5a1

1
Chapter 5 The Data Link Layer

• Our goals
• understand principles behind data link layer
  services
• error detection, correction
• sharing a broadcast channel multiple access
• link layer addressing
• reliable data transfer, flow control done!
• instantiation and implementation of various link
  layer technologies

2
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM and MPLS

3
Link Layer Introduction

• Some terminology
• hosts and routers are nodes
• communication channels that connect adjacent
  nodes along communication path are links
• wired links
• wireless links
• LANs
• 2-PDU is a frame, encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node to adjacent
node over a link
4
Link layer context

• transportation analogy
• trip from Princeton to Lausanne
• limo Princeton to JFK
• plane JFK to Geneva
• train Geneva to Lausanne
• tourist datagram
• transport segment communication link
• transportation mode link layer protocol
• travel agent routing algorithm

• Datagram transferred by different link protocols
  over different links
• e.g., Ethernet on first link, frame relay on
  intermediate links, 802.11 on last link
• Each link protocol provides different services
• e.g., may or may not provide rdt over link

5
Link Layer Services

• framing
• encapsulate datagram into frame, adding header,
  trailer
• link access
• implement channel access if shared medium,
• physical addresses used in frame headers to
  identify source, dest
• different from IP address!
• reliable delivery between two physically
  connected devices
• we learned how to do this already (chapter 3)
• seldom used on low bit error link (fiber, some
  twisted pair)
• wireless links high error rates
• Q why both link-level and end-end reliability?

6
Link Layer Services (cont.)

• 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 detection is a very common service among
  link-layer protocols
• error correction
• receiver identifies and corrects bit error(s)
  without resorting to retransmission
• ATM provides link-layer error correction for the
  packet header rather than for the entire packet
• half-duplex and full-duplex
• with half duplex, nodes at both ends of link can
  transmit, but not at same time

7
Adaptors Communicating

• link layer implemented in adaptor (aka NIC)
• Ethernet card, PCMCI card, 802.11 card
• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control, etc.

• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to rcving node
• adapter is semi-autonomous
• link physical layers

8
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

9
Error Detection

• EDC Error Detection and Correction bits
  (redundancy)
• D Data protected by error checking, may
  include header fields
• Error detection not 100 reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and
  correction

10
Parity Checking

• single bit parity
• odd parity, even parity
• detect single bit errors
• e.g., one-bit even parity

• two dimensional bit parity
• detect and correct single bit errors
• can also detect (but not correct!) any
  combination of two errors
• e.g., two-dimensional even parity
• forward error correction (FEC)
• ability of the receiver to both detect and
  correct errors
• potentially important advantage for real-time
  network applications

11
Internet checksum

• Goal
• detect errors (e.g., flipped bits) in
  transmitted segment
• used at TCP, UDP, IP header

• Sender
• treat segment contents as sequence of 16-bit
  integers
• checksum addition (1s complement sum) of
  segment contents
• sender puts 1's complement of this sum into
  checksum field

• Receiver
• 1's complement sum is computed over the same set
  of octets, including the checksum field.
• if the result is all 1 bits (- 0 in 1's
  complement arithmetic),
• NO - error detected
• YES - no error detected. But maybe errors
  nonetheless?

12
Cyclic Redundancy Check (CRC)

• a.k.a. polynomial codes
• view data bits, D, as a binary number
• choose r1 bit pattern (generator), G
• goal choose r CRC bits, R, such that
• ltD,Rgt exactly divisible by G
• receiver knows G, divides ltD,Rgt by G.
• if non-zero remainder error detected!
• can detect all burst errors less than r1 bits
  and any odd number of bit errors
• modulo 2 arithmetic
• widely used in practice (ATM, HDLC)

13
CRC Example

• want
• D.2r XOR R nG
• equivalently
• D.2r nG XOR R
• equivalently
• if we divide D.2r by G, want reminder R

• 8-bit CRC is used to protect the 5-byte header in
  ATM cells
• CRC-32 32-bit standard, which has been adopted in
  a number of link-level IEEE protocols, uses a
  generator
• GCRC-32 100000100110000010001110110110111

