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EEC484584 Computer Networks


The Ethernet MAC Sublayer Protocol. The Binary Exponential Backoff Algorithm. Switched Ethernet ... Frames received with non-matching destination address is discarded ... – PowerPoint PPT presentation

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Title: EEC484584 Computer Networks

EEC-484/584Computer Networks
  • Lecture 8
  • Wenbing Zhao

  • Manchester Encoding
  • The Ethernet MAC Sublayer Protocol
  • The Binary Exponential Backoff Algorithm
  • Switched Ethernet
  • ARP and DHCP
  • Reminder
  • 10/15 (Monday) Lab 3
  • 10/17 (Wednesday) Quiz 2
  • Graduate students topics for proj1 require my
    approval (proj due on 10/24)

Manchester Encoding
  • Binary encoding
  • Hard to distinguish 0 bit (0-volt) from idle
  • Requires clocks of all stations synchronized
  • Manchester encoding and differential Manchester

Ethernet Frame Structure
  • Preamble for clock synchronization
  • First 7 bytes with pattern 10101010, last byte
    with pattern 10101011
  • The two consecutive 1s indicate the start of a
  • How can the receiver tell the end of the frame?
  • No current on the wire

Not considered as part of the header!
gt 64 bytes
Ethernet Frame Structure
  • Destination address 6 bytes (48 bits)
  • Highest order bit 0 individual, 1 multicast
    all 1s broadcast
  • Frames received with non-matching destination
    address is discarded
  • Type type of network layer protocol
  • Pad used to produce valid frame gt 64 bytes
  • Checksum 32-bit cyclic redundancy check

Minimum Frame Length
  • To ensure the sender can detect collision
  • All frames must take more than 2t to send so that
    transmission is still taking place when the noise
    burst gets back to the sender

Ethernet MAC Sublayer Protocol
  • Uses 1-persistent CSMA/CD
  • Binary exponential backoff
  • Provides unreliable connectionless service

Randomization and Binary Exponential Backoff
  • Time divided into slots
  • Length of slot 2t worst-case round-trip
    propagation time
  • To accommodate longest path, slot time 512 bit
    times 51.2 msec (10Mbps Ethernet)
  • Binary exponential backoff

Randomization and Binary Exponential Backoff
  • After 1st collision, station picks 0 or 1 at
    random, waits that number of slots and tries
  • After 2nd collision, station picks 0,1,2,3 at
    random, waits that number of slots and tries
  • .
  • After i-th collision, station picks 0,1,,2i-1 at
  • If 10 lt i lt 16, station picks 0,1,,210-1 at
  • If i16, controller reports failure to computer

Ethernet Performance
  • Binary exponential backoff results in
  • Low delay when few stations collide
  • Reasonable delay for collision resolution when
    many stations collide
  • When other factors are fixed, channel efficiency
    decreases when
  • Network bandwidth increases
  • Cable length increases
  • Number of stations increases
  • Frame length decreases

Ethernet Performance
  • Efficiency of Ethernet at 10 Mbps with 512-bit
    slot times

Switched Ethernet
  • Switch contains a high-speed backplane and room
    for typically 4 to 32 plug-in line cards, each
    containing 1-8 connectors
  • Possibly each card forms its own collision
    domain, or
  • Full-duplex operation if each input port is

ARP Address Resolution Protocol
  • How do IP addresses get mapped onto data link
    layer addresses, such as Ethernet?

ARP Optimization
  • ARP result is cached (step 5 in figure)
  • When A wants to communicate with B, A includes
    its IP-to-Ethernet mapping in the ARP packet so
    that B knows the mapping right away (step 3 in
  • Have every machine broadcast its mapping when it
    boots, so that everyone else knows the mapping
  • To accommodate changes, entries in the ARP cache
    time out after a few minutes

ARP How to Handle Remote Traffic
  • Proxy ARP A router is configured to answer ARP
    requests on one of its networks for a host on
    another network

ARP Exercise
  • Node 1 wants to send a packet to node 3, what
    will be returned by ARP?
  • Node 1 wants to send a packet to node 2, what
    will be returned by ARP?

RARP Reverse Address Resolution Protocol
  • RARP - Allows a newly-booted diskless-workstation
    (e.g., X terminal) to broadcast its Ethernet
    address and ask for its IP address
  • RARP server responds to a RARP request with the
    assigned IP address

Limitations of RARP
  • RARP uses a link-layer broadcast, RARP requests
    are not forwarded by routers, therefore, an RARP
    server must be present on every network
  • The only thing returned by the RARP server is the
    IP address

BOOTP Bootstrap Protocol
  • BOOTP uses UDP
  • A client broadcasts to
  • The source IP address is set to if client
    does not know its own IP address yet
  • Port number 67 for server, 68 for client
  • BOOTP drawbacks
  • Requires manual configuration of tables mapping
    IP address to Ethernet address at the BOOTP
  • Replaced by DHCP

Dynamic Host Configuration Protocol
  • Allow host to dynamically obtain its IP address
    from network server when it joins network
  • IP address assignment is lease-based (to cope
    with client failure, also enables reuse of
  • Can renew its lease on address in use
  • DHCP overview (UDP is used for communication)
  • Host broadcasts DHCP discover msg
  • DHCP server responds with DHCP offer msg
  • Host requests IP address DHCP request msg
  • DHCP server sends address DHCP ack msg

DHCP Client-Server Scenario
arriving DHCP client needs address in
this network

DHCP Client-Server Scenario
arriving client
DHCP server
DHCP offer
src, 67 dest,
68 yiaddr transaction ID
654 Lifetime 3600 secs
DHCP request
src, 68 dest,
67 yiaddr transaction ID
655 Lifetime 3600 secs
src, 67 dest,
68 yiaddr transaction ID
655 Lifetime 3600 secs
DHCP Replay
  • A DHCP relay agent can be configured on each LAN
  • The agent stores the IP address of the DHCP
    server and forward the request to the server

DHCP with Replay Agent
  • To find its IP address, a newly-booted machine
    broadcasts a DHCP Discover packet
  • The DHCP relay agent on its LAN receives all DHCP
  • On receiving a DHCP Discover packet, the agent
    sends the packet as a unicast packet to the DHCP
    server, possibly on a distant network

  • An IP packet to be transmitted by Ethernet is 60
    bytes long, including all its headers. Is padding
    needed in the Ethernet frame, and if so, how many

  • Consider building a CSMA/CD network running at 1
    Gbps over a 1-km cable. The signal speed in the
    cable is 200,000 km/sec. What is the minimum
    frame size?

  • A switch designed for use with fast Ethernet has
    a backplane that can move 10 Gbps. How many
    frames/sec can it handle? Assume there is an
    endless stream of 64-byte (512-bit) frames.