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Module – 4
Wireless and Mobile Network
Cellular Internet Access
Cellular telephony is now ubiquitous in many areas throughout the world as it support not only voice telephony but wireless Internet access as well. Ideally, this Internet access would be at a reasonably high speed and would provide for seamless mobility, allowing users to maintain their TCP sessions while traveling, for example, on a bus or a train.
An Overview of Cellular Network Architecture
Nearly 80% of total Cellular network users adopt Global System for Mobile communication (GSM) Standard. First generation (1G) systems were analog FDMA systems designed exclusively for voice- only communication. These 1G systems are almost extinct now, having been replaced by digital 2G systems. The original 2G systems were also designed for voice, but later extended (2.5G) to support data (i.e., Internet) as well as voice service. The 3G systems that currently are being deployed also support voice and data, but with an ever increasing emphasis on data capabilities and higher-speed radio access links.
2G Cellular Network
The term cellular refers to the fact that the region covered by a cellular network is partitioned into a number of geographic coverage areas, known as cells. Each cell contains a base transceiver station (BTS) that transmits signals to and receives signals from the mobile stations in its cell. The coverage area of a cell depends on many factors, including the transmitting power of the BTS, the transmitting power of the user devices, obstructing buildings in the cell, and the height of base station antennas.
The GSM standard for 2G cellular systems uses combined FDM/TDM (radio) for the air interface. In combined FDM/TDM systems, the channel is partitioned into a number of frequency sub-bands; within each sub-band, time is partitioned into frames and slots. GSM systems consist of 200-kHz frequency bands with each band supporting eight TDM calls. GSM encodes speech at 13 kbps and 12.2 kbps. A GSM network’s base station controller (BSC) will typically service several tens of base transceiver stations. The role of the BSC is to allocate BTS radio channels to mobile subscribers, perform paging (finding the cell in which a mobile user is resident), and perform handoff of mobile users. The base station controller and its controlled base transceiver stations collectively constitute a GSM base station system (BSS). The mobile switching center (MSC) plays the central role in user authorization and accounting (e.g., determining whether a mobile device is allowed to connect to the cellular network), call establishment and teardown, and handoff. A single MSC will typically contain up to five BSCs, resulting in approximately 200K subscribers per MSC. A cellular provider’s network will have a number of MSCs, with special MSCs known as gateway MSCs connecting the provider’s cellular network to the larger public telephone network.
3G Radio Access Network: The Wireless Edge The Radio Network Controller (RNC) typically controls several cell base transceiver stations. The RNC connects to both the circuit-switched cellular voice network via an MSC, and to the packet-switched Internet via an SGSN. It uses a CDMA technique known as Direct Sequence Wideband CDMA (DS-WCDMA) within TDMA slots. The data service associated with the WCDMA specification is known as HSP (High Speed Packet Access) and promises downlink data rates of up to 14 Mbps.
4G Networks: Long Term Evaluation (LTE)
The 4G Long-Term Evolution (LTE) standards has two important innovations over 3G systems:
1. Evolved Packet Core (EPC) The EPC is a simplified all-IP core network that unifies the separate circuit-switched cellular voice network and the packet-switched cellular data network. It is an “all-IP” network in that both voice and data will be carried in IP datagrams. A key task of the EPC is to manage network resources to provide this high quality of service. The EPC also makes a clear separation between the network control and user data planes, with many of the mobility support features. The EPC allows multiple types of radio access networks, including legacy 2G and 3G radio access networks, to attach to the core network. 2. LTE Radio Access Network. LTE uses a combination of frequency division multiplexing and time division multiplexing on the downstream channel, known as orthogonal frequency division multiplexing (OFDM). In LTE, each active mobile node is allocated one or more 0.5 ms time slots in one or more of the channel frequencies. By being allocated increasingly more time slots a mobile node is able to achieve increasingly higher transmission rates. The maximum data rate for an LTE user is 100 Mbps in the downstream direction and 50 Mbps in the upstream direction, when using 20 MHz worth of wireless spectrum.