14
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

15
Multiple Access Links and Protocols

• Two types of links
• point-to-point link
• PPP for dial-up access
• point-to-point link between Ethernet switch and
  host
• broadcast link shared wire or medium
• traditional Ethernet
• upstream HFC
• 802.11 wireless LAN

16
Multiple Access protocols

• single shared broadcast channel
• two or more simultaneous transmissions by nodes
  interference
• only one node can send successfully at a time
• collision if node receives two or more signals at
  the same time
• multiple access problem how to coordinate the
  access of multiple sending and receiving nodes to
  a shared broadcast channel
• multiple access protocol
• distributed algorithm that determines how nodes
  share channel, i.e., determine when node can
  transmit
• communication about channel sharing must use
  channel itself!
• no out-of-band channel for coordination
• what to look for in multiple access protocols
• synchronous or asynchronous
• information needed about other stations
• robustness (e.g., to channel errors)
• performance

17
Multiple Access Protocols (cont.)

• claim humans use multiple access protocols all
  the time
• as humans, we've evolved an elaborate set of
  protocols for sharing the broadcast channel
• give everyone a chance to speak
• don't speak until you are spoken to
• don't monopolize the conversation
• raise your hand if you have a question
• don't interrupt when someone is speaking
• don't fall asleep when someone else is talking
• you can also "guess" multiple access protocols

18
MAC Protocols a taxonomy

• Three broad classes
• Channel partitioning protocols
• divide channel into smaller pieces (time slots,
  frequency, code)
• allocate piece to node for exclusive use
• Random access protocols
• channel not divided, allow collisions
• recover from collisions
• Taking-turns protocols
• Nodes take turns, but nodes with more to send can
  take longer turns

19
Ideal Multiple Access Protocol

• desirable characteristics
• assume broadcast channel of rate R bps
• when only one node has data to send, that node
  has a throughput of R bps
• when M nodes have data to send, each of these
  nodes has a throughput of R/M bps
• protocol is fully decentralized
• no special node to coordinate transmissions
• no synchronization of clocks, slots
• protocol is simple, so that it is inexpensive to
  implement

20
Channel Partitioning MAC Protocols TDM

• partition of broadcast channel's bandwidth
• TDM (Time Division Multiplexing) channel divided
  into N time slots, one per user
• FDM (Frequency Division Multiplexing) frequency
  subdivided
• TDM time division multiplexing
• each station gets fixed length slot (length
  packet trans time) in each round
• it eliminates collisions and is perfectly fair
• node must always wait for its turn in the
  transmission sequence
• unused slots go idle
• example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle

time slot
21
Channel Partitioning MAC Protocols FDM

• FDM frequency division multiplexing
• channel spectrum divided into frequency bandwidth
• each station assigned fixed frequency bandwidth
• unused transmission time in frequency bands go
  idle
• example 6-station LAN, 1,3,4 have packet,
  frequency bandwidth 2,5,6 idle

time
frequency bands
22
Random Access Protocols

• When node has packet to send
• transmit at full channel data rate R.
• no a priori coordination among nodes
• two or more transmitting nodes ? collision
• each node waits a random delay before
  retransmitting the frame
• random access MAC protocol specifies
• how to detect collisions
• how to recover from collisions (e.g., via delayed
  retransmissions)
• examples of random access MAC protocols
• slotted ALOHA
• ALOHA
• CSMA, CSMA/CD, CSMA/CA

23
Slotted ALOHA

• Assumptions
• all frames same size
• time is divided into equal size slots, time to
  transmit 1 frame
• nodes start to transmit frames only at beginning
  of slots
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes
  detect collision

• Operation
• when node obtains fresh frame, it transmits in
  next slot
• no collision, node can send new frame in next
  slot
• if collision, node retransmits frame in each
  subsequent slot with prob. p until success

24
Slotted ALOHA

• Cons
• collisions, wasting slots
• idle slots
• nodes may be able to detect collision in less
  than time to transmit packet
• clock synchronization

