LAN Technologies

1
LAN Technologies

• Completing Lecture 2

2
Ethernet Technologies 10Base2

• 10 10Mbps 2 under 200 meters max cable length
• thin coaxial cable in a bus topology
• repeaters used to connect up to multiple segments
• repeater repeats bits it hears on one interface
  to its other interfaces physical layer device
  only!

3
10BaseT and 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Hub to which nodes are connected by twisted pair,
  thus star topology
• CSMA/CD implemented at hub

4
10BaseT and 100BaseT (more)

• Max distance from node to Hub is 100 meters
• Hub can disconnect jabbering adapter
• Hub can gather monitoring information, statistics
  for display to LAN administrators

5
Gbit Ethernet

• use standard Ethernet frame format
• allows for point-to-point links and shared
  broadcast channels
• in shared mode, CSMA/CD is used short distances
  between nodes to be efficient
• uses hubs, called here Buffered Distributors
• Full-Duplex at 1 Gbps for point-to-point links

6
Hubs, Bridges and Switches

• Lecture 3

7
Interconnecting LANs

• Q Why not just one big LAN?
• Limited amount of supportable traffic on single
  LAN, all stations must share bandwidth
• limited length 802.3 (Ethernet) specifies
  maximum cable length
• large collision domain (can collide with many
  stations)
• limited number of stations 802.5 (token ring)
  have token passing delays at each station

8
Hubs

• Physical Layer devices essentially repeaters
  operating at bit levels repeat received bits on
  one interface to all other interfaces
• Hubs can be arranged in a hierarchy (or
  multi-tier design), with backbone hub at its top

9
Hubs (more)

• Each connected LAN referred to as LAN segment
• Hubs do not isolate collision domains node may
  collide with any node residing at any segment in
  LAN
• Hub Advantages
• simple, inexpensive device
• Multi-tier provides graceful degradation
  portions of the LAN continue to operate if one
  hub malfunctions
• extends maximum distance between node pairs (100m
  per Hub)

10
Hub limitations

• single collision domain results in no increase in
  max throughput
• multi-tier throughput same as single segment
  throughput
• individual LAN restrictions pose limits on number
  of nodes in same collision domain and on total
  allowed geographical coverage
• cannot connect different Ethernet types (e.g.,
  10BaseT and 100baseT) Why?

11
Bridges

• Link Layer devices operate on Ethernet frames,
  examining frame header and selectively forwarding
  frame based on its destination
• Bridge isolates collision domains since it
  buffers frames
• When frame is to be forwarded on segment, bridge
  uses CSMA/CD to access segment and transmit

12
Bridges (more)

• Bridge advantages
• Isolates collision domains resulting in higher
  total max throughput, and does not limit the
  number of nodes nor geographical coverage
• Can connect different type Ethernet since it is a
  store and forward device
• Transparent no need for any change to hosts LAN
  adapters

13
Backbone Bridge
14
Interconnection Without Backbone

• Not recommended for two reasons
• - single point of failure at Computer Science hub
• - all traffic between EE and SE must path over CS
  segment

15
Bridges frame filtering, forwarding

• bridges filter packets
• same-LAN -segment frames not forwarded onto other
  LAN segments
• forwarding
• how to know on which LAN segment to forward frame?

16
Bridge Filtering

• bridges learn which hosts can be reached through
  which interfaces maintain filtering tables
• when frame received, bridge learns location of
  sender incoming LAN segment
• records sender location in filtering table
• filtering table entry
• (Node LAN Address, Bridge Interface, Time Stamp)
• stale entries in Filtering Table dropped (TTL can
  be 60 minutes)

17
Bridge Operation

• bridge procedure(in_MAC, in_port,out_MAC)
• lookup in filtering table (out_MAC) receive
  out_port
• if (out_port not valid) / no entry found for
  destination /
• then flood / forward on all but the
  interface on which
  the frame arrived/
• if (in_port out_port) /destination is on LAN
  on which frame was received /
• then drop the frame
• Otherwise (out_port is valid) /entry found for
  destination /
• then forward the frame on interface indicated

