5a-1

1
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

2
IEEE 802.11 Wireless LAN

• 802.11b
• 2.4-5 GHz unlicensed radio spectrum
• up to 11 Mbps
• direct sequence spread spectrum (DSSS) in
  physical layer
• all hosts use same chipping code
• widely deployed, using base stations

• 802.11a
• 5-6 GHz range
• up to 54 Mbps
• 802.11g
• 2.4-5 GHz range
• up to 54 Mbps
• All use CSMA/CA for multiple access
• All have base-station and ad-hoc network versions

3
Base station approch

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

4
Ad Hoc Network approach

• No AP (i.e., base station)
• wireless hosts communicate with each other
• to get packet from wireless host A to B may need
  to route through wireless hosts X,Y,Z
• Applications
• laptop meeting in conference room, car
• interconnection of personal devices
• battlefield
• IETF MANET (Mobile Ad hoc Networks) working
  group

5
IEEE 802.11 multiple access

• Collision if 2 or more nodes transmit at same
  time
• CSMA makes sense
• get all the bandwidth if youre the only one
  transmitting
• shouldnt cause a collision if you sense another
  transmission
• Collision detection doesnt work hidden terminal
  problem

6
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
• (ACK is needed due to hidden terminal problem)

7
Collision avoidance mechanisms

• Problem
• two nodes, hidden from each other, transmit
  complete frames to base station
• wasted bandwidth for long duration !
• Solution
• small reservation packets
• nodes track reservation interval with internal
  network allocation vector (NAV)

8
Collision Avoidance RTS-CTS exchange

• sender transmits short RTS (request to send)
  packet indicates duration of transmission
• receiver replies with short CTS (clear to send)
  packet
• notifying (possibly hidden) nodes
• hidden nodes will not transmit for specified
  duration NAV

9
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

10
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

11
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!

12
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 liveness detect, signal link failure
  to network layer
• network layer address negotiation endpoint can
  learn/configure each others network address

13
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!
14
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)

15
PPP Data Frame

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

16
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 01111110gt byte
  after each lt 01111110gt data byte
• Receiver
• two 01111110 bytes in a row discard first byte,
  continue data reception
• single 01111110 flag byte

17
Byte Stuffing
flag byte pattern in data to send
flag byte pattern plus stuffed byte in
transmitted data
18
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

19
Final Exam Review Topics

• Chapters 4 and 5 (plus some global knowledge of
  Chapter 3)

20
Chapter 4 roadmap

• 4.1 Introduction and Network Service Models
• 4.2 Routing Principles
• 4.3 Hierarchical Routing
• 4.4 The Internet (IP) Protocol
• 4.5 Routing in the Internet
• 4.6 Whats Inside a Router

21
Chapter 4 roadmap

• 4.1 Introduction and Network Service Models
• 4.2 Routing Principles
• Link state routing
• Distance vector routing
• 4.3 Hierarchical Routing
• 4.4 The Internet (IP) Protocol
• 4.5 Routing in the Internet
• 4.6 Whats Inside a Router

22
Routing
5
Goal determine good path (sequence of routers)
thru network from source to dest.
3
5
2
2
1
3

• Graph abstraction for routing algorithms
• graph nodes are routers
• graph edges are physical links
• link cost delay, cost, or congestion level

1
2
1

• good path
• typically means minimum cost path
• other defs possible

23
A Link-State Routing Algorithm

• Dijkstras algorithm
• net topology, link costs known to all nodes
• accomplished via link state broadcast
• all nodes have same info
• computes least cost paths from one node
  (source) to all other nodes
• gives routing table for that node
• iterative after k iterations, know least cost
  path to k dest.s

• Notation
• c(i,j) link cost from node i to j. cost infinite
  if not direct neighbors
• D(v) current value of cost of path from source
  to dest. V
• p(v) predecessor node along path from source to
  v, that is next v
• N set of nodes whose least cost path
  definitively known

24
Distance Vector Routing overview

• Iterative, asynchronous each local iteration
  caused by
• local link cost change
• message from neighbor its least cost path change
  from neighbor
• Distributed
• each node notifies neighbors only when its least
  cost path to any destination changes
• neighbors then notify their neighbors if necessary

Each node
25
Hierarchical Routing

• aggregate routers into regions, autonomous
  systems (AS)
• routers in same AS run same routing protocol
• intra-AS routing protocol
• routers in different AS can run different
  intra-AS routing protocol