• Pros
• single active node can continuously transmit at
  full rate of channel
• highly decentralized
• only slots in nodes need to be in sync
• simple

25
Slotted Aloha efficiency
Efficiency is the long-run fraction of successful
slots when there is many nodes, each with many
frames to send

• Q what is max fraction of successful slots?
• A suppose N stations have packets to send
• each transmits in slot with probability p
• probability that a single node has a success is
• S p (1-p)N-1
• because there are N nodes, the probability that
  an arbitrary node has a success is
• S Prob (only one transmits)
• N p (1-p)N-1
• p that maximizes the efficiency for N nodes
• S N p (1-p)N-1 (1 -
  1/N)N-1
• 1/e 0.37 as N ? infinity
• 37 of slots do useful work. thus the effective
  transmission rate of the channel is not R bps but
  only 0.37 R bps!
• 37 of the slots go empty and 26 of slots have
  collisions

26
Pure (unslotted) ALOHA

• unslotted Aloha simpler, no synchronization
• when frame first arrives
• transmit immediately without waiting for
  beginning of slot
• collision probability increases
• frame sent at t0 collides with other frames sent
  in t0-1,t01

27
Pure Aloha (cont.)

• P(success by a given node) P(node transmits)
• 
  P(no other node transmits in p0-1, p0)
• 
  P(no other node transmits in p0, p01)
• 
  p (1 - p)N-1 (1 - p)N-1 p (1 - p)2(N-1)
• P(success by any of N nodes) N p . (1 -
  p)2(N-1)
• 1/(2e) 0.18 . . . choosing optimum p as N ?
  infinity

S throughput (success rate)
28
CSMA Carrier Sense Multiple Access

• CSMA listen before transmit
• if channel sensed idle transmit entire pkt
• if channel sensed busy, defer transmission
• 1-persistent CSMA when a station has data to
  send, it first listens to the channel to see if
  anyone else is transmitting at that moment. if
  the channel is busy, the station waits until it
  becomes idle. when the station detects an idle
  channel, it transmits a frame. if a collision
  occurs, the station waits a random amount of time
  and starts all over again.
• non-persistent CSMA retry after random interval
  if the channel is already busy
• p-persistent CSMA retry immediately with
  probability p when channel becomes idle. it
  applies to slotted channels
• human analogy dont interrupt others!

29
CSMA collisions
spatial layout of nodes
collisions can still occur propagation delay
means two nodes may not hear each others
transmission
collision entire packet transmission time wasted
note role of distance propagation delay in
determining collision probability
30
CSMA/CD (Collision Detection)

• CSMA/CD carrier sense multiple access with
  collision detection
• carrier sensing, deferral as in CSMA
• collisions detected within short time
• colliding transmissions aborted, reducing channel
  wastage
• persistent or non-persistent retransmission
• collision detection
• easy in wired LANs measure signal strengths,
  compare transmitted, received signals
• difficult in wireless LANs receiver shut off
  while transmitting
• human analogy the polite conversationalist

31
CSMA/CD collision detection
32
Taking-turns protocols

• channel partitioning MAC protocols
• share channel efficiently at high load
• inefficient at low load delay in channel access,
  1/N bandwidth allocated even if only 1 active
  node!
• random access MAC protocols
• efficient at low load single node can fully
  utilize channel
• high load collision overhead
• taking-turns protocols
• look for best of both worlds!

