 00000010 00000011 00000100 00000101 00
000110
Duplicate address test Beacon Claim
token Purge Active monitor present Stand by
monitor present
Test if two stations have same address Used to
locate breaks in the ring Attempt to become
monitor Reinitialize the ring Issued
periodically by the monitor Announces the
presence of potential monitors
Token ring control frames
44
FDDI (Fiber Distributed Data Interface)

• 100 Mbps over distances up to 200km up to 1000
  stations.
• Distance between 2 successive nodes cannot exceed
  2km.
• Uses multimode fiber.
• Uses LEDs rather than lasers.
• Design consists of 2 fiber rings.

45
An FDDI ring being used as a backbone to connect
LANs and computers
46
(a) FDDI consists of two counterrotating
rings. (b) In the event of failure of both rings
at one point, the two rings can be joined
together to form a single long ring.
47
FDDI (cont.)

• 2 classes of stations, A and B.
• Class A stations connect to both rings.
• Class B stations only connect to 1 ring.
• Traffic (2 types)
• Synchronous (e.g., audio, video info)
• Asynchronous (e.g., data traffic)
• Uses 4 out of 5 encoding schemes to save
  bandwidth

48
FDDI token ring operation
1. A seizes token and begins transmitting
frame F1 to C 2. A appends token to end of
transmission 3. B seizes token transmits F2
to D 4. B emits token. D copies F2. A
absorbs F1. 5. A lets F2 and token pass. B
absorbs F2. 6. B lets token pass
1
4
2
5
3
6
49
LAN standard MAC frame formats
(a) CSMA/CD
Octets
7 1 2, 6 2, 6
2 0 - 1500
4
Preamble
SFD DA SA Length Data Pad
FCS
(a) Token Bus
1 1 1 2, 6
2, 6 gt 0 4
1
Preamble
SD FC DA SA Data FCS
ED
(a) Token Ring
1 1 1 2, 6
2, 6 gt 0 4
1 1
SD AC FC DA SA Data
FCS ED FS
(a) FDDI
8 1 1 2, 6
2, 6 gt 0 4
1 1
Preamble
SD FC DA SA Data FCS
ED FS
AC Access Control DA Destination Address ED
Ending Delimiter
FC Frame Control FCS Frame Check Sequence FS
Frame Status
SA Source Address SD Starting Delimiter SFD
Start Frame Delimiter
50
Physical Layer Specificationsfor LAN standards
51
Physical Layer Specificationsfor LAN standards
(cont.)
Transmission Medium
Signaling Technique
Data Rate (Mbps)
Max. Segment Length(m)
Not specified 7600 Not specified
Coaxial Cable (75 ohm) Coaxial Cable (75
ohm) Optical fiber
Broadband (AM/PSK) Broadband (FSK) ASK- Manchester
Broadband Carrierband Optical fiber
1, 5, 10 1, 5, 10 5, 10, 20
(b) IEEE 802.4 (Token Bus)
52
Physical Layer Specificationsfor LAN standards
(cont.)
Transmission Medium
Signaling Technique
Data Rate (Mbps)
Max. of Repeaters
Max. distance between repeater
Shielded Twisted Pair
Differential Manchester
Not specified
1, 4
250
(c) IEEE 802.5 (Token Ring)
Transmission Medium
Signaling Technique
Data Rate (Mbps)
Max. of Repeaters
Max. distance between repeater
Optical fiber
ASK-NRZI
100
2000 (m)
1000
(d) Fiber Distributed Data Interface (FDDI)
53
Token ring
24.0
20.0
Token bus
Actual Rate (Mbps)
CSMA/CD bus
4.0
Data Rate (Mbps)
4.0
20.0
Maximum potential data rate for LAN protocols
2000 bits per packet 100 stations active out of
100 stations total
54
24.0
Token ring
20.0
Actual Rate (Mbps)
Token bus
CSMA/CD bus
4.0
4.0
20.0
Data Rate (Mbps)
500 bits per packet 100 stations active out of
100 stations total
55
Token ring
24.0
20.0
Actual Rate (Mbps)
CSMA/CD bus
Token bus
4.0
4.0
20.0
Data Rate (Mbps)
2000 bits per packet 1 station active out of 100
stations total
56
24.0
Token ring
20.0
Actual Rate (Mbps)
CSMA/CD bus
Token bus
4.0
4.0
20.0
Data Rate (Mbps)
500 bits per packet 1 station active out of 100
stations total
57
Token a 0.1
1.0
Token a 1.0
0.8
0.6
CSMA/CD a 0.1
Throughput
CSMA/CD a 1.0
0.2
Number of Stations
5
20
25
Throughput as a function of N for token passing
and CSMA/CD
58
Slotted Ring
59
Medium Access Control Protocols in Wireless
Networks

