Section 8 Local Area Networks

1
Section 8 Local Area Networks

• In this section
• Organization and topology
• IEEE 802.3 Ethernet
• Connecting local area networks with bridges

2
Local Area Networks (LANs)

• Aspects of LANs
• Topology
• Transmission medium
• e.g. twisted-pair, co-axial cable, optical
  fibre, wireless
• Medium access control (MAC) method

bus
ring
mesh
star
3
Star Topologies
Logical topology
Typical actual layout
4
Ring Topologies

• Computers connected in a closed loop
• First passes data to second, second passes data
  to third, and so on
• In practice, there is a short connector cable
  from the computer to the ring
• Ring connections may run past offices with
  connector cable to socket in the office

5
Bus Topology

• Single cable connects all computers
• Each computer has connector to shared cable
• Computers must synchronize and allow only one
  computer to transmit at a time

6
Choice of Topology

• Each has advantages and disadvantages
• Ring eases synchronization
• Unidirectional ring may be disabled if any link
  is cut
• Bidirectional ring can operate with one disabled
  link
• Star easier to manage and more robust requires
  more cables
• Bus requires fewer cables may be disabled if
  link is cut
• The topology most often used in practice
  logical bus, physical star.

7
Hardware Addresses

• Numeric value Length is one to six bytes
• Hardware addresses must be unique on a LAN
• How can those address be assigned and who is
  responsible for uniqueness?
• Static
• Hardware manufacturer assigns permanent address
  to each interface. Manufacturer must ensure every
  interface has a unique address
• Dynamic
• Address can be set by end user, either through
  switches or jumpers on the interface or through
  software
• System administrators must coordinate to avoid
  conflict
• Automatic
• Interface automatically assigns hardware address
  each time it is powered up
• Automatic scheme must be reliable to prevent
  conflicts

8
Broadcasting

• Some applications want to broadcast messages to
  all stations on the LAN
• Shared communication channel can make broadcast
  efficient - message is delivered to all stations
• Special broadcast address used to identify
  broadcast messages, which are captured by all
  stations

9
The Host-LAN Interface
10
LAN Interface Functions

• LAN interface handles all details of frame
  transmission and reception
• Transmission
• Adds hardware addresses, error detection codes,
  etc. to outgoing frames
• May use direct memory access (DMA) to copy frame
  data directly from main memory
• Obeys access rules (e.g., CSMA/CD) when
  transmitting

11
LAN Interface Functions

• Reception
• Checks error detection codes on incoming frames
• May use DMA to copy data directly into main
  memory
• Checks destination address on incoming frames
• If destination address on incoming frame matches
  the local station's address, a copy of the frame
  is passed to the attached computer
• Frames not addressed to the local computer are
  ignored and don't affect the local computer in
  any way

12
Identifying Frame Contents

• Destination must get some clue about how to
  interpret frame data
• Can use
• Explicit frame type - identifying value included
  with frame describes type of included data
• Implicit frame type - receiver must infer type
  from frame data

13
Frames Without Type Fields

• Some LAN technologies do not include a type field
  
• Sender and receiver can agree on interpretation
• Agree on a single data format and use only that
  format
• Limits LAN to one type of data
• All computers on LAN must use one format
• Agree to encode the data format in the first few
  bytes of the data field

14
IEEE 802 LAN standards

• Series of standards for LANs
• Reference model

OSI
IEEE 802
Upper layers
Logical Link Control (LLC)
Medium Access Control (MAC)
Physical
15
Ethernet

• IEEE standard 802.3 series.
• Developed at Xerox PARC in early 1970s
• Versions
• Traditional (classic) 10 Mb/s
• Fast 100 Mb/s
• Gigabit 1Gb/s
• Original intention was that the Ethernet variants
  would be compatible for physical layer
  signalling, but this has not happened.

16
Ethernet Frame Format

• Frame format (size in octets)
• Preamble alternating 0s and 1s to set up
  synchronization
• SFD Start Frame Delimiter 10101011
• Preamble, SFD not included in CRC computation,
  and is not considered part of the frame.
• Source / destination addresses 48-bit hardware
  addresses (assigned by IEEE for Ethernet hardware
  vendors)
• Frame length number of octets in data field OR
  code that indicates upper layer protocol

Preamble
data
CRC
Dest. Addr.
Source. Addr.
Frame length
SFD
7
46 1500
4
6
6
2
1
17
Ethernet Addresses

• 48 bit / 6 octet code, normally written as 6
  pairs of hexadecimal digits separated by dashes
• Example 00-0A-E4-09-85-2B
• The last bit of the first octet of the
  destination address indicates the type of
  transmission
• 0 unicast one source, one destination
• 1 multicast one source, multiple
  destinations
• A multicast address must be set up to include the
  group
• Broadcast to all stations on LAN all
  1sFF-FF-FF-FF-FF-FF

18
Ethernet Medium Access Control

• Medium access control method CSMA/CD
• Use exponential backoff with 16 retries.
• To facilitate collision detection
• Restrictions on maximum cable length
• Restriction of minimum frame size 64 octets (46
  data octets, plus addresses, frame length, and
  CRC), so that frame sufficiently fills the bus.
• If the upper layer data is less than 46 octets,
  extra 0 bits are added to increase the field size
  to 46 octets.