33
Taking Turns MAC protocols

• Token passing
• control token passed from one node to next
  sequentially.
• token message
• concerns
• token overhead
• latency
• single point of failure (token)

• Polling
• master node invites slave nodes to transmit in
  turn
• Request to Send, Clear to Send msgs
• concerns
• polling overhead
• latency
• single point of failure (master)

34
Reservation-based protocols

• Distributed Polling
• time divided into slots
• begins with N short reservation slots
• reservation slot time equal to channel end-end
  propagation delay
• station with message to send post reservation
• reservation seen by all stations
• after reservation slots, message transmissions
  ordered by known priority

35
Summary of MAC Protocols

• what do you do with a shared media?
• channel partitioning, by time, frequency, or code
• time division, frequency division, code division,
  
• random access (dynamic)
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing easy in some technologies
  (wire), hard in others (wireless)
• CSMA/CD used in Ethernet
• taking turns
• polling from a central site, token passing

36
LAN technologies

• Data link layer so far
• services, error detection/correction, multiple
  access
• Next LAN technologies
• addressing
• Ethernet
• hubs, switches
• PPP

37
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

38
LAN Addresses and ARP

• 32-bit IP address
• network-layer address
• used to get datagram to destination network
  (recall IP network definition)
• LAN address
• it is variously called physical address, Ethernet
  address, or MAC (media access control) address
• used to get datagram from one adapter to another
  physically-connected adapter (same network)
• six-bytes long (for most LANs), giving 248
  possible LAN addresses
• these six-byte addresses are typically expressed
  in hexadecimal notation e.g., 49-BD-D2-C7-56-2A
• adapter's LAN address is permanent -- when an
  adapter is manufactured, a LAN address is burned
  into the adapter's ROM.

39
LAN Addresses and ARP (cont.)
Each adapter on LAN has unique LAN address
Broadcast address FF-FF-FF-FF-FF-FF
adapter
40
LAN Addresses and ARP (cont.)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• analogy
• MAC address like social security number
• IP address like postal address
• MAC flat address --gt portability
• can move LAN card from one LAN to another
• IP hierarchical address NOT portable
• depends on network to which one attaches
• LAN broadcast address is a string of 48
  consecutive 1s FF-FF-FF-FF-FF-FF

41
Recall earlier routing discussion

• starting at A, given IP datagram addressed to B
• look up net. address of B, find B on same net. as
  A
• link layer send datagram to B inside link-layer
  frame

frame dest, source address
datagram source, dest address
As IP addr
Bs IP addr
Bs MAC addr
As MAC addr
IP payload
datagram
frame
42
ARP Address Resolution Protocol
Question how to determine MAC address of B given
Bs IP address?

• each IP node (host, router) on LAN has ARP
  module and table
• ARP table IP/MAC address mappings for some LAN
  nodes
• lt IP address1 MAC address1 TTL1gt
• lt IP address2 MAC address2 TTL2gt
• TTL (time to live) time after which address
  mapping will be forgotten (typically 20 min)

43
ARP Protocol

• A wants to send datagram to B, and A knows Bs IP
  address.
• sppose Bs MAC address is not in As ARP table.
• A passes an ARP query packet to the adapter along
  with an indication that the adapter should send
  the packet to the LAN broadcast address
  (FF-FF-FF-FF-FF-FF)
• broadcasts ARP query pkt, containing B's IP
  address
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) physical layer address
• frame sent to As MAC address (unicast)
• A caches (saves) IP-to-physical address pairs
  until information becomes old (times out)
• soft state information that times out (goes
  away) unless refreshed
• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator

44
Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows B IP address
• A creates IP packet with source A, destination B
• in routing table at source host, find router
  111.111.111.110
• A uses ARP to get Rs LAN address for
  111.111.111.110

45

• in ARP table at source, find LAN address
  E6-E9-00-17-BB-4B
• A creates Ethernet frame with R's physical
  address as dest, Ethernet frame contains A-to-B
  IP datagram
• As data link layer sends Ethernet frame to R
• Rs data link layer receives Ethernet frame from
  A
• R removes IP datagram from Ethernet frame, sees
  it is destined to B
• R uses ARP to get Bs physical address
• two ARP tables in router R, one for each IP
  network (LAN)
• R creates frame containing A-to-B IP datagram
• Rs data link layer sends Ethernet frame to B

