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5: DataLink Layer 5a-

Ethernet, Hubs, Bridges,

Switches

5: DataLink Layer 5a-

Ethernet

“dominant” LAN technology:  First widely used LAN technology  Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernet sketch

5: DataLink Layer 5a-

Ethernet Frame Structure

(more)

 Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match

 Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)

 CRC: checked at receiver, if error is detected, the frame is simply dropped

Data Link Layer 5-

Ethernet: Unreliable, connectionless

 connectionless: No handshaking between sending and receiving NICs

 unreliable: receiving NIC doesn’t send acks or nacks to sending NIC  stream of datagrams passed to network layer can have gaps (missing datagrams)  gaps will be filled if app is using TCP  otherwise, app will see gaps

 Ethernet’s MAC protocol: unslotted CSMA/CD

5: DataLink Layer 5a-

Ethernet’s CSMA/CD (more)

Jam Signal: make sure all other transmitters are aware of collision; 48 bits;

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 or more collisions, choose K from {0,1,2,3,4,…,1023}

Data Link Layer 5-

Manchester encoding

 used in 10BaseT  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!  Hey, this is physical-layer stuff!

5: DataLink Layer 5a-

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

5: DataLink Layer 5a-

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: DataLink Layer 5a-

Repeaters

 Physical Layer devices: operating at bit levels: repeat received bits on one interface to all other interfaces

 Extend the range of a signal by amplifying

 Useful when want to connect devices beyond the IEEE 802.3 specifications for distance limitation of 328 feet or 100 meters

 Examples – outdoor installations, mine shafts, remote locations, etc.

5: DataLink Layer 5a-

Hubs 

 Also physical layer device, but may have some management

 Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

 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) Docsity.com

5: DataLink Layer 5a-

Hub limitations

 Single collision domain results in no increase in max throughput  multi-tier throughput same as single segment throughput  Also less secure – hear traffic from/to everyone on the hub

 Individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage

 Difficult to connect different Ethernet types, but can have dual speed hubs (e.g., 10BaseT and 100baseT)

Data Link Layer 5-

Switch

 link-layer device: smarter than hubs, take active role  store, forward Ethernet frames  examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links 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

 Collision domain

 When I speak, who else can I prevent from speaking at the same time  Hub = one collision domain; Switch = collision domain per port

 Broadcast domain

 When I deliberately send a broadcast address, who all hears it  VLANs separate broadcast domains

5: DataLink LayerDocsity.com 5a-

Managed vs Unmanaged

 Switches more likely than hubs or

repeaters to have sophisticated management features

 Log in remotely and configure, get

reports/statistics etc.

 More control over what each port or group

of ports can do (e.g. establish groups of ports into virtual LANs or VLANs that further divide the broadcast domain)

5: DataLink LayerDocsity.com 5a-