19
Frame Length field

• The frame length field also is used as an
  indicator of the upper layer protocol, when
  multiple upper layer protocols are in use.
• If the value is 1516 or less (1500 data octets,
  plus addresses, plus CRC), the value is
  interpreted to be the size of the frame.
• If the value is 1536 or higher, the value is
  interpreted as a protocol code for the type of
  upper layer data contained in the frame.
• Common codes
• 080016 204810 Internet Protocol (IP) packets
• 080616 205410 Address Resolution Protocol
  (ARP) packets

20
Variants of Ethernet

• 10Mb/s (10base)
• Manchester encoding for physical layer
• 10Base5 (IEEE 802.3)
• thick co-axial cable bus
• maximum length 500 metres
• minimum 2.5 metres between stations
• 10Base2 (IEEE 802.3a)
• thin co-axial cable bus
• maximum length 185 metres
• minimum 0.45 metres between stations
• 10BaseT (IEEE 802.3i)
• UTP cable with central hub, star topology
• maximum length 100 metres

21
Variants of Ethernet

• 100Mb/s (100base, Fast Ethernet) (IEEE
  802.3u)
• 100BaseTX Uses 2 wires in UTP cable
• maximum length 100 metres
• Encoding 4B/5B plus MLT-3
• MLT-3 multi-level line transmission, 3-level
• cycles through states in order -, 0, , 0, -,
  etc.
• 0 data bit stay at current state
• 1 data bit move to next state in cycle
• 100BaseFX Uses optical fibre
• maximum length 136 metres
• Encoding 4B/5B plus NRZI
• 100BaseT4
• UTP cable with central hub, star topology
• maximum length 100 metres
• Encoding 8B/6T

22
Variants of Ethernet

• 1000Mbs (1000base, Gigabit ethernet) (IEEE
  802.3u)
• 1000BaseT (IEEE 802.3ab)
• Uses 4 pairs of UTP cable
• maximum length 100 metres
• Encoding complex scheme called 4D-PAM5
• 1000BaseLX (IEEE 802.3z)
• Uses long-wave optical fibre
• maximum length 550 to 5000 metres, depending
  on fibre
• Encoding 8B/10B NRZ
• 1000BaseSX(IEEE 802.3z)
• Uses short-wave optical fibre
• maximum length 220 to 550 metres, depending on
  fibre
• Encoding 8B/10B NRZ

23
Interconnecting LANs (1)

• Why not one big LAN?
• Administrative separation (e.g. separate domain
  for SITE versus university-wide)
• Limited amount of supportable traffic on a
  single LAN, all stations must share bandwidth
• Limited length Ethernet has maximum cable
  length in IEEE802.3 standard
• Limited number of stations token passing delays
  at each station in IEEE802.4, IEEE802.5
• Reliability containment of potential faults
• Security differing levels of security access,
  confidentiality of data

24
Interconnecting LANs (2)

• Bridges versus Repeaters
• Repeater
• Copies (regenerates) bits between LAN segments.
• Physical layer interconnection of LANs.
• Transparent to LAN functionality
• Bridge
• Receives and stores/forwards (when appropriate)
  packets between LANs (usually using same
  technology for physical and data link layers)
• Has several layers of protocol stack physical,
  data link, possibly network layers.
• Acts as a filter.

25
Bridges

• Properties
• No modification to frames
• Need buffer space for anticipated traffic
• Needs to know about addresses and routing
• Specifically, which hardware addresses are on
  which LAN.
• Works at MAC level
• IEEE 802.1D standard

26
Types of Bridge Routing

• Fixed
• Manually create table of addresses for each LAN
  connected to bridge
• Adaptive / Learning
• Bridge eventually determines how to route by
  examining source of sent messages
• Use a lookup table that will match a destination
  address with a LAN segment

27
Adaptive Bridge Routing

• When the bridge boots, the lookup table is
  initially empty
• When a frame arrives at the bridge from a LAN
  segment, the bridge reads the source and
  destination addresses. Then
• If the destination cannot be found in the lookup
  table, or is the broadcast address, the frame is
  stored, and re-transmitted on all other LAN
  segments.
• If the destination is found in the lookup table
• If it is on the same segment from which the frame
  arrived, no further action is taken.
• If on a different segment, the frame is
  re-transmitted on the destinations segment only.
• The frame on which the segment arrived is now
  known to be the segment for the senders hardware
  address, and the lookup table updated.

28
Adaptive Routing Example
1
2
U
V
W
X
Y
Z
B
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Notes on Adaptive Routing

• Algorithm shown here depends on only one path
  between any two nodes
• If topology includes alternate paths (i.e. not a
  tree structure), then a minimum spanning tree
  algorithm (IEEE 802.1) should be used.
• Requires that bridges have identifiers, a cost is
  assigned to each bridge port, and that they
  communicate with each other on startup.
• Periodically, entries in lookup table should
  expire, to allow for updates in topology

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Network that is not a tree
LAN 1
Root of tree
B2
B1
B3
LAN 3
LAN 2
B4
B5
LAN 4
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Hop counts
LAN 1
Root of tree
1
1
1
B2
B1
B3
LAN 3
LAN 2
1
1
B4
B5
LAN 4
32
Hop counts
LAN 1
Root of tree
1
1
1
B2
B1
2
B3
LAN 3
2
LAN 2
1
2
1
2
B4
B5
2
2
LAN 4
33
Hop counts
LAN 1
Root of tree
1
1
1
B2
B1
X
B3
LAN 3
2
LAN 2
1
X
1
X
B4
B5
2
2
LAN 4
Discard edges that produce cycles
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After minimum spanning tree
LAN 1
Root of tree
B2
B1
B3
LAN 3
LAN 2
B4
B5
LAN 4
Bridge will not forward messages incoming from
these ports.