46
DHCP Dynamic Host Configuration Protocol

• Goal allow host to dynamically obtain its IP
  address from network server when it joins network
• can renew its lease on address in use
• allows reuse of addresses
• support for mobile users who want to join network
  (more shortly) dynamically get address
  plug-and-play
• DHCP overview 4-step process
• DHCP sever discovery client broadcasts to locate
  available servers
• DHCP server offer(s) server to client in
  response to DHCP discover with offer of
  configuration parameters. client receives one or
  more DHCP offer messages from one or more servers
  
• DHCP request client message to servers either
  (a) requesting offered parameters from one server
  and implicitly declining offers from all others,
  (b) confirming correctness of previously
  allocated address after, e.g., system reboot, or
  (c) extending the lease on a particular network
  address.
• DHCPACK server to client with configuration
  parameters, including committed network address

47
DHCP client-server scenario
223.1.2.1
DHCP

223.1.1.1
server

223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
arriving DHCP client needs address in
this network
223.1.1.3
223.1.3.27

223.1.3.2
223.1.3.1
48
DHCP client-server scenario
DHCP server 223.1.2.5
arriving client
DHCP offer
src 223.1.2.5, 67 dest 255.255.255.255,
68 yiaddrr 223.1.2.4 transaction ID
654 Lifetime 3600 secs
DHCP request
src 0.0.0.0, 68 dest 255.255.255.255,
67 yiaddrr 223.1.2.4 transaction ID
655 Lifetime 3600 secs
DHCP ACK
src 223.1.2.5, 67 dest 255.255.255.255,
68 yiaddrr 223.1.2.4 transaction ID
655 Lifetime 3600 secs
time
time
49
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

50
Ethernet

• dominant LAN technology, first widely deployed
  high-speed LAN technology
• token ring, FDDI, and ATM are more complex and
  expensive than Ethernet
• kept up with speed race 10 10Gbps
• since it has been so popular, Ethernet hardware
  has become a commodity and is remarkably cheap
  (20 for 100Mbs!)
• the low cost is also due to the fact that
  Ethernet's multiple access protocol, CSMA/CD, is
  completely decentralized, which has contributed
  to a simple design

Metcalfes Etheret sketch
51
Star topology

• Bus topology popular through mid 90s
• Now star topology prevails
• Connection choices hub or switch (more later)

52
Ethernet Frame Structure

• sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• preamble (8 bytes)
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011
• used to synchronize receiver, sender clock rates
• data field (46 to 1500 bytes)
• this field carries IP datagram
• the maximum transfer unit (MTU) of Ethernet is
  1500 bytes
• the minimum size of the data field is 46 bytes.
  if the IP datagram is less than 46 bytes, the
  data field has to be "stuffed" to fill it out to
  46 bytes. the network layer uses the length field
  in the IP datagram header to remove the stuffing

53
Ethernet Frame Structure (more)

• destination address (6 bytes) frame is received
  by all adapters on a LAN and dropped if address
  does not match
• source address (6 bytes)
• type (2 bytes)
• indicates the higher layer protocol (mostly IP
  but others may be supported such as Novell IPX
  and AppleTalk)
• ARP protocol also has its own type number
• cyclic redundancy check (CRC) (4 bytes) checked
  at receiver, if error is detected, the frame is
  simply dropped

54
Ethernet Unreliable, connectionless service

• Connectionless
• no handshaking between sending and receiving
  adapter.
• Unreliable
• receiving adapter doesnt send acks or nacks to
  sending adapter
• stream of datagrams passed to network layer can
  have gaps
• gaps will be filled if app is using TCP
• otherwise, app will see the gaps

55
Ethernet services and transmission

• provide connectionless and unreliable service to
  the network layer
• baseband transmission
• Manchester encoding
• used in 10BaseT, 10Base2
• each bit has a transition
• allows clocks in sending and receiving nodes to
  synchronize to each other
• no need for a centralized, global clock among
  nodes!

