Introduction In computer networking, an ad hoc network refers to a network connection established for a single session and does not require a router or a wireless base station. It is defined as the category of wireless network that utilize multi-hop radio relaying and capable of operating without the support of any fixed infrastructure . hence, is also called infrastructure -less network. Routing and Resource management are done in a distributed manner.
Types of wireless networks Wireless mesh Cellular network Hybrid wireless wireless network network Wireless sensor network Single hop wireless network Multi hop wireless network or Ad hoc network
Types of Ad hoc network1. Wireless mesh network formed to provide an alternate communication infrastructure for mobile or fixed nodes/users, without the spectrum reuse constraints and the requirements of network planning of cellular networks.2. Wireless sensor network used to provide a wireless communication infrastructure among the sensors deployed in a specific application domain.
3. Hybrid wireless network when two nodes in the same cell want to communicate with each other, the connection is routed through multiple wireless hops over the intermediate nodes. the base station maintains the information about the topology of the network for efficient routing.
Cellular network Base station A B E C D Mobile node
Ad hoc wireless network Wireless link A B E C D Mobile node
Difference Cellular network Ad hoc network Fixed infrastructure No infrastructure Single hop wireless Multi hop wireless link link Centralized routing Distributed routing High cost Low cost Seamless connectivity Frequent path breaks due to mobility
Characteristics Operating without a central coordinator Multi-hop radio relaying Frequent link breakage due to mobile nodes Constraint resources (bandwidth, computing power, battery lifetime). Instant deployment
Applications Military applications Collaborative computing Emergency rescue Mesh networks Wireless sensor networks Multi-hop cellular networks Wireless Community Network
General Issue for Ad hoc network Medium access scheme Distributed operation Synchronization Hidden terminal Access delay Fairness Resource reservation Capability of power control
General Issue for Ad hoc network Routing Mobility Bandwidth constraint Error prone Minimum rout acquisition Quick rout reconfiguration Loop free routing Security and privacy
General Issue for Ad hoc network Multicasting Robustness Efficiency Quality of services Efficient group management Scalability Security
General Issue for Ad hoc network Transport layer protocol UDP TCP TORA SMR Pricing Scheme Self organization
General Issue for Ad hoc network Security Denial of services Resource consumption Energy depletion Buffer overflow Interference Addressing and Service Discovery
Infrastructure based versus Ad Hoc Networks Infrastructure Networks contain special nodes called access points(APs), which are connected via existing networks. APs are special in the sense that they can interact with wireless nodes as well as with the existing wired network. The other wireless nodes , also known as mobile stations , communicate via APs. The APs also act as bridges with other networks. Ad hoc LANs do not need any fixed infrastructure. These networks can be set up on the fly at any place. Nodes communicate directly with each other or forward messages through other nodes that are directly accessible.
The design of infrastructure based networks is simpler because most of the network functionality lies within the access point ,whereas the client can remain quite simple. In Ad hoc networks, the complexity of each node is higher because every node has to implement medium access mechanisms to provide certain quality of service. Infrastructure based networks lose some of the flexibility which wireless networks can offer. e.g. They cannot be used for disaster relief in cases where no infrastructure is left, where ad hoc networks can be used.
Architecture of Ad Hoc Network IEEE 802.11 Specifies the most famous family WLAN in which many products are available 802.11 is a set of IEEE standards that govern wireless networking transmission methods. Under 802.11 standards mobile terminals can operate in two modes 1. Infrastructure Mode 2. Ad Hoc Mode
Ad Hoc Mode IEEE 802.11 only covers PHY layer and MAC layer PHY layer is subdivided into 1. Physical Layer Convergence Protocol (PLCP) 2. Physical Medium Dependent sub layer (PMD) PHY management include channel tuning and responsible for higher layer functions (e.g. control of bridging) MAC management controls authentication mechanism and power management to save battery power.
MAC Layer Provides “ Asynchronous Data Service “ and “ Time Bounded Service “ 802.11 only offers the “ Asynchronous Data Service “ in Ad-hoc Mode , but both services can be offered by Infrastructure Mode. Basic access mechanism defined for IEEE 802.11 are 1. Distributed Coordination Function (DCF) 2. Point Coordination Function (PCF) DCF only offers asynchronous service , while PCF offers both asynchronous and time bounded service MAC mechanism also called distributed foundation wireless medium access control (DFWMAC)
Basic DFWMAC using CSMA/CA Mandatory access mechanism for IEE 802.11 is based on CSMA/CA CSMA/CA mechanism shown in figure.
If medium is busy , nodes have to wait for the duration of DIFS , entering a contention phase. Each node now choose a random back off time within a contention window and delays medium access for this random amount of time. As soon as a node senses the channel is busy , it has lost this cycle and has to wait for the next chance. The basic CSMA/CA mechanism is not fair To provide fairness , IEEE 802.11 adds back off timer Each node selects random amount of waiting time If certain station does not get access to the medium in first cycle it stops its back off timer , waits for the channel to be idle again for DIFS and starts the counter again.
DFWMAC with RTS/CTS Hidden terminal problem may occur in 802.11 ,if one station can receive two others, but those cannot receive each other. To deal with this problem, mechanism using two control packets , RTS and CTS . After waiting for DIFS , the sender can issue a request to send (RTS) control packet. Every node receiving this RTS now has to set its net allocation vector (NAV) in accordance with the duration field The NAV than specifies earliest point at which the station can try to access the medium.