• CSMA
• 802.11

60
MAC Protocols Issues in Wireless Networks

• Hidden Terminal Problem
• Reliability
• Collision avoidance
• Congestion control
• Fairness
• Energy efficiency

61
Hidden Terminal Problem

• Node B can communicate with both A and C
• A and C cannot hear each other
• When A transmits to B, C cannot detect the
  transmission using the carrier sense mechanism
• If C transmits, collision will occur at node B

62
Exposed Station Problem

• Node B is transmitting to node A
• Assume node C wishes to transmit to node D
• it will first senses the channel,
• assumes falsely that it cannot transmit to node D
• delays transmission until idle channel is
  detected
• this is not true, collisions only occur at
  receiver, node A

63
MACA Solution for Hidden Terminal/Exposed Station
Problem Karn90

• When node A wants to send a packet to node B,
  node A first sends a Request-to-Send (RTS) to B.
• On receiving RTS, node B responds by sending
  Clear-to-Send (CTS), provided node A is able to
  receive the packet
• When a node (such as C) overhears a CTS, it keeps
  quiet for the duration of the transfer
• Transfer duration is included in both RTS and CTS.

64
Reliability

• Wireless links are prone to errors. High packet
  loss rate detrimental to transport-layer
  performance.
• Mechanisms needed to reduce packet loss rate
  experienced by upper layers

65
A Simple Solution to Improve Reliability

• When node B receives a data packet from node A,
  node B sends an Acknowledgement (Ack). This
  approach adopted in many protocols
  Bharghavan94,IEEE 802.11
• If node A fails to receive an Ack, it will
  retransmit the packet

66
IEEE 802.11 Wireless MAC

• Distributed and centralized MAC components
• Distributed Coordination Function (DCF)
• Point Coordination Function (PCF)
• DCF suitable for multi-hop ad hoc networking

67
IEEE 802.11 DCF

• Uses RTS-CTS exchange to avoid hidden terminal
  problem
• Any node overhearing a CTS cannot transmit for
  the duration of the transfer
• Uses ACK to achieve reliability
• Any node receiving the RTS cannot transmit for
  the duration of the transfer
• To prevent collision with ACK when it arrives at
  the sender
• When B is sending data to C, node A will keep
  quiet

68
Congestion AvoidanceIEEE 802.1 DCF

• When transmitting a packet, choose a backoff
  interval in the range 0,cw
• cw is contention window
• Count down the backoff interval when medium is
  idle
• Count-down is suspended if medium becomes busy
• When backoff interval reaches 0, transmit RTS

69
Congestion Avoidance

• The time spent counting down backoff intervals is
  a part of MAC overhead
• Choosing a large cw leads to large backoff
  intervals and can result in larger overhead
• Choosing a small cw leads to a larger number of
  collisions (when two nodes count down to 0
  simultaneously)

70
GSM (Global System for Mobile Communications)

• Combination of ALOHA, TDM, FDM intertwined in
  complex ways
• Has a max of 200 full duplex channels per cell.
• Each channel has an uplink and a downlink
• Each frequency band has 200kHz wide
• Uses 124 channels and supports 8 separate
  connections, using TDM
• Note Europe GSM is fully digital

71
CDMA (Code Division Multiple Access)

• Channel allocation scheme
• Each station to transmit over the entire
  frequency spectrum all the time
• Multiple simultaneous transmissions are separated
  using coding theory

72
The internal structureof the network layer
(cont.)
73
The internal structureof the network layer
(cont.)
74
IEEE 802 frame formats
75
Problems encountered in building bridges from
802.x to 802.y
Action 1. Reformate the frame and compute new
checksum 2. Reverse the bit order 3. Copy the
priority, meaningful or not 4. Generate a
ficticious priority 5. Discard priority 6. Drain
the ring (somehow) 7. Set A and C bits (by
lying) 8. Worry about congestion (fast LAN to
slow LAN) 9. Worry about token handoff ACK being
delayed or impossible 10. Panic if frame
is too long for destination LAN
Parameters assumed lt802.3gt 1518-byte
frames, 10Mbps (minus collisions) lt802.4gt
8191-byte frames, 10Mbps lt802.5gt
5000-byte frames, 4Mbps
76
Bridges From 802.x to 802.y

• Problems
• Different Frame Format Among LANs
• Interconnected LANs Do Not Run at The Same Data
  Rate
• LANs Have Different Max Frame Length
• Value of Timers in The Higher Layer May Time Out
  too Early When Sending a Long Frame

77
Bridges From 802.x to 802.y (cont.)

• 802.3--802.3
• Fairly straightforward.
• If destination LAN is heavily loaded, then frames
  must be buffered otherwise, they are discarded.

• 802.4--802.3
• Two problems
• priority bits in 802.4 frames.
• 802.4 frames may request an ACK from the
  destination. What should the bridge do?