56
Ethernet uses CSMA/CD

• no slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• before attempting a retransmission, adapter waits
  a random time, that is, random access

57
Ethernets CSMA/CD

• adapter obtains a network-layer PDU, prepares an
  Ethernet frame, and puts the frame in an adapter
  buffer.
• if the adapter senses that the channel is idle,
  it starts to transmit the frame. If the channel
  is busy, it waits until it senses no signal
  energy (plus 96 bit times) and then starts to
  transmit the frame.
• while transmitting, the adapter monitors for the
  presence of signal energy coming from other
  adapters. If the adapter transmits the entire
  frame without detecting signal energy from other
  adapters, the adapter is done with the frame.
• if the adapter detects signal energy from other
  adapters, it stops transmitting its frame and
  instead transmits a 48-bit jam signal.
• after aborting (that is, transmitting the jam
  signal), the adapter enters an exponential
  backoff phase. The adapter then waits K 512 bit
  times and then returns to step 2.

58
Ethernets CSMA/CD (more)

• jam signal
• make sure all other transmitters are aware of
  collision
• 48-bit random bit pattern
• exponential backoff
• Goal adapt retransmission attempts to estimated
  current load
• after experiencing the nth collision in a row,
  the adapter chooses a value for K at random from
  0,1,2, . . ., 2m - 1 where m min(n,10). the
  adapter then waits K 512 bit times
• first collision choose K from 0,1 delay is K
  512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten or more collisions, choose K from
  0,1,2,3,4, ,1023
• if many adapters are involved in the collision,
  it makes sense to choose K from a larger, more
  dispersed set of values
• heavy load random wait will be longer
• bit time 0.1 microsec for 10 Mbps Ethernet
• for K1023, wait time is about 50 msec

59
CSMA/CD efficiency

• Tprop max prop between 2 nodes in LAN
• ttrans time to transmit max-size frame

• efficiency goes to 1 as tprop goes to 0
• goes to 1 as ttrans goes to infinity
• much better than ALOHA, but still decentralized,
  simple, and cheap

See/interact with Java applet on AWL Web site
highly recommended !
60
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!
• without a repeater, the maximum length is 185
  meters, the maximum number of nodes is 30

10Base2 Ethernet
61
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
• 4B5B encoding used in 100BaseT. data byte is
  first broken into two nibbles. if the byte is 0E,
  the first nibble is 0 and the second nibble is E.
  next each nibble is remapped according to the
  following table
• 0000--11110 0001--01001 0010--10100
  0011--10101 010001010
• 0101--01011 0110--01110 0111--01111
  1000--10010 100110011
• 1010--10110 1011--10111 1100--11010
  1101--11011 111011100
• 1111--11101

star topology for 10BaseT and 100BaseT
62
10BaseT and 100BaseT (cont.)

• max distance from node to hub is 100 meters
• hubs are essentially physical-layer repeaters
• bits coming in one link go out all other links
• no frame buffering
• no CSMA/CD at hub adapters detect collisions
• provides net management functionality
• hub can disconnect jabbering adapter
• hub can gather monitoring information, statistics
  for display to LAN administrators

63
Gigabit Ethernet

• Uses the standard Ethernet frame format, and is
  backward compatible with 10BaseT and 100BaseT
  technologies. This allows for easy integration of
  Gigabit Ethernet with the existing installed base
  of Ethernet equipment.
• Allows for point-to-point links as well as shared
  broadcast channels. Point-to-point links use
  switches (see Section 5.6) whereas broadcast
  channels use hubs. In Gigabit Ethernet jargon,
  hubs are called "buffered distributors"
• Uses CSMA/CD for shared broadcast channels. In
  order to have acceptable efficiency, the maximum
  distance between nodes must be severely
  restricted.
• Allows for full-duplex operation at 1 Gbps in
  both directions for point-to-point channels.
• 10 Gbps now !

