Routing and Forwarding - TDMA, FDMA, CDMA - statistical multiplexing: - - PowerPoint PPT Presentation

Communications Research Group

Routing and Forwarding

• Dr. Christian Rohner

Shared Medium

• Broadcast channel
• Multiple nodes sending, multiple nodes receiving.
• Problem 1: Collisions, Errors
• Coordinate access to the medium: multiple access
• TDMA, FDMA, CDMA
• statistical multiplexing: CSMA, CSMA/CD, ALOHA, etc.
• talking turn: TokenRing
• Problem 2: Delay to get access to the medium
• Ideal:
• each station gets a fair share
• get access within reasonable time
• distributed, simple, minimal coordination

Slotted ALOHA

Source: Kurose&Ross

complementary

ALOHA

Source: Kurose&Ross

complementary

Ethernet

Originally: Single wire [1976, Metcalfe] CSMA/CD

• send at any time (no wait)
• do not transmit if others are transmitting (carrier sensing CS)
• stop if collision (collision detection CD)
• random backoff

Connectionless, unreliable

Carrier Sense Multiple Access

Source: Kurose&Ross

CSMA/CD

Source: Kurose&Ross

Efficiency

complementary

Performance Tuning

0.2 0.4 0.6 0.8 1 50 100 150 200 250 300 Vergleich CSMA mit CSMA/CD, beta=0.01 resp. 0.1 G S

CSMA CSMA/CD CSMA CSMA/CD = P*R / L

P: Propagation Time R: Channel Rate L: Frame Length

complementary

Ethernet Frame

Preamble: wake-up, synchronisation (64 bit) Addresses: 48bit Type: IP , ARP , AppleTalk, etc. Data: 46...1500Byte CRC: 32bit - no ACK, no retransmission! connectionless, unreliable

Source: Kurose&Ross

complementary

Hub

• “Ethernet in a Box”

Segment

(Self learning) Switch

Store and Forward Forward to selected port only:

• 1. listen to all traffic
• 2. store source and incomming port in cache
• 3. lookup destination in cache table
• if not found: forward to all but incomming port
• else: forward only to port stored in cache
• 4. age cache

src port A 1 B 2 ... ...

Packet: Cache:

Queue(s) port 1 2 3

src dst

Spanningtree

• election of a root Bridge (lowest ID)
• election of a designated Bridge for every segment

> shortest distance to root > lowest ID

• removal of loops by blocking ports on segments other

than the one closest to the root Bridge or for which the bridge is designated Bridge. 3 27 15

Segment

The Bridges are exchanging HELLO messages including:

• their ID
• ID of the bridge assumed to be root Bridge
• length of best known path to root Bridge
• etc.

closest to root closest to root designated

updated

Algorhyme from Radia Perlman

I think that I shall never see a graph more lovely than a tree. A tree whose crucial property is loop-free connectivity. A tree that must be sure to span so packet can reach every LAN. First, the root must be selected. By ID, it is elected. Least-cost paths from root are traced. In the tree, these paths are placed. A mesh is made by folks like me, then bridges find a spanning tree.

Routing and Forwarding

• Routing: determine the route or path a packet has to follow.
• Forwarding: move a packet from an input link to an

appropriate output link (determined by the routing).

Routing Forwarding

Three Switching Techniques