18
Bridge Learning example

• Suppose C sends frame to D and D replies back
  with frame to C

• C sends frame, bridge has no info about D, so
  floods to both LANs
• bridge notes that C is on port 1
• frame ignored on upper LAN
• frame received by D

19
Bridge Learning example
C 1

• D generates reply to C, sends
• bridge sees frame from D
• bridge notes that D is on interface 2
• bridge knows C on interface 1, so selectively
  forwards frame out via interface 1

20
What will happen with loops?Incorrect learning
21
What will happen with loops?Frame looping
C
2
2
C,??
C,??
1
1
A
22
What will happen with loops?Frame looping
B
2
2
B,2
B,1
1
1
A
23
Introducing Spanning Tree

• Allow a path between every LAN without causing
  loops (loop-free environment)
• Bridges communicate with special configuration
  messages (BPDUs)
• Standardized by IEEE 802.1D
• Note redundant paths are good, active redundant
  paths are bad (they cause loops)

24
Spanning Tree Requirements

• Each bridge is assigned a unique identifier
• A broadcast address for bridges on a LAN
• A unique port identifier for all ports on all
  bridges
• MAC address
• Bridge id port number

25
Spanning Tree ConceptsRoot Bridge

• The bridge with the lowest bridge ID value is
  elected the root bridge
• One root bridge chosen among all bridges
• Every other bridge calculates a path to the root
  bridge

26
Spanning Tree ConceptsPath Cost

• A cost associated with each port on each bridge
• default is 1
• The cost associated with transmission onto the
  LAN connected to the port
• Can be manually or automatically assigned
• Can be used to alter the path to the root bridge

27
Spanning Tree ConceptsRoot Port

• The port on each bridge that is on the path
  towards the root bridge
• The root port is part of the lowest cost path
  towards the root bridge
• If port costs are equal on a bridge, the port
  with the lowest ID becomes root port

28
Spanning Tree ConceptsRoot Path Cost

• The minimum cost path to the root bridge
• The cost starts at the root bridge
• Each bridge computes root path cost independently
  based on their view of the network

29
Spanning Tree Concepts Designated Bridge

• Only one bridge on a LAN at one time is chosen
  the designated bridge
• This bridge provides the minimum cost path to the
  root bridge for the LAN
• Only the designated bridge passes frames towards
  the root bridge

30
Example Spanning Tree
B8
B3
B5

• Protocol operation
• Picks a root
• For each LAN, picks a designated bridgethat is
  closest to the root.
• All bridges on a LANsend packets towards the
  root via the designated bridge.

B7
B2
B1
B6
B4
31
Example Spanning Tree
B8
Spanning Tree
B3
B5
B1
root port
B7
B2
B2
B4
B5
B7
B1
Root
B8
Designated Bridge
B6
B4
32
Spanning Tree AlgorithmAn Overview

• 1. Determine the root bridge among all bridges
• 2. Each bridge determines its root port
• The port in the direction of the root bridge
• 3. Determine the designated bridge on each LAN
• The bridge which accepts frames to forward
  towards the root bridge
• The frames are sent on the root port of the
  designated bridge

33
Spanning Tree AlgorithmSelecting Root Bridge

• Initially, each bridge considers itself to be the
  root bridge
• Bridges send BDPU frames to its attached LANs
• The bridge and port ID of the sending bridge
• The bridge and port ID of the bridge the sending
  bridge considers root
• The root path cost for the sending bridge
• Best one wins
• (lowest root ID/cost/priority)

34
Spanning Tree AlgorithmSelecting Root Ports

• Each bridge selects one of its ports which has
  the minimal cost to the root bridge
• In case of a tie, the lowest uplink (transmitter)
  bridge ID is used
• In case of another tie, the lowest port ID is used

