Hop-centric approach Processes are terminated and re-initiated at every hop Large amount of processing, queuing and contention at each hop, for every packet Each packet processed at 3 layers for header stripping
Bottleneck: Per packet overhead at each relay node
Subway train analogy Its like getting off at each intermediate station en-route to one’s station Going outside the station Waiting in line for fresh ticket Waiting for the next train Boarding it
Unsuitable Physical Layer We still use Physical Layer suited for single-wireless-hop networks (WLAN/Cellular) Current Physical Layer optimized for 2 primitives Receiving Transmitting
In MANETs 3 primitive operations required 1. Relaying( Most common) 2. Transmitting 3. Receiving Currently Relay = Receive -> Store -> Process -> Queue -> Forward -> Contend -> Transmit
Failure to utilize Broadcast We actually try to curb it by imposing wireline-like thinking Most (traditional) routing protocols transmit to a single neighboring node Broadcast can be used To increase signal quality End-to-end path capacity
Next generation MANET architecture Three key features 1. Physical Layer optimized for multi-hop wireless networking 2. Access to medium for entire path (as opposed to single hop) 3. Cooperative transport of packets
1. Physical Layer restructuring Move “Routing” and “Forwarding” – functions to the physical layer! Routing: To determine which set of nodes relay the packet from source to destination Forwarding: To transport along this chosen path
New Physical Layer Has 3 primitive functions Relay Transmit Receive Switching at physical layer itself !
2. Path-Centric hops Atomic unit of operation = multiple hops Medium Access Control is path-oriented Packet does not have to re-contend at every hop
3. Cooperative Transport Harness unused resources to increase capacity of path Concept of “Cooperative Diversity” Nodes simultaneously retransmit the same packet on different frequencies/channels to be diversity combined at receivers
How does this improve performance? Reduced processing and elimination of re- contending at every hop will reduce latency Cooperative transport increases capacity Path diversity increases path robustness
Architecture Notional stack has 3 layers 1. Relay oriented Physical Layer (Relay PL) 2. Path Access Control (PAC) 3. Transport Layer No Network Layer !!
1. Relay-oriented Physical Layer Based on a multi-frequency/multi-band system Full-duplex operation: Simultaneously transmitting and receiving using multiple frequencies Start transmitting while you are still receiving the rest of the packet Transit Routing Table at Physical Layer for routing decisions
Relaying problems Routing & Forwarding Essentially to decide at node X, for a packet destined S -> D, whether to Keep packet (X=D) Discard it (X is not on path S -> D) Re-broadcast (Relay)
Mechanism Extract certain information (destination/signal strength/..) from Front of the packet Use it to decide whether to keep/drop/relay, while still receiving remaining packet Shunt the incoming stream to transmit chain
Routing Decisions ? Transit Control Table at a node X contains mappings from every source (S), destination (D) pair to one of keep/drop/relay Proactive Link-State Routing run at Physical Layer Routing updates and Neighbor discovery probes do not use the MAC layer
Link State Routing Link State Updates (LSU) flooding when a link goes up or down Flooding consists of a multihop network preamble followed by the actual LSU Network Preamble “Captures” all nodes i.e. it gets them to ignore data transmission or reception and tune in to LSU
Routing features “Capturing” of nodes ensures reliable broadcast of LSUs As data rates increase what matters is Propagation time of updates Reliability of updates Not how many control messages were sent!
Infrastructure for Relay PL Hardware components well within scope of current technology Routing logic & algorithms can be placed in Flash ROM (which are increasing in size & decreasing in cost) Flexibility to use Software Radios – switching functionality can be in software
There is no mention of any naming mechanism at the physical layer!! Minor Implementation detail??
2. Path Access Control (PAC) Acquires the floor for multiple hops, namely a segment, within which packets are relayed at physical layer Segment Access Request (SAR) = multi-hop RTS Segment Access Clear (SAC) = multi-hop CTS
Important Issue Setting up of frequencies of each node’s RX and TX to enable full-duplex operation 1. Select TX frequency and let RX “auto-tune” 2. (Less efficient) Always use SAR/SAC and decide a priory in half duplex mode Any path can be full-duplexed using no more than 3 frequencies
3. Cooperative Transport Cooperative Diversity: Operates entirely at the Physical Layer Near simultaneous transmission of the same information by multiple nodes that is coherently combined at the receiver Gives much better SNR at receiver as essentially power of many nodes is added up
Cooperative Diversity Level of synchronization required for decoding depends upon the receiver technology e.g. MIMO MIMO or equivalent technology required to diversity-combine the simultaneous transmissions frequency diversity: receiving multiple versions of the same signal, being transmitted at different carrier frequencies. frequency diversity