78
Bridges From 802.x to 802.y (cont.)

• 802.5--802.3
• Similar problem as before
• 802.5 has frame status byte with A and C bits
  which are set by the destination to tell sender
  whether the frame was copied. What should the
  bridge do?

• 802.3--802.4
• What to put in the priority bits? Assuming
  enough delay has already, bridge may transmit all
  frames at highest priority.

79
Bridges From 802.x to 802.y (cont.)

• 802.4--802.4
• What to do with the temporary token handoff?

• 802.5--802.4
• Same problem as before with the A and C bits.
  Note priority bits are different in the two
  LANs.

• 802.3--802.5
• Bridge must generate priority bits.

80
Bridges From 802.x to 802.y (cont.)

• 802.4--802.5
• Frames may be too long.
• Token handoff problem.

• 802.5--802.5
• What to do with the A and C bits?

81
Transparent Bridges or Spanning Tree (802)

• There Should be No Hardware Changes Required, No
  Software Changes Required, etc. Just Plug in The
  Cable Walk Away
• Each Bridge Has a Hash Table for Looking up
  Destination Addresses
• Initially, All Bridges Hash Tables Are Empty.
  Flooding is Used to Have Bridges Learn
  Destination Addresses
• To Handle Dynamic Topologies, The Arrival Time is
  Noted in Every Hash Table Entry
• Periodically, The Hash Table is Scanned All Old
  Entries Are Purged

82
LANs and Bridges
LAN2
LAN4
LAN1
Bridge 1
Bridge 2
A
B
D
C
LAN3
A Configuration with 4 LANs and 2 Bridges
Connectivity

• Bridge 1 Connected to LAN 1 LAN 2.
• Bridge 2 Connected to LANs ___, ___ and ___.

Note A frame arriving at Bridge 1 on LAN 1
destined for A can be discarded immediately
because it is already on the right LAN.
83
LANs and Bridges (cont.)

• However, a frame arriving on LAN 1 destined for
  ___, ___, or ___ must be forwarded.
• Hash Table (located inside bridge) gt look up
  destination address.
• Example Bridge 2's table would list A as
  belonging to ___.
• Note Bridges learn destinations after the
  initial flooding. By looking at the source
  address, they can tell which machine is
  accessible on which LAN.

84
LANs and Bridges (cont.)

• Example If Bridge 1 sees a frame on LAN 2 coming
  from C, it knows that C must be reachable via
  ___, so it makes an entry in its hash table
  noting this. A subsequent frame addressed to C
  coming in on LAN 2 will be ______ whereas if
  this same frame comes in on LAN 1, it will be
  ______.
• Note Whenever a frame that is already in the
  table arrives, its entry is updated with the
  current time. Periodically, a process in the
  bridge scans the hash table and purges all
  entries more than a few minutes old.

85
Routing Procedure For An Incoming Frame

• If Destination Source LANs Are The Same,
  Discard Frame
• If Destination Source LANs Are Different,
  Forward Frame
• If The Destination LAN is Unknown, Use Flooding

86
Source Routing Bridges

• Note CSMA/CD Token Bus Chose Transparent
  Bridges. The Token Ring Group Chose Source
  Routing
• Source Routing --- Assumes That The Sender of
  Each Frame Knows Whether or Not The Destination
  is on Its Own LAN
• The Frame Header Contains The Exact Path That
  Frame is To Follow A Route is A Seq. of Bridge,
  LAN, Bridge, LAN, .....

87
Source Routing Bridges (cont.)

• When sending a frame to a different LAN, the
  source machine sets the high order bit of the
  destination address to 1 to mark it. Also, it
  includes in the frame header the exact path that
  frame is to follow.
• A route is just a sequence of Bridges, LAN,
  Bridge, ...
• Example Route from A to C in previous
  figure (B1, L2, B2, L3) B1--4bits L2--
  12 bits

88
Source Routing Bridges (cont.)

• This algorithm lends itself to three possible
  implementations
• 1. Software the bridge runs in promiscuous
  mode, copying all frames to its memory to see if
  they have the high-order destination bit set to
  1. If so, the frame is inspected further,
  otherwise, it is not.
• 2. Hybrid the bridge's LAN interface inspects
  the high-order destination bit and only gives its
  frames with the bit set. This interface is easy
  to build into hardware and greatly reduces the
  number of frames the bridge must inspect.

89
Source Routing Bridges (cont.)

• 3. Hardware the bridge's LAN interface not only
  checks the high-order destination bit, but it
  also scans the route to see if this bridge must
  do forwarding. Only frames that must actually be
  forwarded are given to the bridge. This
  implementation require the most complex hardware,
  but wastes no bridge CPU cycles because all
  irrelevant frames are screened out.

90
Discovering Routes

• If destination address is unknown, the source
  issues a broadcast frame (copied by every bridge)
  asking where it is. When the reply comes back,
  the bridges record their identity in it, so that
  the sender can observe routes taken, and choose
  the best route.