If the receiver receives the RTS , it answers with the clear to send (CTS) waiting for SIFS This CTS packet contains duration field and receivers have to adjust their NAV. Now all nodes within receiving distance around sender and receiver are informed that they have to wait more time before accessing the medium. This mechanism reserves the medium for one sender exclusively. It is also called a Virtual Reservation Scheme.
ROUTING ALGORITHM TABLE DRIVEN Maintain the route table The Wireless Routing Protocol localizes the updates to immediate neighbors. The Cluster Gateway Switch Routing protocol reduces the size of the tables and amount of information propagation by having each cluster of nodes select a cluster head.
Source initiated on demand On-demand routing protocols are characterized by a path discovery mechanism which is initiated when a source needs to communicate with a destination that it does not know how to reach.
Classification of protocols Based on the routing information update mechanism Proactive or table-driven routing protocols Reactive or on demand routing protocols Hybrid routing protocols Based on the use of Temporal information for Routing Routing protocols using past temporal information Routing protocols that use future temporal information
Based on Topology Information Organization Flat topology routing protocols Hierarchical topology routing protocols Based on the Utilization of specific Resources Power aware routing Geographical information assisted routing
Table Driven Routing Protocol Destination sequenced distance-vector protocol (DSDV) According to this protocol every node maintains a table that contains the shortest distance and the first node on the shortest path to every other node in the network. It incorporate table update with increasing sequence number tag to prevent loops, to counter to the count-to-infinity problem, and for faster convergences.
DSDV (continue ..) Destination Next Hop Distance Sequence Number A’s routing table : A A 0 S205_A B B 1 S334_B C C 1 S198_C D D 1 S567_D E D 2 S767_E F D 2 S45_F
Advantage of DSDV The availability of the routers to all destination at all times implies that much less delay is involved in the route setup process. The mechanism of incremental updates with sequence number tags makes the existing wired network protocol adaptable to ad hoc networks. Hence wired network protocol can be applied to the ad hoc network by less modification.
Disadvantage of DSDV A small network with high mobility or a large network with low mobility can completely choke the available bandwidth. Hence this protocol suffers from excessive control overhead that is proportional to the number of nodes in the network. In order to obtain information about a particular destination node, a node has to wait for a table update message initiated by the same destination node.
Wireless Routing protocol It is also known as WRP protocols. It is same as DSDV protocol ,but contain some additional flag in table like status of the path which is simple path(correct) or a loop(error), or the destination node not marked(null).
Advantage of WRP It is the faster convergence and involves fewer table updates.
Disadvantage of WRP Complexity of maintenance of multiple tables demands a larger memory and greater processing power from the nodes. At the high mobility, the control overhead involved in updating table entries is almost the same as DSDV. So it is not suitable for highly dynamic and also for very large ad hoc networks.
Cluster head Gateway Switch Routing protocol It is also called CGSR protocols CGSR organizes nodes into clusters, with coordination among the member of each cluster instructed nodes named “cluster head”. CGSR protocol creates a fixed region in the network. Each node in the cluster region is known as cluster member and they all are connected with pivot node which is called cluster-head .Two cluster region are connected via node which are place in intersection region of two cluster region and called cluster-gateway.
Advantage CGSR is a hierarchical routing scheme which enables partial coordination between nodes by electing cluster-heads. Hence , better bandwidth utilization is possible. It is easy to implement priority scheduling scheme with token scheduling and gateway code scheduling.
Disadvantage It increases in path length and instability in the system at high mobility when the rate of change of cluster- heads is high. To avoid gateway conflicts, more resources are required. The power consumption at the cluster-head node is also a matter of concern because the battery-draining rate at the cluster-head is higher than that at a normal node.
On-Demand routing protocols Dynamic Source Routing Protocol (DSR) In this routing protocol does not require periodic ‘hello’ packet transmission, which are used by a node to inform its neighbors of its presence. The basic approach of this protocol during the routing construction phase is to establish a route by flooding Route-Request packets in the network. The destination node, On receiving a Route-Request packet, respond by sending Route-Reply packets back to the resource.
Advantage Do not exchange routing update periodically, so overhead transmission is greatly reduced Can refer to cache for the new route when link fails.
Disadvantages Scalability problem: High route discovery latency for large network. High mobility problem: although the packet dropped may not be substantional, the overhead traffic will increase a lot.
Ad Hoc On-Demand Distance Vector In AODV, a field of the number of hops is used in the route record, instead of a list of intermediate router addresses. Each intermediate router sets up a temporary reverse link in the process of a route discovery. This link points to the router that forwarded the request.
Advantage AODV is loop-free due to the destination sequence numbers associated with routes. Therefore, it offers quick convergence when the ad hoc network topology changes which, typically, occurs when a node moves in the network Disadvantage Poor scalability is a disadvantage of AODV.
Hybrid routing protocols Zone Routing Protocol (ZRP) ZRP is formed by two sub-protocols, the Intrazone Routing Protocol (IARP) and the Interzone Routing Protocol (IERP).
IARP is “a limited scope proactive routing protocol used to improve the performance of existing globally reactive routing protocols”. It relies on the service of a certain neighbor discovery protocol (NDP) to provide neighbor information. IARP may use a scheme based on the time-to-live (TTL) field in IP packets to control the zone range. IERP is the reactive routing component of ZRP. This scheme is responsible for finding a global path. It avoids global queries for destinations that would be sent to surrounding hop neighbors. When global queries are required, “the routing zone based broadcast service can be used to efficiently guide route queries outward, rather than blindly relaying queries from neighbor to neighbor” .
References Mobile Communications - Jochen H. Schiller Ad Hoc Wireless Networks - C. Siva Ram Murthy - B. S. Manoj