计算机英语本科毕业论文Controllable multicast for IPTV over EPON
Abstract The character of 1 Gbps bandwidth and treebased structure make Ethernet passive optical network (EPON) very suitable for broadcast or multicast services such as Internet protocol television video (IPTV). The document proposed a novel scheme, based on the former research for controllable multicast over EPON system, mainly considering system security and maintainability. It can both control the IPTV program source’s and the receiver’s validity, improving the efficiency and precision.
Keywords Ethernet passive optical network (EPON),Internet protocol television video (IPTV), Internet group management protocol (IGMP), controllable multicast,operation, administration and maintenance (OAM), fiber to the home (FTTH)
1 Introduction
Ethernet passive optical network (EPON) is a technology viewed by many as an attractive solution to access networks. All transmissions in a passive optical network (PON) are performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT resides in the local exchange (central office), connecting the optical access network to the metro backbone. The ONU is located at either the curb (fiber to the curb, FTTC) or the end-user location (fiber to the home, FTTH; fiber to the building, FTTB) and provides broadband voice, data, and video services.
In the downstream direction (Fig. 1) [1], an EPON is a point-to-multipoint network in which OLT broadcasts data to every ONU through a downstream channel. In the upstream direction (Fig. 2) [1], an EPON is a multipoint-to-point network in which multiple ONUs transmit time-shared data to the OLT through the passive combiner within a common upstream channel.
Fig. 1 Downstream traffic in EPON
Fig. 2 Upstream traffic in EPON
The advantages of using PONs in subscriber access networks are numerous [1]:
a) PONs allow for long reach between central offices and customer premises, operating at distances over 20 km.
b) PONs minimize fiber deployment in both the local exchange office and the local loop.
c) PONs provide higher bandwidth due to deeper fiber penetration, offering gigabit per second solutions.
d) Operating in the downstream as a broadcast network,PONs allow for video broadcasting as either Internet protocol (IP) video or analog video using a separate wavelength overlay.
e) PONs eliminate the necessity to install active multiplexers at splitting locations, thus relieving network operators of the gruesome task of maintaining active curbside units and providing power to them. Instead of active devices in these locations, PONs use small passive optical splitters, located in splice trays and deployed as part of the optical fiber cable plant.
f) Being optically transparent end to end, PONs allow upgrades to higher bit rates or additional wavelengths.
Figure 3 shows the multicast topology of EPON system. Multicast source [Internet protocol television video (IPTV) server] can send traffic to aggregation node (multicast routers) through IP networks on multicast route protocols [e.g., Internet group management protocol (IGMP)/proto-col independent multicast (PIM)]. Since the EPON is layer-2-based devices, it cannot support IP multicast directly. Most EPON systems adopt IGMP proxy/snooping to support IP multicast. Generally, OLT acts as IGMP proxy device, and ONUs realize IGMP snooping. IGMP proxy/snooping is applied in downstream multicast forwarding but does not concern the security and operation. The multicast router adds specific virtual local access network (VLAN) tag to the multicast flow and then forwards it to the OLT. OLT forwards the flow to ONUs according to the group information established by IGMP join report message. ONU decides to send multicast flows to the receiver by its group membership information, which is collected from set top box (STB) connecting to its user network interface (UNI).
Fig. 3 Topology of EPON
EPON multicast architecture makes it more susceptible. Since OLT and ONUs just replicate multicast flows, they cannot assure the security of IPTV program sources, and the user’s authority of certain channel is not controlled.
In this paper, we focus on security mechanisms that should be available to groups that specifically request it.
We concentrate on two issues: how unauthorized multicast traffic can be prevented and how a group’s susceptibility is likely to be reduced by means of multicast group access control.
1.1 Internet group management protocol (IGMP)
Here, we use IGMPv2 as the main protocol for multicast data forwarding. IGMPv2 is defined by IETF RFC 2236 [2]. The packet format for IGMPv2 is shown in Fig. 4. It is a simple packet format containing the type of packet, a maximum response time (MRT), checksum, and group destination address.
Bit0 7 15 31
Typc MRT Checksum
Group destination address
Fig. 4 IGMPv2 packet format
There are three types of packets for IGMPv2 [3]:
a) membership query, sent by the multicast router to determine if any hosts are listening to a group;
b) membership reports, used by a host to join a group or to respond to membership queries;
c) leave group messages, used by a host to explicitly leave a multicast group.
When an IGMPv2 host needs to join a group (such as the STB wishing to view a new channel), it will send an unsolicited membership report destined to the group it wishes to join. The multicast router will receive the membership report, and if the group is not already being forwarded out that interface, the router will place the IGMP host address in the outgoing interface list for the multicast group and begin the forwarding process out the host’s respective interface./
When an IGMPv2 host leaves a group (such as the STB is no longer watching a channel), it will send out a leave group message to the 224.0.0.2 multicast router address listing the group to leave in the group address field. The router will immediately respond on the logical interface with a group-specific query to the all-hosts 224.0.0.1 multicast address to determine if any hosts are still wishing to receive this specific multicast group query. If no response is received within the MRT interval, the router will no longer forward that multicast group out that specific interface. Some router implementations also support an “immediate leave” mechanism to bypass the group-specific query and immediately remove an interface from an outgoing interface list. Immediate leave overrides the normal checks to see if there are other hosts or proxy devices on the local segment interested in the multicast group. Therefore, it is recommended that immediate leave can only be utilized when only a single host or proxy device is on that interface to ensure that no undesirable group interruptions occur.
The general membership query is used to issue periodic messages to the all-hosts 224.0.0.1 multicast address to determine which groups are currently being used by IGMPv2 hosts. The general membership query is typically used in IPTV to recover from error conditions such as a STB being powered off and not being able to send a leave group message or leave group messages being dropped by the access elements.
计算机本科毕业论文原文出处：Controllable multicast for IPTV over EPON