35
Spanning Tree AlgorithmSelect Designated Bridges

• Initially, each bridge considers itself to be the
  designated bridge
• Bridges send BDPU frames to its attached LANs
• The bridge and port ID of the sending bridge
• The bridge and port ID of the bridge the sending
  bridge considers root
• The root path cost for the sending bridge
• 3. Best one wins
• (lowest ID/cost/priority)

36
Forwarding/Blocking State

• Root and designated bridges will forward frames
  to and from their attached LANs
• All other ports are in the blocking state

37
Bridges vs. Routers

• both store-and-forward devices
• routers network layer devices (examine network
  layer headers)
• bridges are Link Layer devices
• routers maintain routing tables, implement
  routing algorithms
• bridges maintain filtering tables, implement
  filtering, learning and spanning tree algorithms

38
Routers vs. Bridges

• Bridges and -
• Bridge operation is simpler requiring less
  processing
• - Topologies are restricted with bridges a
  spanning tree must be built to avoid cycles
• - Bridges do not offer protection from broadcast
  storms (endless broadcasting by a host will be
  forwarded by a bridge)

39
Routers vs. Bridges

• Routers and -
• arbitrary topologies can be supported, cycling
  is limited by TTL counters (and good routing
  protocols)
• provide firewall protection against broadcast
  storms
• - require IP address configuration (not plug and
  play)
• - require higher processing
• bridges do well in small (few hundred hosts)
  while routers used in large networks (thousands
  of hosts)

40
Ethernet Switches

• layer 2 (frame) forwarding, filtering using LAN
  addresses
• Switching A-to-B and A-to-B simultaneously, no
  collisions
• large number of interfaces
• often individual hosts, star-connected into
  switch
• Ethernet, but no collisions!

41
Ethernet Switches

• cut-through switching frame forwarded from input
  to output port without awaiting for assembly of
  entire frame
• slight reduction in latency
• combinations of shared/dedicated, 10/100/1000
  Mbps interfaces

42
Ethernet Switches (more)
Dedicated
Shared
43
Optional Wireless LAN and PPP
44
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)

45
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

46
IEEE 802.11 MAC Protocol CSMA/CA

• 802.11 CSMA sender
• - if sense channel idle for DISF 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

47
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

48
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

49
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

50
Collision Avoidance RTS-CTS exchange

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

51
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!)

52
PPP Design Requirements 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 livenes detect, signal link failure
  to network layer
• network layer address negotiation endpoint can
  learn/configure each others network address

53
PPP non-requirements

• no error correction/recovery
• no flow control
• out of order delivery OK
• no need to support multipoint links (e.g.,
  polling)

Error recovery, flow control, data re-ordering
all relegated to higher layers!!!
54
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)

55
PPP Data Frame

• info upper layer data being carried
• check cyclic redundancy check for error
  detection

56
Byte Stuffing

• data transparency requirement data field must
  be allowed to include flag pattern lt01111110gt
• Q is received lt01111110gt data or flag?
• Sender adds (stuffs) extra lt 01111101gt byte
  before each lt 01111110gt or lt01111101gt data byte
• Receiver
• Receive 01111101
• discard the byte,
• Next byte is data
• Receive 01111110 flag byte

57
Byte Stuffing
flag byte pattern in data to send
flag byte pattern plus stuffed byte in
transmitted data
58
PPP Data Control Protocol

• Before exchanging network-layer data, data link
  peers must
• configure PPP link (max. frame length,
  authentication)
• learn/configure network
• layer information
• for IP carry IP Control Protocol (IPCP) msgs
  (protocol field 8021) to configure/learn IP
  address

59
Data Link Summary

• principles behind data link layer services
• error detection, correction
• sharing a broadcast channel multiple access
• link layer addressing, ARP
• various link layer technologies
• Ethernet
• hubs, bridges, switches
• IEEE 802.11 LANs
• PPP
• Chapter 5 Kurose and Ross

60
Configuration Messages BPDU