64
Link Layer

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 Link-Layer Addressing
• 5.5 Ethernet

• 5.6 Interconnections Hubs and switches
• 5.7 PPP
• 5.8 Link Virtualization ATM

65
Token Ring IEEE802.5 standard

• 4 Mbps, 16 Mbps
• max token holding time 10 ms, which limits frame
  length

ED

• SD (starting delimiter), ED (ending delimiter)
  mark start, end of packet
• AC (access control)
• token bit value 0 means token can be seized,
  value 1 means data follows FC
• priority bits priority of packet
• reservation bits station can write these bits to
  prevent stations with lower priority packet from
  seizing token after token becomes free

66
Token Ring IEEE802.5 standard (cont.)
ED

• FC frame control used for monitoring and
  maintenance
• source, destination address 48 bit physical
  address, as in Ethernet
• data packet from network layer
• checksum CRC
• FS (frame status) set by dest., read by sender
• set to indicate destination up, frame copied OK
  from ring
• DLC-level ACKing

67
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
• 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

three departmental Ethernets interconnected with
a hub
68
Hubs (cont.)

• 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)
• 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)

69
Link-layer switches

• link layer device
• stores and forwards Ethernet frames
• examines frame header and selectively forwards
  frame based on MAC dest address
• when frame is to be forwarded on segment, uses
  CSMA/CD to access segment
• transparent hosts are unaware of presence of
  switches
• plug-and-play, self-learning
• switches do not need to be configured

three departmental LANs interconnected with a
switch
70
Switch forwarding and filtering

• How do determine onto which LAN segment to
  forward frame?
• Looks like a routing problem...

• filtering ability to determine whether a frame
  should be forwarded to some interface or should
  be just dropped
• forwarding ability to determine the interface to
  which a frame should be directed

71
Self learning

• A switch has a switch table
• entry in switch table
• (MAC address, Interface, Time stamp)
• stale entries in table dropped (TTL can be 60
  min)
• self-learning
• switch learns which hosts can be reached through
  which interfaces
• when frame received, switch learns location of
  sender incoming LAN segment
• records sender/location pair in switch table

72
Filtering/Forwarding

• When switch receives a frame
• index switch table using MAC dest address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
73
Switch example (1)

• Suppose C sends a frame to D

switch
1
3
2
hub
hub
hub
A
F
D
G
B
C
H
E

• switch receives frame from from C
• switch notes that C is on interface 1
• because D is not in table, switch forwards frame
  into interfaces 2 and 3
• frame is received by D

74
Switch example (2)

• Suppose D replies back with frame to C.

• switch receives frame from from D
• switch notes that D is on interface 2
• because C is in the table, switch selectively
  forwards frame only to interface 1
• frame received by C
• switches are plug-and-play devices

75
Switches traffic isolation

• filtering
• same-LAN-segment frames not forwarded onto other
  LAN segments
• segments become separate collision domains
• forwarding
• know the LAN segment on which to forward frame
• switch 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

76
Switches dedicated access

• switch with many interfaces
• hosts have direct connection to switch
• no collisions full duplex
• cut-through switching frame is forwarded from
  input to output port before the entire packet has
  arrived if the buffer becomes empty
• slight reduction in latency
• combinations of shared/dedicated, 10/100/1000
  Mbps interfaces
• Switching A-to-A and B-to-B simultaneously, no
  collisions

77
Institutional network
switch
78
Switches vs. Routers

• both store-and-forward devices
• routers network layer devices (examine network
  layer headers)
• switches are link layer devices
• routers maintain routing tables, implement
  routing algorithms
• switches maintain switch tables, implement
  filtering, learning algorithms

79
Routers vs. Bridges pros and cons

• Switches
• () switches are plug and play
• () switches operation is simpler, has
  relatively high packet filtering and forwarding
  rates
• (-) topologies are restricted with switches a
  spanning tree must be built to avoid cycles since
  bridges do not offer protection against broadcast
  storms
• (-) spanning tree restriction also concentrates
  the traffic on the spanning tree links
• Routers
• () arbitrary topologies can be supported,
  cycling is limited by TTL counters (and good
  routing protocols)
• () provide firewall protection against layer 2
  broadcast storms
• (-) require IP address configuration (not plug
  and play)
• (-) require larger per-packet processing time
• switches do well in small (few hundred
  hosts) while routers used in large networks
  (thousands of hosts)

80
Summary comparison