Mobility Management: Principles
A mobile node is one that changes its point of attachment into the network over time. Several dimensions of mobility:
1. From the network layer’s standpoint, how mobile is a user? A physically mobile user will present a very different set of challenges to the network layer, depending on how he or she moves between points of attachment to the network. Below figure shows various level of mobility. 2. How important is it for the mobile node’s address to always remain the same? With mobile telephony the network-layer address of phone remains the same as you travel from one provider’s mobile phone network to another. If a mobile entity is able to maintain its IP address as it moves, mobility becomes invisible from the application standpoint. There is great value to this transparency—an application need not be concerned with a potentially changing IP address, and the same application code serves mobile and nonmobile connections alike. A less frequent mobile user might simply want to turn off an office laptop, bring that laptop home, power up, and work from home. If the laptop functions primarily as a client in client- server applications (e.g., send/read e-mail, browse the Web, Telnet to a remote host) from home, the particular IP address used by the laptop is not that important. 3. What supporting wired infrastructure is available? It is assumed that there is a fixed infrastructure to which the mobile user can connect—for example, the home’s ISP network, the wireless access network in the office, or the wireless access networks lining the autobahn. What if no such infrastructure exists? If two users are within communication proximity of each other, can they establish a network connection in the absence of any other network-layer infrastructure?
When the mobile node leaves one foreign network and joins another, the new foreign network would advertise a new, highly specific route to the mobile node, and the old foreign network would withdraw its routing information regarding the mobile node. Limitation: Scalability Another Approach is through care-of-address The conceptually simplest approach is to locate foreign agents at the edge routers in the foreign network. One role of the foreign agent is to create care-of address (COA) for the mobile node, with the network portion of the COA matching that of the foreign network. There are thus two addresses associated with a mobile node, its permanent address and its COA, sometimes known as a foreign address. A second role of the foreign agent is to inform the home agent that the mobile node is resident in its network and has the given COA.
Routing to a Mobile Node
1) Indirect Routing to a Mobile Node
In the indirect routing approach, the correspondent simply addresses the datagram to the mobile node’s permanent address and sends the datagram into the network, blissfully unaware of whether the mobile node is resident in its home network or is visiting a foreign network; mobility is thus completely transparent to the correspondent. Home agent is responsible for interacting with a foreign agent to track the mobile node’s COA. Home agent second job is to be on the lookout for arriving datagrams addressed to nodes whose home network is that of the home agent but that are currently resident in a foreign network. The home agent intercepts these datagrams and then forwards them to a mobile node in a two-step process. The datagram is first forwarded to the foreign agent, using the mobile node’s COA, and then forwarded from the foreign agent to the mobile node. Home agent encapsulate the correspondent’s original complete datagram within a new (larger) datagram. This larger datagram is addressed and delivered to the mobile node’s COA. The foreign agent, who “owns” the COA, will receive and decapsulate the datagram—that is, remove the correspondent’s original datagram from within the larger encapsulating datagram and forward the original datagram to the mobile node. Since the mobile node knows the correspondent’s address, there is no need to route the datagram back through the home agent.
query the home agent, assuming that the mobile node has an up-to-date value for its COA registered with its home agent. It is also possible for the correspondent itself to perform the function of the correspondent agent, just as a mobile node could perform the function of the foreign agent. (step 1 and step 2 of below figure). The correspondent agent then tunnels datagrams directly to the mobile node’s COA, in a manner analogous to the tunneling performed by the home agent, (steps 3 and 4 )
While direct routing overcomes the triangle routing problem, it introduces two important additional challenges: A mobile-user location protocol is needed for the correspondent agent to query the home agent to obtain the mobile node’s COA Suppose data is currently being forwarded to the mobile node in the foreign network where the mobile node was located when the session first started (Step 1).We’ll identify the foreign agent in that foreign network where the mobile node was first found as the anchor foreign
agent. When the mobile node moves to a new foreign network (step 2), the mobile node registers with the new foreign agent (step 3), and the new foreign agent provides the anchor foreign agent with the mobile node’s new COA (step 4). When the anchor foreign agent receives an encapsulated datagram for a departed mobile node, it can then re-encapsulate the datagram and forward it to the mobile node (step 5) using the new COA. If the mobile node later moves yet again to a new foreign network, the foreign agent in that new visited network would then contact the anchor foreign agent in order to set up forwarding to this new foreign network.
Mobile IP
Mobile IP is a flexible standard, supporting many different modes of operation (for example, operation with or without a foreign agent), multiple ways for agents and mobile nodes to discover each other, use of single or multiple COAs, and multiple forms of encapsulation. The mobile IP standard consists of three main pieces: Agent discovery: Mobile IP defines the protocols used by a home or foreign agent to advertise its services to mobile nodes, and protocols for mobile nodes to solicit the services of a foreign or home agent.
Important fields in agent advertisement are: Home agent bit (H): Indicates that the agent is a home agent for the network in which it resides. Foreign agent bit (F): Indicates that the agent is a foreign agent for the network in which it resides. Registration required bit (R): Indicates that a mobile user in this network must register with a foreign agent. M, G encapsulation bits: Indicate whether a form of encapsulation will be used. Care-of address (COA) fields: A list of one or more care-of addresses provided by the foreign agent.