• special routers in AS
• run intra-AS routing protocol with all other
  routers in AS
• also responsible for routing to destinations
  outside AS
• run inter-AS routing protocol with other gateway
  routers

26
Chapter 4 roadmap

• 4.1 Introduction and Network Service Models
• 4.2 Routing Principles
• 4.3 Hierarchical Routing
• 4.4 The Internet (IP) Protocol
• 4.4.1 IPv4 addressing
• 4.4.2 Moving a datagram from source to
  destination
• 4.4.3 Datagram format
• 4.4.4 IP fragmentation
• 4.4.5 ICMP Internet Control Message Protocol
• 4.4.6 DHCP Dynamic Host Configuration Protocol
• 4.4.7 NAT Network Address Translation
• 4.5 Routing in the Internet
• 4.6 Whats Inside a Router
• 4.7 IPv6
• 4.8 Multicast Routing
• 4.9 Mobility

27
Internet AS Hierarchy
Intra-AS border (exterior gateway) routers
Inter-AS interior (gateway) routers
28
Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)
• Most common Intra-AS routing protocols
• RIP Routing Information Protocol
• OSPF Open Shortest Path First
• IGRP Interior Gateway Routing Protocol (Cisco
  proprietary)

29
Internet inter-AS routing BGP

• BGP (Border Gateway Protocol) the de facto
  standard
• Path Vector protocol
• similar to Distance Vector protocol
• each Border Gateway broadcast to neighbors
  (peers) entire path (i.e., sequence of ASs) to
  destination
• BGP routes to networks (ASs), not individual
  hosts
• E.g., Gateway X may send its path to dest. Z
• Path (X,Z) X,Y1,Y2,Y3,,Z

30
Router Architecture Overview

• Two key router functions
• run routing algorithms/protocol (RIP, OSPF, BGP)
• switching datagrams from incoming to outgoing link

31
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP

32
Link Layer Services

• Framing, link access
• encapsulate datagram into frame, adding header,
  trailer
• channel access if shared medium
• physical addresses used in frame headers to
  identify source, dest
• different from IP address!
• Reliable delivery between adjacent nodes
• 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?

33
Link Layer Services (more)

• Flow Control
• pacing between adjacent sending and receiving
  nodes
• 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
• Half-duplex and full-duplex
• with half duplex, nodes at both ends of link can
  transmit, but not at same time

34
Parity Checking
Two Dimensional Bit Parity Detect and correct
single bit errors
Single Bit Parity Detect single bit errors
0
0
35
Checksumming Cyclic Redundancy Check

• 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 (modulo 2)
• receiver knows G, divides ltD,Rgt by G. If
  non-zero remainder error detected!
• can detect all burst errors less than r1 bits
• widely used in practice (ATM, HDCL)

36
Multiple Access Links and Protocols

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

37
MAC Protocols a taxonomy

• Three broad classes
• Channel Partitioning
• divide channel into smaller pieces (time slots,
  frequency, code)
• allocate piece to node for exclusive use
• Random Access
• channel not divided, allow collisions
• recover from collisions
• Taking turns
• tightly coordinate shared access to avoid
  collisions

38
Summary of MAC protocols

• What do you do with a shared media?
• Channel Partitioning, by time, frequency or code
• Time Division,Code Division, Frequency Division
• Random partitioning (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

39
LAN Addresses and ARP

• 32-bit IP address
• network-layer address
• used to get datagram to destination IP network
  (recall IP network definition)
• LAN (or MAC or physical or Ethernet) address
• used to get datagram from one interface to
  another physically-connected interface (same
  network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM

40
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
41
ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

42
Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows B IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
43
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

44
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Bit time .1 microsec for 10 Mbps Ethernet for
  K1023, wait time is about 50 msec

• Exponential Backoff
• Goal adapt retransmission attempts to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten collisions, choose K from
  0,1,2,3,4,,1023

See/interact with Java applet on AWL Web
site highly recommended !
45
Interconnecting LAN segments

• Hubs
• Bridges
• Switches
• Remark switches are essentially multi-port
  bridges.
• What we say about bridges also holds for switches!

46
Interconnecting with hubs

• Backbone hub interconnects LAN segments
• Extends max distance between nodes
• But individual segment collision domains become
  one large collision domian
• if a node in CS and a node EE transmit at same
  time collision
• Cant interconnect 10BaseT 100BaseT

47
Bridges

• 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 bridges
• plug-and-play, self-learning
• bridges do not need to be configured

48
Ethernet Switches

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