Registration with the Home Agent
Once a mobile IP node has received a COA, that address must be registered with the home agent. This involves following four steps. Step 1: Following the receipt of a foreign agent advertisement, a mobile node sends a mobile IP registration message to the foreign agent. The registration message is carried within a UDP datagram and sent to port 434. The registration message carries a COA advertised by the foreign agent, the address of the home agent (HA), the permanent address of the mobile node (MA), the requested lifetime of the registration, and a 64-bit registration identification. The requested registration lifetime is the number of seconds that the registration is to be valid. If the registration is not renewed at the home agent within the specified lifetime, the registration will become invalid. Step 2: The foreign agent receives the registration message and records the mobile node’s permanent IP address. The foreign agent then sends a mobile IP registration message to home agent. Step 3: The home agent receives the registration request and checks for authenticity and correctness.
The home agent binds the mobile node’s permanent IP address with the COA; in the future, datagrams arriving at the home agent and addressed to the mobile node will now be encapsulated and tunneled to the COA. The home agent sends a mobile IP registration reply containing the HA, MA, actual registration lifetime, and the registration identification of the request that is being satisfied with this reply. Step 4: The foreign agent receives the registration reply and then forwards it to the mobile node.
- The call is completed, being routed from the correspondent to the home MSC, and from there to the visited MSC, and from there to the base station serving the mobile user.
Handoffs in GSM
A handoff occurs when a mobile station changes its association from one base station to another during a call. There may be several reasons for handoff to occur, including
- The signal between the current base station and the mobile may have deteriorated to such an extent that the call is in danger of being dropped
- A cell may have become overloaded, handling a large number of calls.
Steps:
- The old base station (BS) informs the visited MSC that a handoff is to be performed and the BS (or possible set of BSs) to which the mobile is to be handed off.
- The visited MSC initiates path setup to the new BS, allocating the resources needed to carry the rerouted call, and signaling the new BS that a handoff is about to occur.
- The new BS allocates and activates a radio channel for use by the mobile.
- The new BS signals back to the visited MSC and the old BS that the visited-MSC-to-new-BS path has been established and that the mobile should be informed of the impending handoff. The new BS provides all of the information that the mobile will need to associate with the new BS.
- The mobile is informed that it should perform a handoff. Note that up until this point, the mobile has been blissfully unaware that the network has been laying the groundwork (e.g., allocating a channel in the new BS and allocating a path from the visited MSC to the new BS) for a handoff.
- The mobile and the new BS exchange one or more messages to fully activate the new channel in the new BS.
- The mobile sends a handoff complete message to the new BS, which is forwarded up to the visited MSC. The visited MSC then reroutes the ongoing call to the mobile via the new BS.
- The resources allocated along the path to the old BS are then released.
Inter-MSC handoff The anchor MSC is the MSC visited by the mobile when a call first begins; the anchor MSC thus remains unchanged during the call. Throughout the call’s duration and regardless of the number of inter-MSC transfers performed by the mobile, the call is routed from the home MSC to the anchor MSC, and then from the anchor MSC to the visited MSC where the mobile is currently located. When a mobile moves from the coverage area of one MSC to another, the ongoing call is rerouted from the anchor MSC to the new visited MSC containing the new base station. Thus, at all times there are at most three MSCs (the home MSC, the anchor MSC, and the visited MSC) between the correspondent and the mobile.
received intact; the sender is unaware of whether the segment was lost due to congestion, during handoff, or due to detected bit errors. In all cases, the sender’s response is the same— to retransmit the segment. Bit errors are much more common in wireless networks than in wired networks. Given high bit error rates on wireless links and the possibility of handoff loss, TCP’s congestion-control response could be problematic in a wireless setting. Three broad classes of approaches are possible for dealing with this problem:
- Local recovery: Local recovery protocols recover from bit errors when and where they occur.
- TCP sender awareness of wireless links: In the local recovery approaches, the TCP sender is blissfully unaware that its segments are traversing a wireless link. An alternative approach is for the TCP sender and receiver to be aware of the existence of a wireless link, to distinguish between congestive losses occurring in the wired network and corruption/loss occurring at the wireless link, and to invoke congestion control only in response to congestive wired-network losses.
- Split-connection approaches: In a split-connection approach the end-to-end connection between the mobile user and the other end point is broken into two transport-layer connections: one from the mobile host to the wireless access point, and one from the wireless access point to the other communication end point. The end-to-end connection is thus formed by the concatenation of a wireless part and a wired part. Wireless links often have relatively low bandwidth. As a result, applications that operate over wireless links, particularly over cellular wireless links, must treat bandwidth as a scarce commodity.
